Nondestructive testing and structural health monitoring of...

Marine Applications of Advanced Fibre-Reinforced Composites. http://dx.doi.org/10.1016/B978-1-78242-250-1.00007-7Copyright © Commonwealth of Australia 2016

7Nondestructive testing and structural health monitoring of marine composite structuresM.E. IbrahimMaritime Division, Defence Science and Technology Organisation, Melbourne, VIC, Australia

7.1 Introduction

All structural materials contain imperfections, and the greater their complexity, the more likely flaws will be introduced during manufacture. The sophisticated nature of composite materials employed in the marine environment increases the likelihood of the introduction of flaws within these structures. These can arise both during manu-facture and in-service, and range from human error in design or preparation, to the introduction of anomalies by high temperature and pressure cure of polymer matrices, or damage sustained in the operating environment.

In a component containing multiple internal bondlines and interfaces, a simple vi-sual inspection of the outer surface says much less about the state of the internal struc-ture than it would in a single-phase metallic alloy. This is particularly true for marine composites, which can be hundreds of millimetres in thickness (Mouritz et al., 2000).

Various material tests may be performed on a component to determine its mechan-ical properties, though these inevitably result in the failure of that component, and it must be discarded. Also, such a test is specific to the particular exemplar component, and not necessarily indicative of the properties of a similar composite component to be actually employed on a marine vessel or structure. To understand the likely distri-bution of flaws that will occur during the production of components, a large number must be fabricated and tested, to compare with the intended mechanical properties developed using finite-element codes.

A means of assuring that a component will perform to specification (i.e. that it contains no flaws above a certain size) is to inspect it nondestructively. Nondestructive testing (NDT) is a maturing technology field comprising a variety of mainly physics-based techniques that are used to detect or characterise defects in engineering structures. NDT techniques employ mechanical, chemical and electromagnetic en-ergies to introduce a disturbance into the structure and measure the response, on the basis that an internal anomaly will effect a change in the returned signal.

The original NDT techniques, of unassisted visual and aural (acoustic) inspection have been performed by people for centuries. These graduated to advanced methods of lighting to aid visual inspections, and tap-hammers to detect more subtle acous-tic (and hence localised stiffness) changes. The five traditional NDT techniques developed mainly for the steel industry were radiography, dye penetrant testing,

148 Marine Applications of Advanced Fibre-Reinforced Composites

eddy-current testing, ultrasonics and magnetic particle testing. Additionally, many inspection techniques that fall within the purview of NDT have been developed, mostly during the twentieth century, and those that are suited to the detection and characterisation of defects in marine composite structures are explained in some detail in this chapter.

7.2 Defects occurring in marine composites

In order to design an NDT technique for the inspection of a composite material, we must first understand the test component, and the way in which defects will arise and affect the performance of the overall structure.

7.2.1 Manufacturing defects

Defects that occur in production, where detected, are expected to be repaired or re-placed, if they are above a particular size or severity threshold. The most common production defects are outlined in this section.

The laying up of fibres is a delicate process, especially when done by hand. Unevenly distributed or bunched fibres lead to resin-rich pockets, resulting in non- uniformity of local mechanical properties (Cantwell and Morton, 1992). Shrinkage in the polymer matrix, and in-plane fibre waviness and out-of-plane fibre wrinkling, result from an uneven cure profile (Anastasi, 2008).

Large voids arise from poor resin infusion between woven fibre cloth, and increase rapidly in size and number with composite thickness. They are also created by the entrap-ment of heated air during an autoclave cure. In thicker structures, a uniform cross-sectional cure profile is very difficult to achieve, resulting in relative over-curing of the polymer matrix at the surface and under-curing at the centre of the structure. In other cases, the reverse effect can occur in a matrix that undergoes an exothermic cure, which may give rise to vastly increased temperatures and thus over-cure at the centre of the component.

Other errors result from human factors, such as missing plies, ply damage during cutting and misorientation of plies. In a composite, these errors do not simply alter the total thickness of the part, but affect the symmetry of the layup, and can result in an uneven stress distribution and warping.

The mechanical processes of drilling and bolting cured panels to other structure result in localised crushing and tearing, often leading to edge delaminations that can grow within the panel during service (Saunders et al., 1993). Finally, in sandwich structures, where a relatively thin composite laminate skin is adhered to a thicker foam, timber or honeycomb core, poor adhesive bond strength is a potential problem.

7.2.2 In-service defects

Once production-stage defects have been detected and rectified, the through-life sup-port of composite structures is concerned with in-service damage. This arises from many different events, such as impacts from collision with another vessel or a fixed

Nondestructive testing and structural health monitoring 149

structure, or a simple maintenance accident like the dropping of a tool. It also occurs via the gradual effects of the operating environment, and can thus be separated into discrete (event-based) and non-discrete (global, gradual or distributed) damage.

Discrete damage often occurs via surface impacts that cause delamination and transfer of the impact energy into the separation of a number of subsequent plies, ma-trix cracking, fibre breakage, micro-buckling and fibre waviness (Haque and Hossain, 2003; Kendall and Doud, 2011). The high stiffness of the structure often causes the upper ply to return to its initial position, thus giving the deceptive appearance of being intact, while severe delamination has occurred beneath, in what is termed ‘ barely-visible impact damage (BVID)’. In unpainted glass-fibre-reinforced plastic (GFRP) laminates, the damage progression can be seen by the naked eye (Figure 7.1), though for painted components (more or less always in marine applications), and in carbon-fibre-reinforced plastic (CFRP) composites the damage can be rendered com-pletely invisible. In sandwich structures, this impact behaviour will typically be ac-companied by skin-to-core adhesive disbonding.

Non-discrete damage resulting from the operating environment of in-service com-posite structures includes ingress of moisture and various fluid contaminants, and incipient heat damage at the molecular level from repeated thermal cycling and prox-imity to heated components. The most damaging of these is the combination of mois-ture uptake at elevated temperature (Sala, 2000). Moisture ingress causes a significant reduction in the strength of adhesive bonds, resulting in bondlines that are still in intimate contact, but lack the adhesive power they had at initial manufacture. These are referred to as ‘kissing’ or ‘zero-strength’ bonds.

7.2.3 Size and scale effects

The initiation and growth of defects in thick composite structures is not well understood, compared to thin laminates and metallic alloys. In particular, it has been found that re-sults obtained in the mechanical testing of thin laboratory specimens are not necessarily

(a) (b)

Figure 7.1 A woven glass-fibre-reinforced plastic laminate that has suffered delamination during a water-jet cutting during fabrication. (a) Top side, showing no visible sign of damage and (b) rear side, showing circular delamination at two levels within in the panel. In the unpainted state, the defective region is visible to the naked eye.


transferable to thick structures by simply applying a ‘scaling factor’ to account for the thickness. Rather, unexpected damage modes can occur in otherwise identical thicker structures (Green et al., 2004; Yu and Büyüköztürk, 2008; Wisnom et al., 2010).

A ‘size effect’ will also potentially result in defects filling out a larger volume, due purely to the availability of unbounded material, though whether this actually occurs remains a point of contention (Sutherland et al., 1999). A more certain occurrence is the partial-thickness defect, which may result in damage hidden under tens of milli-metres of material (Belingardi et al., 2008).

7.3 Selection of NDT techniques for marine composites

7.3.1 Inspection challenges

The NDT of marine composites poses unique challenges to the reliable detection and characterisation of defects:

i. The materials used are heavily skewed towards glass-based polymer-matrix composites (PMCs), which are non-conductive, and highly attenuative to some established NDT energies.

ii. The structures to be inspected may be very thick, or of large lateral dimensions. This may render an inspection method impractical, or unreliable, and in thick sections, high stiffness and attenuation can completely preclude the use of some NDT technologies.

iii. There is limited data on reliability of both the composite structures and on the NDT tech-niques designed to inspect them, compared to the vast body of work that has been per-formed for aerospace applications. Miller (2000) indicated that the sizeable safety factors of 2.5–4.0 applied to the fabrication of marine composites by the U.S. Navy are the result of a lack of reliability data on composite structures.

iv. The high level of exposure to moisture increases the likelihood of fluid uptake and reten-tion, as surface coatings are penetrated. The presence of moisture and salt ions will affect the results of measurements made using a number of NDT techniques.

7.3.2 Inspection reliability

A successful NDT inspection requires substantial planning and thought, prior to the arrival of an inspector to ‘push a probe’ on the test article. In many cases, the absolute resolution of a particular NDT technique for a specific inspection is not known, and, in practice, the actual resolution will likely be diminished by a range of factors, such as access practicality, non-ideal setup by the operator, difficulty in coupling the energy to the component, and others (Greene, 2012).

Figure 7.2 shows a damage tolerance curve, which gives the typical growth of a defect in an in-service engineering structure. During routine nondestructive inspec-tion, the defect is likely to be missed until it reaches a sufficient size, usually referred to as ‘a

NDI’.a In practice, an inspection is performed, and the strength of a component

is based on the assumption that no defect larger than aNDI

will be present within the

a The acronym ‘NDI’ is equivalent to ‘NDT’, substituting ‘inspection’ for ‘testing’. It is the more commonly used term in the aerospace industry, in which reliability and probability-of-detection studies were developed.


structure, at the point of the inspection t1 in its lifetime. If the growth characteristics

of the life-limiting defect in the component are known, the time period between t1 and

tcrit

, the time of failure, can be determined.If a large defect is missed at the initial inspection at t

1, the likelihood that the com-

ponent will fail early is greatly increased. The authority responsible for the seawor-thiness of a vessel therefore relies on the NDT test to detect these defects with a high degree of confidence. The most rigorous way to establish confidence in an inspection method is to perform an NDT reliability study, in which the probability-of-detection (POD) for a set of inspection parameters can be obtained. Figure 7.3 shows a POD

Operating time

0 t1

aNDI

a0

t2 tcrit.

acrit

Def

ect s

ize

Figure 7.2 Damage tolerance curve, showing the increasing rate of growth of a defect of size a, and nondestructive detection period from t

1 to t

crit.

Defect size

Pro

babi

lity

of d

etec

tion

0 aNDI

0.0

0.5

1.0

Figure 7.3 Probability-of-detection curve produced from hit-miss NDT data, showing derivation of a

NDI at a

90/95. The light grey line intersects the 95% statistical confidence limit at POD = 0.9.


curve, where aNDI

is identified with the parameter a90/95

, the defect size for which the POD = 0.9, demonstrated with a 95% statistical confidence (dashed line).

7.3.2.1 Acquiring NDT reliability data

The minimum reliably detectable defect size aNDI

, and the form of the POD curve, are determined via a statistical data-gathering process. A simulated defect at a known location in a well-understood calibration standard using laboratory equipment is much more readily detected than the same size defect at an unknown location on a large surface, where the inspector must perform the inspection in an awkward position. Thus, an accurate measurement of an a

NDI requires many dozens of ‘blind’ inspections

(where the inspector does not know the location or true size of the defect, or even whether one is present) using realistic defects and equipment, and qualified inspectors, to obtain an accurate statistical spread of data.

POD trials tend to be lengthy and expensive processes, and are thus normally re-served for fracture-critical components on military platforms that are being managed on a safety-by-inspection philosophy.

Understanding the purpose of inspection reliability is key to the correct appli-cation of POD data. A few important points that are commonly misunderstood or misinterpreted by both researchers and practitioners in NDT and related fields are as follows:

i. A POD curve is not transferable. The results of an NDT reliability trial are specific to the inspection, flaws, specimens, inspection conditions and NDT equipment used. The only generalisation made by such a study is that the range of inspectors used encom-passes the typical range of human factors that are likely to occur in future inspections. Hence, a single curve cannot be presented as a ‘POD of ultrasonic inspection in marine composites’, as thousands of potential curves can be generated that would fulfil this statement;

ii. The POD of a specific inspection can vary if applied using different means. For example, the mechanical raster scanning of a small probe across a large area significantly improves the reliability compared to an inspector positioning an ultrasonic probe manually at many hundreds of locations, and reading a voltage versus time trace, despite inspecting the same component at the same frequency; and

iii. The value of aNDI

is normally chosen to be a90/95

. While it may be tempting for the structural engineer to translate this value to a 100% guarantee that no flaws above this size will be present in the structure, this is not the case, as, according to the statistics, 90% of defects of size a

NDI will be detected by that NDT technique.

As a result, an NDT technique should not be selected on the basis of the smallest defect it can detect, but the largest one it might miss, as this larger defect will be the life-limiting factor for the component. There are no published reliability results for specific marine or thick-section composites, and the size effects apparent in thick sections make estimating the resolution targets a non-trivial exercise. The author has suggested elsewhere that a technique able to inspect a thick marine composite should be able to at least resolve a 5% anomaly in the through-thickness direction of the struc-ture (Ibrahim, 2014b) with high reliability.


7.3.3 Testing methodologies

There are three major approaches that may be selected for the detection and character-isation of defects, depending on the type of structure being inspected, ease of access and the area to be covered by the inspection.

i. Traditional ‘point inspection’ or ‘local-area’ NDT, which assesses a small area of a test compo-nent. When coupled to an encoded scanning system, the probe or inspection device may be ras-ter scanned across the surface so that a 2D image (‘C-scan’) of the probe response at each point can be built up. This permits a more intuitive and comprehensive presentation of the results;

ii. ‘Wide-area’ or ‘full-field’ techniques cover greater areas (e.g. 400 mm × 400 mm or higher) in a single measurement, allowing rapid coverage of large components. A number of more modern NDT techniques fall into this category. The ability for rapid inspection tends to come with a trade-off in sensitivity or penetration, though, coupled with a point-inspection technique, can form a useful part of a two-stage NDT process;

iii. Structural health monitoring (SHM) in which a large number of sensors are permanently mounted to the test structure, in order to monitor pointers to failure (e.g. surface-strain lev-els), or to detect the gradual accumulation of damage (Mouritz, 2003).

The NDT techniques of interest to the inspection of fibre-reinforced marine composites are presented in Section 7.4, and applicable SHM techniques are discussed in Section 7.5.

7.4 Overview of NDT techniques

This section discusses the application of advanced NDT techniques to the inspection of marine composites. Of the original techniques discussed in Section 7.1, only ultra-sonics and radiography have any significant application to marine composites. Where high-conductivity composites, such as high-density CFRP are used, eddy-current testing and electromagnetic techniques can also be applied. A demonstration has been given of the application of fluorescent penetrant to the detection of substantial weathering at the surface of GFRP, in which the dye is absorbed rapidly at a porous matrix and gives a large reflection to the UV light source compared to a newly fabricated panel (Bar-Cohen and Crane, 1980). Other than this, no other work of report has been performed using the NDT techniques typically targeted at surface-breaking crack detection in metal.

7.4.1 Vibration analysis

Vibrational techniques acquire a global acoustic response for a structure over a broad frequency spectrum, and repeat measurements at specified intervals, or in conjunction with dynamic analyses of the entire structure, where the characteristic response will change over time due to changes in structural stiffness (Cawley and Adams, 1979). These techniques are performed at relatively low elastic-wave frequencies (<20 kHz).

The inherent complexity of composite structures means they exhibit far more com-plicated resonance modes than their metallic counterparts. Thus, elastic modelling is not currently able to be applied to the determination of the expected response of non-simplistic geometries (Chandra et al., 1999; Gagneja et al., 2001). Figure 7.4a


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(b)

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pons

e (d

B) –20

0

20

40

100 200 300 400 500Frequency (Hz)

600 700 800

Healthy

Delam

Crush

900 1000

Data acqusition system

PVDF sensorImpulsehammer

Figure 7.4 (a) Changes to the vibration response of a GFRP skin/cellulite honeycomb paper core sandwich composite, showing the healthy, delaminated and core-crush states. (b) The GFRP composite undergoing testing.Reproduced from Lestari and Qiao (2005) with permission.


shows the change in the vibration response of a healthy GFRP-skinned cellular-paper honeycomb core panel of 127 mm thickness (Figure 7.4b), after it sustains a delamina-tion and crushed honeycomb core (Lestari and Qiao, 2005).

Localised through-thickness vibration measurements can be performed using low- frequency ‘bond-testing’ techniques, which produce localised standing waves through the thickness of the structure. These methods typically work best for thin-skinned sand-wich structures, but struggle to detect voids or delaminations in monolithic structures of any significant thickness. ‘Resonance’ testing is low-frequency resonant ultrasound performed at 20–300 kHz, using a single send–receive piezoelectric transducer.

Another test is mechanical impedance analysis, in which a metallic probe tip loads the structure at a particular audible frequency in the range 2–10 kHz, and receiving elements sense variations in the amplitude and phase of the probe. This test achieves slightly deeper penetration than the resonance technique. Pitch–catch techniques use two dry-coupled transducer contacts at a fixed distance, and transmit and receive frequencies in the range 10–100 kHz across the surface. Changes in local stiffness between the probe, caused notably by disbonding, will cause a change in energy ab-sorption, affecting the received signal.

Figure 7.5 shows the results of applying a swept-frequency pitch–catch bond-tester to the delaminated GFRP panel shown in Figure 7.1. From the reverse side, where no damage is visible, the technique is unable to satisfactorily detect the change in stiff-ness. However, from the delaminated side, a clear loss of energy occurs, providing a high-amplitude response.

Laser vibrometry, in which the natural resonance modes are measured excited by a shaker at kHz frequencies, and measured using a laser, is routinely performed in 2D for out-of-plane displacement patterns on vibrating structures, and, with greater system complexity, in 3D for in-plane displacement measurements (Castellini et al., 2006). This has been used to demonstrate gradual changes in local surface stiffness over time, though measurements are somewhat painstaking.

(a) (b) (c)X X

Y Y Y

X

Figure 7.5 Swept-frequency pitch-catch inspection at 20–40 kHz of the delaminated GFRP panel shown in Figure 7.1. (a) Probe placed over sound region of the panel, (b) inspection over the most severely delaminated section of the panel from the blind side, showing little indication and (c) clearly visible disturbance to the probe response when inspecting from the delaminated side.


7.4.2 Conventional ultrasonics

Ultrasonic testing (UT) utilises the piezoelectric effect, in which a permanently po-larised and appropriately cut crystal can be used to both generate elastic waves in structures at specific frequencies, and convert those waves into electrical energy upon return. The original transducers employed quartz crystals, which were later replaced by piezoceramics such as barium titanate and lead zirconate titanate (‘PZT’). UT re-lies on a mismatch in acoustic impedance between structures and internal defects, to detect the presence of anomalies.

In normal inspections, pulses of ultrasound are introduced into the structure at a high repetition rate, and at operating frequencies anywhere from the lower ultrasound limit of 20 kHz in timber structures (Emerson et al., 1999) to 50 MHz in high-stiffness metallic alloys. In marine composites, successful inspections are typically performed at the lower end of the frequency band, in the range 0.5–2.25 MHz.

Transducers may be arranged in a variety of geometries, though the most common configurations are the ‘through-transmission’, in which two axially aligned transduc-ers measure the degree of attenuation of the signal through the specimen, and ‘pulse-echo’, in which a single transducer both sends and receives the elastic waves. The advantage of the latter is that the time of each echo may be used to determine the depth in the structure at which a defect occurs, and thus the precise location of the damage in the through-thickness direction may be inferred.

The many internal interfaces in a composite structure scatter ultrasound, and the acoustic impedance mismatch between glass fibres and polymer matrices, in par-ticular, is high. Therefore, glass is typically highly reflective to ultrasound, though not impregnable to the trained inspector. Figure 7.6 shows the losses occurring in the inspection of a thick GFRP composite when using elastic waves at ultrasonic frequencies.

Figure 7.7 shows the ultrasonic C-scan image developed by mechanically raster- scanning a 2.25 MHz ultrasonic transducer over an approximately 200 mm × 200 mm area of the panel shown in Figure 7.1, to reveal the delamination in an easily understood format. In Figure 7.7a, the inspection was performed using a through- transmission arrangement, indicating that no sound could travel through the delaminated region. Figure 7.7b shows the results of a pulse-echo inspection, which shows the two levels of delamination clearly.

When used to its potential, UT is a truly quantitative NDT technique that can be used to determine elastic constants, and thus characterise the anisotropy of a material (Kinra and Ganpatye, 2005). It can also be used to detect a range of structural features in composites, including ply-by-ply fibre orientation and layup stacking sequence (Smith and Clarke, 1994; Hsu et al., 2002; Smith et al., 2009). A particular challenge, and the focus of several decades of research, has been the effort to characterise weak or zero-strength adhesive bonds using waves that travel along thin plate-like structures (‘Lamb’ waves). While demonstrated under laboratory conditions, this inspection has proved most challenging experimentally, and is still limited to the study of thin plates with parallel sides (Bar-Cohen, 2001; Yang et al., 2001; Ren and Lissenden, 2013; Vijaya Kumar et al., 2013).


Frequency (MHz)

–200

Diffraction loss

Usabledynamic range

at 2 MHz

Attenuation loss

Transducer loss

Total

–150

Loss

(dB

)

–100

–50

0

0.10.01 1 10

Figure 7.6 Ultrasonic inspection losses occurring due to diffraction, attenuation and internal to the transducer.Adapted from Fortunko and Fitting (1991).

(a) (b)

Figure 7.7 (a) An ultrasonic C-scan image of a 2.25 MHz scan across the surface of the delaminated specimen shown in Figure 7.1. A clear loss of signal at the delamination site occurs. (b) A C-scan image generated using a pulse-echo inspection from one side of the panel, showing the increased reflection caused by the high acoustic impedance air gap inside the delamination.


An ultrasonic inspection is currently the NDT technique of choice to ensure bon-dline integrity in bonded composite repairs consisting of hybrid CFRP/GFRP layers that are applied to metallic ship decks to add strength or restrict crack growth in the al-loy plate (Grabovac and Whittaker, 2009). It has also been successfully demonstrated for the detection of disbonding in the thick rubber acoustic cladding tiles that are bonded onto the exterior of submarines (Brown and Green, 1997).

7.4.3 Advanced ultrasonics

A major criticism of UT has been the severe bottlenecks it can create, both in manu-facturing facilities, and in costly maintenance downtime for in-service components. A range of modifications have been developed to overcome this problem, mainly by introducing ultrasound into the structure using alternative means, as outlined below.

7.4.3.1 Air-coupled UT

Consistent mechanical coupling between the transducer and test component is crucial in ultrasonic inspection. This is usually achieved via water coupling, either by im-mersing the transducers and test part in a water tank, or by a water-squirter arrange-ment. Localised (point) inspections are achieved using a water-based gel or grease. Achieving and maintaining constant coupling via these means adds considerable time to the successful completion of a UT inspection.

Air-coupled UT is performed without the need for acoustic couplants, greatly im-proving inspection efficiency. Overcoming the high impedance mismatch that occurs at the transducer/air and subsequent air/component interfaces is not trivial, and re-quires several modifications. For applications replicating water-coupled UT inspec-tions, this is achieved largely by

i. The use of modified piezoelectric transducers that accept greater input potential; ii. Performing all measurements in through-transmission (send–receive) mode, necessitating

access to both sides of the structure under test; and iii. Restricting measurements to the lowest portion of the ultrasonic band (typically ≤500 kHz),

to allow sufficient penetration.

In marine composites of substantial thickness (of the order of 50 mm), air- coupled inspections are particularly suited to the detection of disbonds and voids in low- density foam-sandwich structures (Borum, 2006).

7.4.3.2 Laser-based ultrasound

A novel method of producing elastic waves of ultrasonic wavelength is via the use of high-frequency pulsed lasers, usually the solid-state neodymium-doped yttrium aluminium garnet (‘Nd:YAG’), which taps the surface at megahertz frequencies. A detailed overview of the method is given by Scruby and Drain (1990) and Monchalin (2004). While the ability to perform a non-contact and stand-off inspection is highly attractive, in composite materials it must be delicately applied, as the ultrasonic waves are produced thermo-elastically, that is, via laser ablation, or minute localised melting


at the surface. Too much power can result in overheating of the polymer matrix, caus-ing macroscopic surface damage.

The ultrasound detection function can also be performed using lasers, and a com-mon method has been to use a heterodyne Michelson interferometer, one arrangement of which is shown in Figure 7.8. Robotic laser UT has become an established NDT technique for the inspection of aerospace composites, and inspections for sandwich skin-to-core debonding in particular should translate well to the sphere of marine com-posites testing.

7.4.3.3 Phased-array UT

Phased arrays have revolutionised UT, by forming an ultrasonic beam using 64 or 128 small piezoelectric elements, allowing an almost unlimited combination of arrays (Drinkwater and Wilcox, 2006), and improving inspection times by an order of mag-nitude, as well as improving resolving power using innovative algorithms (Simonetti, 2006). Larger arrays have been designed, for example, by Frankle and Rose (1995), which allow areas as large as 200 mm × 200 mm to be inspected in a single shot, in the frequency range 0.4–1.0 MHz, and in marine composites of the order of 25 mm thick-ness. An image of this array is shown in Figure 7.9.

7.4.4 Microwave-band inspection

Microwave-band NDT employs variations in complex electric permittivity, ε, be-tween test media in order to detect flaws. The total usable bandwidth is very broad (300 MHz to 300 GHz), and, where the source is sufficiently flexible, an inspection can be frequency-optimised to provide the penetration-resolution balance desired. Permittivity has historically been referred to as the ‘dielectric constant’, though this is a misleading term as it in fact varies with frequency, and hence this terminology is

Optical detector

Interference filterVariable attenuator

Beam splitter

Oscillator

Laser

M5

L2L1

M3

M4

M2

M1

Acousto-opticcell

Ultrasoundemission

Figure 7.8 Laser-based ultrasonic detection using a heterodyne Michelson interferometer detection method.Adapted from Monchalin (1986).


being gradually phased out. Figure 7.10 shows the frequency dependence of the real and complex components of ε for water, in the range 2–70 GHz.

The intensity of the electric field at a distance z from the source decays according to the function E i z

0e- +( )a b , where E0 is the maximum intensity value at the source,

and α and β are the absorption (real) and phase (imaginary) constants for the medium,

respectively. The loss coefficient α is related to ε by the relation ap

le= ( )2

0

Im r (Zoughi and Ganchev, 1995).

Fowler and Hatch (1966) were among the first to perform microwave NDT for the detection of voids in GFRP, and were able to detect a 3.2 mm diameter hole in a 6.4-mm-thick panel. Subsequently, Ganchev et al. (1994) detected thin aluminium squares embedded 12.7 mm below the surface of a polyester matrix GFRP composite at 10.5 and 24 GHz, with a high degree of accuracy. Zoughi and Ganchev (1995) also demonstrated the detectability of flat-bottomed holes up to 25.5 mm from the surface.

Good electrical conductors reflect microwave radiation almost perfectly, and the technique therefore has limited applications to the inspection of CFRP components. However, the majority of marine composites can be inspected using microwaves. A major challenge in the inspection of in-service structures is determining the dif-ference in transmission due to the presence of water, which will cause differences between the component and a reference standard, and make absolute quantitative mea-surements more or less impossible.

Figure 7.11 shows the results of a microwave inspection of a GFRP block 38 mm thick, containing three aluminium foreign objects of varying size, at the mid-plane. The vastly different electrical properties of the simulated flaws compared to the composite material is clear from the C-scan, which shows an increasing reflected signal with the increased size of each inclusion. At this depth in monolithic GFRP

Figure 7.9 A large, conformable ultrasonic phased-array probe consisting of 1024 piezoelectric elements for performing wide-area inspections.Reproduced from Frankle and Rose (1995) with permission.


laminates, detectability is limited to between 6.35 and 12.7 mm in edge length, at 10.5 GHz (Zoughi et al., 1996). Marine GFRP sandwich composites have been in-spected down to a depth of 25 mm, for flat-bottomed holes (Greenawald et al., 2000), as well as for cracking in foams beneath GFRP skins, and of crushed core honeycomb (Green et al., 2004). For extremely thick GFRP as is used in some vessels such as mine-hunting ships, attempts to penetrate the entire thickness have not shown success at microwave frequencies (Mouritz and Luescher, 1997).

7.4.4.1 Ground-penetrating radar (GPR)

Microwave inspections in the bandwidth 1–3 GHz have been used within the geosci-ences for some time, where they successfully penetrate tens of metres into the earth to perform soil surveys, or for the detection of large rocks and buried infrastructure such as pipes. It is only more recently that this technique has come to light as a potential means for the inspection of composite structures hundreds of millimetres thick, many of which are found in the marine environment.

As an NDT technique, ground-penetrating radar has so far been applied to the inspection of civil structures such as bridge decks and pylons, in some cases these

20

10

20

30

400

20

40e�

(n)

e� (n

)

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Figure 7.10 Kaatze's (1989) measurements of the frequency dependence of the complex permittivity of water. The real part ε′ refers to energy stored within the medium, whereas the imaginary part ε″ refers to energy dissipation.Reproduced with permission.


consist of concrete structures reinforced with thin GFRP wraps. Inspection resolu-tion appears to be limited to 10–20 mm in depth and the in-plane directions, but this is compensated for by penetrating power of over 500 mm in concrete (Maierhofer, 2003; Hing and Halabe, 2010).

7.4.4.2 Terahertz spectroscopy

The terahertz (1012 Hz) band had been largely unexplored until the last two decades, as there was a lack of sources and detectors capable of producing and detecting the picosecond-length pulses required to achieve energy at the right frequency. At the lower end (0.1–4 THz), this technology is effectively an extension of microwave test-ing, permitting high-resolution imaging. As expected, the trade-off is a difficulty in inspecting through significant thickness, except at high power, and to-date, little work has been reported on the inspection of marine composites. At the higher (optics and

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Aluminum inclusions

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Figure 7.11 (a) A GFRP composite panel of 38 mm thickness, containing three aluminium inclusions of varying size buried at the mid-plane. (b) Microwave C-scan of the panel, showing the increased resolution of the response from the three simulated flaws.Reproduced from Ganchev et al. (1994) with permission.


photonics) end of the electromagnetic spectrum, ultrafast semiconductor lasers have been developed that have lowered normal laser frequencies into the THz band.

Terahertz-frequency NDT has been demonstrated on mainly high-porosity thick foams (Redo-Sanchez et al., 2006; White and Zimdars, 2007), through which it pro-vides good penetration and would be suited to the inspection of GFRP/foam sandwich composites. An advantage of employing electromagnetic radiation over ultrasonics is its ability to transmit through air gaps, permitting inspection beneath a near-surface delamination. This is particularly useful for thick composite sections.

Figure 7.12 shows a 12.4-mm-thick GFRP laminate in which two air gaps have been created, using a circular saw blade. In this case, the smaller-diameter cut-out is located on the far-side of the component, and the larger cut-out must be inspected through to detect it (Chiou et al., 2011). As shown in the figure, this low-frequency THz inspection is successful in identifying both of the simulated flaws, and a spatial resolution approximating that of an ultrasonic inspection.

7.4.5 Infrared thermography

Thermography is an advanced NDT technique that employs electromagnetic radiation over the infrared (IR) spectrum, lying just above the visible band, at wavelengths of approximately 700 nm to 1 mm. As the name suggests, this bandwidth is sensed by the human body as ‘warmth’, and the technique has thus found great application in the industrial detection of over-heating, in energised electrical circuits, or in pipes and connectors in petrochemical processing plants. Such measurements can be made ‘passively’, as the IR radiation is produced by the source in its routine operating state. However, insufficient heat is dissipated from the internal structure of a composite

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Figure 7.12 C-scan of saw cuts in a thick GFRP block using raster-scanned low-frequency THz. The smaller diameter cut on the left, while located beneath the larger cut, is clearly visible.Reproduced from Chiou et al. (2011) with permission.


component to allow for such measurements to be made on composite structures, and an active heating source is required, in order to perform an accurate inspection of these structures.

Active heating for thermography is achieved via quite a few methods. Most com-monly, this is via the use of heating lamps, either at the inspection surface, or on the non-inspected side to create a through-thickness heat diffusion. Other methods include heating via acoustic mechanical vibrations (‘sonic/vibro-thermography’) to give im-proved depth resolution (Cawley, 2006), or via the application of eddy-currents in elec-trically conductive components. The internal components of a composite will absorb and then radiate heat out to a sensor. Where a structural anomaly such as a void is present, the different rate at which it stores and dissipates heat from a sound region will result in a change in the intensity of the IR signal, permitting detection at the sensor.

Active heating methods may be applied as a continuous, single-frequency signal, or as a transient signal. The use of a transient signal is also referred to as pulsed or ‘flash’ thermography, and, via the use of additional signal processing, permits an estimate of the flaw depth (Rajic, 2002). Thermography is a ‘wide-area’ technique, in that it does not require to be mechanically scanned to capture an image of a large area, facilitating the rapid inspection of large surfaces. Additionally, thermal camera images are pre-sented in a highly intuitive way to the operator, which makes for easy interpretation of the results. Figure 7.13 shows a continuous-wave thermograph of an impact event in a thin GFRP laminate, and the actual visual indentation zone it is representing (Meola and Carlomagno, 2014).

The major drawback of thermography is its limited applicability to thick structures. The physical constraints of heat transfer in the majority of structures causes a rapid decrease in sensitivity in components of more than a few millimetres thickness. As a re-sult, there are very few examples of application to marine composite structures, though it has been used to inspect the state of insulation behind deck-mounted brackets (Jones and Lindgren, 1994). Uneven heating rates also create difficulty in the lateral sizing of defects, resulting in a different heat dissipation rate across the flaw, meaning that in most cases, only qualitative data can be gleaned from an inspection, and the type of de-fect, and its volumetric geometry are unable to be defined (Montanini and Freni, 2012).

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Figure 7.13 (a) Image produced by a continuous-wave thermography system of an impact in a thin GFRP laminate. (b) A photograph of the impacted region, defined by the heavy black line.Reproduced from Meola and Carlomagno (2014) with permission.


7.4.6 Optical interferometry

The application of laser light to study component surfaces has resulted in a range of NDT techniques, useful in determining local strain and stiffness in thin structures. Techniques including holography, electronic speckle-pattern interferometry (ESPI) and shearography have seen rapid development in the past few decades (Butters and Leendertz, 1971; Leendertz and Butters, 1973; Hung, 1982). Their various capa-bilities for the detection and characterisation of disbonds and delaminations in the thin CFRP laminates commonly found in thin aerospace skins have been well docu-mented (Pezzoni and Krupka, 2001; Newman, 2005; Gryzagoridis and Findeis, 2008; Heslehurst, 2011). The extremely rapid wide-area capability they provide has also been able to reduce inspection times by over an order of magnitude, producing huge cost savings over conventional production-line ultrasonics, where substitution of the methods was feasible (Gregory, 2002).

A hologram is produced by measuring the reflection of coherent light on a surface, then comparing subsequent reflections after the surface is altered in some way. A small deformation in the surface results in interference fringes, and minute anomalies in the surface displacement (corresponding to light-wavelength dimensions) may be detected. In ESPI, laser light is used, and a reference beam and measurement beam are shone on the inspection surface, with a charge-coupled device (CCD) camera used to store and process the data as a digital image, allowing for far advanced image post-processing.

Shearography operates on a similar concept to ESPI, though the reference beam is removed, and the image is sheared using a lens, so that the reference and live laser speckle patterns may be compared. In so doing, shearography takes the gradient of the displacement field, thus reporting anomalies in the strain field, rather than directly measuring surface displacement (Hung et al., 2003).

Figures 7.14 and 7.15 show the key differences in the laser measurement system for ESPI and shearography. The schematic diagrams are adapted from Kim et al. (2003) and are combined with a hologram and shearogram of three disbonds of varying geometry

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Figure 7.14 (a) Schematic diagram of an ESPI NDT system, adapted from Kim et al. (2003). (b) Holographic image captured over three disbonds located in the midline of a laminate plate (Yang and Hung, 2004).Reproduced with permission.


in a laminate plate of unspecified layup from Yang and Hung (2004). The dual intensity field becomes clear when viewing the shearogram as a positive- and negative-going peak, compared to the intensity function in the image produced by holography/ESPI.

Of the three techniques, holography is the most sensitive to physical movement, requiring the surface to remain extremely still during subsequent measurements. Thus, it is not easily performed in an industrial environment. Shearography is demonstrably the most field-portable of the techniques, as it does not require an additional reference beam, and bulk transport of the object does not create additional strains, so it is less sensitive to slight movements during testing.

The major drawback of interferometric techniques is that any internal structural anomaly must render itself visible by a change in the displacement or strain at the visible surface. Ongoing research has determined that a severe reduction in inspection resolution occurs with increasing stiffness, and thus, these techniques are often not able to detect damage in structures of any significant thickness. However, disbonding and significant core damage at the near side of thin-skinned sandwich structures is a likely target application for this group of wide-area techniques.

A related strain technique that is typically confined to the laboratory is moiré in-terferometry, in which a fine grating on the component surface is monitored as a load is applied, and compared with the unloaded reference image. A further technique that is more broadly applicable to the measurement of geometry changes is digital image correlation. This technique uses digital imaging of a surface illuminated with (usually) white light, and a post-processing algorithm tracks the movement of a random speckle-type pattern applied to, or naturally occurring on, the surface of the object.

7.4.7 Radiography and tomography

X-radiography is one of the longest established NDT techniques, having the benefit of over a century of development since the initial production of X-rays in 1895, and early application to medical imaging. Despite this, radiography has seen relatively

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Figure 7.15 (a) Schematic diagram of a digital shearography system, adapted from Kim et al. (2003). (b) Image generated showing strains at the surface, caused by the three disbonds located in the laminate plate (Yang and Hung, 2004).Reproduced with permission.


little application to the inspection of composite materials, due to multiple challenges. Firstly, the danger of ionising radiation places a significant cost impost on inspec-tion through the many safety precautions required, and secondly, the low atomic weight of the molecules on which polymer composite materials are based signifi-cantly reduces the contrast, compared to inspections of metallic alloys. Despite this, thin laminates have been inspected (Baidya et al., 2001), where larger air voids are readily detectable. In the lateral plane, delaminations are not as easily detectable, and porosity and fibre volume fraction inspections have always been considered a difficult proposition (Prakash, 1980).

Where X-radiation has proved quite successful for composite NDT is in the micro-computed tomography (μCT) of composite microstructure in post-failure analysis. This has also been extended to the penetrating and high-resolution images obtainable via the use of synchrotron radiation at X-ray wavelengths (Bull et al., 2013), although even in these cases inspection thicknesses are limited to approxi-mately 5 mm.

A medical CT scanner has also proven effective in the confirmation of bondline regularity for a thick marine composite, as demonstrated by Lambrineas et al. (1991), for composites of thickness 78 mm, with a 8- to 9-mm-thick GFRP upper and lower skin, and a 5-mm-thick slice of the bondline (Figure 7.16). However, this again re-quires the removal of a suitably sized component to fit within the CT machine's phys-ical parameters.

The main drawback of X-radiation is its danger to humans, which has made at-tempts to create portable or handheld systems difficult at best. In cases where the equipment can be transported to the inspection site, entire workshops or hangars must be cleared in order to ensure no unintended harm occurs to other personnel working in the vicinity.

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Figure 7.16 A 78-mm-thick GFRP/foam composite marine structure inspected using a medical CT scanner. (a) Schematic diagram showing 8- to 9-mm-thick GFRP skin and the two 30 mm-thick sections of foam core and (b) CT slice in the XY plane, showing a 5-mm-thick image of the foam–foam bondline at the centre of the panel.Reproduced from Lambrineas et al. (1991) with permission.


7.4.7.1 Backscattered X-ray

An advanced single-sided form of X-ray inspection has been developed, operating on the principle of Compton (inelastic) scattering, via which it is possible to effectively perform a send–receive single-sided X-ray measurement. While this technology is frequently used for large structures such as truck inspections in the transport indus-try, the same limitations of penetration in laminate composites apply as for through- transmission techniques, and inspections are generally limited to thinner structures.

However, the backscattered X-ray technique has proven useful in the detection of thin delaminations, which is normally not reliably performed by standard X-ray sys-tems (Carriveau, 1993). It has also been used to detect large voids within thin-skinned foam-core sandwich structure, although only for gross defects at any significant depth (Dugan et al., 2004).

7.4.7.2 Neutron radiography

Collimated neutron beams are usually employed for their powerful penetration, which would suggest they should be an attractive proposition for inspection of thick struc-tures. While this is true for some materials, particularly metallic alloys, neutrons exhibit a strong preferential incoherent scattering by hydrogen in preference to other elements.

In composites, the attenuation coefficient in hydrogen due to incoherent scattering is of the order of 50 times that of oxygen, carbon and nitrogen (Berger, 1971). This means that inspections through thick monolithic laminates are not easily achieved. However, this also means that neutron radiography is the ideal inspection solution for water ingress, and can be readily applied to the inspection of this type of defect in foam sandwich composites. The weak contrast between air and the strongly scat-tering polymer matrix makes the detection of voids difficult, except when inspecting the integrity of adhesive bondlines between composite or metallic layers (Dance and Middlebrook, 1979).

Neutron radiography remains a very niche inspection technique, due to the ex-orbitant cost and technical difficulty of operating the equipment. Portability is nor-mally out of the question, though, some field-transportable equipment employing Californium-252 has been constructed and successfully demonstrated (John, 1979).

7.4.8 Electrical and magnetic techniques

A range of NDT techniques that exploit electrical conductivity, or permittivity have been demonstrated for composite inspection, though in most cases, they require either an electrically conductive (i.e. carbon-fibre) composite, or they are used to detect the presence of moisture.

7.4.8.1 Eddy-current testing

Eddy-current testing, a mainstay in aerospace surface-crack detection and sizing tech-niques, has seen little successful application to composites, as it requires the electro-magnetic induction of electrical current in the surface of the test component. It has


been successfully employed in the characterisation of ply-orientation in CFRP, by Prakash and Owston (1976) and others. This is possible via a correlation between ply orientation and electrical conductivity, though the variation in the polymer ma-trix layer separating the probe from the fibres causes a sizeable uncertainty in such measurements.

7.4.8.2 Capacitive imaging

Capacitive imaging employs two charged electrodes that emit and receive an electric field, averaged over the material volume it interacts with. Disturbances to the electric field from internal structural anomalies cause changes in the response. This technique may be applied to a wider range of material applications than eddy-currents, and gives relatively clear indications of the presence of cracking and impacts, heat damage, and moisture and fluid contamination. However, the non-intuitive nature of the volume- averaged field measurement precludes its use in quantitative analysis, and in most cases, reliable 2D sizing or characterisation of the detected anomaly is not possible. Yacht surveyors typically employ such devices as ‘moisture meters’ in the inspection of GFRP hulls, although there is significant variability between devices, and they can be overly sensitive to near-surface moisture, while insensitive to moisture within the actual laminate (Summerscales, 1994).

Figure 7.17 (Yin and Hutchins, 2012) shows a capacitive image of a 2-mm-thick GFRP laminate containing a through-thickness crack of approximately 40 mm length. Dielectric spectroscopy uses similar physics, examining changes in the complex elec-tric permittivity with frequency.

7.4.8.3 Nuclear magnetic resonance

Nuclear magnetic resonance, often better known by its acronym, NMR, is a technique that has been successfully applied in the medical industry as ‘Magnetic Resonance Imaging (MRI)’, operating as a 3D computed tomography system. The excellent sen-sitivity of NMR to the presence of water provides the potential for the prediction of

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Figure 7.17 The disturbance created in the response of a capacitive imaging probe when scanned over a through-thickness crack in a thin GFRP laminate. (a) Good region and (b) cracked region.Reproduced from Yin and Hutchins (2012) with permission.


drying rates in different composites with either physically absorbed or chemically bound moisture. It has been applied as a demonstrator to the detection of water trapped within the cells of a thin-skin honeycomb sandwich composite (LaPlante et al., 2005; Marble et al., 2009). The technique would likely be useful only for moisture-related inspections in low-conductivity composites, i.e. glass-fibre (Green, 1995).

In the marine environment, and despite modern surface treatment systems, protective coatings break down over time, particularly where a damage event has occurred. Thus, inspected in-service composite structures may be either immersed in, or repeatedly in the presence of, water when inspected, and this may have a debilitating influence on those techniques that are most sensitive to it (such as NMR and microwave-band inspection), as it can significantly hamper the contrast of the technique to other types of flaw.

7.5 Structural health monitoring

The effective application of NDT measurements typically requires knowledge about the component being inspected, including (i) its structural geometry, (ii) its behaviour under load, and hence the regions of stress concentration and likely damage initiation and (iii) its history of damage, and the location of such events. The more contextual information that is available to the inspector, the higher the inspection reliability.

SHM employs a different methodology; of attaching a large number of perma-nently mounted sensors, and monitoring them at routine intervals. This provides gen-eral information about the behaviour of the structure over time, which can yield an indication of its useful life, in addition to detecting damage events that might happen to within the measurement zone of the sensors. The boundary between the two philos-ophies is not entirely sharp, and in some cases both methodologies employ identical measurement physics. A major factor in SHM is the need for long-term in-service stability of sensors and systems in order to maintain calibration when exposed to the elements over many months or years.

7.5.1 Strain monitoring

Strain gauges are a common feature on many structural test articles. The ability to monitor strain at a large number of sites on a component's surface gives the operator an indication of the likelihood of early failure, and its location on the structure. Strain gauges are commonly small, planar metallic foils that are adhesively bonded to the test surface. As the surface is strained, the resistivity of the gauge either increases (tension) or decreases (compression), and the degree of change can be related to the surface strain measurement.

An innovative application of strain sensing utilises fibre-Bragg gratings (FBGs) rather than resistivity sensors to measure the strain variation at the test surface. Many FBG can be scribed onto a single optic fibre, and these act as a notch filter on the incident laser light. Changes in the local strain can then be correlated with a shift in reflectance or transmittance of the light, to give a sensing resolution <1 microstrain.


In marine applications, optical fibres containing FBG have been wrapped over the surface of GFRP pressure vessels (Degrieck et al., 2001), on a GFRP rudder (Davis et al., 2012), between foam-core sandwich panels with GFRP skins (Majumder et al., 2008), and embedded along the length of 57-m-long carbon-fibre masts on a luxury superyacht (Black, 2007). A challenge for this technology in service will be to develop reliable methods of wide-area inspection, and to protect the fibres and their associated optical connections.

Figure 7.18 shows a photograph of the GFRP rudder with a series of FBG attached, and the impacting hammer that was used to create damage, accompanied by the stiff-ness map produced by the FBG sensors and associated processing algorithm. This technique, applied by Davis et al., is an inverse implementation of the interpolation method developed by the United States Navy (Ratcliffe et al., 2004), and which has been implemented for wide-area stiffness measurements on large GFRP structures (Yoon et al., 2004).

7.5.2 Acoustic emission

Acoustic emission (AE) is the passive version of UT, in which a series of piezo-electric transducers are permanently attached to a structure and monitored. The difference is in the source of the acoustic waves, which, rather than emanating from a driver transducer, come from the internal structure itself: matrix cracking and fibre breakage both give rise to ultrasonic-frequency disturbances, and the rate and frequency of occurrence are measured to assess the rate of damage growth, often in a structure experiencing dynamic loading. An algorithm is then used to

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Figure 7.18 (a) Impact damage arrangement on a GFRP marine rudder containing surface-mounted fibre-Bragg grating arrays. (b) Stiffness irregularity index contour map overlaid on the rudder geometry, post-impact.Reproduced from Davis et al. (2012) with permission.


measure the timing of receipt of the signal at the various transducers, and estimate the location of the source (Summerscales, 2013).

AE has been widely applied to the inspection of GFRP pressure vessels and pet-rochemical storage tanks (Narisawa and Oba, 1984; Le Floc'h, 1986; Giordano et al., 1998; Ramirez, 2012), which are of similar construction to many marine laminates. Typically, quite some amplification of the signals is required, as they are of much lower amplitude than traditional ultrasonic signals.

A structure that undergoes multiple loading cycles gives rise to different patterns of AE than one enduring a single load excursion, due to the behaviour of internal dis-continuities. This phenomenon is known as the Kaiser effect, which prevents further emission until the previous high load mark has been reached. At high load levels, a further complication is the Felicity effect, which results in some emission of sound prior to the previous peak load level being surpassed. There is no published work on the depth resolution of the technique for thick laminates.

Figure 7.19 shows a typical AE measurement setup, in which a cracking event at the centre of the plate creates an elastic wave disturbance that emanates outwards, reaching two of the sensors that then display a received waveform.

7.5.2.1 Acousto-ultrasonics

A hybrid version of the technique using an advanced signal-processing method is acousto-ultrasonics, also called the acoustic-emission-simulation technique or the stress-wave factor method. Developed in the late 1970s by Vary and Bowles (1979), Vary and Lark (1979), and Vary (1990), it utilises active, offset pitch-catch transduc-ers, rather than relying on energy release from within the structure, and is normally

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Figure 7.19 Schematic of an acoustic emission measurement. A disturbance emanating from the centre of the plate has impacted on sensors ‘1’ and ‘2’, with the resultant waveforms, depending on the proximity of the sensor to the original disturbance.


conducted in the high kHz frequency range. This improved control over the transmit-ted energy and excited wave modes permits greater depth penetration, making the technique suitable for inspecting thick timber sections (Beall, 2002), though not all authors have found this to be the case in woven composite structures (Konstantinidis et al., 2006; Majumder et al., 2008).

7.5.3 Guided-wave testing

Guided waves is a collective term to describe techniques employing particular elas-tic wave modes at low ultrasonic frequencies: Lamb waves, which can be sustained in thin plates, and which are also the predominant mode measured in AE testing; Rayleigh waves, which travel along the surface of a structure; and horizontally polar-ised shear waves, elastic waves that oscillate transverse to the direction of propagation.

The aim of such testing is to interrogate a large length of structure using a single send–receive transducer, or a single pitch–catch pair, usually at frequencies below 500 kHz (Su et al., 2006). Transducers are typically fixed permanently to the struc-ture, and interrogated on a periodic basis. The term ‘guided’ indicates that the inspec-tion must take into account the physical geometry of the structure, and the resultant changes in wave behaviour.

The most common application of guided waves has been to the detection of metal-lic pipe-wall thinning by corrosion, though its viability for inspection in composites has been studied in detail: see, for example, Cawley and Adams (1979) and Ren and Lissenden (2013). A useful review of the physical basis, technology and additional system considerations of guided wave testing is given by Raghavan and Cesnik (2007). Certain modes, notably Lamb waves, apply to plates, and are therefore not able to be sustained in many of the thicker composites employed in marine applications.

7.5.3.1 Other in situ techniques

The remaining SHM techniques are mainly related to electrical impedance measure-ments, which are effectively in situ eddy-current, such as using conformable spiral-coil sensors that can be unobtrusively mounted on surfaces, beneath coatings, to monitor cracking in electrically conductive structures (Ditchburn et al., 2011). However, these are limited to electrically conductive structures, and have lower power than typical eddy-current sensors, but could be used near boltholes and other likely areas of defect initiation in CFRP.

7.6 General comparison of techniques

The NDT techniques presented here have all demonstrated capability in the detection of some defects in marine composites. In application to true structures, the physical in-spection limitations are but one aspect of a number in assessing the structural integrity of a component. Other important factors in deciding an NDT methodology are the cost of equipment, technicians and training; ease of application, inspection repeatability


and other human factors; type of damage being searched for, and whether it is known to be present; the area to be covered, and whether access to one or both sides of the component is available.

A list of defects occurring in marine composites together with appropriate NDT and SHM techniques for their detection is provided in Table 7.1. The techniques are

Defect Technique Composite

Manufacturing Fibre bunching, waviness

Ultrasonics Monolithic laminateRadiographyMicrowave

Layup irregularities, ply orientation

Ultrasonics Eddy-current CFRP only

Fibre volume fraction

Ultrasonics Microwave Eddy-current CFRP only

Voids/porosity Ultrasonics Radiography Thermography

Foreign inclusions Microwave GFRP onlyRadiography Ultrasonics

Bondline integrity Radiography Ultrasonics Thermography Near-surfaceOptical interferometry Near-surface

In-service Delamination Ultrasonics Thermography Near-surfaceOptical interferometry Near-surface

Fibre breakage Acoustic emission Skin-to-core disbonding

Optical interferometry Sandwich structureThermographyResonanceUltrasonics

Core crush Radiography Sandwich structureThermography

Water presence Microwave Honeycomb sandwichRadiographyThermographyCapacitive imaging

Global strain state Vibration analysis Strain sensing

Surface-breaking cracks

Most techniques

Table 7.1 A list of NDT techniques applicable to the detection of various types of manufacturing defects or in-service damage in the different marine composite structure classes


listed on the basis of effectiveness at defect detection alone, not including the other factors discussed above. A more comprehensive comparison that takes thick-section structures, and non-fibre-reinforced composites into account is provided elsewhere by the author (Ibrahim, 2014b). The United States Ship Structures Committee also provides a range of charts on NDT technique effectiveness in a report by Greene (2012).

Each NDT technique and every inspection methodology has applications to which it is ideally suited, and others to which it is less so. Some general observations that can be made in summarising the applicability of NDT techniques to marine composite structures are

i. Techniques that operate on the ‘global’ structure, such as vibrational, strain sensing and AE should provide reliable sensing of significant damage, though have difficulty in locating it precisely, or characterising it. However, they are the most useful way to detect gradual changes that take a long time to detect, and where there is no clear region of high stress concentration.

ii. Wide-area techniques such as thermography and optical interferometry, despite significant equipment cost and setup arrangements, can inspect large components rapidly, allowing initial identification of near-surface defects for further analysis. However, they are unable to successfully inspect thick structures.

iii. Low-ultrasonic frequency resonance techniques are relatively inexpensive, and simple to apply, and suitable to the detection of disbonds in honeycomb and foam-sandwich compos-ites, but, again, are limited in their application to solid laminates.

iv. Moisture ingress is best inspected using neutrons, X-rays, thermography and microwaves, though these all struggle to characterise the volume of water contained within honeycomb or other open-structure internal cells. High-resolution imagery is possible using THz-frequency measurements in thin or low-density components.

v. Penetration of truly thick sections, particularly of monolithic PMC laminates appears pos-sible on a reliable basis only using pulsed-ultrasonic inspections at the lower part of the frequency band, and some of the sub-bands within the microwave band, in particular the GPR inspections at 1–3 GHz.

7.7 Future trends

The fundamental physical basis of many aspects of the techniques applied in NDT and SHM techniques has been studied in detail, and can be found in the technical literature. The increasing ease of integrating data acquisition hardware and software interfaces, together with the constant improvement in computer processing and data storage has led to greater flexibility and intuitive design for the NDT technician or structural engineer to assess inspection results. The field will continue to harness the convenience provided by powerful and portable technology, for example, the use of smart-phone processors to remotely control SHM systems, and to acquire and anal-yse data.

The human eye and ear continue to be extraordinarily capable instruments, and are often formally employed to perform a wide-area pre-inspection for damage events. Data manipulation continues to be designed in a way to permit the human


eye/brain processing system to best acquire the data and determine an appropri-ate course of action. This, coupled with miniaturisation will continue to decrease the gap between inspection reliability of robotic or laboratory systems and field- portable equipment.

Despite demonstrations being performed on a range of simpler components, par-ticularly thin and light-weight composite structures, the physical limitations of defect characterisation for most techniques have yet to be clarified for thick composite struc-tures (Ibrahim, 2014b). Further work is expected here, as more large marine vessels and key propulsion components such as propeller blades and control surfaces are fab-ricated from composite materials, with less and less conservative design (Ibrahim, 2014a). Sensors will be added to high-strain regions of key components as a standard part of the manufacturing process, rather than add-on demonstrators for expensive or security-sensitive assets.

Finally, improved linkages between NDT and structural integrity management will mean that the structural engineer can use NDT to a more quantitative degree. Currently, either the entire part would be condemned, or the defective region would be considered to have been removed and replaced by empty space. Quantitative 3D NDT data will allow a finite-element structural model to take into account a percentage reduction in strength in a damaged component, as determined by the nondestructive inspection, and then calculate the residual strength of the structure.

7.8 Further reading

The worldwide NDT&E (nondestructive testing and evaluation) research community is relatively small, but active in addressing inspection challenges and determining ca-pabilities for a wide variety of NDT technology areas.

7.8.1 Books and monographs

Further information on the application of a variety of techniques to the inspection of mainly aerospace composites may be found in Karbhari (2013). Summerscales has also edited two volumes discussing the application of a range of early NDT techniques, and following the development of techniques emerging in the 1980s (Summerscales, 1987, 1990). An early review on the application of NDT technology to the inspection of fibreglass marine structures was written by Bar-Cohen (1986), then at the Douglas aircraft corporation. The United States Ship Structure Committee has also produced a large report on inspection techniques for marine composite con-struction and NDT (Greene, 2012). A comprehensive overview of SHM technology is found in the ‘Encyclopedia of structural health monitoring’ (Boller et al., 2009). The handbooks on the application of NDT produced by the American Society for Nondestructive Testing are also recommended to the reader. A detailed introduction to NDT reliability assessment can be found in the United States Department of Defense handbook, MIL-HDBK-1823 (1999).


7.8.2 Scientific journals related to composites NDT

A subjective list of some leading scientific journals that give good coverage to NDT&E (non-destructive testing and evaluation), and its applications to marine composite ma-terials follows, with their publisher and International Standard Serial Number for the online version of the journal:

i. Composites Part A: Applied science and manufacturing, Elsevier, 1359-835X ii. Composites Part B: Engineering, Elsevier, 1359-8368 iii. Composites Science and Technology, Elsevier, 0266-3538 iv. Composite Structures, Elsevier, 0263-8223 v. Journal of Composite Materials, Sage, 1530-793X vi. Journal of Nondestructive Evaluation, Springer, 1573-4862 vii. NDT&E International, Elsevier, 0963-8695 viii. Nondestructive Testing and Evaluation, Taylor and Francis, 1477-2671 ix. Polymer Composites, Wiley, 1548-0569 x. Research in Nondestructive Evaluation, Springer, 1432-2110 xi. Ultrasonics, Elsevier, 0041-624X

Additionally, journals in the subject areas of applied physics, electromagnetics, fatigue, fracture mechanics, materials testing, materials characterisation, and struc-tural integrity, among others, all contain sizeable contributions by the NDT and SHM communities.

7.8.3 Technical meetings and conferences

The international NDT community hosts a range of recurring meetings and confer-ences in various countries, and many of these publish papers in proceedings. The peak international body, the International Committee for NonDestructive Testing (ICNDT) maintains a comprehensive worldwide events calendar at their website, www.icndt.org, as well as links to partner organisations in a number of countries or continents.

Acknowledgement

The helpful advice and suggestions of Robert Smith, Professor of NDT and High Value Manufacturing in the Department of Mechanical Engineering, University of Bristol, are grate-fully acknowledged, in the development of the initial concept for this chapter, formulation of its structure, and revision during its preparation.

References

Anastasi, R.F., 2008. Investigation of fiber waviness in a thick glass composite beam us-ing THz NDE. In: 15th International Symposium on Smart Structures and Materials & Nondestructive Evaluation and Health Monitoring, SPIE, pp. 69340–69348.

Baidya, K.P., Ramakrishna, S., Rahman, M., Ritchie, A., 2001. Quantitative radiographic analy-sis of fiber reinforced polymer composites. J. Biomater. Appl. 15 (3), 279–289.

http://www.icndt.org

http://www.icndt.org

http://refhub.elsevier.com/B978-1-78242-250-1.00007-7/rf0010






Bar-Cohen, Y., 1986. NDE of fiber-reinforced composite materials – a review. Mater. Eval. 44 (4), 446–454.

Bar-Cohen, Y., 2001. NDE of composites using leaky lamb waves (LLW). Nondestruct. Test. Eval. 17 (2), 91–120.

Bar-Cohen, Y., Crane, R.L., 1980. NDE of hygrothermal affected glass fiber-reinforced plastics. In: ASNT Spring Conference, March 1980. American Society for Nondestructive Testing, Philadelphia, PA, USA.

Beall, F.C., 2002. Overview of the use of ultrasonic technologies in research on wood proper-ties. Wood Sci. Technol. 36 (3), 197–212.

Belingardi, G., Cavatorta, M.P., Paolino, D.S., 2008. A new damage index to monitor the range of the penetration process in thick laminates. Compos. Sci. Technol. 68 (13), 2646–2652.

Berger, H., 1971. Neutron radiography. Annu. Rev. Nucl. Sci. 21, 335–364.Black, S., 2007. Megayacht composite masts get smart. High Perf. Compos. 15 (1), 44–46.Boller, C., Chang, F.-K., Fujino, Y., 2009. Encyclopedia of Structural Health Monitoring. Wiley,

West Sussex, UK.Borum, K.K., 2006. Evaluation of the quality of thick fibre composites using immersion and

air-coupled ultrasonic techniques. In: 9th European Conference on Nondestructive Testing, ECNDT, 25–29 September 2006, Berlin, Germany.

Brown, D.T., Green, G., 1997. Development of an ultrasonic system for submarine hull acoustic cladding inspection. NDT&E Int. 30 (3), 123–133.

Bull, D.J., Helfen, L., Sinclair, I., Spearing, S.M., Baumbach, T., 2013. A comparison of multi-scale 3D X-ray tomographic inspection techniques for assessing carbon fibre composite impact damage. Compos. Sci. Technol. 75, 55–61.

Butters, J.N., Leendertz, J.A., 1971. Speckle pattern and holographic techniques in engineering metrology. Opt. Laser Technol. 3 (1), 26–30.

Cantwell, W.J., Morton, J., 1992. The significance of damage and defects and their detection in composite materials: a review. J. Strain Anal. Eng. Des. 27 (1), 29–42.

Carriveau, G.W., 1993. Benchmarking of the state of the art in nondestructive testing/ evaluation for applicability in the composite armored vehicle (cav) advanced technology demonstra-tor (atd) program dla900-90-d-0123. Report No. 7304–103:6WC-D172.7, U.S. Army Tank-Automotive Command Research, Development, and Engineering Center, Warren, MI, USA.

Castellini, P., Martarelli, M., Tomasini, E.P., 2006. Laser doppler vibrometry: development of advanced solutions answering to technology's needs. Mech. Syst. Signal Process. 20 (6), 1265–1285.

Cawley, P., 2006. Inspection of composites – current status and challenges. In: 9th European Conference on Nondestructive Testing, ECNDT, 25–29 September 2006, Berlin, Germany.

Cawley, P., Adams, R.D., 1979. A vibration technique for non-destructive testing of fibre com-posite structures. J. Compos. Mater. 13 (2), 161–175.

Chandra, R., Singh, S.P., Gupta, K., 1999. Damping studies in fiber-reinforced composites – a review. Compos. Struct. 46 (1), 41–51.

Chiou, C.P., Hsu, D.K., Barnard, D., Im, K.H., Thompson, R.B., 2011. Signal modeling in the far-infrared region for nondestructive evaluation applications. Rev. Prog. Quant. Nondestruct. Eval. 30 (2011), 581–588, AIP.

Dance, W.E., Middlebrook, J.B., 1979. Neutron radiographic nondestructive inspection for bonded composite structures. In: Pipes, R.B. (Ed.), Nondestructive Evaluation and Flaw Criticality for Composite Materials, 696. ASTM International, West Conshohocken, PA, USA, pp. 57–71.










































http://refhub.elsevier.com/B978-1-78242-250-1.00007-7/rfbb0110








Davis, C.E., Norman, P., Ratcliffe, C., Crane, R., 2012. Broad area damage detection in compos-ites using fibre Bragg grating arrays. Struct. Health Monit. 11 (6), 724–732.

Degrieck, J., De Waele, W., Verleysen, P., 2001. Monitoring of fibre reinforced composites with embedded optical fibre Bragg sensors, with application to filament wound pressure vessels. NDT&E Int. 34 (4), 289–296.

Ditchburn, R.J., Ibrahim, M.E., Burke, S.K., 2011. Measurement of fatigue crack growth using low-profile eddy-current sensors. Non-Destr. Test. Aust. 48 (2), 46–48.

Drinkwater, B.W., Wilcox, P.D., 2006. Ultrasonic arrays for non-destructive evaluation: a re-view. NDT&E Int. 39 (7), 525–541.

Dugan, E.T., Jacobs, A.M., Shedlock, D., Ekdahl, D., 2004. Detection of defects in foam ther-mal insulation using lateral migration backscatter X-ray radiography. In: Penetrating Radiation Systems and Applications VI. SPIE, vol. 5541, pp. 47–57.

Emerson, R.N., Pollock, D.G., Mclean, D.I., Fridley, K.J., Ross, R.J., Pellerin, R.F., 1999. Nondestructive testing of large bridge timbers. In: 11th International Symposium on Nondestructive Testing of Wood, September 9–11, 1998. Forest Products Society, Madison, WI.

Fortunko, C.M., Fitting, D.W., 1991. Appropriate ultrasonic system components for NDE of thick polymer-composites. In: Thompson, D.O., Chimenti, D.E. (Eds.), Review of Progress in QNDE, 10. Springer, Dordrecht, The Netherlands, pp. 2105–2112.

Fowler, K.A., Hatch, H.P., 1966. Detection of voids and inhomogeneities in fiber glass reinforced plastics by microwave and beta-ray backscatter techniques. Report No. SA-TR19-1519, US Army, Springfield, MA, USA.

Frankle, R.S., Rose, D.N., 1995. Flexible ultrasonic array system for inspecting thick composite structures. In: Nondestructive Evaluation of Aging Maritime Applications, SPIE, 6–8 June 1995, Oakland, CA, USA.

Gagneja, S., Gibson, R.F., Ayorinde, E.O., 2001. Design of test specimens for the determination of elastic through-thickness shear properties of thick composites from measured modal vibration frequencies. Compos. Sci. Technol. 61 (5), 679–687.

Ganchev, S.I., Carriveau, G.W., Qaddoumi, N., 1994. Microwave detection of defects in glass- reinforced polymer composites. In: Advanced Microwave and Millimeter-Wave Detectors. SPIE, vol. 2275, pp. 11–20.

Giordano, M., Calabro, A., Esposito, C., D'amore, A., Nicolais, L., 1998. An acoustic- emission characterization of the failure modes in polymer-composite materials. Compos. Sci. Technol. 58 (12), 1923–1928.

Grabovac, I., Whittaker, D., 2009. Application of bonded composites in the repair of ships struc-tures – a 15-year service experience. Compos. A: Appl. Sci. Manuf. 40 (9), 1381–1398.

Green Jr., R.E., 1995. Nondestructive evaluation of thick-composite fatigue damage. In: Nondestructive Evaluation of Aging Maritime Applications, SPIE, 6–8 June 1995, Oakland, CA, USA.

Green, G.A., Campbell, P., Zoughi, R., 2004. An investigation into the potential of microwave NDE for maritime application. In: 16th World Conference of Non-Destructive Testing, ICNDT, 30 August–03 September 2004, Montreal, Canada.

Greenawald, E.C., Levenberry, L.J., Qaddoumi, N., Mchardy, A., Zoughi, R., Poranski, J.C.F., 2000. Microwave NDE of impact damaged fiberglass and elastomer layered composites. In: Thompson, D.O., Chimenti, D.E. (Eds.), Review of Progress in QNDE, 19. Springer, Dordrecht, The Netherlands, pp. 1263–1268.

Greene, E., 2012. Inspection techniques for marine composite construction and NDE. Report No. SSC-463, United States Ship Structure Committee, Washington, DC.


















































Gregory, R., 2002. Production inspection of aerospace composites using laser shearography. Insight 44 (4), 220–223.

Gryzagoridis, J., Findeis, D., 2008. Benchmarking shearographic NDT for composites. Insight 50 (5), 249–252.

Haque, A., Hossain, M.K., 2003. Effects of moisture and temperature on high strain rate behav-ior of s2-glass-vinyl ester woven composites. J. Compos. Mater. 37 (7), 627–647.

Heslehurst, R.B., 2011. Assessment of local damage in composite structures by optical meth-ods. In: SAMPE Spring Technical Conference and Exhibition, SAMPE, 23–26 May 2011, Long Beach, CA, USA.

Hing, C.L.C., Halabe, U.B., 2010. Nondestructive testing of GFRP bridge decks using ground penetrating radar and infrared thermography. J. Bridge Eng. 15 (4), 391–398.

Hsu, D.K., Dong, F., Zhanjie, L., 2002. Ultrasonically mapping the ply layup of composite laminates. Mater. Eval. 60 (9), 1099–1106.

Hung, Y.Y., 1982. Shearography: a new optical method for strain measurement and nondestruc-tive testing. Opt. Eng. 21 (3), 391–395.

Hung, M.Y.Y., Shang, H.M., Yang, L., 2003. Unified approach for holography and shearography in surface deformation measurement and nondestructive testing. Opt. Eng. 42 (5), 1197–1207.

Ibrahim, M.E., 2014a. Nondestructive evaluation of mechanically loaded advanced marine composite structures. Adv. Mater. Res. 891–892, 594–599.

Ibrahim, M.E., 2014b. Nondestructive evaluation of thick-section composite and sandwich structures: a review. Compos. A: Appl. Sci. Manuf. 64, 36–48.

John, J., 1979. Neutron radiography for nondestructive testing. In: Sagamore Army Materials Research Conference. Plenum Press, Melville, New York, NY, USA, pp. 151–182.

Jones, T.S., Lindgren, E.A., 1994. Thermographic inspection of marine composite structures. In: International Conference on Thermal Sensing and Imaging Diagnostic Applications, SPIE, pp. 173–175.

Kaatze, U., 1989. Complex permittivity of water as a function of frequency and temperature. J. Chem. Eng. Data 34 (4), 371–374.

Karbhari, V.M. (Ed.), 2013. Non-Destructive Evaluation (NDE) of Polymer Matrix Composites. Woodhead, Cambridge, UK.

Kendall, C., Doud, G.R., 2011. Characterizing tensile strength properties in an E-glass compos-ite with fiber waviness. In: International Mechanical Engineering Congress and Exposition, ASME vol. 8, pp. 263–272.

Kim, K.S., Kang, K.S., Kang, Y.J., Cheong, S.K., 2003. Analysis of an internal crack of pres-sure pipeline using ESPI and shearography. Opt. Laser Technol. 35 (8), 639–643.

Kinra, V.K., Ganpatye, A.S., 2005. Ultrasonic Ply-by-Ply Detection of Matrix Cracks in Laminated Composites. 24 Springer, The Netherlands. p. 1065.

Konstantinidis, G., Drinkwater, B.W., Wilcox, P.D., 2006. The temperature stability of guided wave structural health monitoring systems. Smart Mater. Struct. 15 (4), 967.

Lambrineas, P., Davis, J.R., Suendermann, B., Wells, P., Thomson, K.R., Woodward, R.L., Egglestone, G.T., Challis, K., 1991. X-ray computed tomography examination of inshore minehunter hull composite material. NDT&E Int. 24 (4), 207–213.

Laplante, G., Marble, A.E., Macmillan, B., Lee-Sullivan, P., Colpitts, B.G., Balcom, B.J., 2005. Detection of water ingress in composite sandwich structures: a magnetic resonance ap-proach. NDT&E Int. 38 (6), 501–507.

Le Floc'h, C., 1986. Acoustic emission monitoring of composite high-pressure fluid storage tanks. NDT Int. 19 (4), 259–262.

Leendertz, J.A., Butters, J.N., 1973. An image-shearing speckle-pattern interferometer for mea-suring bending moments. J. Phys. E: Sci. Instrum. 6 (11), 1107–1110.



















































Lestari, W., Qiao, P., 2005. Damage detection of fiber-reinforced polymer honeycomb sandwich beams. Compos. Struct. 67 (3), 365–373.

Maierhofer, C., 2003. Nondestructive evaluation of concrete infrastructure with ground pene-trating radar. J. Mater. Civ. Eng. 15 (3), 287–297.

Majumder, M., Gangopadhyay, T.K., Chakraborty, A.K., Dasgupta, K., Bhattacharya, D.K., 2008. Fibre Bragg gratings in structural health monitoring – present status and applica-tions. Sens. Actuators A: Phys. 147 (1), 150–164.

Marble, A.E., Laplante, G., Mastikhin, I.V., Balcom, B.J., 2009. Magnetic resonance detection of water in composite sandwich structures. NDT&E Int. 42 (5), 404–409.

Meola, C., Carlomagno, G.M., 2014. Infrared thermography to evaluate impact damage in glass/epoxy with manufacturing defects. Int. J. Impact Eng. 67, 1–11.

Miller, P.H., 2000. Durability of Marine Composites: A Study of the Effects of Fatigue on Fiberglass in the Marine Environment (Ph.D. thesis). University of California, Berkeley, CA, USA.

Monchalin, J.-P., 1986. Optical detection of ultrasound. IEEE Trans. Ultrason. Ferroelectr. Freq. 33 (5), 485–499.

Monchalin, J.-P., 2004. Laser-ultrasonics: from the laboratory to industry. In: Thompson, D.O., Chimenti, D.E. (Eds.), Review of Progress in QNDE, 23. Springer, Dordrecht, The Netherlands, pp. 3–31.

Montanini, R., Freni, F., 2012. Non-destructive evaluation of thick glass fibre-reinforced com-posites by means of optically excited lock-in thermography. Compos. A: Appl. Sci. Manuf. 43 (11), 2075–2082.

Mouritz, A.P., 2003. Non-destructive evaluation of damage accumulation. In: Harris, B. (Ed.), Fatigue in Composites. Woodhead, Cambridge, UK, pp. 242–266.

Mouritz, A.P., Luescher, R.E., 1997. Preliminary ultrasonic inspection of the Huon Class Minehunter Coastal. Report No. DSTO-TR-0592, Defence Science and Technology Organisation, Melbourne, Australia.

Mouritz, A.P., Townsend, C., Shah Khan, M.Z., 2000. Non-destructive detection of fatigue dam-age in thick composites by pulse-echo ultrasonics. Compos. Sci. Technol. 60 (1), 23–32.

Narisawa, I., Oba, H., 1984. An evaluation of acoustic emission from fibre-reinforced com-posites – part 1 acoustic emission interpretation of epoxy matrix and model composites containing glass beads, carbon and glass fibres. J. Mater. Sci. 19 (6), 1777–1786.

Newman, J.W., 2005. Holographic and shearographic NDT applications in aerospace manufac-turing. Mater. Eval. 63 (7), 746–750.

Pezzoni, R., Krupka, R., 2001. Laser-shearography for non-destructive testing of large-area composite helicopter structures. Insight 43 (4), 244–248.

Prakash, R., 1980. Non-destructive testing of composites. Composites 11 (4), 217–224.Prakash, R., Owston, C.N., 1976. Eddy-current method for the determination of lay-up order in

cross-plied CFRP laminates. Composites 7 (2), 88–92.Raghavan, A., Cesnik, C.E.S., 2007. Review of guided-wave structural health monitoring.

Shock Vib. Digest 39 (2), 91–116.Rajic, N., 2002. Principal component thermography for flaw contrast enhancement and flaw

depth characterisation in composite structures. Compos. Struct. 58 (4), 521–528.Ramirez, G., 2012. Use of acoustic emission to evaluate residual strength in FRP pipes after

impact damage. Res. Nondestruct. Eval. 23 (4), 207–220.Ratcliffe, C.P., Crane, R.M., Gillespie, J.W., 2004. Damage detection in large composite struc-

tures using a broadband vibration method. Insight 46 (1), 10–16.Redo-Sanchez, A., Karpowicz, N., Xu, J., Zhang, X.C., 2006. Damage and defect inspection

with terahertz waves. In: 4th International Workshop on Ultrasonic and Advanced Methods
















































for Nondestructive Testing and Material Characterization, 19 June 2006. University of Massachusetts, Dartmouth, MA, USA.

Ren, B., Lissenden, C.J., 2013. Ultrasonic guided wave inspection of adhesive bonds between composite laminates. Int. J. Adhes. Adhes. 45, 59–68.

Sala, G., 2000. Composite degradation due to fluid absorption. Compos. B 31 (5), 357–373.Saunders, D.S., Galea, S.C., Deirmendjian, G.K., 1993. The development of fatigue damage

around fastener holes in thick graphite/epoxy composite laminates. Composites 24 (4), 309–321.

Scruby, C.B., Drain, L.E., 1990. Laser Ultrasonics: Techniques and Applications. Taylor & Francis Group, New York.

Simonetti, F., 2006. Localization of pointlike scatterers in solids with subwavelength resolution. Appl. Phys. Lett. 89 (9), 094105–094105-3.

Smith, R.A., Clarke, B., 1994. Ultrasonic C-scan determination of ply stacking sequence in carbon-fibre composites. Insight 36 (10), 741–747.

Smith, R.A., Nelson, L.J., Mienczakowski, M.J., Challis, R.E., 2009. Automated analysis and advanced defect characterisation from ultrasonic scans of composites. Insight 51 (2), 82–87.

Su, Z., Ye, L., Lu, Y., 2006. Guided lamb waves for identification of damage in composite struc-tures: a review. J. Sound Vib. 295 (3), 753–780.

Summerscales, J. (Ed.), 1987. Non-Destructive Testing of Fibre-Reinforced Plastics Composites, 1. Elsevier Applied Science, Barking, England.

Summerscales, J. (Ed.), 1990. Non-destructive Testing of Fibre-Reinforced Plastics Composites, 2. Elsevier Science Publishers, Barking, England.

Summerscales, J., 1994. Non-destructive measurement of the moisture content in fibre rein-forced plastics. Br. J. Nondestr. Test. 36 (2), 64–72.

Summerscales, J., 2013. Acoustic Emission Source Location in Fibre-Reinforced Composite Materials. Advanced Composites Manufacturing Centre, Plymouth University, Plymouth, UK.

Sutherland, L.S., Shenoi, R.A., Lewis, S.M., 1999. Size and scale effects in composites: I. Literature review. Compos. Sci. Technol. 59 (2), 209–220.

United States Department of Defense, 1999. Nondestructive Evaluation System: Reliability Assessment. MIL-HDBK-1823, Wright-Patterson Air Force Base, OH, USA. 106 p.

Vary, A., 1990. Acousto-ultrasonics. In: Summerscales, J. (Ed.), Non-destructive Testing of Fibre-Reinforced Plastics Composites, 2. Elsevier Applied Science, Barking, UK, pp. 1–54.

Vary, A., Bowles, K.J., 1979. An ultrasonic-acoustic technique for nondestructive evaluation of fiber composite quality. Polym. Eng. Sci. 19 (5), 373–376.

Vary, A., Lark, R.F., 1979. Correlation of fiber composite tensile strength with the ultrasonic stress wave factor. ASTM J. Test. Eval. 7 (4), 185–191.

Vijaya Kumar, R.L., Bhat, M.R., Murthy, C.R.L., 2013. Some studies on evaluation of deg-radation in composite adhesive joints using ultrasonic techniques. Ultrasonics 53 (6), 1150–1162.

White, J., Zimdars, D., 2007. Time domain terahertz non destructive evaluation of water in-trusion in composites and corrosion under insulation. In: Quantum Electronics and Laser Science Conference.

Wisnom, M.R., Hallett, S.R., Soutis, C., 2010. Scaling effects in notched composites. J. Compos. Mater. 44 (2), 195–210.

Yang, L.X., Hung, Y.Y., 2004. Digital shearography for nondestructive evaluation and appli-cation in automotive and aerospace industries. In: World Conference on Nondestructive Testing, ICNDT, August 30–September 3, 2004, Montreal, Canada.



















































Yang, S., Gu, L., Gibson, R.F., 2001. Nondestructive detection of weak joints in adhesively bonded composite structures. Compos. Struct. 51 (1), 63–71.

Yin, X., Hutchins, D.A., 2012. Non-destructive evaluation of composite materials using a ca-pacitive imaging technique. Compos. B 43 (3), 1282–1292.

Yoon, M.K., Krauthauser, C., Heider, D., Gillespie Jr., J.W., Ratcliffe, C.P., Crane, R.M., 2004. Damage detection in large-scale composite structures via vibration technique using MEMS accelerometers. In: International SAMPE Technical Conference, SAMPE, Long Beach, CA, USA.

Yu, T.Y., Büyüköztürk, O., 2008. A far-field airborne radar NDT technique for detecting debonding in GFRP-retrofitted concrete structures. NDT&E Int. 41 (1), 10–24.

Zoughi, R., Ganchev, S.I., 1995. Microwave nondestructive evaluation: state-of-the-art review. Report No. NTIAC 95–01, Fort Collins, CO, USA.

Zoughi, R., Ganchev, S.I., Carriveau, G.W., 1996. Overview of microwave NDE applied to thick composites. Mater. Sci. Forum 210–213, 69–76.















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