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Guided wave crack detection andsize estimation in stiffened structures

Md Yeasin Bhuiyan, Mohammad Faisal Haider, BanibrataPoddar, Victor Giurgiutiu

Md Yeasin Bhuiyan, Mohammad Faisal Haider, Banibrata Poddar, VictorGiurgiutiu, "Guided wave crack detection and size estimation in stiffenedstructures," Proc. SPIE 10599, Nondestructive Characterization andMonitoring of Advanced Materials, Aerospace, Civil Infrastructure, andTransportation XII, 105991Y (27 March 2018); doi: 10.1117/12.2309975

Event: SPIE Smart Structures and Materials + Nondestructive Evaluation andHealth Monitoring, 2018, Denver, Colorado, United States

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Guided wave crack detection and size estimation in stiffened structures Md Yeasin Bhuiyan1*, Mohammad Faisal Haider2, Banibrata Poddar3, Victor Giurgiutiu4

1,2,4Department of Mechanical Engineering, University of South Carolina Columbia, SC, USA

3Research Scientist, Intelligent Automation Inc., Rockville, MD, USA *Corresponding author: [email protected]

ABSTRACT

Structural health monitoring (SHM) and nondestructive evaluation (NDE) deals with the nondestructive inspection of defects, corrosion, leaks in engineering structures by using ultrasonic guided waves. In the past, simplistic structures were often considered for analyzing the guided wave interaction with the defects. In this study, we focused on more realistic and relatively complicated structure for detecting any defect by using a non-contact sensing approach. A plate with a stiffener was considered for analyzing the guided wave interactions. Piezoelectric wafer active transducers were used to produce excitation in the structures. The excitation generated the multimodal guided waves (aka Lamb waves) that propagate in the plate with stiffener. The presence of stiffener in the plate generated scattered waves. The direct wave and the additional scattered waves from the stiffener were experimentally recorded and studied. These waves were considered as a pristine case in this research. A fine horizontal semi-circular crack was manufactured by using electric discharge machining in the same stiffener. The presence of crack in the stiffener produces additional scattered waves as well as trapped waves. These scattered waves and trapped wave modes from the cracked stiffener were experimentally measured by using a scanning laser Doppler vibrometer (SLDV). These waves were analyzed and compared with that from the pristine case. The analyses suggested that both size and shape of the horizontal crack may be predicted from the pattern of the scattered waves. Different features (reflection, transmission, and mode-conversion) of the scattered wave signals are analyzed. We found direct transmission feature for incident A0 wave mode and mode-conversion feature for incident S0 mode are most suitable for detecting the crack in the stiffener. The reflection feature may give a better idea of sizing the crack.

Keywords: NDE, SHM, Lamb waves, Horizontal crack, Scanning laser Doppler vibrometer.

1. INTRODUCTION

The researchers of structural health monitoring (SHM) and nondestructive evaluation (NDE) thrives on developing new techniques for detecting, localizing and quantifying damage in the structures. The various kinds of damage such as fatigue crack, dibonds, delamination, corrosion, leakage may occur in the engineering infrastructures depending on the material and nature of application [1]–[3]. One of the most common type of damage is the formation of a fatigue crack in the structures. These cracks are difficult to detect by conventional schedule based inspection method, but if undetected, can grow to a critical size and jeopardize the integrity of the structure [4], [5]. In the past, many research have been conducted to detect, localize and quantify the fatigue crack by using a condition-based SHM technique using ultrasonic Lamb waves [6], [7].

Chang et al. [8] characterized and compared the sensitivity of the acousto-ultrasound-based SHM techniques and NDE techniques to the damage detection and determined the most influencing parameters to its sensitivity. They used piezo enabled spectral element analysis (PESEA) to characterize the ultrasound waves from a lead zirconate titanate (PZT) transmitter accompanied by three receivers located around the damage. Schubert et al. computed the probability of detection (POD) for SHM on a wing attachment lug by loading at the end of wing spur [9] and on multiple-rivet hole [10]. Bergman et al. [11] introduced an integrated hybrid SHM using PZT for damage detection and static/dynamic-load monitoring capabilities. Built-in-sensor network has been used for composite structure monitoring [12]. The above research have used some algorithms based on the guided wave interaction with damage.

Nondestructive Characterization and Monitoring of Advanced Materials, Aerospace, Civil Infrastructure,and Transportation XII, edited by Peter J. Shull, Andrew L. Gyekenyesi, Tzu-Yang Yu, H. Felix Wu, Proc.of SPIE Vol. 10599, 105991Y · © 2018 SPIE · CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2309975

Proc. of SPIE Vol. 10599 105991Y-1


The acoustic emission (AE) monitoring have been performed for various application including crack monitoring [13]–[17]. Some researchers focused on AE-hit based analysis using amplitude and signal strength [18] and some are on AE-waveform based analysis [19]–[21]. New strategies have been developed incorporating wireless technology in AE measurement [22]. However, the AE approach, a passive SHM technique that uses only sensors and it is mostly used for in-situ damage detection purposes. Guided wave interaction study is performed mostly in active SHM setting which consists of transmitter and receiver sensors [23]–[25].

The aerospace and civil infrastructures often use stiffeners to the load-bearing structural components [26], [27]. Often the stiffeners are manufactured as an integral part of the mother structural components to reduce the fasteners. However, cracks may develop in between the stiffener and the structural component under prolonged and/or critical load applications. The crack formation may follow various orientation such as horizontal crack, vertical crack, inclined crack. Since the critical stress may occur in the interface of the stiffener and mother component, the horizontal cracks are more likely to develop as a result. The development of a proper SHM technique to detect, localize these crack as well as to determine the shape and size of the horizontal cracks are the current research challenge.

In this paper, an SHM technique based on PZT actuators and non-contact laser measurement for sensing has been proposed. Experiments were conducted to develop the understanding of the guided wave interaction with stiffener. A stiffener was manufactured as an integral part of plate-like structures. A half-penny shaped horizontal crack is manufactured using electric discharge machining (EDM). The stiffener is itself a structural discontinuity in the plate hence, produce scattered waves. We found that the crack also produces scattered wave in addition to original scattered waves due to the pristine stiffener. The scattered wavefield was measure by using scanning laser Doppler vibrometer (SLDV). Trapped wavefield was observed in the cracked stiffener. Both S0 and A0 Lamb wave excitations were selectively used. A waveform analysis was also carried out to study the propagating scattered waves.

2. MANUFACTURING OF PLATE WITH STIFFENER

The manufacturing of the plate with a stiffener is little bit tricky and worth discussing for the reader. An aircraft grade Aluminum 6061 material is used to manufacture the plate specimen. A square plate of 24-inch side is used. The initial thickness of the plate was 0.5 inch. In order to make a continuous stiffener in the mid-section of the plate, a milling process is carried out on the entire plate except for the stiffener portion. The milling process is carried in several steps to remove one-third of an inch of materials from the plate. This process has given a continuous stiffener on the plate. Thus, the stiffener is an integral part of the plate and does not require any extra bonding agents. The isometric view of the plate with stiffener is shown in Figure 1a. The side view of the plate is shown in Figure 1b.

The dimension of the stiffener is 0.333 inches thick, 0.333 inches wide and 24 inches long. After the milling process, the plate thickness is 0.167 inches everywhere. A small half-penny-shaped crack is manufactured at a certain location of the stiffener. Practically, a slit of a finite dimension has been made by using a ram electric discharge method (EDM). Since the thickness of the slit is kept very small (0.01 inch, about 3% of the stiffener thickness), it may be referred to as a “crack” in ultrasonic applications. This very fine thin slit may exhibit similar ultrasonic wave scattering behavior as a crack of similar dimension. The diameter of the penny-shaped crack is 0.333 inches.

3. EXPERIMENTAL SETUP

A test plan has been made for the ultrasonic inspection of the plate with stiffener using a non-contact sensing approach. A schematic diagram of the test plan is illustrated in Figure 2. The plate is sub-divided into two sections. One section contains the half-penny-shaped crack and the other section contains no crack. The section that contains the crack is, hereafter, referred to as “cracked” and the section without crack is, hereafter, referred to as “pristine”.



.333

.010

- .333

.333

.500

24 inch

24 inch

Dimensions are in inchStiffener

(b) Side view of the plate with stiffener

Stiffener

(a) Isometric view of the plate with stiffener

(c) Zoomed-in view of the stiffener with dimension

Slit

Slit



Figure 1. Geometric dimension of the plate with stiffener used for manufacturing.

Two lead zirconate titanate (PZT) wafer type (also termed as piezoelectric wafer active sensor, PWAS) transmitters are used in each section of the plate. These PWAS transducers are bonded back to back on the opposite surfaces of the plate. This has performed by a careful measurement of the transmitter location since both of them are to be bonded at the same location: one at the top and other at the bottom of the plate. These PWAS transducers are used to excite the structure with a selective type of Lamb wave mode.

Figure 2. Test plan for the ultrasonic inspection of the plate to detect crack in the stiffener using non-contact laser measurement.

A wave absorbing clay boundary is used around the plate to minimize the plate edge reflections. This wave absorbing boundary has been put very carefully by maintaining a linearly varying thickness profile from inner edge to outer edge of the boundary. In the past, it has been found that a linearly varying profile of the wave absorbing boundary successfully minimized the edge reflections. This allowed us to analyze the wave signals that are solely from the stiffener and the crack.

Wave absorbing boundary

6”

LASER receiver

8”

6”

X

14”

Excitation signal: 3 count toneburst at 200 kHz

PWAS Transmitter Half-penny

shaped crack

PWAS Transmitter

Stiffener without crack (pristine)

Stiffener with Crack (cracked)

1 2 3



Figure 3. (a) Experimental setup for scanning using laser Doppler vibrometer (LDV), (b) The test specimen with a laser beam at the center of the scanning area.

A non-contact sensing methodology has been adopted in this research. A laser receiver has been used to capture the waveforms at different locations. Laser Doppler vibrometry (LDV) is used in this experiment. The experimental setup is

(a)

(b)

Test specimen

LDV Controller

Function Generator

Oscilloscope

LDV laser head

Test specimen

Laser beam

PWAS exciter



Time =74/ís

Time =74/Ls

illustrated in Figure 3a. A single point laser beam from the interferometer is directed to a certain location of the plate specimen and the reflected laser beam is compared with an internal reference beam. The out-of-plane motion of the surface changes the frequency and phase of the reflected beam which is measured by the interferometer. The out-of-plane velocity is measured by using the Doppler shift in frequency of the reflected laser beam with an internal process of the LDV. The laser beam is directed at desired locations by using a motion controller. The laser measurements are performed over an area by using suitable discretization. The out-of-plane velocity measurements at certain locations (1, 2, 3 in Figure 2) are also performed. The test specimen with a laser beam is shown in Figure 3b. A very thin reflective tape is used for better laser measurement with high signal-to-noise ratio.

4. RESULTS AND DISCUSSION

The laser scanning has been performed over an area of 15 inches by 2 inches. Both S0 and A0 Lamb wave excitation were used by the PWAS transducers.

Figure 4. LDV scanning results for comparison of the wavefield due to the pristine and cracked stiffener. In both cases, S0 Lamb waves are incident waves. The wavefields are captured at the same time of 74-µs. (a) The wavefield due to the pristine stiffener: reflected and transmitted waves are straight-crested waves, (b) the wavefield due to the cracked stiffener: reflected and transmitted waves are no more straight-crested waves; trapped wavemodes are generated within the half-penny-shaped crack; the effect of crack is carried by the propagating waves in the plate.

Straight crested reflected waves

Locally distorted reflected waves

Reflected waves Transmitted waves

Transmitted waves Reflected waves

(a) Pristine stiffener

(b) Cracked stiffener

S0

S0



0 20 40 60 80 100 120 140 160 180

time, µs

0 20 40 60 80 100 120 140 160 180

time, pS

i0 20 40 60 80 100 120 140 160 180

time. Fis

4.1 Wavefield analysis for crack detection, shape and size estimation

The laser measurement results for pristine and cracked stiffener with S0 Lamb wave excitation are illustrated in Figure 4a and Figure 4b, respectively. In case of the pristine stiffener, there are reflected waves coming from the stiffener and some wave are transmitted through the stiffener. But, the reflected and transmitted waves maintain the straight-crested nature. For the cracked stiffener, there are reflected waves and transmitted waves from the stiffener. In addition, there are reflected and transmitted waves from the crack in the stiffener. Thus, the straight-crested nature of the reflected waves and transmitted waves are broken. Interestingly, the straight-crested nature in the reflected waves is broken more than the transmitted waves as observed from the wavefield shown in Figure 4b.

Figure 5. Transmitted waveform comparison between pristine and cracked stiffener due to A0 incident Lamb wavemode. The waveforms are captured at three different locations on the transmission side as marked by 1, 2, 3 in Figure 2. The points 1, 2, 3 are located at 4.2, 4.6, 4.8 inches from the stiffener. Due to the presence of a horizontally oriented crack, more transmission of A0 mode is observed.

1

2

3

___ Pristine ___ Cracked

Transmitted A0



0.4

E 0.2

N0

o -0.2

-0.40 20 40 60 80 100 120 140 160 180

time, ic.s0.4

>

o -0.2 -TD

-0.4

é

0 20 40 60 80 100 120 140 160 180

time, As

0.4cn

0.2

N> oT--o -0.24)>

-0.40 20 40 60 80 100

time, ps120 140 160 180

In addition, there are trapped wave modes exist within the half-penny-shaped crack. The trapped waves originated from multiple reflections within the semi-circle boundary of the crack since this is the line of discontinuity. The back and forth motion of the trapped waves may produce additional waves that can travel in reflected and transmitted side of the stiffener.

The shape of the crack can be visualized from the isolines (isoline - line with equal signal amplitude and phase) of the wavefields as shown by the dotted lines. In addition, the size of the deformed isoline is pretty much similar to the size of the crack. It is interesting to note that these isolines deformed more in the refection side than the transmission side.

Figure 6. Transmitted waveform comparison between pristine and cracked stiffener due to S0 incident Lamb wavemode. The waveforms are captured at three different locations on the transmission side as marked by 1, 2, 3 in Figure 2. The points 1, 2, 3 are located at 4.2, 4.6, 4.8 inches from the stiffener. Due to the presence of a horizontally oriented crack, less mode-conversion of the S0 model is observed.

___ Pristine ___ Cracked

1

2

3

Mode converted A0



4.2 Waveform analysis for Incident A0 Lamb Waves

In SHM, it is important to study the propagating wavemodes that would give an idea how far the effect of the crack can exist. In other words, by studying this one knows how far can one put sensors to detect the crack in the stiffener.

The transmitted wave packets due to A0 incident Lamb wave are illustrated in Figure 5. It shows the comparison between the pristine case and cracked case. Three waveforms are captured at three different locations such as 4.2, 4.6, 4.8 inches from the stiffener. It can be observed that the amplitude of the transmitted A0 wave packet is a distinguishing factor between the pristine and cracked stiffener. The amplitude of the transmitted A0 packet is increased in each of the signal obtained from cracked stiffener. Since this is a horizontally oriented crack, the A0 waves find an easier path to go directly through the plate with less interruption in the crack area of the stiffener.

At a smaller distance from the stiffener, it would be much stronger signals. As the distance becomes larger, the geometric spreading of the waves causes lower amplitudes. Despite that, we observed that at about 5 inches away from the stiffener, we can clearly see the difference between the pristine and cracked signals.

4.3 Waveform analysis for Incident S0 Lamb Waves

The transmitted wave packets due to S0 incident Lamb wave are illustrated in Figure 6. It shows the comparison between the pristine case and cracked case. Three waveforms are captured at three different locations such as 4.2, 4.6, 4.8 inches from the stiffener. It can be observed that the amplitude of the first S0 packet is more or less same for pristine and cracked stiffener which was not the case for incident A0 as discussed earlier.

For S0 incident wave, it can be observed that mode conversion is the distinguishing factor between the cracked and pristine signal. For cracked stiffener, there is less mode conversion (from S0 to A0) than the pristine one. Since this is a horizontally oriented crack and there is a discontinuity in the semi-circular area, the S0 waves experience less mode conversion. The stiffener is the source of asymmetry in the plate, which causes the mode conversion from S0 to A0. The crack area in the stiffener eases the path of S0 wave propagation without being mode converted from the stiffener.

At a smaller distance from the stiffener and in the reflection side of the stiffener, there would be much stronger distinguishing signals between the pristine and cracked stiffener. The transmission side is less sensitive than the reflection side. Despite that, we still clearly see the difference between the pristine and cracked signals when we look at the right distinguishing feature (mode conversion in this case) at about 5 inches away from the stiffener.

5. SUMMARY AND CONCLUSIONS

A more complicated, but realistic structure has been used for the experimental study of ultrasonic guided Lamb wave interaction. A non-contact type laser sensing approach has been adopted in this experiment. A plate with a stiffener and a horizontal half-penny-shaped crack in the stiffener in the experiment. The presence of a crack in the stiffener produces additional scattered waves and trapped waves. These scattered waves and trapped waves were experimentally measured by a scanning laser Doppler vibrometer (SLDV). The scattered waves from the pristine stiffener and cracked stiffener were analyzed and compared with each other. The analyses suggested that both size and shape of the horizontal crack may be predicted from the pattern of the scattered waves. In addition, we found that certain feature of the scattered waves may be more suitable than others. Identification of an appropriate feature of the scattered waves may provide with a better information of the damage. For instance, in this experiment, the direct transmission feature may be used to detect the crack for incident A0 Lamb wave. The mode-conversion feature may be used to detect the crack for incident S0 Lamb wave. The reflection feature may be used to determine the shape and size of the crack.



6. FUTURE WORK

The reflection side of the stiffener can be studied more to determine the shape of the crack. A quantitative study can be performed to estimate the size of the crack from the measured waveforms. A frequency-wavenumber analysis may be useful to distinguish the different modes of propagating waves. The trapped waves within the crack area can be studied more to extract more features of the damage.

7. ACKNOWLEDGEMENT

Support from the National Aeronautics and Space Administration (NASA) and Intelligent Automation, Inc. (IAI) are thankfully acknowledged.

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