Fatigue crack growth monitoring

in multi-layered tensile specimenMr. Erik Kostson, Dr Paul Fromme

DEPARTMENT OF MECHANICAL ENGINEERING

Introduction Results

Conclusions

Aircraft structures are subject to various types of cyclic loading

conditions. Stress concentration at fastener holes connecting

multiple layers, can lead to the development of fatigue

cracks [1]. When only one side of the structure is accessible, the

detection of fatigue cracks located in other layers (figure 1) can

sometimes be difficult.

The excitation and wave propagation of low frequency guided

ultrasonic waves in multi-layered tensile specimens was studied.

FE simulations show the existence of multiple flexural modes for

the excitation used. Both experiments and 3D FE simulations show

a shift in the amplitude pattern due to a fatigue crack at a fastener

hole. Experimental fatigue monitoring of three tensile specimens

shows an amplitude increase with increasing crack length. This is

in good agreement with results obtained from FE simulations

predicting a similar increase. Future work involves optimising the

excitation in order to obtain a single mode. Fatigue experiments

will be carried out and the scattering phenomena of low frequency

guided ultrasonic waves at a fastener hole will be studied.

1 C. Boller, “Next generation structural health monitoring and its integration in

aircraft design”, Int. J. Sys. Sci. 31 (11), 1333-1349 (2000)

2. Inside an aircraft wing, (10/ 2007), www.zenithair.com\stolch701\7-construc.html3. M. Castaings, et al., “Wave propagation along transversely periodic structures”,

J. Acoust. Soc. Am. 121 (4), 1935-1944 (2007)

4. E. Kostson and P. Fromme, “Defect detection in multi-layered structures

using guided ultrasonic waves”, in Rev. of Prog. in Quantitative

Nondestructive Evaluation 28A, AIP Conference Proceedings 1096, 209-216 (2009)

5. E. Kostson and P. Fromme, “ Fatigue crack growth monitoring in multi-layered structure using guided ultrasonic waves”, submitted to the Journal of Physics;

Conference Series.

Methodology

Experimentally the waves were excited (centre frequency: 115 kHz)

using three piezoelectric discs. The out of plane displacement of the

wave was measured using a laser interferometer (figure 3). Three

specimens were fatigued (figure 4) and low frequency ultrasonic

waves were used to monitor the fatigue crack growth and the

influence of a fatigue crack on the scattered field.

Objectives

The main objective of the PhD thesis was to employ low frequency

guided ultrasonic waves (similar to a bending wave) for the

detection of fatigue cracks located in one of the layers of a multi-

layered tensile specimen (figure 2). The wave propagation and

scattering of low frequency ultrasonic bending waves in multi-

layered tensile specimens was studied.

Figure 7. Optically measured crack length against

number of fatigue cycles for multi-layered model

structure.

Figure 8. Change in amplitude during the

monitoring of a single point against optically

measured crack length; FE: blue curve.

Figure 5. Wave field obtained from 3D

FE simulations (ABAQUS).

Figure 1. Fatigue cracks inside the wing of an aircraft [2].

Figure 3. Experimental setup Figure 4. View of fatigue setup

The tensile specimens consist of two aluminium plate strips (3 mm

thick, 600 mm long and 40 mm wide) bonded together by a 0.2 mm

thick adhesive layer (figure 2). A 6 mm diameter fastener hole was

introduced in the centre of the specimen.

Model Structure

Figure 2. Multi-layered model structure (600 x 40 x 3 mm).

Cyclic tensile loading of three tensile specimens was carried out.

During the fatigue experiments, crack initiation occurred at about

25 000 cycles. After 35 000 cycles the fatigue crack had grown

through the bottom layer. The measured amplitude in front of the

crack (top layer) increases with increasing crack length. FE

simulations (blue curve, figure 7) show a similar amplitude

increase for increasing crack length [5].

The wave propagation obtained from 3D FE simulations includes

multiple bending modes [4] with an amplitude pattern across the

width (figure 5). Good agreement was found between the scattered

field obtained from measurements and FE simulations (figure 6). In

both experiments and FE simulations it was seen that the

presence of a defect (5 mm long fatigue crack, bottom layer)

produces an asymmetry in the scattered field (top layer) and an

increase in amplitude of about 10-15 % in front of the defect (figure

6 b, d). This location was monitored during the fatigue crack

growth monitoring.

In order to understand the wave propagation in the model structure,

dispersion curves and mode shapes were obtained from Semi-

Analytical Finite Element (SAFE) calculations [3]. Assuming a wave

propagation along the length of the specimen, the 3D problem

simplifies to an equivalent 2D problem. By solving the eigenvalue

problem for a certain frequency, the corresponding wave number

and mode shape are calculated. 3D FE simulations

(ABAQUS/Explicit) were performed in order to study the wave

propagation resulting from the excitation used in the experiments

and the scattering at the fastener hole.

Figure 6. Amplitude of scattered field (f = 115 kHz);

a) No crack (FE); b) 5 mm crack (FE);

c) No crack (Exp.); d) 5 mm crack (Exp.).

(a) (b)

(c) (d)

Crack

Crack