Residual Strength and Fatigue Lifetime of ... - Solid Mechanics
Residual Strength and Fatigue Lifetime of ... - Solid Mechanics Residual Strength and Fatigue Lifetime of ... - Solid Mechanics
toughness using MMB fracture toughness results and differing crack tip details between the panels and the MMB specimens due to mechanical releasing of the debonded area in the panels. However, in most cases an acceptable deviation (a minimum deviation of 9%) was obtained. It was observed that in the panels and the MMB specimens with H130 and PMI cores the debond initially kinks into the core and propagates beneath the face/core interface, but in the panels and the MMB specimens with H250 core the debond propagates directly in the interface. Finally, based on experimental and numerical results, the strength reduction factor Rl was plotted against the debond diameter. The plot is tentative because of the uncertainties regarding the intact strengths as well as the differences between test and analysis results. 6.2 Fatigue Crack Growth in Bimaterial Interfaces After developing a methodology for analysis of interface crack propagation in sandwich structures exposed to quasi-static loading, it was applied to simulation of fatigue crack growth in bimaterial interfaces. However, in order to achieve acceptable computational efficiency, the problem of very high computational time due to simulation of many cycles and the need for a heavy element mesh density at the crack tip (front) must first be solved. To overcome the above-mentioned problem a cycle jump method for accelerating the simulation of fatigue crack growth in a bimaterial interface was developed. The proposed method is based on finite element analysis for a set of cycles to establish a trend line, extrapolating the trend line spanning many cycles, and use the extrapolated state as an initial state for additional finite element simulations. Two finite element routines were developed in order to simulate fatigue crack growth in bimaterial interfaces. The first routine is suitable for 2D crack growth and the second is applicable to any 3D fatigue crack growth simulation with an arbitrary crack front shape. To examine the computational efficiency and accuracy of the developed numerical schemes, they were applied to simulation of face/core interface fatigue crack growth in sandwich beams (2D) and sandwich panels (3D). The results of the simulations were compared with reference analyses simulating all individual cycles. Using the cycle jump method, fatigue crack growth in the interface of a sandwich beam was simulated for 500 cycles and verified against a reference analysis. The computational efficiency and accuracy of the cycle jump method were discussed on the basis of three parameters: crack length, difference between maximum and minimum energy release rate in a cycle (G), and mode-mixity phase angle. The effect of the control parameters governing the computational efficiency and accuracy of the developed cycle jump method was studied. The results suggest that the computational efficiency of the simulations increases considerably by increasing the control parameters. However, the accuracy of the simulations decreases. It was shown that with an appropriate choice of control parameters more than 65% savings in computational time can be achieved with reasonably good accuracy. 130
Fatigue debond propagation in sandwich panels with an elliptical face/core debond at the centre of the panels was simulated by means of the second finite element routine (3D). The distribution of the mode III energy release rate, GIII, along the crack front was studied for different elliptical debonds. However, only mode I and II components of the strain energy release rate were used in the crack growth routine due to the present lack of experimental methods for characterisation of the effect of GIII on fatigue crack growth. Results show that the mode III crack tip loading is significant close to the longer radius of the ellipse for an elliptical debond with large a/b radius ratios. A reference simulation, simulating all individual cycles, and simulations exploiting the cycle jump method with different control parameters were performed to examine the accuracy and computational efficiency of the developed 3D cycle jump method. Use of the cycle jump method shows good accuracy and leads to a reduction in computational time of more than 70%. 6.3 Face/Core Interface Fatigue Crack Growth in Sandwich Structures After the initial analysis of the developed accelerated fatigue crack growth simulation scheme, it was validated in the last chapter of this thesis against experimental testing and used to study interface fatigue crack growth in sandwich composites. The first finite element routine (2D) was utilised to study face/core fatigue crack growth in cracked sandwich X-joints. Sandwich Tear Test (STT) specimens with a face/core debond representing a cracked sandwich X-joint, were tested under cyclic loading. Fatigue tests were conducted on STT specimens with H45, H100 and H250 PVC cores and glass/polyester face sheets. Digital Image Correlation (DIC) technique was used to locate the crack tip and monitor the crack growth. Different fatigue crack growth paths were observed during the fatigue experiments: For the specimens with H45 core the crack grew unstably in the beginning up to a length of 150 mm in a few cycles. The crack initially propagated unstably in the core underneath the resin-rich cells. After the unstable crack growth, stable crack growth was observed in all specimens. During the stable crack growth the growing crack approached the interface, but never kinked into the interface. This can be attributed to the very low fracture toughness of H45 core compared to the interface. For the specimens with H100 core, the crack initially propagated in the core in a stable manner and then kinked into the interface. The kinked crack continued to propagate in the interface until the end of the experiments where the crack growth eventually stopped due to decreasing energy release rate at the crack tip. 131
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<strong>Fatigue</strong> debond propagation in s<strong>and</strong>wich panels with an elliptical face/core debond at the centre<br />
<strong>of</strong> the panels was simulated by means <strong>of</strong> the second finite element routine (3D). The distribution<br />
<strong>of</strong> the mode III energy release rate, GIII, along the crack front was studied for different elliptical<br />
debonds. However, only mode I <strong>and</strong> II components <strong>of</strong> the strain energy release rate were used in<br />
the crack growth routine due to the present lack <strong>of</strong> experimental methods for characterisation <strong>of</strong><br />
the effect <strong>of</strong> GIII on fatigue crack growth. Results show that the mode III crack tip loading is<br />
significant close to the longer radius <strong>of</strong> the ellipse for an elliptical debond with large a/b radius<br />
ratios.<br />
A reference simulation, simulating all individual cycles, <strong>and</strong> simulations exploiting the cycle<br />
jump method with different control parameters were performed to examine the accuracy <strong>and</strong><br />
computational efficiency <strong>of</strong> the developed 3D cycle jump method. Use <strong>of</strong> the cycle jump method<br />
shows good accuracy <strong>and</strong> leads to a reduction in computational time <strong>of</strong> more than 70%.<br />
6.3 Face/Core Interface <strong>Fatigue</strong> Crack Growth in<br />
S<strong>and</strong>wich Structures<br />
After the initial analysis <strong>of</strong> the developed accelerated fatigue crack growth simulation scheme, it<br />
was validated in the last chapter <strong>of</strong> this thesis against experimental testing <strong>and</strong> used to study<br />
interface fatigue crack growth in s<strong>and</strong>wich composites.<br />
The first finite element routine (2D) was utilised to study face/core fatigue crack growth in<br />
cracked s<strong>and</strong>wich X-joints. S<strong>and</strong>wich Tear Test (STT) specimens with a face/core debond<br />
representing a cracked s<strong>and</strong>wich X-joint, were tested under cyclic loading. <strong>Fatigue</strong> tests were<br />
conducted on STT specimens with H45, H100 <strong>and</strong> H250 PVC cores <strong>and</strong> glass/polyester face<br />
sheets. Digital Image Correlation (DIC) technique was used to locate the crack tip <strong>and</strong> monitor<br />
the crack growth. Different fatigue crack growth paths were observed during the fatigue<br />
experiments:<br />
For the specimens with H45 core the crack grew unstably in the beginning up to a length<br />
<strong>of</strong> 150 mm in a few cycles. The crack initially propagated unstably in the core underneath<br />
the resin-rich cells. After the unstable crack growth, stable crack growth was observed in<br />
all specimens. During the stable crack growth the growing crack approached the<br />
interface, but never kinked into the interface. This can be attributed to the very low<br />
fracture toughness <strong>of</strong> H45 core compared to the interface.<br />
For the specimens with H100 core, the crack initially propagated in the core in a stable<br />
manner <strong>and</strong> then kinked into the interface. The kinked crack continued to propagate in<br />
the interface until the end <strong>of</strong> the experiments where the crack growth eventually stopped<br />
due to decreasing energy release rate at the crack tip.<br />
131
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