Residual Strength and Fatigue Lifetime of ... - Solid Mechanics
Residual Strength and Fatigue Lifetime of ... - Solid Mechanics
Residual Strength and Fatigue Lifetime of ... - Solid Mechanics
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S<strong>and</strong>wich Tear Test (STT) specimens with face/core debonding exposed to cyclic loading have<br />
been tested to study the fatigue behaviour <strong>of</strong> the interface cracked s<strong>and</strong>wich X-joints. <strong>Fatigue</strong><br />
tests were performed on the STT specimens with H45, H100 <strong>and</strong> H250 PVC cores <strong>and</strong><br />
glass/polyester face sheets. The following fatigue crack growth paths were observed during the<br />
experiments:<br />
For the specimens with H45 core, unstable crack growth took place initially. After the<br />
unstable propagation the crack propagated in the core underneath the resin-rich cell layer<br />
moving towards the interface. However, the crack never kinked into the interface due to<br />
low fracture toughness <strong>of</strong> the H45 core compared to the interface.<br />
For the specimens with H100 core, the crack propagated initially in the core <strong>and</strong> then<br />
kinked into the interface <strong>and</strong> continued to propagate in the interface.<br />
For the specimens with H250 core, the crack initially propagated in the core <strong>and</strong> kinked<br />
into the interface. The interface crack eventually kinked into the face sheet, resulting in<br />
large-scale fibre bridging.<br />
By application <strong>of</strong> the finite element method mode-mixity phase angles at the crack tip <strong>of</strong> the STT<br />
specimens were evaluated at different crack lengths using the maximum fatigue load amplitude.<br />
To characterise the fatigue response <strong>of</strong> the interface <strong>of</strong> the STT specimens, fatigue tests were<br />
performed on Mixed Mode Bending (MMB) specimens at similar mode-mixity phase angles.<br />
The da/dN vs. G relations measured by the MMB fatigue tests were used to simulate fatigue<br />
crack growth in the STT specimens by the finite element method. To accelerate the simulation,<br />
the cycle jump method was exploited. Control parameters were introduced to control the<br />
accuracy <strong>of</strong> the cycle jumps. Simulations with different control parameters <strong>and</strong> a convergence<br />
analysis were carried out to choose the most accurate <strong>and</strong> efficient control parameters.<br />
Simulations <strong>of</strong> the specimens with H100 core showed fair accuracy compared to the fatigue<br />
experiments. However, the simulation <strong>of</strong> H45 specimens was found to be less accurate due to<br />
unstable crack growth observed in the fatigue experiments <strong>of</strong> the H45 STT specimens. This<br />
inaccuracy can be attributed to the interface fatigue characterisation. Since the interface fatigue<br />
characterisation was only performed for the stable linear part <strong>of</strong> the crack growth rate diagram<br />
(the Paris regime), the resulting da/dN vs. G relation is not valid for unstable crack growth <strong>and</strong><br />
produces incorrect results.<br />
The numerical scheme developed in Chapter 4 to simulate 3D fatigue crack growth in bimaterial<br />
interfaces was used to simulate fatigue crack growth in s<strong>and</strong>wich panels with a circular debond.<br />
<strong>Fatigue</strong> experiments were conducted on debonded s<strong>and</strong>wich panels to validate the numerical<br />
scheme. S<strong>and</strong>wich panels with a circular face/core debond at the panel centre were exposed to<br />
cyclic loading. <strong>Fatigue</strong> tests were performed on the debonded panels with H45 PVC core <strong>and</strong><br />
glass/polyester face sheets. It was observed that the debond initially kinked into the core <strong>and</strong><br />
continued to propagate underneath the resin-rich cell layer <strong>of</strong> the core. Using the finite element<br />
method, the energy release rate <strong>and</strong> mode-mixity phase angle at the crack tip <strong>of</strong> the debonded<br />
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