Residual Strength and Fatigue Lifetime of ... - Solid Mechanics

For the specimens with H250 core the crack first propagated in the core due to the initial
crack location after pre-cracking. The crack consequently kinked into the interface due to
the presence of negative mode-mixity phase angle and large fracture toughness of the
H250 core. The interface crack eventually kinked into the face sheet, which resulted in
large-scale fibre bridging.
A 2D finite element model of the STT specimen was developed to determine the mode-mixity
phase angle and the energy release rate at the crack tip of the STT specimens. To characterise the
interface fatigue behaviour of the STT specimens, fatigue tests were conducted on Mixed Mode
Bending (MMB) specimens at a mode-mixity phase angle similar to that of the STT specimens.
The resulting da/dN vs. G relations generated by the MMB fatigue tests were utilised in the
developed crack growth finite element routine to simulate fatigue crack growth in the STT
specimens. To choose appropriate control parameters, simulations with different control
parameters were performed. A convergence analysis was conducted and an appropriate control
parameter was chosen. Simulations of the H45 STT specimens showed a very high dependency
on the control parameters. During the initial cycles, simulations using different control
parameters showed small differences, but as the unstable crack growth zone was approached the
deviation became larger. This dependency is attributed to the extrapolations in the transition
from stable to unstable crack growth zone and the extreme non-linearity of this transition, which
implies the importance of appropriate choice of control parameters in the case of highly nonlinear
problems. With smaller control parameters the crack growth diagrams converged to one
diagram since the cycle jump scheme was able to extrapolate accurately the stable-unstable crack
transition zone by performing small or no jumps. H100 specimens, due to less non-linearity and
stable crack growth, showed much less dependency to the control parameters. The developed
finite element models were validated against the conducted fatigue tests. Simulations of the
specimens with H100 core showed fair accuracy compared to the fatigue experiments. However,
the simulation of the H45 specimens was much less accurate due to unstable crack growth
observed in the fatigue experiments of the H45 STT specimens. Since the interface fatigue
characterisation using the MMB specimens was only conducted for the stable linear part of the
crack growth rates diagram (Paris’ regime), the resulting da/dN vs. G relation was not valid for
unstable crack growth observed during the experiment of the H45 STT specimens and produced
incorrect results.
To validate the 3D fatigue crack growth numerical scheme, it was used to simulate fatigue crack
growth in sandwich panels with a circular debond. Fatigue tests were carried out on a limited
number of debonded sandwich panel specimens with a circular face/core debond at the centre
with H45 PVC core and glass/polyester face sheets. It was observed that the crack initially kinks
into the core and continues to propagate below the resin-rich core cells at the core. Because of a
similar mode-mixity at the crack tip and similar face/core materials, da/dN vs. G relations from
the MMB tests obtained previously were employed as input to the crack growth routine. A
convergence analysis was conducted for different control parameter values to choose appropriate
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