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

due to the proximity of the debond propagation load and the compression failure load of the face
sheets. Bifurcation type buckling of the debonded face sheet was not observed and the debond
opening occurred gradually, which can be attributed to large initial imperfections. Slight kinking
of the debond into the core was observed in the columns with a low-density H45 and H100 core.
Modified Tilted Sandwich Debond (TSD) specimens were tested under different tilt angles to
measure the fracture toughness of the interface at the calculated mode-mixity phase angles for
the column specimens associated with the debond propagation.
The measured interface fracture toughness was used to determine crack propagation loads from
the finite element model of the columns. Instability and crack propagation loads of the columns
were predicted on the basis of a geometrically non-linear finite element analysis and linear
elastic fracture mechanics. Fair agreement was achieved for the comparison of the measured outof-plane
deflection, instability, and debond propagation loads from the experiments and the finite
element analysis. For most of the investigated column specimens, it was shown that the
instability and debond propagation loads are very reasonable estimates of the ultimate failure
load, unless the other failure mechanisms occur prior to buckling instability.
To examine the accuracy of the developed scheme in case of sandwich panels, debond
propagation in sandwich panels with a circular debond at the centre was modelled. To validate
the finite element model of the debonded panels, intact and debonded sandwich panels with
glass/polyester face sheets and H130, H250 and PMI foam cores were tested under static inplane
compressive loading. The following damage mechanisms were observed during the
experiments:
1) All debonded panels failed by the propagation of the debond to the edges of the panels.
2) All intact panels with H130 and H250 core failed by the compressive failure of a face
sheet very close to the wooden inserts, which can be attributed to additional peeling
stresses arising due to the junction between the wooden insert and the core and to a slight
unintentional mismatch between the core and the thicknesses of the insert.
3) Intact panels with PMI core failed by a combination of shear crimping and global
buckling.
This time instead of using the TSD specimen, characterisation tests were performed on Mixed
Mode Bending (MMB) specimens to measure the fracture toughness of the face/core interface
for a span of mode-mixity phase angles. As expected it was shown that the fracture toughness is
increasing with increasing magnitude of the mode-mixity phase angle. The obtained fracture
toughness data was used to determine the crack propagation load in the debonded sandwich
panels. Instability and crack propagation loads of the panels were estimated on the basis of
geometrically non-linear finite element analysis and linear elastic fracture mechanics. It was
shown that the FEA predictions in few cases are much higher than the experimental ones (a
maximum deviation of 46%), which can be attributed to the large scatter in the measured fracture
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