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

3.6 Conclusion
In this chapter the compressive failure of foam cored sandwich panels containing a face/core
circular debond was experimentally and numerically investigated. Sandwich panels with
glass/polyester face sheets and H130, H250 and PMI foam cores were tested in a specially
designed test rig. All debonded panels failed by the propagation of the debond to the edges of the
panels. All intact panels with H130 and H250 cores 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 insert and the core and to a slight unintentional mismatch
between the core and insert thicknesses. Intact panels with PMI core failed by a combination of
shear crimping and global buckling. MMB characterisation tests were conducted to measure the
fracture toughness of the face/core interface for a span of mode-mixity phase angles. Results
showed increasing fracture toughness for increasing magnitude of the phase angle. A large
scatter was observed in the fracture toughness results due to brittleness of the core material,
different manufacturing defects and dissimilarity in crack propagation paths at the face/core
interface.
Instability and crack propagation loads of the panels were estimated based on geometrically nonlinear
finite element analysis and linear elastic fracture mechanics. A numerical scheme similar
to the one developed in Chapter 2, based on submodelling was used for the simulations. In some
of the panels the FEA predictions are up to 46% higher than the experimental ones, which can be
attributed to the large scatter in the measured fracture 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. However, in most of the panels better agreement, up
to 20% deviation, is observed between numerical and experimental results. To investigate the
debond propagation path in the tested panels, some of the panels were cut and it was observed
that the debond propagation path is similar between the panels and the MMB specimens. It was
observed that in the panels and the MMB specimens with PMI core and panels with H130 core,
the debond kinks into the core and propagates beneath the face/core interface. However, in the
MMB specimens with H130 core two different crack growth paths were observed. In the H130
MMB specimens with the mode-mixity phase angle of 0° >> -20° (similar to the panels), the
crack path was located below the face/core interface. However, when the magnitude of the
mode-mixity phase angle was increased to -25° >> -65°, the crack path was directly in the
face/core interface. In the panels and the MMB specimens with H250 core the debond
propagates directly in the interface. This may be explained by the higher fracture toughness of
the H250 core compared to H130 and PMI cores. Fibre bridging was observed after more than 4-
5 mm crack growth in the MMB specimens and panels with H250 core. The similarity between
the debond propagation paths in the MMB specimens and the panels refutes the role of the
different propagation paths in the inaccuracy of the determined debond propagation loads.
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