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

Since the observed large-scale fibre bridging in the STT specimens with H250 core violates the
initial assumptions of linear elastic fracture mechanics in the developed numerical fatigue crack
growth scheme, the H250 specimens were discarded and characterisation of the interface was
only performed for the specimens with H100 and H45 core. MMB sandwich specimens of each
core type were manufactured with 20 mm core and 2 mm face sheet thickness. An initial 20 mm
long start crack was defined in the face/core interface of the MMB specimens by inserting a
Teflon film, 30 m thick, during the manufacturing process. Similar face sheets, core materials
and manufacturing processes as for the STT specimens were used in the manufacturing of the
MMB specimens. The specimens were 35 mm wide with a span length (2L) of 160 mm.
Figure 5.19: Mixed mode bending rig with the MMB sandwich specimen.
To determine the mode-mixity at which the face/core interface fatigue behaviour should be
characterised by MMB tests, the mode-mixity phase angle at the crack tip of the STT specimens
was evaluated by the finite element method at a load corresponding to the maximum fatigue load
in the STT fatigue tests (to be presented in the next section). In all the STT specimens the modemixity
phase angle for different crack lengths is between -5 to -20 , which implies mode I
dominant loading at the crack tip. The MMB lever arm distances (c) resulting in similar modemixities
as those of the STT specimens were determined from the finite element model of the
MMB specimen shown in Figure 5.20. The FE model was developed using PLANE42 elements
in the commercial finite element code ANSYS. The phase angle and the energy release rate are
determined from relative nodal pair displacements along the crack flanks obtained from the finite
element analysis using the CSDE method as outlined in Chapter 1. The characteristic length h is
arbitrarily chosen as the face sheet thickness. Figure 5.21 shows the variation of the mode-mixity
phase angle vs. the lever arm distance (c) in the MMB specimens. At small level arm distances
the mode-mixity phase angle increases significantly and mode II dominant loading is present at
the crack tip. Increasing the c distance, the phase angle converges to around -20 for the present
specimen geometry. It appears that with the current design of the test rig and the MMB specimen
it is not possible to reach mode-mixity phase angles more than -20. Therefore, only a -20
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