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

Figure 3.5 shows the face/core debond fracture toughnesses vs. phase angle for specimens with
H130, H250 and PMI 51 IG cores. The mechanical properties for these materials were listed in
Tables 3.2 and 3.3. It should be noted that the DBL reinforcement in lay-up C was oriented with
the 0º plies parallel to the load direction in the panel tests and transverse to the beams in the
MMB fracture specimens, thus simulating the fibre orientation in the 3 and 9 o’clock positions
along the circular debond in the panels. Figure 3.5 reveals that the face/core interface fracture
toughness is strongly dependent on the mode-mixity at the crack tip for all sandwich composites
examined herein, especially in mode II dominated loading, i.e. increased shear loading at the
crack tip. However, this dependency is weak in mode I dominated loadings, i.e. low magnitudes
of mode-mixity, where roughly constant fracture toughness is observed, see Figure 3.5. A
phenomenological relationship between fracture toughness and mode-mixity proposed by
Hutchinson and Suo (1992) is used to fit the experimental data by Equation (3.14). A fitted curve
(by visual inspection) is represented by a continuous line in Figure 3.5, for the three sandwich
composites being examined, i.e. specimens with H130, H250 and PMI cores. In addition, the
upper and lower bounds are shown as dashed lines. A large scatter in the fracture toughness vs.
phase angle results is observed, which is expected in sandwich composites due to different
manufacturing defects and crack propagation paths at the face/core interface.
2
tan
G 1 1k Gc IC
(3.14)
In Equation (3.14), GIC is the interface fracture toughness in pure mode I and k is a nondimensional
curve fitting parameter. Here, since in mode I dominated loading, a roughly constant
fracture toughness is observed, GIC is assumed to be the minimum fracture toughness value for
the material being evaluated. Thus, when =0, Gc() = GIC.
(a) (b) (c)
Figure 3.5: Fracture toughness vs. phase angle results and curves for specimens with (a)
H130 (b) H250 and (c) PMI cores.
Based on the face/core debond fracture toughness vs. phase angle results shown in Figure 3.5,
the parameters for Equation (3.14) are provided in Table 3.4 for each sandwich configuration.
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