surfaces. The CFRP also presented more of a skip start behavior but this was likely due to larger resin rich regions since it has a coarser weave than the GFRP. Unlike the bulk material DCB tests, the ADCB shows a significant temperature dependence (Fig. 16.8). As the temperature drops, theΔT increases which leads to higher residual stresses at the bi-material interface. Since higher residual stresses are correlated with higher apparent strain energy release rate it is assumed that there is a complex stress state at the crack tip. With the help of computational methods, the stress fields at the crack tip can be investigated further to determine whether it is simply the residual stresses producing a stress state that is counter to the applied loading or perhaps the in-plane shear stresses along the interface are producing mixed-mode behavior. It is also worth pointing out that in the bulk material DCB tests, the critical energy release rate for the GFRP was slightly under 300 J/m2 and the CFRP is between 250 and 350 J/ m2. However, in the ADCB tests, the elevated temperature, least tough experiment showed a critical energy release rate of around 400 J/m2, higher than that of any of the bulk material tests. This further suggests that the small amount of residual stress even at elevated temperature is beneficial to the crack propagation resistance. Fig. 16.7 Energy release rate vs crack length for (a) GFRP/ GFRP and (b) CFRP/CFRP laminates 16 Effect of Process Induced Stresses on Measurement of FRP Strain Energy Release Rates 163
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