Fracture mechanics testing of non-unidirectional FRP composites

Trakas and Kortschot [76] used carbon fibre epoxy (AS4/3501-6) specimens modified with Teflon edge delaminations in a study comparing unidirectional and 0°/90° and 90°/90° interfaces under mode 1, mode 11 and mode HI loading. Delamination resistance decreased for all modes when going from unidirectional to cross-ply and 90°/90°. The tests were complemented with scanning electron microscopy of the fracture surfaces, but they did not yield a direct quantitative correlation with the measured values of Gc- It was noted that Gc could be used as a material property for design as long as the layup (ply orientation) and mode of fracmre corresponded to those in the intended use. 

Prombut et al. [77] used carbon fibre epoxy (T7(X)/M21) laminates in asymmetric double cantilever beam (ADCB) and asymmetric mixed-mode flexure (AMMF) specimens besides standard mode I, mode II and mixed mode I/II flexure specimens in their investigation, and they complemented the experiments with numerical simulations. Although they succeeded in preventing jumps in the delamination plane and achieved reasonable crack front profiles, they concluded that more work would be needed to establish a crack propagation criterion for the 0°/45° interface. 

Data plotted from ESIS TC4 round-robin analysis [74]. 

In all of the laminates mentioned in this chapter, the number of phes with a given orientation can be varied and optimized for the strength or stiffness requirements for the FRP composite elements or stractures, resulting in a different thickness of the respective layer. 

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