Experimental assessment of mixed-mode partition theories

The propagation of mixed-mode interlaminar fractures is investigated using existing experimental results from the literature and various partition theories. These are (i) a partition theory by Williams (1988) based on Euler beam theory; (ii) a partition theory by Suo (1990) and Hutchinson and Suo (1992) based on 2D elasticity; and (iii) the Wang-Harvey partition theories of the authors based on the Euler and Timoshenko beam theories. The Wang-Harvey Euler beam partition theory seems to offer the best and most simple explanation for all the experimental observations. No recourse to fracture surface roughness or new failure criteria is required. It is in excellent agreement with the linear failure locus and is significantly closer than other partition theories. It is also demonstrated that the global partition of energy release rate when using the Wang-Harvey Timoshenko beam or averaged partition theories or 2D elasticity exactly corresponds with the partition from the Wang-Harvey Euler beam partition theory. It is therefore concluded that the excellent performance of the Wang-Harvey Euler beam partition theory is either due to the failure of materials generally being based on global partitions or that for the specimens tested, the through-thickness shear effect is negligibly small. Further experimental investigations are definitely required.