Approach for analysing off-axis tunnelling cracks in biaxially loaded laminates

A novel approach is presented on how the growth of off-axis tunnelling cracks in a composite laminate subjected to biaxial loads can be studied on an equivalent laminate with an another off-axis angle under uniaxial load. The primary objective is to determine the off-axis angle of the equivalent laminate such that the mode mixity of tunnelling cracks is identical to that of the tunnelling cracks in the original biaxially loaded laminate. The situation of steady state cracking is considered for both the laminates and the stress state at the crack front is expressed in terms of the mode mixity and the steady state energy release rate, both averaged across the cracked ply thickness. The approach is based on an energy accounting method that uses the crack opening displacements, obtained from a finite element analysis, and the principle of superposition to account for biaxial loads. Results are presented in terms of average energy release rates and average mode mixity for an isolated crack in an arbitrary oriented off-axis ply subjected to uniaxial and biaxial loads. The design of the equivalent uniaxially loaded laminate is analysed with reference to the general off-axis laminates subjected to a range of biaxial load ratios. With respect to testing, the presented approach can replace a complex biaxial fatigue test method with a more simple uniaxial fatigue test.