Propagation of tunnelling cracks in composite materials under strain and force-controlled cyclic loading

Abstract

In this paper, the propagation of tunnelling cracks in composite laminates subjected to uniaxial force-controlled and strain-controlled cyclic loading is analysed considering both interacting and non-interacting cracking scenarios. Glass fibre reinforced polymer laminates with ${\left[0/60/0/-60\right]}_S$ layup are used in the analysis. For non-interacting cracks, a strain-controlled test under a single strain value is found to be sufficient to produce the slope of the Paris-Erdogan law type curve relating the crack front growth rate (CGR) with the energy release rate (ERR), but within a limited range of ERR. Whereas, a force-controlled test is found more suitable when characterising the variation of the CGR at a certain ERR value. For closely spaced cracks, commonly seen crack interaction scenarios are identified and the CGR of the crack front is investigated for both loading control modes. It is found that the CGR decreases when volume averaged stresses are reduced ahead of the crack front due to shielding effect at high crack densities. A relationship between these two parameters is presented. It is further found that the CGR of two collinear crack fronts growing towards each other for coalescence do not influence each other. But when the two crack fronts are not collinear the CGR of the crack fronts is significantly affected after they have grown past each other.