An appropriate stiffness degradation parameter to monitor fatigue damage evolution in composites

This article confirms, both computationally and experimentally, the well known observation that damage caused by tensile loading in fiber reinforced polymeric composite laminates may induce only minor variations in the longitudinal stiffness, thereby rendering this stiffness a poor measure for estimating the residual strength of such materials. On the other hand, it is shown herein that the torsional stiffness is highly sensitive to the forms of damage that arise and evolve in the aforementioned composites under tension and appears to correlate very well with the extent of damage. Since the tensile failure scenarios under static and fatigue loads are similar to each other, the latter stiffness can serve as a sensitive parameter for predicting residual life. Furthermore, the current results suggest that the resistance of these materials to twist may be greatly degraded despite their purported insensitivity to reduction in tensile response.