Structural composite laminates are often subjected to flexural loads which induce significant shear stresses in ply interfaces, leading to delamination growth in mode-II. In certain laminates, delamination growth is accompanied by the fiber-bridging phenomenon in which fibers form bridge-like micro-structures between crack faces, hindering crack growth. Such a phenomenon delays the onset of steady-state delamination growth, requiring greater energy inputs to advance the interlaminar crack growth, which is known as the R-curve effect in Fracture Mechanics. While mode-I failure in the presence of fiber-bridging has received considerable attention within the research community, mode-II failure has received disproportionately less attention despite its significance. In the present work, a novel regressive cohesive damage model is developed, and a combination of numerical and experimental studies, as well as three-dimensional (3D) DIC analysis is employed to validate the model as well as study the mechanics and mechanism of mode-II delamination growth in a GFRP laminate, considering the fiber-bridging phenomenon. Due to the confluence of several energy dissipation mechanisms during damage evolution in the presence of fiber bridging, accurate simulation of delamination evolution requires the implementation of multi-linear or exponential softening. In the present work, the total energy dissipated during damage evolution is considered to be made up of multiple components, each corresponding to a certain physical observation. As such, the regressive exponential softening law is formulated such that it accounts for the variety of dissipation mechanisms involved in the damage evolution process. Numerical prediction of experimental data indicates high accuracy of the model with less than 2 and 5% errors in the prediction of structural response and delamination-crack growth, respectively.
Koloor et al. (Thu,) studied this question.