Experimental and computational study of the damage process in CFRP composite beams under low-velocity impact

Damage process in composites subjected to low-velocity impact is investigated both experimentally and numerically. Drop-weight impact experiments are carried out, in which a model unidirectional [05/903]s CFRP laminate beam is impacted by a cylindrical head creating an almost uniform two-dimensional loading condition. Initiation and progression of damage, consisting of matrix cracks and delamination, are visualized in real-time via ultra-high-speed camera at rates up to 60,000fps and the sequence of failure events are clearly captured. Evolution of dynamic strain fields in the laminate is then quantified by a Digital Image Correlation (DIC) analysis and the resulting final failure patterns are characterized by a digital microscope. In the computational part, a three-dimensional finite element analysis is performed using ABAQUS/Explicit to simulate the experiments. In these simulations, the intraply matrix damage in the middle 90° layers is modeled using a Continuum Damage Mechanics (CDM) based composite failure theory with LaRC04 initiation criterion and implemented via a user-written VUMAT subroutine. Delamination is modeled using cohesive interface elements that are introduced between the 0°/90° interfaces. Damage initiation time, location and the interaction of failure modes are compared with the experimental data. Real-time observations of the sequentially occurring diagonal matrix cracking followed by dynamic delaminations are made. In addition to the major diagonal matrix cracks, existence of multiple diagonal micro-matrix cracks near the upper interface are shown which are also predicted by the simulations. Finally, experimentally obtained real-time strain field values, failure mechanisms and the failure sequence are shown to be in good agreement with the simulations. We believe that the elaborate experimental results presented here for an idealized composite layup can serve as a benchmark test case to validate composite and interface damage modeling methods.