Failure mechanisms and damage evolution of laminated composites under compression after impact

Presently, Carbon Fiber Reinforced Polymers (CFRP) are gradually replacing metals for various application in structural components. In comparison to their counterparts, CFRP has distinct mechanical properties such as strength, fatigue resistance, good corrosion performance among others which make them favorable for such applications. However, CFRP materials are in most cases susceptible to failures caused by out-of-plane loading, which includes quasi-static indention and low-velocity impact. This result due to the absence of reinforcement in the through-thickness direction in the laminated CFRP.

Usually, this kind of damage is difficult to detect and access during inspection as it does not leave any significant traces on the fiber composite surfaces. Therefore, to avoid such challenges, design capability needs to assume such damage already exists in the material and demonstrate satisfactory performance under ultimate loading.

Dr. Xiaochuan Sun and Professor Stephen Hallett at the Bristol Composites Institute (ACCIS) of University of Bristol, in the United Kingdom investigated the damage evolution and failure mechanisms of a laminated composite containing Barely Visible Impact Damage (BVID). They used both advanced numerical analysis and data rich experimentation to study damage and failure of composite materials under indention, impact and compression after impact tests. This study was aimed at providing the information regarding the behavior of the material during the compression after impacts tests that could be used in design of safer composite structures. Their work is published in the research journal, Composites Part A: Applied Science and Manufacturing.

In brief, their experiments involved two different setups, one for the static indention and the other one for low-velocity impact, both using quasi-isotropic laminates. The full-field displacement of the specimens during the compression after impact (CAI) tests was captured using 3D Digital Image Correlation (DIC). DIC was especially applied during the moment of failure using high speed video cameras. Finally, they evaluated the accuracy of their models by comparing their experimental results. The authors successfully observed that the results recorded using the digital image correlation provided excellent data for model validation, as well as the understanding of the mechanisms behind the CAI failure.

Based on numerical analysis developed in previous studies, the authors were able to investigate the CAI failure mechanisms through buckling behaviors, surface damage growth and delamination propagation. They were able to show that the critical stages were growth of the initial dent and coalescence of the existing delamination damage, which was then followed by a rapid propagation of the buckled and delaminated regions in the lateral direction, towards the plate edges. The numerical analysis performed was able to provide unique insight into these mechanisms, that could not be easily obtained from the experimental testing alone. Sun and Hallett are optimistic that their study will lead to better understanding of failure and damage evolution in laminated composites, and thus ultimately better and safer designs of structures made from CFRP materials.