Experimental and Computational Studies of the Low Velocity Impact and Compression After Impact of Carbon Fiber Reinforced Polymer Composites

Carbon fiber reinforced polymer (CFRP) composites have been widely used in many industrial sectors because of their high strength-to-weight ratio, cost advantages, and the possibility of tailoring the design. During the lifetime of a composite structure, low velocity impact (LVI) tends to happen in many scenarios, including manufacturing, service, and maintenance operations. A small event such as a tool drop may cause barely visible impact damage (BVID) with extensive internal cracking while without any noticeable marks on the outer surface of a composite structure. LVI-induced damage can lead to a significant reduction in post-impact compressive strength. Therefore, the LVI and compression after impact (CAI) problems have received continued attention for decades. Considerable effort on experimental and numerical investigations persists. Generations of accurate, efficient, versatile, and robust numerical tools have been developed to tackle the LVI and CAI problems virtually to save experimental costs and accelerate the verification and validation (V&V) process. Experimental and computational studies on the LVI and CAI damage of CFRP laminated composites are presented in this dissertation. Effects of laminate stacking sequence, impact energy, and panel size on the LVI and CAI are investigated. In the experimental part, LVI test results are reported. Non-destructive inspection (NDI) techniques, including ultrasound C-scanning and micro computed tomography (µCT), are conducted to characterize the impact damage. With the high-resolution µCT scanning, damage mechanisms are analyzed. Following the LVI tests and NDI characterization, CAI tests are done to relate the impact energy to the degradation of compressive strength. CAI fixtures for standard-size composite panels are modified to customize to panels with increased sizes. Post-buckling responses during the CAI of panels with increased sizes are presented. Parallel with the experimental results, computational results are obtained with the enhanced Schapery Theory (EST) model, which is a seamless combination of the Schapery theory (ST) and the crack band (CB) method. EST based on 2D plane stress is extended to a 3D stress state. A novel mixed-mode cohesive law is integrated to model the degradation of stress components progressively and simultaneously. The capability of EST to capture matrix inelasticity is implemented. 2D and 3D EST applied to LVI and CAI modeling are verified against the experimental results. A high-fidelity and high-efficiency LVI-CAI computational framework is established based on 2D EST to accurately predict the LVI damage and CAI strength with significantly improved efficiency. The computational results agree well with the experimental results in terms of the load responses, damage morphology, and CAI strength. This dissertation provides detailed LVI and CAI results concerning the effects of stacking sequence, impact energy, and panel size. The EST model developed is validated and will be useful in the V&V process of aeronautical composite structures. Furthermore, the high-fidelity and high-efficiency LVI-CAI computational framework enables obtaining the CAI strength with significantly reduced computational time, which will help enlarge the design space of CFRP laminated composites with increased impact resistance.PHDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/171345/1/sylinae_1.pd