Dynamic Fracture of Advanced Composites With Nanoreinforced Interfaces

Advanced composites have been developed since 1960s and are now widely used in military, aerospace, and civilian applications. The main advantages of composites are their high specific strength and stiffness coupled with tailorability of properties. Laminated composites with unidirectionally reinforced plies are the most popular form of composites that allows high volume fraction of fibers and therefore achieves high utilization of fiber properties. However, laminated composites are prone to delaminate due to edge stresses and under impact. Existing methods of delamination suppression resulted in significant weight or cost penalties. The goal of this dissertation was systematic evaluation and analysis of dynamic fracture behavior of novel laminated composites with nanoreinforced interfaces.

Novel methods were developed for quantitative dynamic interlaminar fracture analysis under Mode I, Mode II and Mixed Mode loadings. Crack propagation was evaluated in specimens with and without nanofiber reinforcement. Significant improvements in fracture resistance were observed as a result of nanoreinforcement. Several unusual dynamic effects were observed for the first time. In particular, a possibility of suppression of ultrafast intersonic Mode II crack into the subsonic regime as a result of nanoreinforcement was demonstrated. It was also shown that a single dynamic experiment on a newly developed Mixed Mode test specimen may provide critical dynamic fracture resistance parameters for a broad range of mode mixities, a potentially highly efficient new method of dynamic material fracture characterization. Demonstration of applicability of the dynamic results from such a test for complex transient laminate delamination analysis was performed. A numerical model for composite laminate subjected to side impact was developed. The model utilized the data from the new Mixed Mode test. Laminate delamination under impact was studied numerically and significant reduction in delamination area was demonstrated as a result of nanoreinforcement. Simulated delamination areas and shapes correlated well with the ones obtained by ultrasonic analysis. The results of this work provide new insights into dynamic fracture behavior of advanced structural composites. The developed experimental methods and numerical models can be used as tools for fundamental fracture studies and further optimization of novel hierarchical composites with nanoreinforced interfaces.