Testing and simulation of composite laminates under impact loading

Owing to their high stiffness-to-weight and high strength-to-weight ratios, fiber-reinforced polymer-matrix composite laminates are excellent materials for high-performance structures. However, their properties in the thickness direction are very poor as they are weakly bonded by polymeric matrices through laminate interfaces. Accordingly, when a composite laminate is subjected to impact loading, high interlaminar stresses along with the low interlaminar strengths could easily result in interlaminar damage such as delamination. This thesis investigated the response of composite laminates under low-velocity impact and presented numerical techniques for impact simulation. To begin with, instrumented drop-weight impacts ranging from subperforation to perforation levels were introduced to composite laminates having various dimensions and thicknesses. Damaged composite laminates were then subjected to compression-after-impact tests for evaluations of residual properties. Experimental results revealed that perforation was an important damage milestone since impact parameters such as peak force, contact duration, maximum deflection and energy absorption, and residual properties such as compressive stiffness, strength and energy absorption all reached critical levels as perforation took place. It was also found that thickness played a more important role than in-plane dimensions in perforation process. In order to understand more about the relationship between laminate thickness and perforation resistance and to present an economical method to improve perforation resistance, thick laminated composite plates and their assembled counterparts were investigated and compared. An energy profile correlating the impact energy and absorbed energy at all energy levels for each type of composite plates investigated was established and found to be able to address the relationship between energy and damage. Experimental results concluded that increasing thickness was more efficient than improving assembling stiffness in raising perforation resistance. As a first step to simulate composite response to impact loading, LS-DYNA3D was used for numerical analysis. However, due to its inability to describe interlaminar stresses, no delamination simulation could be achieved. As delamination played a very important role in damage process, a computational scheme capable of identifying interlaminar stresses and considering both numerical accuracy and computational efficiency was required for impact simulation. Accounting for interlaminar shear stress continuity and having degrees of freedom independent of layer number, a laminate theory named Generalized Zigzag Theory was formulated into a finite element subroutine and integrated into ABAQUS code. The computational scheme was able to present reasonable interlaminar shear stresses via an updated Lagragian algorithm. Combining the calculated interlaminar stresses with a delamination failure criterion, the computer program was able to predict the response of composite laminates up to the onset of delamination. Further computational simulation involving all damage modes should be considered in future studies.