| The research work reported in this dissertation is primarily divided into two categories of studies relating to contact impact responses of composite materials. Initially, the contact impact response has been investigated as a nondestructive testing (NDT) parameter for identifying localized defects and damage in composite laminates, and the corresponding NDT method is known as the Acoustic Impact Technique (AIT). The sensitivity of the Acoustic Impact Technique has been investigated for detecting embedded delaminations, impact, static compressive, fatigue, and moisture induced damage in laminated composites. The results indicated that the AIT method can be successfully applied in identifying localized micro-defects in laminated composites, provided the test parameters are maintained the same in each location. This research established a model of the contact impact response on the basis of nonlinear Hertz's contact law with the introduction of a localized damping parameter. Such formulations incorporated more variables related to the test object and the experimental set-up of the AIT.; The second category of study is focussed on the impact damage tolerance of composites. In studying the impact damage tolerance of composites, attempts have been made to improve the impact resistance and compression strength after impact (CAI) of graphite-based composites through the hybridization technique. Although the studies have been conducted to increase the impact resistance of graphite composites using high strain fibers such as Kevlar, glass, and Spectra individually, the combined effect of two high strain fibers were rarely studied. The hybrid system with 'spectra shield' as an outer layer of graphite/epoxy (hybrid-1) and graphite/kevlar/epoxy (hybrid-2) is a new laminate architecture which is studied in the present research for improved impact resistance and compression strength after impact (CAI). The present study emphasizes a thorough understanding of the impact damage mechanisms, post-impact residual properties, and impact damage resistance of the hybrid system and compares them to graphite/epoxy composites. The hybrid-1 system showed the highest compression strength after impact (CAI) and hybrid-2 showed the minimum damage in the core materials as compared to the graphite/epoxy system. The impact induced delamination in the form of a penny-shaped crack has also been modeled by the finite element method using a 3-D layered structural element. The response of the composite plate under the influence of a compressive load with various sizes of delamination have been studied. |
