ANALYSIS AND TESTING OF COMPOSITE LAMINATES WITH TRANSVERSE CRACKS

Abstract

The analysis and testing of transverse crack damage and evolution in composite laminates are described in this thesis. The background, problem statement and motivation of the research topic are presented in Chapter 1. Chapter 2 reviews the damage and evolution theories available in the open literature. Salient research challenges are discussed and the direction for the present research work is outlined. Chapter 3 documents the experimental work undertaken on the Fibredux 913C-HT A crossply and angleply laminates. Crossply and angleply Fibredux 913C-HT A laminates are fabricated and tested for transverse crack damage. The laminates' in-plane property changes and crack density evolutions arising from transverse crack damage are analysed and discussed. Chapter 4 presents a set of second order tensor Internal State Variables (ISV's) that are embedded within the constitutive equations derived based on the principles of Continuum Damage Mechanics. The ISV's represents the micromechanics of internal damage caused by the presence of transverse cracks in individual off-axis plies of composite laminates. An energy based evolution criterion is proposed to govern the initiation and evolution of transverse cracks in individual off-axis plies. The Finite Element (FE) analysis to evaluate the ISV's and stress states of crossply laminates sustained transverse crack damage are described in Chapter 5. The ISV's for transverse crack damage under coupled Mode I and III loads are derived for general off-axis plies using the geometry and kinematics of transverse cracks extracted from finite element (FE) analyses. A constraint parameter is introduced to account for the constraint scenarios of off-axis plies due to constraint factors such as ply thickness, orientation and stacking sequence of a laminate. The ISV's account for the effects of general constraints through the proposed constraint parameter. Stress states of crossply laminates from the FE analysis are analysed and discussed. An additional evolution criterion is proposed to govern the transverse crack interactions effect at higher crack densities. In Chapter 6, a damage analysis algorithm based on the proposed damage theory of Chapters 4 & 5 is used to study the damage characteristics, constraint effects and in-plane property changes of crossply and angleply laminates sustaining transverse crack damage. The theoretical predictions are verified with experimental data of Chapter 3 and data from other researchers. The damage evolution analysis of transverse cracking in general composite laminates is undertaken in Chapter 7. The damage theory, transverse crack evolution and transverse crack interaction criteria of Chapters 4 & 5, form the integrated damage and evolution theory for transverse crack evolution and the associated damage analysis. Two damage evolution analysis algorithms are coded based on the damage and evolution theory. The algorithms are used to analyse the evolution of transverse cracks and the corresponding laminate's in-plane property changes subjected to mechanical loading and temperature gradients respectively. The theoretical predictions are compared with experimental results of Chapter 3, as well as published data from other researchers. Conclusion on the topic of study and issues for further research are described in Chapter 8.