Analysis of Fatigue Damage in Unidirectional Carbon Fibre Reinforced Polymer Material

Carbon fibre reinforced polymer (CFRP) is widely used across different industries, mainly aviation, marine, automotive, sports equipment, and many more. This is attributed to its outstanding high strength-to-weight ratio compared to other materials such as metals. In addition, this material also features design customizability to fit various applications, which makes it one of the most versatile materials today. In real-life applications, CFRP structures are subjected to fatigue loading. The material response towards this loading is very complex. This is due to the evolution of damage within the material that occur through various failure mechanisms. The intricacy of fatigue damage in CFRP material is also contributed by the individual failure mechanisms and the synergy effects by these failure modes that occur concurrently at the microscopic level. Due to this complexity, macroscopic and microscopic material level response knowledge is required to comprehensively understand fatigue
damage in CFRP. However, this crucial knowledge is not easily obtainable from standard testing alone.

This thesis discusses the analysis of damage methods for unidirectional (UD) CFRP under fatigue loading through experimental and modelling work. ... mehrA new experimental method to study fatigue on micro-level material response was introduced. The fatigue test requires a unique sample preparation process that involves micro-specimens of different fibre orientations (0°, 30°, 45°, 60° and 90°). The test was done under room temperature and $R=0.1$ stress ratio. The results obtained from the test are presented as an S-N curve, and in-situ observations of the micro-specimens are analysed. The fracture surface of selected
broken micro-specimens is also presented and discussed here. Based on the comparison with standard macro-specimens testing, it can be deduced that the micro-fatigue test is of comparable quality to the macro-specimens standard testing. It also offers additional advantages in supplying a direct possibility to observe several critical phenomena under loading.

A new continuum damage mechanics model (CDM) for fatigue and degradation of CFRP has also been proposed. The model is formulated based on anisotropic linear elastic Hooke’s law brittle model. The model includes three damage variables, each representing damage effect oriented with respect to the three coordinate axes. The formulation concept based on the dissipation of microplastic work within the material gives a physical sense of how the damage evolves in CFRP under fatigue. It is found to be numerically efficient due to the possible estimation of microplastic work from the elastic components of stress and strain. The model has been implemented in Abaqus user-subroutine and validated against two different types of fatigue specimens (macro-specimens and micro-specimens). In the case of macro-specimens, the model has been validated against filament wound carbon fibre epoxy matrix for the case of UD and multidirectional (MD) composite under tension, compression and alternating cyclic loading. On micro-specimens, a UD composite was analysed under tension fatigue load. The model proves a good prediction which indicates the model capability to accurately predict the fatigue lifetime of CFRP material at both macro and micro-level response. The thesis also discusses a new formulation to improve the current fatigue damage model further. The combination of experimental and modelling methods presented here provides high potential tools in the analysis of fatigue damage on CFRP material.