A damage prediction of high temperature polymer matrix composites

Abstract

This work for prediction of damage and life is based on a new micromechanics of failure theory (MMF) and was started a few years ago. Instead of using the unidirectional ply as the building block, it was found that starting with the fiber, matrix and their interface would lead to a much simpler theory. The constituent properties will include two tensile and compressive strengths of matrix and fiber, plus normal and shear strengths covering the interface. The measured storage modulus of the viscoelastic matrix can be linked to the stiffness and strengths of plies and laminates from which creep rupture and fatigue strengths can be derived. One advantage of this approach is that the failure process of plies and laminates can be explicitly defined in terms of fiber, matrix and interfacial failures. No empirical factor is needed. The effect of moderately high temperature is fully integrated in the basic viscoelastic formulation, not accounted for by a knockdown factor. In addition, macro level properties of idealized square, hexagonal and random fiber arrays can be modeled and the resulting medium and scatter of predicted stiffness and strength are in reasonable agreement with available data. This modeling process is generic and can be applied to different material systems at room and elevated temperatures. Examples of metal matrix composites and hybrid composites like Tigr and Glare can be systematically described with no additional constants except the six basic strengths (four for fiber and matrix and two for the interface). First and last ply failures of laminates (FPF and LPF) can be similarly described with explicit definition of the failure modes. The failure process of matrix does not necessarily define the ultimate strengths of plies and laminates. Sophisticated modeling of the local degradation of matrix by micro cracking, ply delamination and fiber micro buckling will be explored with appropriate software packages developed for use by design engineers. In the mean time master curves covering the constituent stiffness and strength as functions of time and temperatures will be developed, measured and analyzed. In the end, failure envelopes of composite laminates in creep rupture and fatigue strength will be illustrated and their use for design be explained. In particular, the failure and life of a composite wind turbine blade is a good test of the predictive power of MMF.