As already mentioned in Chapter 2, the strength of composite materials is examined by two main approaches: phenomenological and micromechanical methods. The phenomenological approach, as studied in Chapter 2, applies the local stress or average stress failure criteria to form strength models. These models are based on fitting experimentally acquired failure data. The alternative approach studies destruction from the viewpoint of micromechanics. The strength characteristics of a unidirectional composite are explained in this chapter from the positions of failure mechanisms for fibers and matrix. For this purpose, the micromechanical models of both components are developed. This approach provides qualitative understanding of failure and explains the hypothetical mechanisms on a structural level of the composite material. The explanations propose the ways for optimization and proper material selection. However, since micromechanical criteria are based on several hypotheses, they cannot be as precise as phenomenological criteria, which are grounded on direct strength measurements. This chapter deals with the modeling of the fracture process of a fiber composite material with parallel fiber orientation. The problem of fiber pullout from the matrix is investigated. The elastic behavior of the fibers and elastic‐plastic behavior of the matrix are presumed. The zones around and far from the crack tip are considered independently. The problem of a crack whose surfaces are bridged by undamaged fibers is solved. The effect of fiber draw from the surface of the crack is associated with the jump of the effective displacements over the crack surface. The equations of a plane and a rotationally symmetric problem of crack propagation are grounded on the fiber pullout solution. The results of the numerical solutions and the fracture toughness evaluations are described. Further, debonding on the surface of fibers under the action of tensile load in the fiber direction is rigorously studied. The debonding region is modeled as the cylindrical crack on the surface of the fiber. Closed form solutions are achieved. The main result is as follows: for a relatively long debonding zone, the stress intensity factor at the tip of the debonding crack does not depend on the crack length. Thus, for cyclic load application the elongation of the debonding crack remains constant and crack length is proportional to the number of load cycles. This effect leads to continuous degradation of a material with increase in cycle number. This explains the near linear reduction of longitudinal module with the duration of cyclic load.
Micromechanical Failure Criteria of Composites
2019-05-13
59 pages
Article/Chapter (Book)
Electronic Resource
English
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