Evaluation of fracture toughness of orthotropic composite materials

Abstract

The fracture toughness of composites depends on the fracture of fibers, accumulation of damage, growth of microcracks, breaking of adhesion bonds, plastic deformation of the matrix, etc. In different materials, the roles and importance of these factors are different. The characteristics of fracture toughness of composites are determined for a conventionally homogeneous material with specified effective moduli. This approach enables us to describe the process of crack growth in a composite but may turn out to be incorrect. This is largely explained by the fact that the fracture process may be not collinear with the initial line of the crack. Moreover, if the material undergoes fracture near the crack tip, the degree of stress concentration significantly decreases. It is believed that the methods of linear fracture mechanics developed for homogeneous brittle materials cannot be applied directly to composites due to the existence of mechanisms of fracture of absolutely different nature, their low sensitivity to notches, and the absence of through cracks [1,2]. At the same time, if the critical value of the stress intensity factor KI ~ does not depend on the crack length, this application is possible [3][4][5][6][7][8]. Furthermore, the stress field in the vicinity of the crack tip determined on the macrolevel is equal for homogeneous and composite materials, although the mechanisms of cracking in these materials are different.We propose a method for the evaluation of the fracture toughness of composite materials including the choice of the shape of the specimen, the analysis of the stressed state of a notched specimen, the choice of the fracture load, and the statistical processing of data. As a distinctive feature of testing for crack resistance, we can mention the existence of the effect of broken fibers. Therefore, round, trapezoidal, and other similar specimens cannot be used for composites because they contain fibers of different length and are characterized by a pronounced dispersion of strength. In this case, it is much better to use rectangular specimens containing a single central crack or two edge cracks. Rectangular specimens containing a single edge notch and loaded by two concentrated forces have not only advantages (such as the simplicity of making notches and monitoring of a single tip of a moving crack) but also certain disadvantages (e.g., the rotation of crack lips under loading promoting the formation of compression in the notch-free part of the specimen and high sensitivity to the conditions of fastening). This leads to the loss of stability of the material (bulging of fibers, fracture of the matrix, and adhesion cracking under compression) and the appearance of additional defects. However, these effects are, as a rule, neglected.At the same time, the specimens containing either a single central notch or two edge notches are more difficult for manufacturing but much better for testing. This is why we focus our attention on a rectangular specimen whose faces parallel to the central notch are subje...