A Unified Micromechanical Model for the Mechanical Properties of Two Constituent Composite Materials. Part III: Strength Behavior

Abstract

This series of papers reports a new, general, and unified micromechanical model for estimating the three-dimensional mechanical properties of a composite made from two constituent materials, i.e., continuous fiber and matrix. The linear elastic and elasto-plastic behaviors have been studied in Parts I and II respectively. The present paper investigates the tensile strength and failure mode of the composite. The tensile strength of the composite is predicted based on the ultimate stresses in the constituent phases, as the states of stresses in both phases are explicitly represented as the function of the applied overall stress field. The maximum normal stress theory for isotropic materials is incorporated with this prediction. As long as the maximum normal stress in either the fibers or the matrix attains its ultimate value, the composite is considered to fail, and the corresponding strength and failure mode are thus automatically defined. This makes determination of the composite strength as well as the failure mode extremely general, yet particularly easy. The composite can be subjected to any kind of load combination, whereas the corresponding strength behavior is determined through the same simple procedure. Comparisons between predicted and measured tensile strengths for several unidirectional composites under off-axial loads and for a plain weft-knitted fabric-reinforced composite under different uniaxial loads confirm that the present micromechanical strength theory is very efficient.