Matrix-limited fatigue properties in fibre composite materials

Abstract

Cyclic stressing of fibre composites in the elastic-plastic region is discussed under the assumption of equal strains in the fibre and the matrix. Under conditions of alternating load, it is found that the strain decreases and the stress on the matrix increases with increasing number of cycles. Such a phenomenon appears to be a macroscopic version of the Orowan model of fatigue.

Within certain stress-levels, it appears that the matrix may work-harden to its failure stress and the composite will fail by this mechanism. Equations are developed which relate the number of cycles to matrix failure to the composite stress. A characteristic of the equation is the existence of a matrix “endurance limit”, a stress below which the matrix will not fall by progressive work-hardening, but rather by conventional fatigue mechanisms.

The analysis is applied to Ag-steel and Cu-W composites. It shows that, for stresses in excess of the “endurance limit”, the number of cycles to induce failure is rather limited. At high volume-fraction of fibres in these systems, the “endurance limit” is only a small fraction of the composite tensile strength.