A method to determine composite material residual tensile strength in the fibre direction as a function of the matrix damage state after fatigue loading

The phenomenon of residual strength in composite laminates after fatigue loading has been studied for decades but is generally expressed as a function of the number of cycles applied to the specimens. Phenomenological laws deduced from these tests are therefore loading dependent and identifying a valid model for any loading condition of a given laminate requires long fatigue testing campaigns. To overcome this difficulty, a testing procedure is proposed to express the residual tensile strength of a unidirectional ply in the fibre direction, as a function of its matrix damage. The resulting behaviour is then independent of the imposed loading and can be determined by only a few fatigue tests. Experimental results obtained on glass-epoxy non-crimp fabric laminates are presented and analysed.