Monotonic and fatigue behaviour of chopped-strand-mat/polyester composites with rigid and flexibilised matrix

Monotonic and tension-tension fatigue tests were carried out on E-glass chopped-strand-mat/polyester composites, varying the flexibiliser content by weight in the matrix in the range 0-30%. The flexibilising action was due to the adipic acid monomers present in the flexibiliser. In monotonic tests, the most marked effect of resin flexibility was in the transverse cracks formed during loading, whose critical density (i.e., the density at failure) was very high for the rigid matrix, resulting in a highly non-linear stress-strain curve, and in the largest apparent strain to failure. With suitably increasing the flexibiliser content, the transverse crack formation was nearly suppressed, and the overall stress-strain curve approached linearity. In fatigue, the critical crack density decreased with increasing fatigue life in the case of the rigid matrix. For the flexibilised resins, the crack density at failure was independent of the maximum applied stress, larger than observed in monotonic tests, and higher the higher was the flexibiliser content, up to about 80% of the tensile strength. Beyond this limit, it converged through the material monotonic behaviour. The evolution of the residual elastic modulus with elapsing fatigue cycles was qualitatively consistent with the number of transverse cracks observed. The more flexible the matrix, the lower was the fractional modulus loss in fatigue. However, the highest elastic modulus along all the fatigue life pertained to the composite with rigid matrix, due to the flexibiliser adversely affecting the initial rigidity. Despite the differences in monotonic response and crack formation features, all the materials tested exhibited very similar S-N curves at moderately high fatigue lives. Nevertheless, appropriately treating the experimental results in terms of fatigue sensitivity, it was found that this parameter tends to increase with increasing matrix flexibility.