Influence of the Flexibilizer Content on the Monotonic and Fatigue Behaviour of a Polyester Resin for Composites

Monotonic tensile tests were carried out on a polyester orthophthalic resin with various contents of flexibiliser, using two loading speeds. All the main mechanical parameters characterising the tensile material behaviour, i.e. the elastic modulus, strain to failure, and strength, were affected by the flexibiliser content and strain rate. In particular, increasing the flexibiliser content and decreasing the strain rate resulted in a higher strain to failure and a lower elastic modulus. On the contrary, the tensile strength reached a maximum in correspondence of a flexibiliser content specific of the loading speed adopted. The different resin formulations were characterised in tension–tension fatigue. Many qualitative correlations were found between the experimental S–N curves of the resins and those of composite materials. In fact, their trend was well described by a fatigue model assessed for composites, and their scatter in fatigue was consistent with the strength-life equal rank assumption. The fatigue sensitivity was affected by the flexibiliser content, but did not vary monotonically with the latter. The curves of fatigue life were very different depending on the fact that the stress or the strain was used as the ordinate. Nevertheless, the feature common to both cases was the seeming convergence of all the fatigue curves when 106 cycles were approached, similarly to what usually occurs for composites.