Fatigue and fracture in polymer networks and their composites

Abstract

Fatigue is encountered in wide spectrum applications varying from biomaterials to energy storage devices. As we explore novel materials to meet stringent performance demands of new applications, strategies to tackle fatigue damage must be explored as well. Polymer networks like thermosets are ubiquitous as structural components and their fiber composites are equally popular because of their superior strength to weight ratio. Hence, improving fatigue and fracture performance of these polymer networks and their composites is an important research question.

We demonstrate fracture performance improvement in a thermoset modified by nanofillers. A field of stiffness heterogeneity is observed in this nanocomposite which is created by stochastic dispersion of nanofillers. This heterogeneous filed activates a mesoscale toughening mechanism which is confirmed by a continuum scale simulation. Fatigue testing of the nanocomposite shows that small scale interactions created by nonofillers increases fatigue crack propagation threshold force. However, the crack propagation in large cracks is independent of nanofiller loading. Fatigue perforrmance of carbon fiber composite made with nanomodified thermoset shows improvement in high cycle fatigue regime which increases with nanofiller loading fraction.

Fatigue performance improvement in the thermoset networks and their composites observed thus far is irreversible, making the eventual rapid fatigue failure inevitable. Vitrimers, which are novel epoxy based networks with reversible crosslinking ability, can potentially possess ability to heal fatigue damage. As vitrimers are crosslinked netwroks, they can possess healing ability while also retaining strength and stiffness comparable to conventional thermosets. We demonstrate that a vitrimer system can indeed undergo reversal of small scale fatigue damage when subjected to periodical heating to the characteristic temperature. Furthermore, carbon fiber composites made with the vitrimer also showed reversal of fatigue damage when subjected to similar intermittent heating strategy. This finding may pave way for composites which potentially have ultra high fatigue life.