Innovative research on understanding stress relaxation behaviour

Short glass and carbon fibres are added to polymers, such as epoxy, to provide stiffness and strength. As the fibres are elastic, they behave like metal springs, but the polymers are viscoelastic, meaning that they undergo creep and stress relaxation. When elastic fibres are added to viscoelastic polymers, the fibres seem to strangely alter the behaviour of the polymer itself. Previous researchers have suggested that the polymer structure must be altered near the fibre surfaces.

To further understand the stress relaxation behaviour of short fibre composites, Numaira Obaid (ChemE PhD student) co-supervised by Professors Mark Kortschot (ChemE) and Mohini Sain (MIE), have taken a simple but innovative approach.

“We assumed the polymer structure was unchanged, but that the efficiency of the reinforcing fibres was changing over time because of the existing and well-understood polymer viscoelasticity,” says Kortschot. “With this in mind, we were able to design a model that predicts the behaviour of composites in a very simple way without needing to speculate about polymer structural changes. Using only independent measurements or properties of the polymer and fibres, we are able to determine how the composites will behave.”

This is significant as short fibre composites are widely used, including in many applications where there is long-term exposure to stress.

“The societal impact of the research is indirect since the main users are materials engineers. The semi-empirical models that were developed in this study can be used to predict the stress relaxation and creep of any short-fiber reinforced composite of any matrix/fiber combination. This is quite useful for the manufacturing sector, for example, it can help predict the rate at which residual stresses are released from manufactured composites to minimize component warpage. The application also extends to research in the biomedical field when designing and selecting synthetic composites to replace biological tissues where viscoelasticity may be a core functional characteristic, for example, in the design of artificial valves or arteries,” says Obaid.

This fundamental study, funded by NSERC and OGS, has been published in a series of three papers in the open-source journal Materials, and Composites Science and Technology.