From aerospace industry to sports and leisure, and wind energy: long or continuous fibre reinforced composites are an increasingly pervasive material group. While carbon fibres garner a lot of attention due to their superb strength-to-weight ratio, glass fibres dominate the industry in terms of production volume – and will continue to do so for the foreseeable future. The glass surface is, however, not inherently compatible with the polymer matrix and load transfer inside the composite material necessitates the creation of an interphase – the ph instead of f being very relevant here. This separate phase connecting the two materials is created by intermixing of the polymer matrix and the surface treatment of the fibres, generally called a sizing, based on nomenclature inherited from the broader textile industry.
“This is occasionally a source of some confusion as the, let’s say more traditional textile industry term is well-defined but somewhat insufficient in this case, whereas for the composite industry the distinction is less defined or understood,” Pekka Laurikainen clarifies.
The chemical structure and experimental characterisation of this interphase has been an active topic of discussion in the scientific composite community for decades, leading to a constant, if somewhat slow, progress in the research.
“It does require a specific brand of stubborn, along with an ample supply of patience, to study something that exists solely in a nanometre-scale, and between two materials, and thus remains unobservable directly. The first step was indeed figuring out how the interphase can and should be studied,” says Laurikainen.
The interphase is especially important for long-term durability
The dissertation work was carried out in close collaboration with a Tampere University spin-off company, Fibrobotics Oy. Two out of the four publications of the compilation dissertation present the collaborative work with the company to develop and validate the experimental setup for microscale mechanical tests used to characterise the interphase.
“As a particularly impactful topic I would point out the interfacial fatigue test in microscale. A worrisome trend in interphase studies utilising microscale methods, even globally, is the chasing of maximum strength values for debonding of the fibre and the matrix. This kind of quasi-static – meaning low strain rate up to ultimate failure – test is a poor point of comparison for predicting material performance and product lifetime in mainly dynamic or cyclic loading cases such as aeroplane wings or wind turbine blades but nevertheless drives development of new surface treatments,” says Pekka Laurikainen.
The validation of the microscale tests included solving challenges related to the materials themselves. A problem with properties of the polymer matrix materials in samples prepared at different length-scales was detected early in the work. Identifying and mitigating the issue required exploring the polymer chemistry and structure property relationships of the resin – in which atomistic scale modelling became an important tool.
“In this regard I feel the work only scrapes the surface. I came across some very inspiring work in modelling various types of interfaces and interphases at atomistic scale and I foresee significant potential for these methods in further exploring and visualising the structure of the fibre-matrix interphase,” Laurikainen elaborates.
Public defence on Friday 1 September
The doctoral dissertation of M.Sc. (Tech) Pekka Laurikainen in the field of materials science titled Characterisation and Validation of the Microscale testing of Fibre Matrix Interphases will be publicly examined at the Faculty of Engineering and Natural Sciences at Tampere University on Friday 1 Sep, 2023 at 12 o‘clock at Hervanta Campus in the auditorium Pieni Sali 1 of the Festia building. (Korkeakoulunkatu 1, 33720 Tampere). The Opponents will be Doctor Liu Yang, University of Strathchlyde, and Associate Professor Andrea Dorigato, University of Trento. The Custos will be Associate Professor Essi Sarlin, Faculty of Engineering and Natural Sciences, Tampere University.
The doctoral dissertation is available online.
The public defence can be followed via remote connection.
Photograph: Tuire Marin