Characterisation and Validation of the Microscale Testing of Fibre Matrix Interphases

P. Laurikainen


The critical contribution of the interface – or interphase – to the final composite properties is a widely acknowledged pillar of the composite industry. However, many aspects of the composition and optimisation of interfacial performance remain closely guarded secrets in the halls of fibre manufacturers. Additionally, studies of the interphase, commonly through microcomposite characterisation methods such as the fibre pull-out or the microbond test, remain hindered by the overly argumen- tative discussion and haphazard approaches to studying them – the former being quite understandable considering the latter. Microcomposite testing has been fre- quently cited as suffering from large and unpredictable scatter of the experimental results, which can often – if not always – originate from poorly designed experimen- tation. While many great successes exist, they tend to get lost amidst a vast amount of unremarkable or even misguided work. This work aims to lift these aforementioned successes, from our research and that of others, to the forefront. To use the data and analysis therein – supported by se- lected approaches from outside the topic of interphases and microcomposites – to build a more comprehensive overview of the topic of the interphase and its testing while simultaneously hoping to begin to solve the identified key issues limiting the microcomposite characterisation methods. The majority of the discussion presented in the work revolves around this field of topics: the methodology of microcompos- ite testing in quasi-static and cyclic loading, and the limitations and advantages of microcomposite testing. These are presented along with the improvements made to achieve the current state-of-the-art, developed in parallel to – and in part based on – this work. The primary aim was to achieve statistical reliability within microbond testing. This work demonstrates that the issue of experimental scatter could be mit- igated effectively by a properly designed testing setup. What scatter remained in the data originated mainly from variations at the interphase and in the resin prop- erties. The contributions from resin should be mitigated, and significant effort was vii afforded to addressing this issue in the work. Variation of the interphase within the same fibre matrix combination was expected and represents an important aspect of understanding the overall performance of the interphases in composites. For studies on the interphase, the dependency on resin properties remains an is- sue. To fully realise the potential of microcomposite testing, the methods should be applicable to most, if not all, fibre matrix combinations and offer comparable results at least for the same method. As the interphase cannot exist without the presence of resin, this problem can never be fully resolved. Instead, it needs to be understood and mitigated. This highlights the second major topic of this study: The degree of cure of the resin, specifically as a part of a microcomposite sample. Many methods are available to characterise the degree of cure at the macroscale but, as many research groups have recently demonstrated, the curing of picolitre volume droplet-on-fibre systems differs significantly from those of bulk resin. The easiest potential explana- tionwas identified as surface-to-volume ratio and vapour pressure causing the volatile components of the resin to evaporate, and as a result the degree of cure of the droplet system remains at a much lower level compared to bulk. This theory holds for both two-component (resin/hardener) systems and reactive solvent systems such as epoxy resins and polyester resins, respectively. Both types of thermoset resins have been shown to suffer from similar issues in microcomposite curing. As direct characteri- sation of the droplet degree of cure is very challenging, the approach focused more on understanding the underlying phenomena and structure property relationships of the resin. Atomistic scale modelling along with experimental thermal analysis were used to understand the c ring process and how it translates to resin proper- ties, while the experimental comparison for mechanical performance was provided through nanoindentation – as close to scale as possible. The primary conclusion was that while the degree of cure has some effect on the final properties at all levels, above a critical threshold the effect becomes sufficiently minor to not affect the measure- ment itself. The differences in the curing states of the resin were noted to go beyond the expected dependence on degree of cure and evaporation-related changes in stoi- chiometry, highlighting the need for further understanding of the development of morphology in the curing resins. Based on this work, several possible and quite universally applicable steps could be suggested to minimise the effect of resin conversion during microcomposite sam- ple preparation. In order to have the most accurate analysis possible, the conversion- viii related properties, such as modulus, strength and curing shrinkage, should be ad- dressed droplet by droplet, but this goes beyond the scope of this work and remains an interesting future challenge. Another future topic highlighted by the results of this work is how the aspects of the resin conversion affect the overall structure of the interphase. The atomistic simulations presented in the work highlight the im- portance of non-covalent bonding to both the curing kinetics and network structure formation, and most of these findings have significant implications to the formation of the interphase as well