Viscoelastic response of carbon fibre reinforced polymer during push-out tests
Graphical abstract
Introduction
Carbon fibre reinforced polymers are composed of two phases: the carbon fibres, which provide strength and stiffness, and the polymeric matrix, which holds the reinforcing fibres in place and distributes the load among individual fibres [1]. The interfacial strength (IFS) between carbon fibres and polymeric matrices has important implications for the mechanical properties of composite materials [2], [3]. In spite of the extensive research efforts, the appropriate assessment of the IFS and its link to the macroscopic properties of composites remains challenging. Several testing methods have been developed to assess the IFS [4], including: micro-bond [5], pull-out [6], [7] or push-out tests [8], [9], fragmentation tests [10], [11] and Raman spectroscopy measurements of specimens under stress [12]. Even though each of these methods can be used to effectively rank the IFS, the results obtained by different methods are not directly comparable [13]. Moreover, it is difficult to link the microscopic test results with the macroscopic properties of composites. The inconsistencies are frequently attributed to the different stress states developed in microscopic and macroscopic conditions [13].
In addition, there is no general agreement on the operating interfacial failure modes. Several models have been proposed with this regard, mainly based on stress and energy failure criteria [9], [14]. However, the limitations of both approaches have been discussed by several authors [15], [16], which could be attributed to the different load–displacement responses of ceramic-matrix and polymer-matrix composites. Clearly, further advance in scientific understanding of the debonding mechanism is important to improve the IFS and to predict the behaviour of composites materials in real operating environments. In this article, the results of push-out tests, conducted at multiple loading rates and temperatures, are presented and discussed. The micro-mechanical tests were supported by microscopic observations to identify the debonding mechanisms.
Section snippets
Sample preparation
The samples used for the tests were extracted from a rod of Toray Rebar S12 unidirectional composite, consisting of T700 carbon fibres in an epoxy matrix (Table 1). Discs, 700 µm thick (Fig. 1), were extracted from the rod of Rebar S12 using a Struers Accutom 5 precision cut-off machine, fitted with an Al2O3 abrasive disc and applying abundant cooling fluid. At least 300 µm of material were removed from each side of the discs by wet grinding with #1200 SiC abrasive paper. Finally, both sides of
Validity and limitations of the push-out tests
The SEM observations conducted after the push-out tests clearly revealed single fibres which were pushed-in on the loading side of the samples, i.e. the surface of these fibres was at a lower plane compared to the nearest neighbours (Fig. 4a). The pushed-in fibres exhibited minimum surface damage, although the edges of the Berkovich indenter clearly made contact with the surrounding material, causing damage to the neighbouring fibres. In addition, the observations conducted on the back side of
Discussion
The results presented in the previous section are in agreement with the literature [2], [9], [14], [15], [16]. However, the time-dependent response as a function of the loading rate and temperature, as well as the occurrence of push-out events during the dwell time at constant load, are noteworthy (Fig. 8a, inset). These phenomena seem to be associated with viscoelastic response of the interface between the carbon fibres and the matrix, or the viscoelastic deformation of the polymer in the
Conclusions
The push-out tests conducted on carbon fibre reinforced epoxy using different loading rates and testing temperatures revealed the following conclusions:
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It was possible to push-out single carbon fibres from the matrix with a Berkovich indenter under different loading rates and temperatures (below and above Tg).
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The deviation in the force–displacement curve from the linear behaviour, which is often associated with debonding, could occur at different stresses depending on the loading conditions,
Acknowledgements
The research leading to these results has received funding from the European Union (GA604248 & GA685844). SCG would like to thank Alan Riley, from Toray International UK Ltd, for providing samples of RebarS12.
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