Viscoelastic response of carbon fibre reinforced polymer during push-out tests

doi:10.1016/j.compositesa.2018.06.003

Composites Part A: Applied Science and Manufacturing

Volume 112, September 2018, Pages 178-185

https://doi.org/10.1016/j.compositesa.2018.06.003 Get rights and content

Highlights

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Push out tests were conducted at different loading rates and temperatures.
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The debonding occurred at different stresses depending on the loading conditions.
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A time-dependent response was observed and described with simple viscoelastic models.

Abstract

Push-out is one of the available techniques to assess the bond strength between the reinforcing fibres and the matrix in composite materials. The test is conducted on thin sections of composite, and a small indenter is used to apply increasing load on single fibres while measuring the displacement, until the debonding occurs. This study used push-out tests to assess the debonding mechanism of carbon fibres in an epoxy matrix. The tests were conducted at multiple loading rates (0.1 mN/s, 1 mN/s and 10 mN/s) and temperatures (24 °C and 125 °C). The results were analysed and contrasted with scanning electron microscopy and atomic force microscopy observations. The data showed evidence of push-out events and provided new insights into the contribution of the viscoelastic behaviour of the fibre/matrix interface and/or the matrix. This finding could pave new pathways for improving the bond strength between the carbon fibres and the matrix in composite materials.

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 T_g).
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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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View full text

Viscoelastic response of carbon fibre reinforced polymer during push-out tests

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Sample preparation

Validity and limitations of the push-out tests

Discussion

Conclusions

Acknowledgements

Prog Mater Sci

Comp Mater Sci

Compos Sci Technol

J Eur Ceram Soc

Compos Part B-Eng

Compos Sci Technol

Compos Part A-Appl S

Compos Part A-Appl S

J Eur Ceram Soc

Compos Part A-Appl S

Compos Sci Technol

Mat Sci Eng A

Compos Sci Technol

Ceramic composites: A review of toughening mechanisms and demonstration of micropillar compression for interface property extraction

J Mater Res