Tensile fatigue behaviour of glass plain-weave fabric composites in on- and off-axis directions

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Abstract

The tension–tension fatigue behaviour of woven fabric composites was investigated in the on-axis and off-axis directions. Similar to the static tensile properties of woven fabric composites, the tension–tension fatigue behaviour of woven fabric composites was also observed to be anisotropic. The fatigue damage development in both the directions was studied using scanning electron microscopy and acoustic emission techniques. The influence of the applied fatigue frequency and the fatigue stress ratio in both the directions was investigated. At a fatigue frequency of 5 Hz, the hysteresis heating will occur more severely in the off-axis direction than in the on-axis direction. Increasing fatigue stress ratio will lead to a lower hysteresis heating.

Introduction

Textile fabrics are used as reinforcements for composites in addition to uni-directional fibre laminates. The use of textile fabrics offers a lower cost to composite manufacturing and a better damage tolerance for impact loading. Woven fabrics are probably the most commonly used textiles in structural applications. The mechanical properties of woven fabric composites, such as strength and stiffness, are strongly determined by the weave parameters (weave geometry, yarn size, yarn spacing and yarn crimp), and the laminate parameters (fibre orientations and overall fibre volume fraction) and the inherent material properties of fibre and matrix [1], [2].

Although woven fabric composites show to have the optimal in-plane mechanical properties compared to other textile fabric composites with similar materials, the mechanical properties of woven fabric composites are highly anisotropic. In the on-axis directions (warp and weft), woven fabric composites possess the highest tensile and stiffness. The tensile properties such as strength and stiffness decrease drastically when woven fabric composites are subjected to an off-axis tensile loading. In the bias direction, the tensile strength and stiffness of woven fabric composites are comparable to those of knitted fabric composites.

During service, composite structures may be subjected to fatigue loads. The damage due to fatigue, will gradually decrease the mechanical properties of the composite (strength and stiffness). The tensile–tensile fatigue properties of glass-woven fabric composites in the on-axis directions (i.e. warp and weft directions) have been reported in [3], [4], [5], [6], [7], [8]. Failure in woven fabric composites is initiated by cracks in fibre bundles oriented transversely to the loading direction. The cracks subsequently grow either into matrix-rich areas or into the interface between longitudinal and transverse fibre bundles within the same layer (the so-called meta-delamination). The fatigue properties of woven fabric composites are also influenced by the geometry of the specimen and the ductility of the matrix. However, the information about the tensile fatigue properties of woven fabric composites in the off-axis directions found in literature is very limited.

In real structures, the fibre orientation of woven fabric composites can vary from one position to other position. The variation in the fibre direction can be generated by the complex shape of the desired structure and also by misalignments in laying up woven fabrics during composite production. As a consequence, important loads in off-axis directions can occur. This has inspired us to further investigate the tension–tension fatigue (dynamic) properties also in off-axis directions. The aims of this paper are to make a review of the tension–tension fatigue properties in on-axis directions and to investigate the tension–tension fatigue properties in off-axis directions. This paper focuses on the characterisation of the damage at several stages of fatigue life and their correlation with the change in the mechanical properties during fatigue loading. Scanning electron microscopy (SEM) and acoustic emission are used to identify several damage mechanisms.

Section snippets

Experimental

Plain-weave fabric composites were produced in an autoclave under an external pressure of 3 bar and a curing temperature of 125°C. In this research, E-glass plain-weave fabrics were used, coded as R420, and supplied by the company SYNCOGLAS. The linear density (tex) of the yarns is 610 tex. Woven fabrics (five layers) were impregnated using epoxy film F533 from Hexcel. The thickness of composites is 2 mm. The obtained fibre volume fraction of the woven fabric composites is 50%.

Fatigue tests were

Static tensile properties

Before investigating the fatigue behaviour of a particular material, its static behaviour needs to be discussed first. The average static tensile properties of plain-weave fabric composites are listed in Table 1. Fig. 1 shows the static tensile properties of the investigated woven fabric composites. In the off-axis tensile tests, i.e. 60 and 45° direction, the non-linearity occurs at very low applied stresses. The non-linearity becomes worse with strain increase. When strain reaches about 5%,

Conclusion

The tensile–tensile fatigue behaviour of woven fabric composites is anisotropic. The fatigue performance of woven fabric composites is worse in the off-axis directions than in the on-axis directions. Fatigue damage in the on-axis directions is initiated by cracks in the transverse fibre yarn. The cracks are then growing into the matrix or are deflected along the longitudinal yarn (meta-delamination). As the most fatigue damage takes place in the transverse fibre bundle, the residual properties

Acknowledgements

This text presents research results of the Belgian programme on Inter-university Poles of Attraction, funded by the Belgian state, Prime Minister's Office, Science Policy Programming. The scientific responsibility is assumed by its authors. S.D. Pandita is financed through grants of the Governmental Agency for Co-operation with Developing Countries (ABOS-VLIR) and the Concerted Research Action (GOA).

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