Effect of fabric biaxial prestress on the fatigue of woven E-glass/polyester composites Nawras H. Mostafa a,b, ⁎, Z.N. Ismarrubie a , S.M. Sapuan a,c , M.T.H. Sultan d a Department of Mechanical and Manufacturing Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia b Department of Mechanical Engineering, Faculty of Engineering, University of Babylon, Babylon Province, Iraq c Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia d Aerospace Manufacturing Research Centre (AMRC), Level 7, Tower Block, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia abstract article info Article history: Received 8 October 2015 Received in revised form 18 December 2015 Accepted 19 December 2015 Available online 21 December 2015 In this study, woven E-glass/polyester composites are prepared with different equi-biaxially fabric prestressing levels. Fatigue tests were carried out to assess the effect of fabric pretension on the life of the prestressed compos- ite under cyclic loading. The proposed prestressing method was implemented by applying an identical tension load to the warp and fill bundles and maintaining it until the matrix has cured. The monotonic quasi-static tensile tests were first conducted on the specimens with fabric prestressing level up to 100 MPa to estimate the optimum level of fabric prestressing. Tension–tension fatigue tests were then conducted to the samples with three differ- ent cases of prestressing, such as non-prestressed (pristine), prestressed at 50 MPa (optimum) and prestressed at 100 MPa (over-prestressed). For the composite system used throughout this work, the fatigue life of the samples prestressed at the optimum level was extended up to ~43%. The S–N f relationships showed that the fabric prestressing method could be used to extend the fatigue life of composites in the intermediate and low-stress re- gions. The improvement in fatigue life due to fabric prestressing could decrease with increasing the off-axis fabric orientation. Fabric prestressing method was not recommended when the normalized peak stress is higher than ~0.6. © 2015 Elsevier Ltd. All rights reserved. Keywords: Equi-biaxially prestressing Composite fatigue life Residual stress Prestressing level Plain-weave fabric orientation 1. Introduction Recently, composite materials are extensively used in aerostructures, renewable energy, oil piping industries, automotive and marine industries for their higher stiffness and strength to weight ratios in comparison with common metals [1,2]. Woven-fabric rein- forced composites have been widely used in primary and secondary load-bearing structures [3–5]. Moreover, composite materials rein- forced by woven fabric could exhibit better inplane mechanical proper- ties in comparison with other textiles made from the same raw material; however, woven fabric composites characterized by their comparatively high anisotropic properties with respect to fabric orien- tation [6]. Polymer–matrix composites are widely subjected to dynamic loads during their service life, and fatigue can be considered the most common failure type in composite components. The significance of studying fatigue life of engineering components was related to the fact that failure could be occurred at stress levels lower than those needed to induce static failure. Several factors can affect the fatigue life of fibre-reinforced composites such as material constituents, volume fraction, fibre orientation, loading frequency, stress level and environ- ment conditions [7]. Fabric-reinforced composites are frequently fabricated in the plane or shell shapes. The spatial variation of fibre orientation is very common in fibre-reinforced composite structures with complex profiles [6]. This implies that, these components are locally subjected to on-axis and more often to off-axis loading during their service life [8]. Unlike fatigue damage in metals, fatigue in composite materials might be induced due to many reasons such as fibre breakage, fibre splitting, matrix microcracking, fibre/matrix debonding, delamination, void growth or a combination of them [9]. Thereby, fatigue damages in woven fabric- reinforced composites are difficult to predict [10]. In the off-axis load- ing, fatigue damage characteristics are more complicated than on-axis loading [11]. Moreover, mechanical properties of fibre-reinforced com- posites can decrease extremely with increasing the off-axis angle [12]. The development of fatigue damage when tension–tension fatigue is applied in the on-axis (warp or fill direction) of woven-fabric compos- ites could be divided into three distinct stages [13]. The first stage oc- curred at comparatively low cycles and could be represented by initiation of cracks within the fibre bundles that arranged transversely to the loading direction at or close to the crossover sites. This stage is rel- atively fast (~10% of fatigue life) and characterized by a rapid decrease in composite stiffness. The second stage was characterized by cracks growing either in the rich area of the matrix or into the interface Materials and Design 92 (2016) 579–589 ⁎ Corresponding author at: Department of Mechanical and Manufacturing Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia. E-mail address: nawras1980@gmail.com (N.H. Mostafa). http://dx.doi.org/10.1016/j.matdes.2015.12.109 0264-1275/© 2015 Elsevier Ltd. 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