Determination of the critical crack length of notched weft-knitted glass fiber variable width composite plates Mevlu ¨t Tercan a,1 , Osman Asi b,1 , Alaattin Aktas ß b, * a Department of Textile Engineering, Us ßak University, Us ßak, Turkey b Department of Mechanical Engineering, Us ßak University, Us ßak, Turkey Available online 5 June 2006 Abstract In this study, the critical crack length (CCL) of the knitted composite plates with different central holes and different plate width were predicted by means of point stress criterion, (PSC), and average stress criterion, (ASC). The knitted fabrics used for the study are fab- ricated from the 200 tex glass yarn with epoxy resin and cured in a hot press machine. Specimens with 10, 20 or 40 mm width (W) and containing 2, 3, 4 or 5 mm diameter holes (D) were prepared for two configurations and then tested. The CCL found through the stress criteria from the experimental data. The CCL data are plotted with respect to D/W ratios and then logarithmic curves were fitted to the data by using curve fitting technique. The results show that the CCL depends on the hole size as well as width of the plate. In addition, the CCL curves obtained by curve fitting technique give the same character and also it can be observed that there is a correlation between the curves. As a result of that the CCL can be predicted by anyone by using the less experimental data. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Textile composite; Weft knitting; Glass fiber; Stress concentration; Critical crack length 1. Introduction Fiber mats, braided and woven fabrics have been used in composite materials for a long time. Knitted fabrics were not considered because of two reasons: the first reason is that people think of knitted fabrics as very loose structures with low load carrying ability, secondly the achievable fiber content in knitted fabric composites was assumed to be insufficient [1]. But, it is understood that, both arguments about the knitted fabrics were invalid and knitted fabrics are used extensively in composite materials today. Fig. 1 shows the situation of some features of knitted fabric com- posites with respect to other composites [2]. The increasing use of knitted fabrics in composite materials has motivated the investigators to understand their behaviors. Falconnet et al. [2] have researched the mode I inter-laminar fracture toughness of advanced knitted textile composites. They have selected two complex weft-knitted glass fabrics which were triple rib knit and milano knit, impregnated with a tough epoxy resin and tested using double cantilever beam geometry. They have studied the influence of the growth direction for both knitted composites investigating crack propagation in both the wale and course direction and quantified the fracture toughness determining the critical strain energy release rate using the modified beam theory. Their results showed that the origin of the high energy val- ues, which are superior to woven or UD laminates, lies in the complex fabric structure. Leong et al. [3] have analyzed the mechanical performance of resin transfer molded com- posites comprising a weft-knit glass fabric of milano rib architecture. A number of mechanical properties i.e. ten- sion compression, bearing etc. were considered for lami- nates containing up to 12 layers of knitted fabric by them. They found that the knitted composites have relatively poor tension and compression properties but comparable 0263-8223/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2006.04.072 * Corresponding author. Tel.: +90 276 2634195x215; fax: +90 276 263 4196. E-mail addresses: tercan@aku.edu.tr (M. Tercan), osmanasi@aku. edu.tr (O. Asi), aaktas@aku.edu.tr (A. Aktas ß). 1 Tel.: +90 276 2634195x206. www.elsevier.com/locate/compstruct Composite Structures 77 (2007) 111–119