Experimental investigations on the response of stitched/unstitched woven S2-glass/SC15 epoxy composites under single and repeated low velocity impact loading M.V. Hosur * , M.R. Karim, S. Jeelani Center for Advanced Materials, Tuskegee University, Tuskegee, AL 36088, USA Abstract In the current investigation, damage resistance of stitched/unstitched S2-glass/epoxy composites is studied. Five layer stitched/ unstitched plain weave S2-glass woven fabric composite laminates were manufactured using toughened SC15 epoxy resin system. For stitching, two configurations: one with 25.4 mm grid and other with 12.7 mm grid, were used with 6 mm pitch. Damage re- sistance was evaluated by subjecting 100 Â 100 mm samples to low velocity impact loading at energy levels ranging 10–80 J using DYNATUPModel8210,aninstrumentedimpacttestsystem.Threesamplesweretestedateachenergylevel.Theextentofdamage was evaluated using ultrasonic C-scan.Resultsofthestudyindicatethatstitchingconfinedthedamagesize.Asthedamagewaslow over most of the energy range, the study was further extended to determine the effect of repeated impact loading. Under this study, laminatesweresubjectedtorepeatedimpactloadinguptomaximumof40impactsatenergylevelsrangingfrom10to50J.Results of repeated impact study are reported in terms of peak load, absorbed energy and projected damage area. All the laminates, sus- tained repeated impact loading up to 30 J. Beyond 30 J, laminates failed to carry repeated loading beyond certain numbers, which depended on the laminate configuration. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: VARIM; Low velocity impact; Damage resistance; Stitching; Ultrasonics 1. Introduction Though composites have excellent mechanical prop- erties over conventional metallic materials, their use in advanced technological applications has been limited due to their poor translaminar properties. Catastrophic propagation of delamination in various composite structures is of serious concern. Delamination can occur in composite materials due to various scenarios like low velocity, high strain rate and ballistic impact. Such im- pacts may occur during fabrication, normal mainte- nance operations or during service conditions. Such damages are not detectable to the normal eye (the damage being in a subsurface region or being too small tobevisibletothenakedeye),whichaddstothecritical nature of low velocity impact damage. Delamination is of major concern because there is significant loss of stiffness and compressive strength of the composite ma- terial. On the other hand, during routine maintenance activities and during operation, composite components may be subjected to repeated impact at localized sites. The resultant damage may seriously impair the subse- quent mechanical performance of the composites. Thus, it is appropriate to investigate the effects of repeated impact on structural composite materials. There remains considerable need for improvement of damage tolerance and arresting delamination of com- posite structures. One of the techniques of arresting delamination is providing through-the-thickness rein- forcement like stitching. The objective of this study is to understand the influence of stitching and stitch spacing on single and repeated low velocity impact (RLVI) loading. Conventional laminates are made with unidirectional prepregs using autoclave-molding process. Prepregs as well as autoclave molding method are expensive. How- ever, with the advent of low-cost liquid molding pro- cesses the cost of production has been drastically reduced. Resin transfer molding (RTM) and vacuum * Corresponding author. Address: Center for Advanced Materials, TuskegeeUniversity,Tuskegee,AL36830,USA.Tel./fax:+1-334-724- 4220/727-8236. E-mail address: hosur@tuskegee.edu (M.V. Hosur). 0263-8223/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0263-8223(03)00032-1 Composite Structures 61 (2003) 89–102 www.elsevier.com/locate/compstruct