Assessment of Residual Tensile Strength of Carbon/Epoxy Composites Subjected to Low-Energy Impact Ayman Mosallam 1 ; Joel Slenk 2 ; and Jesa Kreiner 3 Abstract: This paper presents the results of an experimental study to investigate the effects of impact loading on the residual tensile strength of woven graphite epoxy laminates with a toughened resin system. In this study, both cross-ply, 0 / 90 6 , and angle-ply, 45 6 , laminate lay-up configurations were studied and compared. Test specimens were subjected to various levels of impact loading, after which tensile pull tests were performed to determine the residual tensile strength properties. The study results demonstrated that impact damage causes a significant reduction in tensile strength properties of woven cross-ply 0°/90° laminates. For example, cross-ply laminated composite specimens subjected to the lowest impact level, 6.8 J, exhibited a 25% decrease in ultimate tensile strength. Angle-ply woven laminates 45° , however, exhibited an 18% increase in ultimate tensile strength after being subjected to 6.8 J impact. This charac- teristic of increasing tensile strength in 45°  specimens is an example of increased fiber reorientation in composite laminates with limited fiber damage. DOI: 10.1061/ASCE0893-1321200821:4249 CE Database subject headings: Impact; Residual Strength; Epoxy compounds; Composite materials; Tensile strength. Introduction General For many years, composite laminates made from unidirectional tape have been popular in aerospace applications. The micro- and macromechanical analytical tools for modeling the elastic behav- ior of composites is long established and well documented; how- ever, elastic analysis of woven fabric laminates is relatively limited. Published articles that address the effects of low-velocity impact on unidirectional tape composite laminates are plentiful; however, research in the area of impact damage of woven fabric composite panels received little attention until the mid-1990s Siow and Shim 1998. Woven fabrics consist of two sets of interlacing threads. De- pending upon the pattern of yarn interlacing, the fabric is denoted as plain, twill, or satin weave. The fabric construction is one of the main parameters on which the elastic and strength properties of woven fabric composites depend Naik 1991. In this study, specimens cut from an eight-harness satin weave fabric laminate were tested and analyzed see Fig. 1. In this weave style, the warp fibers cross over seven fill fibers and under one in a repeated pattern. Because satin weave fabrics have much less crimp than plain or twill fabrics, they drape easily. These weaves are pre- ferred when laying up parts with complex contours because they are so much easier to mold over the forming tool. Woven fabrics demonstrate some very beneficial material characteristics. These include good toughness, increased resistance to impact damage, dimensional stability, and low fabrication cost. The effects of impact damage on tensile strength are not stud- ied as frequently as the effects on compressive strength. Various studies have revealed that residual tensile strength is more af- fected by fiber breakage as opposed to matrix cracking and ply delaminations Cairns and Lagace 1990. Impact Damage and Composites Composite laminates vary in sensitivity to impact loading. Lami- nates containing Kevlar aramid fibers offer excellent resistance to impact damage, whereas laminates made from E-glass are less effective. Graphite/epoxy composite laminates are the most sen- sitive to impact damage. Even low-velocity impact from a dropped tool or rough handling can result in matrix cracking, fiber breakage, and delaminations between adjacent plies Cairns and Lagace 1990. Impact damage is often located below the surface of the laminate and is rarely visible to the naked eye this is particularly true in case of ply delaminations. Damage produced by low-velocity impacts can be very detrimental to the strength properties of a laminate, and the ability to predict the residual strength of damaged structures is of major importance for an ef- fective damage tolerant design. The level of impact damage in a composite laminate can be measured using nondestructive test methods NDE. If a designer can correlate the amount of impact damage measured by NDE to the residual strength value of the laminate, then it will be easier to 1 Dept. of Civil and Environment Engineering, Univ. of California, E4130 Engineering Gateway, Irvine, CA 92697-2175 corresponding author. E-mail: mosallam@uci.edu 2 Boeing Company, Phantom Works, Huntington Beach, CA 92647. 3 Mechanical Engineering Department, California State Univ., Fuller- ton, CA 92834-9480. Note. Discussion open until March 1, 2009. Separate discussions must be submitted for individual papers. The manuscript for this paper was submitted for review and possible publication on March 26, 2007; ap- proved on August 13, 2007. This paper is part of the Journal of Aero- space Engineering, Vol. 21, No. 4, October 1, 2008. ©ASCE, ISSN 0893-1321/2008/4-249–258/$25.00. JOURNAL OF AEROSPACE ENGINEERING © ASCE / OCTOBER 2008 / 249 J. Aerosp. Eng. 2008.21:249-258. Downloaded from ascelibrary.org by California,Univ Of Irvine on 03/04/14. Copyright ASCE. For personal use only; all rights reserved.