An experimental investigation of the impact response of composite laminates Mehmet Aktas ß a, * , Cesim Atas b , Bu ¨ lent Murat _ Ic ßten b , Ramazan Karakuzu b a Usak University, Department of Mechanical Engineering, 64300 Usak, Turkey b Dokuz Eylul University, Department of Mechanical Engineering, 35100 Izmir, Turkey Available online 13 February 2008 Abstract In this study, the impact response of unidirectional glass/epoxy laminates has been investigated by considering energy profile dia- grams and associated load–deflection curves. Damage modes and the damage process of laminates under varied impact energies are dis- cussed. Two different stacking sequences, [0/90/0/90] s and [0/90/+45/45] s , were chosen in tests for comparison. An alternative method, based on variation of the excessive energy (E e ) versus impact energy (E i ), is presented to determine penetration threshold (Pn). The pen- etration threshold for stacking sequence [0/90/+45/45] s is found to be smaller than that of [0/90/0/90] s . The primary damage mode was found to be fiber fracture for higher impact energies; whereas, it was indentation resulting in delamination and matrix cracks for smaller impact energies. Contour plots of the overall damage areas are also depicted for several impact energies. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Unidirectional glass/epoxy; Impact response; Damage modes 1. Introduction There has been a growing interest, particularly in the few last decades, in the use of composite materials in struc- tural applications ranging from aircraft and space struc- tures to automotive and marine applications. However, their behavior under impact loading is one of the major concerns [1], since impacts do occur during manufacture, normal operations, maintenance and so on. Especially, uni- directional laminated plates are highly susceptible to the transverse impact loads resulting in significant damages such as matrix cracks, delaminations, and fiber fractures. Therefore, a lot of studies have been carried out to help understand and improve the impact response of composite materials and structures [2–6]. In the literature, a lot of experimental, numerical, and analytical studies on the impact response of laminated composite structures in many aspects can be found. Among them, Sadasivam and Mallick [7] have studied on the low energy impact characteristics of four different E-glass fibers reinforced thermoplastic and thermosetting matrix com- posites. Caprino et al. [8] have performed low velocity impact tests on carbon/epoxy laminates of different thick- nesses. They have examined the force and absorbed energy at the onset of delamination, the maximum force and related energy, and penetration energy. Some experimental investigations have been carried out by Hosur et al. [9] to determine the response of four different combinations of hybrid laminates subjected to low velocity impact loading. They have indicated that there was considerable improve- ment in the load carrying capability of hybrid composites as compared to carbon/epoxy laminates with slight reduc- tion in stiffness. Datta et al. [10] have investigated the effects of variable impact energy and laminate thickness on the low velocity impact damage tolerance of GFRP composite laminates. Critical values of impact energy and laminate thickness were also defined. Baucom and Zikry [11] have addressed an experimental study to understand the effects of reinforcement geometry on damage progress in woven composite panels under repeated impact loading. Fuoss et al. [12,13] have worked on the effects of key stack- 0263-8223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2008.02.003 * Corresponding author. Tel.: +90 276 2634195; fax: +90 276 2634196. E-mail address: mehmet.aktas@usak.edu.tr (M. Aktas ß). www.elsevier.com/locate/compstruct Available online at www.sciencedirect.com Composite Structures 87 (2009) 307–313