The eects of glass-®ber sizings on the strength and energy absorption of the ®ber/matrix interphase under high loading rates M. Tanoglu a,1 , S.H. McKnight b , G.R. Palmese a , J.W. Gillespie Jr. a,c, * a Department of Materials Science and Engineering, Center for Composite Materials, University of Delaware, 201 Composites Manufacturing Laboratory, Newark 19716-3144, DE 19716, USA b Army Research Laboratory, Aberdeen Proving Ground, MD 21005, USA c Department of Civil and Environmental Engineering, Center for Composites, University of Delaware, 201 Composites Manufacturing Laboratory, Newark, 19716-3144, DE 19716, USA Received 8 February 2000; received in revised form 20 July 2000; accepted 15 August 2000 Abstract The interphases of various sized E-glass-®ber/epoxy-amine systems were tested at displacement rates in the range 230±2450 mm/s by a new experimental technique (dynamic micro-debonding technique). By this method, the rate-dependent interphase properties, apparent shear strength and absorbed energies due to debonding and frictional sliding, were quanti®ed. The systems include unsized, epoxy-amine compatible, and epoxy-amine incompatible glass ®bers. The high displacement rates that induce high-strain-rate interphase loading were obtained by using the rapid expansion capability of piezoelectric actuators (PZT). The results of dynamic micro-debonding experiments showed that the values of interphase strength and speci®c absorbed energies varied in a manner that is dependent on the sizing and exhibited signi®cant sensitivity to loading rates. The unsized ®bers exhibit greater frictional sliding energies that could provide better ballistic resistance, while the compatible sized ®bers show higher strength values that improve the structural integrity of the polymeric composites. In addition, signi®cantly higher amounts of energy are absorbed within the fric- tional sliding regime compared to debonding. By using the experimental data obtained, a case study was performed to reveal the importance of the interphase related micro damage modes on energy absorption (and therefore ballistic performance) of glass/ epoxy composite armor. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Coupling agents; A. Polymer-matrix composites (PMCs); B. Interphase: B. Fiber/matrix bond; B. Impact behavior 1. Introduction Composite materials are playing a key role in the development of lightweight integral armor for military vehicles such as tanks or armored personnel carriers. For future applications, revolutionary approaches are required to signi®cantly reduce (up to 50%) the mass of these systems and improve their mobility and trans- portability without sacri®cing survivability or main- tainability. Recent advances in lightweight armor include the development of composite/ceramic integral armor systems as represented in Fig. 1. The lightweight integral armor design includes multiple layers of glass ®ber reinforced-polymeric composite to meet the structural and ballistic requirements. Sized glass ®bers are being used with epoxy, vinyl-ester and polyester resin systems for these applications. Interphases in composites form in the vicinity of ®ber surfaces and may exhibit signi®cantly dierent material characteristics than the bulk resin properties [1±6]. The properties of the interphase and degree of adhesion between the ®ber and matrix govern load transfer between the composite constituents. Also, the properties of the interphase are critical to global composite per- formance such as strength, toughness, durability and impact/ballistic resistance [6±14]. An optimum balance of structural, ballistic/impact and durability perfor- mance from the composite armor can be achieved by tailoring the ®ber/matrix interphase. In addition to the composite performance, it has been recognized in recent years that the properties of the interphase may aect the energy absorption in the composites used for lightweight armor applications. The 0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(00)00195-0 Composites Science and Technology 61 (2001) 205±220 www.elsevier.com/locate/compscitech * Corresponding author. 1 Current address: Izmir Institute of Technology, Urla, Izmir, Turkey. E-mail address: gillespie@ccm.udel.edu (M. Tanoglu).