Contents lists available at ScienceDirect Composite Structures journal homepage: www.elsevier.com/locate/compstruct Mode-II interlaminar fracture and crack-jump phenomenon in CFRP composite laminate materials S.S. R. Koloor ⁎ , M.N. Tamin ⁎ Computational Solid Mechanics Laboratory, Department of Applied Mechanics and Design, School of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM, Johor Bahru, Johor, Malaysia ARTICLE INFO Keywords: CFRP composite laminates Interlaminar damage and fracture Crack-jump event End-notched flexure test Cohesive zone model ABSTRACT An interlaminar crack-jump event is one of the complex failure phenomena in laminated composite structures. This paper examines the mechanics of the interlaminar damage process leading to crack-jump event in CFRP composites under mode-II loading condition. A series of end-notched flexure tests of CFRP composite were conducted to create a standard interface failure, and an unstable interface fracture that led to crack-jump event. FE simulation of the composite tests was created using a new FE model-based construction and CZM theory in combination with a hybrid experimental-computational approach to assess the interface damage and crack-jump events. The FE model of the standard test predicted a short range crack-jump event instead of a gradual interface crack growth, coincided with the load drop in the structural response. A constant value of interface damage dissipation was predicted at the time of fracture for all composite cases. The unstable crack-jump event occurred due to the release of high strain energy in the composite structure while the interface underwent cracking process. 1. Introduction Advanced materials such as carbon fiber-reinforced polymer (CFRP) composite laminates have drawn attention in many industries such as transport, marine, aerospace, etc., due to their superior properties such as high strength, low weight and durability features in the design of vehicle components [1]. In aerospace, automotive, etc. applications, the manufacturing methods such as automated fiber placement, autoclave moulding, hot-press process, etc., are used for high quality production of CFRP composite structures in the form of multidirectional laminated panels with high fiber volume fractions and excellent specific stiffness and strength properties [2,3]. FRP composites are normally used as the airframe structure which designed to resist various types of loads in- cluding bending, torque, compressive, etc. [4]. Composite structures under such loads normally face multiple damage modes such as matrix and fiber failures in laminae or interface sections [4–7]. Physical da- mage events in CFRP composite laminates are classified as matrix cracking/crushing in micro-to-meso scales, interface delamination that could be induced by matrix failure, fiber breakage/buckling in micro- to-macro scales, lamina failure and laminate rupture in macro-scale [6,8–11]. Poor interface strength in CFRP composites is one of the limiting factors in the design of CFRP composite structures. In a phy- sical view, the inherent low strength of the interface is due to the typical creation of the interface as the discontinuous region between laminae during the assembly process of semi-cured prepreg layers. Therefore, investigation into the improvement of interface character- istics is one of the important topics in the enhancement of the inter- laminar fracture toughness as well as the resistance to the onset of impact and fatigue damage phenomena [6,7,12–15]. Interface damage evolution is often analyzed using physically-based models that were derived from numerical modeling of delamination. Cohesive zone model (CZM) is recognized as one of the foremost used models in the interpretation of interface damage and delamination of laminated composite structures [16]. A precise physical interpretation of the CZM model can be shown in independent experimental processes that mea- sures the normal and shear displacements of interface in the FRP composite laminate. The hypotheses and relevant equations for inter- face failure criteria and the constitutive damage models for FRP com- posites can be found in the open literature [6,16–19]. Interlaminar fracture phenomena of laminated composites have been classified in modes I, II and III or mixed-mode conditions, which are studied using end-notched flexure (ENF), double cantilever beam, edge crack torsion, mixed-mode bending and etc., tests [20–25]. Studies about the interface failure due to excessive shear deformation, is of prime interest for the design of composite structures under flexural loading condition [1,7,26]. The ENF test has been designed to initiate a https://doi.org/10.1016/j.compstruct.2018.07.132 Received 8 May 2017; Received in revised form 29 June 2018; Accepted 31 July 2018 ⁎ Corresponding authors. E-mail addresses: s.s.r.koloor@gmail.com (S.S. R. Koloor), taminmn@fkm.utm.my (M.N. Tamin). Composite Structures 204 (2018) 594–606 Available online 02 August 2018 0263-8223/ © 2018 Elsevier Ltd. All rights reserved. T