Comparative experimental analysis of the effect caused by artificial and real induced damage in composite laminates Marco A. Pérez a,b,⇑ , Lluís Gil a,b , Montserrat Sánchez a,b , Sergio Oller b,c a Laboratory for the Technological Innovation of Structures and Materials (LITEM), Colon 11, TR45, Terrassa, 08222 Barcelona, Spain b Departament de Resistència de Materials i Estructures a l’Enginyeria, Universitat Politècnica de Catalunya – BarcelonaTech, Jordi Girona 31, 08034 Barcelona, Spain c International Center for Numerical Methods in Engineering (CIMNE), Gran Capità s/n, Edifici C1, 08034 Barcelona, Spain article info Article history: Available online 22 February 2014 Keywords: Composite laminates Low-velocity impact Ultrasonic testing Experimental modal analysis Quasi-static loading Compression after impact abstract This paper presents the results of an extensive experimental campaign aimed to examine the effect upon the vibration response and on the residual load-bearing capacity caused by both: isolated artificially induced interlaminar damage and low-velocity impact induced damage in composite laminates. The experimental programme included modal testing, drop-weight impact testing, ultrasonic inspection, transverse quasi-static loading testing and compression testing conducted on a set of 72 carbon fibre- reinforced composite laminated coupons. Both types of damage caused measurable changes in laminate performance, however marked divergent trends were observed. The results allowed for conclusions to be drawn regarding the adequacy of the artificial damage approach and highlighted the importance and role of other forms of degradation upon damage tolerance of laminated composites containing damage. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction With the increasing use of composite materials in industry comes an increasing need of a better understanding of their behaviour and improving their performance. Over the years, a tre- mendous amount of activity has been devoted to developing accu- rate and fast non-destructive evaluation techniques as well as numerical methodologies, which can quantitatively predict the performance and durability of composite structures. This is due to the fact that composite materials represent a departure from the way that conventional materials are used, and, consequently, they require unconventional approaches to dealing with them. During the last decades, a significant research effort has been particularly devoted to the study of impact and post-impact behav- iour of laminate composite structures, since this is a phenomenon which has greatly hindered their widespread application. Experimental studies consistently indicate that impact induced damage is a mixture of three main failure modes: matrix cracking, delamination and fibre breakage, among which delamination the most severe because it may severely degrade the stiffness and strength of composite structures Reid and Zho [1], Hodgkinson [2]. Consequently, several studies assessed and quantified composite damage resistance and damage tolerance in terms of delaminated area. Numerous contributions have been made to numerically model the damage in composite materials Orifici et al. [3]. Continuum damage mechanics approaches, which are based on material deg- radation models, have proved to be successful to predict different composite failure modes, including matrix cracking, delamination and fibre breakage Hinton et al. [4]. However, major efforts have been focused on the treatment of delamination Pagano and Scho- eppner [5]: two common approaches are the virtual crack closure technique (VCCT) Krueger [6] and the cohesive zone models (CZMs) Camanho et al. [7], Lopes et al. [8]. In parallel, numerous experimental studies dealing with composite damage detection have also been published. It is notable that a large number of experimental works were based on the analysis of the influence of an isolated artificially induced damage Ooijevaar et al. [9], Wei et al. [10], Kessler et al. [11], Yam [12]. It is common practice to in- duce delamination by inserting a polyimide film before consolidat- ing a composite specimen in an autoclave. As shown, the importance of delamination is well recognized. However, for the particular case of an impact, delamination does not occur in isolation. While numerous studies on the relationship between delamination and damage tolerance have been reported, little has been found in the literature concerning the role of other forms of damage, such as fibre breakage or indentation, in the residual load-bearing capacity. This issue could be addressed http://dx.doi.org/10.1016/j.compstruct.2014.02.017 0263-8223/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Laboratory for the Technological Innovation of Structures and Materials (LITEM), Colon 11, TR45, Terrassa, 08222 Barcelona, Spain. Tel.: +34 937398289. E-mail address: marco.antonio.perez@upc.edu (M.A. Pérez). Composite Structures 112 (2014) 169–178 Contents lists available at ScienceDirect Composite Structures journal homepage: www.elsevier.com/locate/compstruct