Fatigue damage accumulation and residual strength assessment of CFRP laminates K.I. Tserpes a , P. Papanikos b , G. Labeas b , Sp. Pantelakis a, * a Laboratory of Technology and Strength of Materials, Department of Mechanical Engineering and Aeronautics, University of Patras, Panepistimioupolois Rion, Patras 26 500, Greece b ISTRAM, Institute of Structures and Advanced Materials, Patron-Athinon 57, Patras 26 441, Greece Abstract The method of progressive damage modelling has been used to assess fatigue damage accumulation and residual strength of carbon-fibre reinforced plastic (CFRP) laminates under fully reversed cyclic loading (R ¼ r min =r max ¼1). The accumulation of different damage modes has been assessed, as a function of number of cycles, using a three-dimensional fatigue progressive damage model (FPDM). The residual strength of the CFRP laminates has been assessed through the combined use of the FPDM with a static three-dimensional progressive damage model (PDM). By simulating the experimental procedure, the FPDM has been applied up to certain number of cycles, to estimate the accumulated fatigue damage and then, the static PDM has been applied (quasi-static tensile loading) to predict final tensile failure of the laminates, which corresponds to the residual strength of the laminate, after it has been exposed at the specific cycles. The models comprised the components of stress analysis performed using finite elements, failure analysis performed using polynomial stress-based failure criteria and material property degradation performed using degradation rules. The analysis has been validated experimentally (a) by assuming a laminate free of internal defects, and (b) by considering the initial defects, which were determined experimentally for certain laminates. The analysis has resulted in an accurate simulation of the experimentally determined fatigue damage accumulation and residual strength. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Composite laminates; Progressive damage modelling; Fatigue damage; Residual strength; Delamination; Initial damage 1. Introduction Composite materials are being widely used today in civil and military aeronautical applications due to their advantages over conventional materials. Civil aircraft are expected to increase their use of composites, bene- fiting from their specific strength and stiffness, tolerance of temperature extremes, high resistance to corrosion and fatigue and weight saving. Nevertheless, the use of composites as primary structural materials has remained limited and the advantages they offer are not being fully exploited. Designers are reluctant to adopt composites in load bearing applications because aircraft structures are subjected to cyclic loading in service and unlike metallic structures, there is a lack of reliable methodol- ogies for assessing the fatigue life and residual strength of composite structures. This is mainly due to their late introduction in aircraft structures and partly due to the early belief that composites are not prone to fatigue. This might be true for the service stresses that are low. However, in the last few years both academic research and industrial investigations have weakened this belief. Extensive problems have been reported in many cases of usage of composites in aircraft structures; e.g. bolted connections between the composite and metallic parts. These kind of problems has been investigated exten- sively in the frame of European aeronautical research projects such as EDAVCOS [1] and BOJCAS [2,3]. For designing metallic aircraft structural components against fatigue, three approaches have been mainly used: the safe-life, the fail-safe and the damage tolerance ap- proach. The same approaches, with small modifications, have been also used for the fatigue design of composite aircraft structural components. In recent years, the damage tolerance approach has been eventually adopted by the aircraft industry, due to its economic and safety advantages (higher fatigue life combined with higher damage tolerance capability, higher corrosion resistance * Corresponding author. Tel.: +30-2610-991027/61-991027; fax: +30-2610-997190/61-997190. E-mail address: pantelak@mech.upatras.gr (Sp. Pantelakis). 0263-8223/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0263-8223(03)00169-7 Composite Structures 63 (2004) 219–230 www.elsevier.com/locate/compstruct