JOURNAL OF COMPOSITE MATERIALS Article Quantitative analysis of fatigue cracks in laminated carbon fibre-reinforced polymer composites using micro-computed tomography Naglaa ElAgamy 1 , Jeremy Laliberte ´ 1 and Fred Gaidies 2 Abstract A phenomenological study was carried out on laminated carbon fibre-reinforced polymer composites subjected to constant amplitude fatigue loading. Visualization of damage progression was performed using a high-resolution Skyscan micro-computed tomography unit which provided detailed information on propagation of initially occurring cracks throughout fatigue life at specific intervals. Quantitative analysis of image sequences of virtual cross-sections throughout the three orthogonal planes of the sample resulted in defining fatigue crack growth rates, da/dn for each plane, which was interpreted in terms of the three damage modes: opening (mode I), in-plane shear (mode II) and out-of- plane shear (mode III). By applying linear elastic fracture mechanics laws, strain energy release rates were calculated and then used in a cohesive zone model formulation to define model parameters. Considering a bi-linear triangular cohesive zone model curve, maximum traction and maximum separation were calculated for each of the three damage modes, differentiating between modes II and III in a novel manner. Keywords CFRP, fatigue, microCT Background Advanced laminated carbon fibre-reinforced polymers (CFRP) composites continue to replace metal parts in aircraft structures 1 to improve fuel efficiency, reduce airframe weight and lower part numbers. In order to maintain flight safety, progressive damage in critical parts commonly experiencing constant or variable amp- litude fatigue loading are required to lie within stand- ards of damage tolerance requirements for composites, 2 hence increasing the necessity of accurate damage pre- diction methods. Propagation of existing cracks in materials subjected to fatigue loading is conventionally studied using Linear Elastic Fracture Mechanics (LEFM) and the well-known Paris law in which crack propagation rate is directly proportional to strain energy release rate in a linear relation. Published expressions of Paris law for composites which account for contribution of both fracture modes I (opening) and II (in-plane shear) were summarized by Blanco et al. 3 The contribution of mode III (out-of-plane shear) was found to be negligible in most studies. 3–5 Another approach, the Virtual Crack Closure Technique (VCCT) was intro- duced by Singh and Talreja. 6 VCCT is an energy- based model in which it is assumed that the work done to open new cracks between the previously exist- ing cracks is critical to initiate cracks in other plies by crack sliding displacement. However, the assumption of symmetrical crack propagation associated with this technique limited its popularity in addition to the com- monly encountered condition in both LEFM and VCCT laws of defining a pre-existing crack length. 1 Department of Mechanical and Aerospace Engineering, Carleton University, Ottawa, ON, Canada 2 Department of Earth Sciences, Carleton University, Ottawa, ON, Canada Corresponding author: Naglaa ElAgamy, Carleton University, 1125 Colonel By drive, Ottawa, ON K1S 5B6, Canada. Email: naglaaelagamy@cmail.carleton.ca Journal of Composite Materials 0(0) 1–18 ! The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0021998315608252 jcm.sagepub.com by guest on October 2, 2015 jcm.sagepub.com Downloaded from