Fiber bundle push-out test and image-based finite element simulation for 3D carbon/carbon composites Rajneesh Sharma, Puneet Mahajan * , Ramesh Kumar Mittal Department of Applied Mechanics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India ARTICLE INFO Article history: Received 20 December 2011 Accepted 11 February 2012 Available online 19 February 2012 ABSTRACT The interfacial properties such as debond strength, fracture energy release rate in Mode-II and coefficient of friction play important roles in determining the mechanical properties and strength of carbon/carbon (C/C) composites. Push-out tests were conducted on 3D C/C composites and the experimental results were fitted to the shear lag model to deter- mine these interfacial properties. X-ray tomography was used to explore the internal mate- rial structure of the composite. The fiber bundle and matrix interfaces were observed as being partially damaged in the tomographic images and the crack network was explored in detail. The tomographic images were also used to reconstruct a finite element (FE) mesh for simulating push-out tests. The interface of the fiber bundle and matrix in the FE mesh was represented by cohesive surfaces with frictional contact. The cohesive surface proper- ties were obtained by matching FE results with the experimental results. The simulations had a good agreement with experiments and values of 0.75 for coefficient of friction, 2–5 N/mm 2 for debond stress, 1–4 N/mm 2 for clamping stress and 3–6 N/m for fracture energy release rate were obtained as interfacial parameters for the composite. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Multidirectional C/C composites are commonly employed in thermo-structural applications in the aerospace industry, such as re-entry cones, rocket nozzles [1] or airplane brakes, due to their remarkable properties at elevated temperatures. Their properties depend on the fiber volume fraction, archi- tecture of composites, material properties of the constituents and interfacial behavior between the constituents. Rao et al. [2] carried out asymptotic homogenization using finite element method to determine the properties of these com- posites for different architectures both for perfectly and imperfectly bonded interfaces between the fiber bundle and matrix. The imperfectly bonded interfaces were modeled with cohesive elements the properties for which were taken from the pull-out test data reported by Sakai et al. [3]. The push- out and pull-out test are mostly used to find the interfacial properties such as friction coefficient, radial stress, debond stress and fracture toughness of a fiber reinforced composite. Gao et al. [4], Hutchinson and Jensen [5], and Kerans and Par- thasarathy [6] have established the expressions to find out the interfacial parameters from experimentally obtained load displacement curves. Domnanovich et al. [7] performed pull-out tests on satin weave C/C composites and used two approaches (Gao et al. [4], and Hutchinson et al. [5]) to obtain the interfacial parameters. The values of fracture energy were reported in the range of 3–9 J/m 2 depending on the graphitiz- ing temperature of the composite. Sakai et al. [3] performed pull-out and double notch compression tests on C/C compos- ites to determine debond stress and fracture energy release rate. Valette et al. [8] conducted bundle pull-out test on spe- cial specimens to calculate the interfacial pull-out shear stress for 4D C/C composites and they considered roughness of bundle surfaces and abrasion in their analysis. Micro-Vick- ers indenter was used by Furukawa et al. [9] for the interfacial characterization of the C/C composite by fiber push-in, fiber 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2012.02.030 * Corresponding author: Fax: +91 11 26581119. E-mail address: mahajan@am.iitd.ac.in (P. Mahajan). CARBON 50 (2012) 2717 – 2725 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon