X-ray tomographical observations of cracks and voids in 3D carbon/carbon composites Rajneesh Sharma, Vinit V. Deshpande, Atul R. Bhagat, Puneet Mahajan * , Ramesh K. Mittal Department of Applied Mechanics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India ARTICLE INFO Article history: Received 11 September 2012 Accepted 14 April 2013 Available online 19 April 2013 ABSTRACT X-ray tomography was used to reconstruct the microstructure of a carbon/carbon (C/C) composite to clearly reveal cracks and voids. The voids are divided into two categories (bub- bles and micro-pores), and the cracks are classified into three categories (matrix, intra-bun- dle, and interfacial). The bubbles were found to account for 2–3% volume in the composite and intra-bundle porosity was in the range of 5–8%. The bubbles are spherical and intra- bundle pores are cylindrical corresponding to structural model index (SMI). A three dimen- sional (3D) image of the network of cracks and voids was reconstructed for the better visu- alization. Interfacial cracks on a circular bundle/matrix interface were firstly segmented manually from the composite and special algorithms were used for segmentation and mea- surement of bundle/matrix interfacial a cracks of circular bundle in the composite. The crack thickness is obtained in the range of 0.012–0.018 mm and percentage debonding 20–50%. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction C/C composites are in demand in aerospace industry, due to excellent mechanical properties at higher temperatures be- sides being light in weight. C/C composites are manufactured by impregnation of the carbonaceous matrix in the preforms of carbon fibers followed by high temperature graphitization. Although many technologies are possible for matrix impreg- nation such as liquid route, gas route or combination of these two, in the present work liquid route using coal-tar has been used for fabrication of composite. During the graphitization process, the composites experience high thermal stresses due to the mismatch in the expansion coefficients of the con- stituents. These thermal stresses lead to significant internal damage in the composites. The mechanical properties of these composites depend upon the internal material struc- ture and damage of C/C composite [1,2]. Therefore, much ef- fort has been put by researchers in the past to characterize the internal structure of the C/C composite. Jortner [3] studied the morphology of 3D C/C composite with fibrous reinforce- ment placed according to a repeated pattern in three mutu- ally orthogonal directions. He observed shrinkage and thermal-stress cracks, and bubble like pores produced during processing. The imperfections were divided in two categories (1) big voids (2) dry zone [3]. The dry zone contained the cracks and micro voids. Siron and Lamon [2] studied the post failure crack network in these composites. The micro-cracks detected in the specimens during testing were grouped into three families as (1) intra-bundle cracks parallel and perpen- dicular to the loading direction, (2) inter bundle cracks paral- lel and perpendicular to the loading direction, (3) intra bundle cracks propagated through the matrix and adjacent bundle. X-ray tomography is a non-destructive method to explore the internal material structure and is frequently used by the researcher. More et al. [4] were the first to introduce X-ray tomography to study the variation of the density in a C/C 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.04.046 * Corresponding author: Fax: +91 11 26581119. E-mail address: mahajan@am.iitd.ac.in (P. Mahajan). CARBON 60 (2013) 335 – 345 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon