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Furui, Mitsuaki

Kagawa, Yutaka

Kiyohara, Takumi

Sato, Mitsuhiko

English

Engineering Conferences International

MEASUREMENT METHOD OF IN-SITU TENSILE STRENGTH OF SIC FIBER IN SIC/SIC COMPOSITE  Yoshihisa Tanaka, The Center for Ceramic Matrix Composites, Katayanagi Advanced Research Laboratory, Tokyo University of Technology, Japan tanakayh@stf.teu.ac.jp Takumi Kiyohara, Faculty of Engineering, Department of Mechanical Engineering, Tokyo University of Technology, Japan Mitsuhiko Sato, The Center for Ceramic Matrix Composites, Katayanagi Advanced Research Laboratory, Tokyo University of Technology, Japan Yutaka Kagawa, The Center for Ceramic Matrix Composites, Katayanagi Advanced Research Laboratory, Tokyo University of Technology, Japan Mitsuaki Furui, Faculty of Engineering, Tokyo University of Technology, Japan   Key Words: Tensile test, Fractography, SiC fiber, In-situ strength, SiC matrix, SiC/SiC, Heat-exposure  Measurement of in situ tensile strength of SiC fiber in SiC matrix is important to understand mechanical behavior of SiC/SiC. Fractographic approach is one possible way to measure in-situ tensile strength of the fiber. According to the approach, tensile strength of SiC fiber, 𝜎𝜎𝐹𝐹, is given by 𝜎𝜎𝐹𝐹 = 𝜙𝜙𝜙𝜙 �𝑎𝑎𝑖𝑖⁄ , where 𝜙𝜙 is a constant depend on fracture toughness of SiC fiber, 𝜙𝜙 is a numerical constant, and 𝑎𝑎𝑖𝑖 is the characteristic crack length, usually used “mirror-mist-hackle” zone approach. It seems reasonable that both intrinsic (heat exposure, chemical degradation etc.) and extrinsic (stress states, effective volume etc.) factors affect to the fracture toughness of SiC fiber when the fiber is incorporated in SiC matrix. Definition and determination procedure of 𝑎𝑎𝑖𝑖 also affects to the constant 𝜙𝜙.  However, effects of such the factors on the constant 𝜙𝜙  and 𝜙𝜙 are not known.  The major attention of this study is to develop measurement method of in-situ SiC fiber strength in SiC matrix, using fractographic approach. Here, only degradation during fabrication process (heat-exposure history) is considered because chemical degradation is inhibited by BN interphase layer. Under this condition, in-situ tensile strength of the fiber is roughly given by 𝜎𝜎𝐹𝐹 = 𝐶𝐶(∆𝑇𝑇,∆𝑡𝑡) �𝑎𝑎𝑐𝑐⁄ + 𝜎𝜎𝑟𝑟𝑟𝑟𝑟𝑟 , where 𝜎𝜎𝑟𝑟𝑟𝑟𝑟𝑟 is the residual stress in SiC fiber, ∆𝑇𝑇 and ∆𝑡𝑡 are temperature and time, respectively. Stress states is assumed to be unidirectional and residual stress of SiC fiber in SiC matrix is neglected (𝜎𝜎𝑟𝑟𝑟𝑟𝑟𝑟~0). It should be noted that within this study, both 𝜙𝜙 and 𝜙𝜙 are included in a constant 𝐶𝐶(∆𝑇𝑇,∆𝑡𝑡).  Tensile strength of as received Hi-NicalonTM SiC fiber and heat exposed Hi-NicalonTM SiC fiber (1450℃ and 1500℃ for 0.5ℎ in Ar) are carried out. Fracture surfaces of tensile tested fiber are observed by SEM. Fracture surface observations reveal well identifiable lengths, 𝑎𝑎1 and 𝑎𝑎2 (𝑎𝑎1 < 𝑎𝑎2) are available. This characteristic fracture lengths are observed both before and after heat-exposure. Relationship between the length, 𝑎𝑎2, and tensile strength of the as-received SiC fiber, 𝜎𝜎𝐹𝐹, follows 𝜎𝜎𝐹𝐹 ∝ 1 √𝑎𝑎2⁄ . Therefore, the length 𝑎𝑎2 is assumed to be equal to the critical crack size 𝑎𝑎𝑐𝑐 (= 𝑎𝑎2) and the length is used for prediction of the fiber strength.   Tensile strengths of as received and after heat-exposed SiC fibers follow the Weibull probability distribution function: 𝑙𝑙𝑙𝑙𝑃𝑃𝐹𝐹(𝜎𝜎) = −�𝐿𝐿𝐿𝐿0� ∙ � 1𝜎𝜎0𝐶𝐶(∆𝑇𝑇,∆𝑡𝑡)�𝑎𝑎𝑐𝑐�𝑚𝑚, 𝑃𝑃𝐹𝐹(𝜎𝜎) is the probability of failure under applied tensile stress 𝜎𝜎, 𝐿𝐿 is the fiber length, 𝜎𝜎 is the stress, and 𝑚𝑚 is a constant. Subscripts “0” indicates characteristic values. Assuming that the strength follows the Weibull statistical distribution, and numerical curve fitting of a set of experiments, the parameters 𝐶𝐶(∆𝑇𝑇, 𝑡𝑡) are obtained. The constants 𝐶𝐶(∆𝑇𝑇,∆𝑡𝑡) for the original SiC fiber (∆𝑇𝑇 = ∆𝑡𝑡 = 0) is ~5.4 and heat exposed at 1450°C (∆𝑇𝑇 = 1450℃,∆𝑡𝑡 = 0.5ℎ) is ~5.2, heat exposed at 1500°C (∆𝑇𝑇 = 1500℃,∆𝑡𝑡 = 0.5ℎ) is ~5.1. This change means that application of 𝜎𝜎𝐹𝐹 = 𝐶𝐶(∆𝑇𝑇,∆𝑡𝑡) �𝑎𝑎𝑐𝑐⁄  needs selection of 𝐶𝐶(∆𝑇𝑇,∆𝑡𝑡) according to heat exposure history of the SiC fiber. This evidence suggests that care should be taken for application of 𝜎𝜎𝐹𝐹 =𝜙𝜙 �𝑎𝑎𝑖𝑖⁄  for measurement of in-situ fiber strength: modification of 𝜙𝜙  is necessary and physical meaning of 𝑎𝑎𝑖𝑖  should be considered. Further study is needed to examine physical meaning of 𝑎𝑎2 and 𝐶𝐶(∆𝑇𝑇,∆𝑡𝑡), and the effect of extrinsic factors, e.g. residual stress of SiC fiber in SiC matrix.   This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Materials Integration for revolutionary design system of structural materials.”  

Measurement method of in-situ tensile strength of SiC fiber in SiC/SiC composite

https://dc.engconfintl.org/cgi/viewcontent.cgi?filename=0&article=1058&context=cmcii&type=additional

Measurement method of in-situ tensile strength of SiC fiber in SiC/SiC composite

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