Availableonlineatwww.sciencedirect.com SCIENCE DIRECT S materials letters ELSEVIER Materials Letters 59(2005)3246-3251 ww.elsevier. com/locate/malet Damage analysis of 2D C/Sic composites subjected to thermal cycling in oxidizing environments by mechanical and electrical characterization Hui Mei", aifei Cheng National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi'an Shaanxi 710072, PR China Received 2 April 2005: accepted 3 May 2005 Available online 5 July 2005 Abstract Damage in 2D C/SiC composites subjected to thermal cycling in wet oxygen atmosphere was analyzed by both mechanical testing electrical resistance measurement. The microstructures were observed on a scanning electron microscope and a novel monitoring syste was developed to acquire automatically and analyze the resistance of composites during their dynamic damage. Results show that under cyclic temperature, matrix cracks and delaminates transversely as regular spacing and superficial fibers are susceptible to oxidation through the opening cracks, leading to two types of major fiber failure pattern: (i) physical fracture under thermal cycling and (ii) chemical recession in oxidizing atmosphere. They are considered to be responsible for mechanical degradation with increasing thermal cycles In electrical testing, resistance decreases upon heating and increases with cooling in each cycle. As cycling progresses, the baseline resistance decreases continuously and then levels off after about 35 cycles, which is consistent with the statistical result(about 38 times) of mechanical testing. C 2005 Elsevier B.v. All rights reserved. Keywords: Fiber; Ceramic composites; Mechanical properties; Electrical resistance change 1. Introduction and analysis of the real time electrical resistance of continuous carbon fiber reinforced Sic matrix composites Carbon-fiber-reinforced SiC-matrix composites(C/SiC) in the presence of combined mechanical, thermal, and fabricated by the chemical vapor infiltration process(CVi) environmental applied conditions have been proposed as advanced materials suitable for Many efforts have been devoted to acquisition of aerospace and gas turbine engine parts [1, 2]. Because of the electrical resistance of Carbon-fiber-reinforced polymers brittleness of carbon fiber composites, the monitoring of their (CFRP)[3-7 by the traditional measurement with multi damage during use is desirable so as to provide remedies or meter and electrical behavior of a RuO2 dispersed glass hanges in service conditions before catastrophic failure composites during tensile loading has been studied [8] takes place. Use of changes of electrical resistance to monitor However, real time monitoring of resistance of carbon fiber the structural integrity of carbon fiber reinforced composites reinforced ceramic matrix composites in controlled environ- has attracted considerable research interest in the last decade ments has not been obtained. In this paper, Damage of 2D 3, 4]. Changes in resistance can be caused by a wide range of C/SiC composites subjected to thermal cycling in oxidizing mechanisms such as fiber breakage, delamination, mechan- environments was characterized by both mechanical proper- ical strain and temperature. The present work aims toward the ties testing and electrical resistance measurement. On the application of an innovative methodology for the acquisition basis of Matlab programming language and its data acquisition toolbox, a convenient, economical and effective monitoring system was designed and developed to acquire Corresponding author. Tel +86 29 88494616; fax: +862988494620. and analyze the resistance of composites during their huimei@yahoo.com(H.Mei). dynamic damages. The results of mechanical testing and 0167-577X/S- see front matter o 2005 Elsevier B V. All rights reserved. doi:10.1016malt.2005.05.052Damage analysis of 2D C/SiC composites subjected to thermal cycling in oxidizing environments by mechanical and electrical characterization Hui Mei *, Laifei Cheng National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an Shaanxi 710072, PR China Received 2 April 2005; accepted 3 May 2005 Available online 5 July 2005 Abstract Damage in 2D C/SiC composites subjected to thermal cycling in wet oxygen atmosphere was analyzed by both mechanical testing and electrical resistance measurement. The microstructures were observed on a scanning electron microscope and a novel monitoring system was developed to acquire automatically and analyze the resistance of composites during their dynamic damage. Results show that under cyclic temperature, matrix cracks and delaminates transversely as regular spacing and superficial fibers are susceptible to oxidation through the opening cracks, leading to two types of major fiber failure pattern: (i) physical fracture under thermal cycling and (ii) chemical recession in oxidizing atmosphere. They are considered to be responsible for mechanical degradation with increasing thermal cycles. In electrical testing, resistance decreases upon heating and increases with cooling in each cycle. As cycling progresses, the baseline resistance decreases continuously and then levels off after about 35 cycles, which is consistent with the statistical result (about 38 times) of mechanical testing. D 2005 Elsevier B.V. All rights reserved. Keywords: Fiber; Ceramic composites; Mechanical properties; Electrical resistance change 1. Introduction Carbon-fiber-reinforced SiC-matrix composites (C/SiC) fabricated by the chemical vapor infiltration process (CVI) have been proposed as advanced materials suitable for aerospace and gas turbine engine parts [1,2]. Because of the brittleness of carbon fiber composites, the monitoring of their damage during use is desirable so as to provide remedies or changes in service conditions before catastrophic failure takes place. Use of changes of electrical resistance to monitor the structural integrity of carbon fiber reinforced composites has attracted considerable research interest in the last decade [3,4]. Changes in resistance can be caused by a wide range of mechanisms such as fiber breakage, delamination, mechan￾ical strain and temperature. The present work aims toward the application of an innovative methodology for the acquisition and analysis of the real time electrical resistance of continuous carbon fiber reinforced SiC matrix composites in the presence of combined mechanical, thermal, and environmental applied conditions. Many efforts have been devoted to acquisition of electrical resistance of Carbon-fiber-reinforced polymers (CFRP) [3 – 7] by the traditional measurement with multi￾meter and electrical behavior of a RuO2 dispersed glass composites during tensile loading has been studied [8]. However, real time monitoring of resistance of carbon fiber reinforced ceramic matrix composites in controlled environ￾ments has not been obtained. In this paper, Damage of 2D C/SiC composites subjected to thermal cycling in oxidizing environments was characterized by both mechanical proper￾ties testing and electrical resistance measurement. On the basis of Matlab\ programming language and its data acquisition toolbox, a convenient, economical and effective monitoring system was designed and developed to acquire and analyze the resistance of composites during their dynamic damages. The results of mechanical testing and 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.05.052 * Corresponding author. Tel.: +86 29 88494616; fax: +86 29 88494620. E-mail address: phdhuimei@yahoo.com (H. Mei). Materials Letters 59 (2005) 3246 – 3251 www.elsevier.com/locate/matlet