J.Am. Ceram.Soc,902135-2142(2007) DOl:10.161551-2916.2007.01669x c 2007 The American Ceramic Society urna Real-Time Monitoring of Thermal Cycling damage in Ceramic Matrix Composites Under a Constant Stress Hui Mei, Laifei Cheng, Litong Zhang, Peng Fang, Zhixin Meng, and Chidong Liu National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xian haanxi 710072. China Under a constant stress of 50 MPa. a thermal strain with a and partial debonding stress were investigated quantita ange of 0. 2% was measured on a carbon-fiber-reinforced Sic by morscher and colleagues. 24-27Kishi and Enoki, and matrix composite(C/siC) subjected to thermal cycling between Yu et al. also successfully established a detection system of 700 and 1200 C. Acoustic emission(AE) technology was im- microcracks to estimate matrix cracking behavior in the CMCs. plemented to assist in monitoring the occurrence of damag Moreover, some analytical approaches/algorithms for AE data during testing. The monitored AE signals, together with the lave been proposed to predict the loss of mechanical propertie measured strain, were shown to have a significant dependence on and identify the different damage mechanisms in the process of repetitive temperature. In a single cycle the cycled specimens Nevertheless, only limited work has been published concern- emitted fewer acoustic emissions during heating, but as the cool- ing fiber-reinforced CMCs in the direction of the ae character ing stage approached, the emission rate increased dramatically ization of thermal shock or thermal cycling damage,35-37 As the cycle proceeded, the AE energy increased stepwise, although under similar conditions, monolithic ceramic38-4 whereas this stepwise increment per cycle continuously de- and metal matrix composites(MMCs) have received con- creased until finally it nearly disappeared at 15 cycles, after siderable attention. Unfortunately, the ae technology in these which no further increase in thermal cycle creep strain was few published papers was mainly used to monitor damage de- observed with a rate of zero, and the measured coating crack velopment or microstructural changes of the thermally shocked density reached a stable value of about 5.0 CMCs during the monotonic tensile tests for the different ther mal shock temperature and or after different cycle numbers. A comprehensive understanding of the real-time damage proces L Introduction of the CMC materials subjected to thermal cycling on the basis of AE methodology has not yet been reported in a detailed FIBER-R EINEOiRC D e ramt ic madex dm Ites(CMCs) have manner. Efforts were made in this investigation to characterize of a typical CMC applications in a variety of technological fields. The wide ran (i.e, C/SiC) by the subtle AE response, and to correlate repetitive temperatures of applications for these composites are likely to include rocket strain responses, and AE readings with the dynamic damage The monitored AE signals were mainly conducted to better understand the underlying damage mechanisms derived from these structural components are like temperature thermal cycling processes changes combined with significant mechanical loading. Even ture changes may produce thermal stresses that exceed the matrix yield stress, leading to microstructural chang es, and nonreversible damage in the composites such as matrix Il. Experimental Procedure cracking. fiber-matrix debonding and fiber fracture. The acous- (1) Materials tic emission(AE) technique provides a means to monitor the Three-dimensional (3D) preforms were braided by a four-step in situ damage evolution in the material without interrupting the method using I K carbon fibers(T-300", Toray, Tokyo, Japan) test procedure Low-pressure Isothermal-CVI was used to deposit a pyrolytic Fracture process and mechanics of the fiber-reinforced CMCs carbon interphase on the fibers and the silicon carbide matrix at during mechanical loading have been widely investigated by 1000C. The volume fractions of fibers were about 40%, and the many previous researchers, mainly based on continuous AE braiding le was about 20. Finally, test specimens were cut tative discussion of the raw data. The AE studies of damage SiC by I-CVI under the same conditions until a thicknes on monitoring of the loading procedure and qualitative or quanti from the fabricated composite plates and further coated w evolutions and failure mechanisms in the CMCs have been c50 um Fiber architectures and Pyc interphase m xperimentally conducted under monotonic tension, hyster- he fabricated unloa stepwise inc spectively. The dimensions of the dog-bone-shat pression,bending, cyclic fatigue, and even creep stress are 185 mm x3 mm x 3 mm. The properties of the as-received vated C/SiC composite specimens are listed in Table I of Mei et al basis of data analysis of acoustic emission, interfacial mechan- cal parameters between fiber and matrix such as interfacial shear strength, interfacial debonding length, interfacial sliding (2) Thermal Cycling Test As shown in Fig. 2 of reference Mei et al. thermal cycling ex E. Lara-Curzo-contributing editor periments were conducted on an integrated system, which included an induction heating furnace controlled by a pro- grammable microprocessor and a servo-hydraulic machine Model 8801, Instron Ltd, High Wycombe, UK). The micro- Manuscript No. 22247. Received September processor was set to run thermal cycles between T,=700C and National Yo No 50425208). T2=1200C with a period of 210 s, temperature difference △T≈500°C. Only the middle part of the specimens was kept 213Real-Time Monitoring of Thermal Cycling Damage in Ceramic Matrix Composites Under a Constant Stress Hui Mei,w Laifei Cheng, Litong Zhang, Peng Fang, Zhixin Meng, and Chidong Liu National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an Shaanxi 710072, China Under a constant stress of 50 MPa, a thermal strain with a range of 0.2% was measured on a carbon-fiber-reinforced SiC￾matrix composite (C/SiC) subjected to thermal cycling between 7001 and 12001C. Acoustic emission (AE) technology was im￾plemented to assist in monitoring the occurrence of damage during testing. The monitored AE signals, together with the measured strain, were shown to have a significant dependence on temperature in a single cycle and to change periodically with repetitive temperature. In a single cycle, the cycled specimens emitted fewer acoustic emissions during heating, but as the cool￾ing stage approached, the emission rate increased dramatically. As the cycle proceeded, the AE energy increased stepwise, whereas this stepwise increment per cycle continuously de￾creased until finally it nearly disappeared at 15 cycles, after which no further increase in thermal cycle creep strain was observed with a rate of zero, and the measured coating crack density reached a stable value of about 5.0 mm-1. I. Introduction FIBER-REINFORCED ceramic matrix composites (CMCs) have found, during the last two decades, numerous industrial applications in a variety of technological fields. The wide range of applications for these composites are likely to include rocket engine nozzles, thrusters, combustion chambers, divergent/con￾vergent flaps, turbomachinery, and aircraft brakes,1–3 where these structural components are likely to undergo temperature changes combined with significant mechanical loading. Even slight temperature changes may produce thermal stresses that exceed the matrix yield stress, leading to microstructural chang￾es, and nonreversible damage in the composites such as matrix cracking, fiber-matrix debonding, and fiber fracture. The acous￾tic emission (AE) technique provides a means to monitor the in situ damage evolution in the material without interrupting the test procedure. Fracture process and mechanics of the fiber-reinforced CMCs during mechanical loading have been widely investigated by many previous researchers, mainly based on continuous AE monitoring of the loading procedure and qualitative or quanti￾tative discussion of the raw data. The AE studies of damage evolutions and failure mechanisms in the CMCs have been experimentally conducted under monotonic tension,4–6 hystere￾sis reloading/unloading,7–9 stepwise incremental loading,10 com￾pression,11 bending,12,13 cyclic fatigue,14–20 and even creep stress rupture both at room and elevated temperatures.21–23 On the basis of data analysis of acoustic emission, interfacial mechan￾ical parameters between fiber and matrix such as interfacial shear strength, interfacial debonding length, interfacial sliding stress, and partial debonding stress were investigated quantita￾tively by Morscher and colleagues.24–27 Kishi and Enoki,28 and Yu et al. 29 also successfully established a detection system of microcracks to estimate matrix cracking behavior in the CMCs. Moreover, some analytical approaches/algorithms for AE data have been proposed to predict the loss of mechanical properties and identify the different damage mechanisms in the process of mechanical degradation of CMCs.15,30–34 Nevertheless, only limited work has been published concern￾ing fiber-reinforced CMCs in the direction of the AE character￾ization of thermal shock or thermal cycling damage,35–37 although under similar conditions, monolithic ceramic38–41 and metal matrix composites (MMCs)42–45 have received con￾siderable attention. Unfortunately, the AE technology in these few published papers was mainly used to monitor damage de￾velopment or microstructural changes of the thermally shocked CMCs during the monotonic tensile tests for the different ther￾mal shock temperature and/or after different cycle numbers. A comprehensive understanding of the real-time damage process of the CMC materials subjected to thermal cycling on the basis of AE methodology has not yet been reported in a detailed manner. Efforts were made in this investigation to characterize thermal cycling damage of a typical CMC (i.e., C/SiC) by the subtle AE response, and to correlate repetitive temperatures, strain responses, and AE readings with the dynamic damage. The monitored AE signals were mainly conducted to better understand the underlying damage mechanisms derived from thermal cycling processes. II. Experimental Procedure (1) Materials Three-dimensional (3D) preforms were braided by a four-step method using 1 K carbon fibers (T-300t, Toray, Tokyo, Japan). Low-pressure Isothermal-CVI was used to deposit a pyrolytic carbon interphase on the fibers and the silicon carbide matrix at 10001C. The volume fractions of fibers were about 40%, and the braiding angle was about 201. Finally, test specimens were cut from the fabricated composite plates and further coated with SiC by I-CVI under the same conditions until a thickness of
50 um. Fiber architectures and PyC interphase morphology of the fabricated composites are shown in Figs. 1(a) and (b), respectively. The dimensions of the dog-bone-shaped specimens are 185 mm 3 mm 3 mm. The properties of the as-received C/SiC composite specimens are listed in Table 1 of Mei et al. 46 (2) Thermal Cycling Test As shown in Fig. 2 of reference Mei et al. 46 thermal cycling ex￾periments were conducted on an integrated system, which included an induction heating furnace controlled by a pro￾grammable microprocessor and a servo-hydraulic machine (Model 8801, Instron Ltd., High Wycombe, UK). The micro￾processor was set to run thermal cycles between T1 5 7001C and T2 5 12001C with a period of 210 s, temperature difference DT
5001C. Only the middle part of the specimens was kept in E. Lara-Curzio—contributing editor This work was supported by the Natural Science Foundation of China (Contract No. 90405015) and National Young Elitists Foundation (Contract No. 50425208). w Author to whom correspondence should be addressed. e-mail: phdhuimei@yahoo.com Manuscript No. 22247. Received September 14, 2006; approved February 23, 2007. Journal J. Am. Ceram. Soc., 90 [7] 2135–2142 (2007) DOI: 10.1111/j.1551-2916.2007.01669.x r 2007 The American Ceramic Society 2135