J.Am.cerm.Soc.836]1469-75(200 urna Rate of Strength Decrease of Fiber- Reinforced Ceramic-Matrix Composites during Fatigu Bent f. sorensen Materials Research Department, Riso National Laboratory, DK-4000 Roskilde, Denmark John W. holmes Ceramic Composites Research Laboratory, Depart Mechanical Engineering and Applied Mechanics, of Michigan, Ann Arbor, Michigan 48109-2125 Eddy L. Vanswijgenhoven Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, De Croy laan 2, B-3001 Heverlee E An experimental investigation ormed to study the rate resistance must be sufficiently low, such that interfacial sliding can at which strength-controlling amage evolves in a readily take place ceramic-matrix composite Te pecimens of a unidirec- The stress-strain behavior of CMCs at high temperature may tional Sic-fiber-reinforced calcium aluminosilicate matrix differ from the behavior observed at room temperature since creep omposite were cycled to failure or to a preselected number of and oxidation damage may occur.9-lI If oxidation occurs at the cycles under similar loading histories. The residual strength of per/matrix interface, an interphase with strong bonding may he precycled specimens was found to be similar to that of form. 10, 12, 13 This hinders interfacial sliding, resulting in a loss of virgin specimens. Microstructural investigations showed that damage-tolerant behavior, the composite then fails in a brittle the fracture surfaces of the specimens cycled to failure had a manner central region where fiber pullout was negligible. It is pro- Most experimental studies of CMCs subjected to cyclic loading posed that frictional heating (due to interfacial sliding) is the have been conducted at room temperature. 2, 14-19 Typically, the ause of fatigue failure. High interfacial temperatures are assumed to cause the formation of a strong interface bond, composite stiffness decreases rapidly in the early cycles, reaching leading to internal embrittlement minimum within 10-10 cycles. ,The number of cycles to failure may be significantly higher than the number of cycles at which the modulus reaches a minimum. 7 Damage evolution L. Introduction during cyclic loading has been found to be similar to that found for monotonic tension(multiple matrix cracking, fiber/matrix debond- emperature load-carrying components, such as turbine blades or Macroscopically, this slip results in hysteresis in the stress-strain heat exchangers. Before CMCs can be used in such applications, behavior and a temperature rise of the specimens( frictional temperatures, and environments must be understoauons of loads, heating, 17,20). At the microscale, cyclic slip may result in their long-term behavior under complex combinati interfacial wear wering the interfacial frictional slid The monotonic stress-strain behavior of unidirectional compos- shear stress, T. It has been proposed that a decrease in the tes has been studied extensively at room temperature nterfacial shear stress may decrease the composite strength and underlying damage mechanisms have been identified as the initi cause fatigue failure. For instance, for 2D SiC/SiC, Rouby and ation and growth of multiple matrix cracks. The matrix cracks are Reynaud found a fatigue limit( maximum allowable stress, o bridged by intact fibers, with debonding and sliding occurring at giving run-out) at 2.5 x 10 cycles, which corresponded nicely the fiber/matrix interface. Composite failure occurs when the with predictions based on a decrease in T. In their study, fatigue fibers fail. This distinction between matrix cracking and composite failures all occurred within 2 x 10 cycles. This is consistent with fracture provides damage-tolerant behavior, that is attractive from the decrease in T, which occurs within a low number of cycles desensineering point of view. Models have been developed to Thus, for high stresses, a mechanism for low cycle fatigue failure these mechanisms. The models predict that in order to appears to be the cyclic-induced decrease in T. However, for other have a damage-tolerant behavior, the interface bonding and sliding CMCs, the number of cycles to failure can exceed by orders of magnitude the number of cycles at which the interfacial shear itional fatigue damage mechanisms exist. F. W. Zok-contributing editor Experimental fatigue studies conducted at high temperature have shown that embrittlement due to oxidation damage of the nterphase layer may be the most severe problem. 023-26Holmes found that the life of cyclically loaded SCS-6 SiC/Si, speci- 999nuscrip No. 189843. Received septemder u, 98, approved Decem mens at 1200C was shorter than the creep life. The fatigue life Support for B. F. Sorensen was decreased with decreasing stress ratio. This provides clear evi- o551 support was dence of a high-temperature fatigue-life controlling mechanism orce Office of Scientific Research( Grant No. F49620-95-1-0206) Thus, fatigue interactions with oxidation and creep are damage Member, American Ceramic Society mechanisms that must be understood 1469Rate of Strength Decrease of Fiber-Reinforced Ceramic-Matrix Composites during Fatigue Bent F. Sørensen Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark John W. Holmes Ceramic Composites Research Laboratory, Department of Mechanical Engineering and Applied Mechanics, The University of Michigan, Ann Arbor, Michigan 48109-2125 Eddy L. Vanswijgenhoven Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, De Croylaan 2, B-3001 Heverlee, Belgium An experimental investigation was performed to study the rate at which strength-controlling fatigue damage evolves in a ceramic-matrix composite. Tensile specimens of a unidirec￾tional SiC-fiber-reinforced calcium aluminosilicate matrix composite were cycled to failure or to a preselected number of cycles under similar loading histories. The residual strength of the precycled specimens was found to be similar to that of virgin specimens. Microstructural investigations showed that the fracture surfaces of the specimens cycled to failure had a central region where fiber pullout was negligible. It is pro￾posed that frictional heating (due to interfacial sliding) is the cause of fatigue failure. High interfacial temperatures are assumed to cause the formation of a strong interface bond, leading to internal embrittlement. I. Introduction BECAUSE of their damage-tolerant behavior, ceramic-matrix composites (CMCs) have the potential for use in high￾temperature load-carrying components, such as turbine blades or heat exchangers. Before CMCs can be used in such applications, their long-term behavior under complex combinations of loads, temperatures, and environments must be understood.1 The monotonic stress–strain behavior of unidirectional compos￾ites has been studied extensively at room temperature.2–4 The underlying damage mechanisms have been identified as the initi￾ation and growth of multiple matrix cracks. The matrix cracks are bridged by intact fibers, with debonding and sliding occurring at the fiber/matrix interface. Composite failure occurs when the fibers fail. This distinction between matrix cracking and composite fracture provides damage-tolerant behavior; that is attractive from an engineering point of view. Models have been developed to describe these mechanisms.5–8 The models predict that in order to have a damage-tolerant behavior, the interface bonding and sliding resistance must be sufficiently low, such that interfacial sliding can readily take place. The stress–strain behavior of CMCs at high temperature may differ from the behavior observed at room temperature, since creep and oxidation damage may occur.9–11 If oxidation occurs at the fiber/matrix interface, an interphase with strong bonding may form.10,12,13 This hinders interfacial sliding, resulting in a loss of damage-tolerant behavior; the composite then fails in a brittle manner. Most experimental studies of CMCs subjected to cyclic loading have been conducted at room temperature.2,14–19 Typically, the composite stiffness decreases rapidly in the early cycles, reaching a minimum within 103 –105 cycles.15,17 The number of cycles to failure may be significantly higher than the number of cycles at which the modulus reaches a minimum.17 Damage evolution during cyclic loading has been found to be similar to that found for monotonic tension (multiple matrix cracking, fiber/matrix debond￾ing and sliding). In addition, during cyclic loading, repeated forward and reverse slip can occur at the fiber/matrix interface. Macroscopically, this slip results in hysteresis in the stress–strain behavior and a temperature rise of the specimens (frictional heating16,17,20). At the microscale, cyclic slip may result in interfacial wear,17,19,21 lowering the interfacial frictional sliding shear stress, t. It has been proposed that a decrease in the interfacial shear stress may decrease the composite strength and cause fatigue failure.19 For instance, for 2D SiC/SiC, Rouby and Reynaud19 found a fatigue limit (maximum allowable stress, smax giving run-out) at 2.5 3 105 cycles, which corresponded nicely with predictions based on a decrease in t. In their study, fatigue failures all occurred within 2 3 104 cycles. This is consistent with the decrease in t, which occurs within a low number of cycles. Thus, for high stresses, a mechanism for low cycle fatigue failure appears to be the cyclic-induced decrease in t. However, for other CMCs, the number of cycles to failure can exceed by orders of magnitude the number of cycles at which the interfacial shear stress reaches a minimum value.17,20,22 This disparity suggests that additional fatigue damage mechanisms exist. Experimental fatigue studies conducted at high temperature have shown that embrittlement due to oxidation damage of the interphase layer may be the most severe problem.10,23–26 Holmes27 found that the life of cyclically loaded SCS-6 SiCf /Si3N4 speci￾mens at 1200°C was shorter than the creep life. The fatigue life decreased with decreasing stress ratio. This provides clear evi￾dence of a high-temperature fatigue-life controlling mechanism. Thus, fatigue interactions with oxidation and creep are damage mechanisms that must be understood. F. W. Zok—contributing editor Manuscript No. 189843. Received September 30, 1998; approved December 14, 1999. Support for B. F. Sørensen was provided by the Risø Engineering Science Center for Structural Characterization and Modeling of Materials. Additional support was provided by the National Science Foundation (Grant No. DMR-9257557) and the Air Force Office of Scientific Research (Grant No. F49620-95-1-0206). *Member, American Ceramic Society. J. Am. Ceram. Soc., 83 [6] 1469–75 (2000) 1469 journal