can Ceramic SocietSoren Vol. 83. No 6 Fatigue failure occurs when the residual strength of the com-(3) Microstructural Characterization posite, O, has decreased to the maximum applied cyclic stress, gmax. It is the aim of the present study to investigate t After the tensile or fatigue tests, the average matrix crack the rate at which the strength-controlling damage evolves in a CMC during spacing was measured at the ished face by optical microscopy prior work had shown that well-developed surface cracks typi he experimental ap- cally span the entire cross section of a specimen), as follows:The proach is straightforward. The tensile strength of virgin and number of cracks was counted along 20-30 mm long lines parallel refatigued specimens is determined experimentally. Other speci- with the fiber direction. Usually more than 300 cracks were nens are cycled under similar conditions until fatigue failure counted. All measurements were taken away from the localized ccurs. Then, a diagram of composite strength as a function of region associated with the fracture surface Fracture surfaces were number of load cycles can be constructed inspected with optical and environmental sca electron micro- scopes(Model E-3, ElectroScan Corp, Wilmington, MA). In order to observe debris at the fracture surfaces, the specimens were II. Experimental Methods neither cleaned nor coated before being investigated. Finally, using (I Specimen Preparation a conventional scanning electron microscope, overview pictures The material used in this study was an 8-ply unidirectional were taken of gold-coated fracture surfaces Nicalon SiC-fiber-reinforced calcium aluminosilicate composit denoted SiC/CAS II, from Corning Inc. The nominal fiber volume Il. Results and discussion fraction was 35-40%. The composite was processed by hot pressing. During processing, an approximately 0 I um thick (I Monotonic Tensile Tests of virgin Specimens arbon-rich interphase layer developed around the SiC fibers, Typical stress-strain curves for the virgin material are shown in (The thickness of the carbon layer, which depends on processing Fig. I. The shape of the stress-strain curves is typical of damage conditions, was not measured in the present study. )The carbon- tolerant CMCs, , The shape reflects elastic response at low interphase layer is known to enable debonding and frictional strain(stage D), multiple matrix cracking(stage ID), large-scale sliding along the interfaces. The fiber diameters were in the range interfacial slip along the interfaces of intact fibers(stage II),and distributed fiber failures(stage IV) prior to localization of damage Edge-loaded tensile specimens, with the Some characteristic parameters of the virgin composite are sum parallel with the fiber direction, were cut from rec marized in Table I. E. is the elastic modulus within the first linear portion of the stress-strain curve, Uo.oz is the stress level where the crack spacing to be measured. The polishing was performed using axial strain, e, deviates 0.02% from linear elasticity(note that a 38 mm mandrel rotating at 1500 rpm. The following polishing matrix cracks develop below oo.o? T. and E. are the failure procedure was used: (1)600-grit SiC paper for 5 min, (2)45 um stress and strain, respectively, and s is the average matrix crack in(nylon cloth), (4)1. 0 um diamond paste for 10 min(nylon s was calculated by the rule of mixtures, utilizing a Young's modulus of 200 and 98 GPa, respectively, for the fibers and matrix.",A similar value of v was found by the area fraction method (i. e, by using micrographs to estimate the cross-sectional (2) Mechanical Testing area fraction of fibers For all specimens, the fracture was located within the gage Four specimens were loaded in monotonic uniaxial tension(the section. The fracture surface was macroscopically flat, but dis ading rate was 100 MPa/s)to establish the stiffness and strength of the virgin material. Four other specimens were cycled under played considerable fiber pull-out(Fig. 2). In the region near the identical conditions(o 240 MPa, omin 10 MPa, 200 Hz) fracture surface, the matrix cracks were opened significantly more until fatigue failure occurred. Four additional specimens were opening is assumed to have occurred by fibers that had broken and cycled under the same conditions, but for only 10 cycles, which subsequently pulled out. Thus, the length of this localization zone, vas slightly lower than the number of cycles to failure found for L(Table I), is an indirect measure of the interfacial slid the previous samples. These specimens were then loaded in stress monotonic tension(100 MPa/s) in order to measure the residual strength of the composite after 10 cycles nts were conducted on a mTS servohydrau- (2) Specimens Cycled to Failure ic test frame(Model No 331, MTS Systems Corp, Minneapolis, The shape of the cyclic stress-strain curves changed during MN). The fatigue tests were performed inside a 0. 1 m'water ycling. These changes were accompanied by the development of cooled chamber, where the temperature of the walls and grips was a permanent offset strain, e", and an increase in the surface kept constant at 22.0+0. 1C(see Holmes and Cho for details) The temperature rise of the specimen surface(caused by frictional ating)was measured with an infrared pyrometer(Model No 5402, Everest Interscience Inc, Fullerton, CA), focused at a 5 mm 600 [o] SiC/CAS pot size within the specimen gauge section. In order to achieve 100 PAls table temperature conditions, the chamber temperature was al lowed to stabilize for at least 2 h before the fatigue tests were arted. The axial strain data were measured with an extensometer (Model 632.27B-20, MTS System Corp )with a 33 mm gauge length. For the cyclic tests, the extensometer was mounted along a specimen edge by O-rings and fixed to the specimen with Super-Glue. Stress-strain data were recorded at regular intervals, and from these data the hysteresis modulus(averaged over one cycle) was calculated as a function of the number of load cycles The specimens were cycled between omin= 10 MPa an omax= 240 MPa with a sinusoidal waveform at a frequency of 200 02040.60.811.21.4 Hz. In order to prevent overshooting during the first few load Strain cycles, the load span was increased (linearly with time) from zero Fig. 1. Two typical monotonic stress-strain curves obtained from vi to the maximum stress within o8sFatigue failure occurs when the residual strength of the com￾posite, su, has decreased to the maximum applied cyclic stress, smax. It is the aim of the present study to investigate the rate at which the strength-controlling damage evolves in a CMC during high cycle fatigue at room temperature. The experimental ap￾proach is straightforward. The tensile strength of virgin and prefatigued specimens is determined experimentally. Other speci￾mens are cycled under similar conditions until fatigue failure occurs. Then, a diagram of composite strength as a function of number of load cycles can be constructed. II. Experimental Methods (1) Specimen Preparation The material used in this study was an 8-ply unidirectional Nicalon SiC-fiber-reinforced calcium aluminosilicate composite, denoted SiCf /CAS II, from Corning Inc. The nominal fiber volume fraction was 35–40%. The composite was processed by hot pressing. During processing, an approximately 0.1 mm thick carbon-rich interphase layer developed around the SiC fibers12,13 (The thickness of the carbon layer, which depends on processing conditions, was not measured in the present study.) The carbon￾interphase layer is known to enable debonding and frictional sliding along the interfaces. The fiber diameters were in the range of 10–20 mm. Edge-loaded tensile specimens,28 with the tensile direction parallel with the fiber direction, were cut from rectangular plates. A minor face of each specimen was polished to allow the matrix crack spacing to be measured. The polishing was performed using a 38 mm mandrel rotating at 1500 rpm. The following polishing procedure was used: (1) 600-grit SiC paper for 5 min, (2) 45 mm diamond paste for 5 min (nylon cloth), (3) 6 mm diamond paste for 5 min (nylon cloth), (4) 1.0 mm diamond paste for 10 min (nylon cloth). (2) Mechanical Testing Four specimens were loaded in monotonic uniaxial tension (the loading rate was 100 MPa/s) to establish the stiffness and strength of the virgin material. Four other specimens were cycled under identical conditions (smax 5 240 MPa, smin 5 10 MPa, 200 Hz) until fatigue failure occurred. Four additional specimens were cycled under the same conditions, but for only 105 cycles, which was slightly lower than the number of cycles to failure found for the previous samples. These specimens were then loaded in monotonic tension (100 MPa/s) in order to measure the residual strength of the composite after 105 cycles. All fatigue experiments were conducted on a MTS servohydrau￾lic test frame (Model No. 331, MTS Systems Corp., Minneapolis, MN). The fatigue tests were performed inside a 0.1 m3 water￾cooled chamber, where the temperature of the walls and grips was kept constant at 22.0 6 0.1°C (see Holmes and Cho17 for details). The temperature rise of the specimen surface (caused by frictional heating) was measured with an infrared pyrometer (Model No. 5402, Everest Interscience Inc., Fullerton, CA), focused at a 5 mm spot size within the specimen gauge section. In order to achieve stable temperature conditions, the chamber temperature was al￾lowed to stabilize for at least 2 h before the fatigue tests were started. The axial strain data were measured with an extensometer (Model 632.27B-20, MTS System Corp.) with a 33 mm gauge length. For the cyclic tests, the extensometer was mounted along a specimen edge by O-rings and fixed to the specimen with Super-Glue. Stress–strain data were recorded at regular intervals, and from these data the hysteresis modulus (averaged over one cycle) was calculated as a function of the number of load cycles. The specimens were cycled between smin 5 10 MPa and smax 5 240 MPa with a sinusoidal waveform at a frequency of 200 Hz. In order to prevent overshooting during the first few load cycles, the load span was increased (linearly with time) from zero to the maximum stress within 0.8 s. (3) Microstructural Characterization After the tensile or fatigue tests, the average matrix crack spacing was measured at the polished face by optical microscopy (prior work had shown that well-developed surface cracks typi￾cally span the entire cross section of a specimen), as follows: The number of cracks was counted along 20–30 mm long lines parallel with the fiber direction. Usually more than 300 cracks were counted. All measurements were taken away from the localized region associated with the fracture surface. Fracture surfaces were inspected with optical and environmental scanning electron micro￾scopes (Model E-3, ElectroScan Corp., Wilmington, MA). In order to observe debris at the fracture surfaces, the specimens were neither cleaned nor coated before being investigated. Finally, using a conventional scanning electron microscope, overview pictures were taken of gold-coated fracture surfaces. III. Results and Discussion (1) Monotonic Tensile Tests of Virgin Specimens Typical stress–strain curves for the virgin material are shown in Fig. 1. The shape of the stress–strain curves is typical of damage￾tolerant CMCs.3,4,17 The shape reflects elastic response at low strain (stage I), multiple matrix cracking (stage II), large-scale interfacial slip along the interfaces of intact fibers (stage III), and distributed fiber failures (stage IV) prior to localization of damage. Some characteristic parameters of the virgin composite are sum￾marized in Table I. Ec is the elastic modulus within the first linear portion of the stress–strain curve, s0.02 is the stress level where the axial strain, ε, deviates 0.02% from linear elasticity (note that matrix cracks develop below s0.02 3,4), su and εu are the failure stress and strain, respectively, and s is the average matrix crack spacing measured after the tensile test. The fiber volume fraction, vf , was calculated by the rule of mixtures, utilizing a Young’s modulus of 200 and 98 GPa, respectively, for the fibers and matrix.3,4 A similar value of vf was found by the area fraction method (i.e., by using micrographs to estimate the cross-sectional area fraction of fibers). For all specimens, the fracture was located within the gage section. The fracture surface was macroscopically flat, but dis￾played considerable fiber pull-out (Fig. 2). In the region near the fracture surface, the matrix cracks were opened significantly more than at locations distant to the fracture site. This enhanced crack opening is assumed to have occurred by fibers that had broken and subsequently pulled out. Thus, the length of this localization zone, L (Table I), is an indirect measure of the interfacial sliding shear stress. (2) Specimens Cycled to Failure The shape of the cyclic stress–strain curves changed during cycling. These changes were accompanied by the development of a permanent offset strain, ε*, and an increase in the surface Fig. 1. Two typical monotonic stress–strain curves obtained from virgin specimens. 1470 Journal of the American Ceramic Society—Sørensen et al. Vol. 83, No. 6