3728 HENAGER et al. SUBCRITICAL CRACK GROWTH: PART I posite creep rates. They observed extensive matrix exponents if the understanding produced by the pre- cracking due to fiber creep and pointed out that this vious studies is correct. present authors have observed [4, 33). Zhu et al. [2 performed creep tests in air and argon at 1573 K on 2. EXPERIMENTAL APPROACH Hi-Nicalon-fiber SiC/SiC and Nicalon-CG-fiber The experimental approach has been described SiC/siC and made many of the same observations. more detail elsewhere [3] and will only be summar They did measure a slower creep rate and longer ized here. Subcritical crack growth was obtained by time-to-rupture for the Hi-Nicalon-fiber composite loading single-edge-notched beam(SENB)specimens attributed to a greater creep resistance of Hi-Nicalon (typically 50x55x4 mm)in 1/4 four-point bending in fibers compared to Nicalon-CG fibers [34]. Tressler a fully articulated silicon-carbide fixture with an outer al. [26] crept Hi-Nicalon-fiber SiC/SiC microcom- span length of 40 mm. We incorporated the specimen posites at 1473 to 1673 K and demonstrated a model and fixture in a vertically oriented mullite tube con that used explicit fiber and matrix creep data to tained within a high-temperature furnace mounted to account for creep deformation with and without an electromechanically controlled mechanical test matrix cracks. They observed transient creep curves frame. t The SENB specimens contained an initial for all materials and conditions in their study. A rule- notch with a depth-to-width ratio(all) of approxi- of-mixtures creep model was used for the material mately 0. 2. The notch was made by a high-speed dia- without matrix cracks, while a different approach mond saw and was typically 3.9x10-4 m wide at the suitable for a crack bridging fiber in tension was tip of the notch and 5.9x10-4 m wide at the mouth adopted for the cracked microcomposites No attempt was made to sharpen the notch. Speci- However, few studies have compared creep or mens were heated to the test temperature at a rate of crack-growth rates in materials with similar com- about 0. 25 K/s and were allowed to equilibrate for posite architecture but different fibers [13, 25, 35]. 1200 s at temperature. Then a load calculated [38]to Studies by the present authors [2-4, 13] have demon- provide an initial applied stress intensity of 9 to 10 strated that thermal activation energies for crack MPa m 2 was applied to the sample and held for the growth in inert environments are consistent with acti- duration of the test. This stress intensity was chosen vation energies measured for these same fibers in sin- to induce some initial crack extension and crack e-fiber creep or stress relaxation tests [34, 36]. bridging and it falls between that required for matrix However, this strong evidence for a crack growth cracking and the peak load fracture toughness, Ko, mechanism controlled by fiber creep would be bols- reported in Table 1. In addition, we periodically tered by supporting evidence in similar materials with unloaded to 95% of the constant applied load and then different fibers. Additional data would provide an reloaded; at the time of the initial loading and 5, 25 opportunity for testing our ability to model crack and 50 h after the initial loading to generate hysteresis growth in a specific specimen geometry using loops(for the Hi-Nicalon materials only) detailed fiber properties where the fiber type was the The atmosphere inside the mullite tube was con- most significant variable trollable and maintained at atmospheric pressure Moreover, since these composites are being con- (1.01x10 Pa). We used gettered argon, initially sidered for use in high-temperature, gas-cooled reac- 99.999% pure, for testing, with an oxygen content tor concepts [10-12], we are interested in lifetimes in reduced to less than 0.01 Pa by passing the gas ert environments or in low oxygen concentrations. through a titanium-gettering furnace. The deflection Many of the studies discussed above were performed of the specimen midpoint was measured by using ar in air, which is a very degrading environment, parti- alumina pushrod, also containing a thermocouple, cularly for CFCCs with a carbon-based fiber-matrix attached to a strain-gauge extensometer. The dis- interphase. This teaches us little about life prediction placements were corrected for differences between and failure mechanisms in environments where these the load-point and midpoint and for the compliance materials are seriously being considered for use. of the test apparatus 3] Therefore, we began the present study using similar An SiC/SiC CFCC containing Hi-NicalonTM fibers 0/90-woven, chemical vapor infiltration(CVI)-SiC was examined in this study and compared to the matrix CFCCs reinforced with Hi-Nicalon fibers to results of previous studies [2-4, 13] on materials con- compare with the previous Nicalon-CG fiber CFCCs taining "Ceramic-grade" Nicalon TM(Nicalon-CG) 24, 13]. This study is the first to critically compare fibers. Data obtained using the previous Nicalon-CG the effect of fiber creep characteristics on crack- materials were extended over a temperature range growth kinetics and activation energies in an inert from 1173 to 1398 K. The Hi-Nicalon materials were environment. Hi-Nicalon fibers have higher creep fabricated from two-dimensional, plain weave fiber activation energies and a reduced creep rate compared to Nicalon-CG fibers [34, 36, 37]. Composites made with Hi-Nicalon fibers should exhibit crack-growth ates and activation energies consis on fiber creep rates, activation energies, and creep t Instron1125,Instron Corp,Canton,MA,USA.3728 HENAGER et al.: SUBCRITICAL CRACK GROWTH: PART I posite creep rates. They observed extensive matrix cracking due to fiber creep and pointed out that this cracking facilitates environmental ingress, as the present authors have observed [4, 33]. Zhu et al. [25] performed creep tests in air and argon at 1573 K on Hi-Nicalon-fiber SiCf/SiC and Nicalon-CG-fiber SiCf/SiC and made many of the same observations. They did measure a slower creep rate and longer time-to-rupture for the Hi-Nicalon-fiber composite attributed to a greater creep resistance of Hi-Nicalon fibers compared to Nicalon-CG fibers [34]. Tressler et al. [26] crept Hi-Nicalon-fiber SiCf/SiC microcom￾posites at 1473 to 1673 K and demonstrated a model that used explicit fiber and matrix creep data to account for creep deformation with and without matrix cracks. They observed transient creep curves for all materials and conditions in their study. A rule￾of-mixtures creep model was used for the material without matrix cracks, while a different approach suitable for a crack bridging fiber in tension was adopted for the cracked microcomposites. However, few studies have compared creep or crack-growth rates in materials with similar com￾posite architecture but different fibers [13, 25, 35]. Studies by the present authors [2–4, 13] have demon￾strated that thermal activation energies for crack growth in inert environments are consistent with acti￾vation energies measured for these same fibers in sin￾gle-fiber creep or stress relaxation tests [34, 36]. However, this strong evidence for a crack growth mechanism controlled by fiber creep would be bols￾tered by supporting evidence in similar materials with different fibers. Additional data would provide an opportunity for testing our ability to model crack growth in a specific specimen geometry using detailed fiber properties where the fiber type was the most significant variable. Moreover, since these composites are being con￾sidered for use in high-temperature, gas-cooled reac￾tor concepts [10–12], we are interested in lifetimes in inert environments or in low oxygen concentrations. Many of the studies discussed above were performed in air, which is a very degrading environment, parti￾cularly for CFCCs with a carbon-based fiber-matrix interphase. This teaches us little about life prediction and failure mechanisms in environments where these materials are seriously being considered for use. Therefore, we began the present study using similar 0/90-woven, chemical vapor infiltration (CVI)-SiC matrix CFCCs reinforced with Hi-Nicalon fibers to compare with the previous Nicalon-CG fiber CFCCs [2–4, 13]. This study is the first to critically compare the effect of fiber creep characteristics on crack￾growth kinetics and activation energies in an inert environment. Hi-Nicalon fibers have higher creep activation energies and a reduced creep rate compared to Nicalon-CG fibers [34, 36, 37]. Composites made with Hi-Nicalon fibers should exhibit crack-growth rates and activation energies consistent with Hi-Nica￾lon fiber creep rates, activation energies, and creep exponents if the understanding produced by the pre￾vious studies is correct. 2. EXPERIMENTAL APPROACH The experimental approach has been described in more detail elsewhere [3] and will only be summar￾ized here. Subcritical crack growth was obtained by loading single-edge-notched beam (SENB) specimens (typically 50×5.5×4 mm) in 1/4 four-point bending in a fully articulated silicon-carbide fixture with an outer span length of 40 mm. We incorporated the specimen and fixture in a vertically oriented mullite tube con￾tained within a high-temperature furnace mounted to an electromechanically controlled mechanical test frame.† The SENB specimens contained an initial notch with a depth-to-width ratio (a/W) of approxi￾mately 0.2. The notch was made by a high-speed dia￾mond saw and was typically 3.9×10
4 m wide at the tip of the notch and 5.9×10
4 m wide at the mouth. No attempt was made to sharpen the notch. Speci￾mens were heated to the test temperature at a rate of about 0.25 K/s and were allowed to equilibrate for 1200 s at temperature. Then a load calculated [38] to provide an initial applied stress intensity of 9 to 10 MPa m1/2 was applied to the sample and held for the duration of the test. This stress intensity was chosen to induce some initial crack extension and crack bridging and it falls between that required for matrix cracking and the peak load fracture toughness, KQ, reported in Table 1. In addition, we periodically unloaded to 95% of the constant applied load and then reloaded; at the time of the initial loading and 5, 25, and 50 h after the initial loading to generate hysteresis loops (for the Hi-Nicalon materials only). The atmosphere inside the mullite tube was con￾trollable and maintained at atmospheric pressure (1.01×105 Pa). We used gettered argon, initially 99.999% pure, for testing, with an oxygen content reduced to less than 0.01 Pa by passing the gas through a titanium-gettering furnace. The deflection of the specimen midpoint was measured by using an alumina pushrod, also containing a thermocouple, attached to a strain-gauge extensometer. The dis￾placements were corrected for differences between the load-point and midpoint and for the compliance of the test apparatus [3]. An SiCf/SiC CFCC containing Hi-Nicalon fibers was examined in this study and compared to the results of previous studies [2–4, 13] on materials con￾taining “Ceramic-grade” Nicalon (Nicalon-CG) fibers. Data obtained using the previous Nicalon-CG materials were extended over a temperature range from 1173 to 1398 K. The Hi-Nicalon materials were fabricated from two-dimensional, plain weave fiber † Instron 1125, Instron Corp., Canton, MA, USA