3732 HENAGER et al: SUBCRITICAL CRACK GROWTH: PART I Crack her argon and agree with fiber-creep activation energies for both fiber types. Optical microscopy reveals multiply cracked damage zone, and crack lengths are obtained for comparison with model results. We esti- mate the total plastic strain from the specimen dis- placements and find that these values compare favor A ably with estimates of single-fiber creep strains under Notch S Damage similar conditions. Crack velocities are calculated from the optical measurements of crack lengtH 4.1. Fiber creep properties and activation energies Previous investigations of subcritical crack growth 0.2 mm in CG-C composites by Henager and colleagues [2 4, 13] hypothesized that fiber creep is the rate-con- Cyy =100 MPa trolling deformation mechanism for crack extension cracking stress in an inert environment This is not th case for those materials tested in oxygen-containing environments [4, 33, 42, 44]. However, in inert environments at temperatures above about 1000 K, cracks bridged with fine-grained ceramic fibers width,D extend under applied loads, apparently due to a decrease in crack shielding caused by fiber creep in the crack wake. Both the crack-tip stress intensities and crack-opening displacements are shown to be time dependent because of the relaxation of the crack- wake bridging forces resulting from creep of the bridging fibers [2-4, 13]. Matching the measured Multiple Cracked Damage Zone temperature dependence of the cracking, using an activation-energy analysis, with the activation energy fter 6 2x10 s at 1373 K in argon. (b)Sche. for creep in Nicalon-CG fibers was a significant step micrograph of polished sections of tested zone showing parameters discussed in text in identifying the mechanism that dominates the rate and in ontour of constant o,, of 100 MPa of width of cracking [4]. However, this hypothesis would be 1.2 mm is super this schematic strengthened by including crack-growth rates in materials with other fibers, comparing the results in ms of test time, test temperature, number of cracks terms of published fiber-creep parameters as was done he damage zone and crack are listed in previously [4]. This is useful in understanding the dif Table 3. The cracks were hard to follow from the ferences between crack growth controlled by fiber notch through the 0o-plies, but in several cases, the creep versus other time-dependent relaxation pro- entire set of cracks(in that one plane)could be cesses, such as viscous sliding or oxidative interface maged and measured. Although the cracks were dif. removal mechanisms [4, 5, 45] ficult to image it appears that crack shedding is occur- Previous scg data were presented by computing ring as the cracks propagate away from the notch, crack velocities from displacement-time data using with the number of cracks decreasing by a factor of standard linear-elastic relationships between displace two. The crack length data for the CG-C specimens ments in bending and crack lengths. If our hypothes are more complete since only a single Hi-C specimen Is correct, the time-dependent displacement of the was sectioned specimens is proportional to the creep rate of the fib- ers and the displacement-time curves can be used to determine activation energies for crack growth 4. DISCUSSION AND ANALYSIS OF RESULTS Therefore, measured displacement rates are adequate This paper describes time-dependent crack-growth for this study. The shapes of the displacement-time experiments performed with two types of fibers and curves are similar for the CG-C and Hi-C materials these results are used to explore the effects of fiber and exhibit a power-law form(Figs 1-4). DiCarlo et reep rates on the subcritical-crack-growth behavior al. [34, 36, 37, 4648] have performed the most sig- f SiC/SiC composites. The experimental findings nificant amount of single fiber-creep testing and have support the hypothesis that fiber creep controls the developed an extensive database of creep properties rate of crack extension in these composite materials. and empirical fitting parameters to a Sherby-Dorn Activation energit3732 HENAGER et al.: SUBCRITICAL CRACK GROWTH: PART I Fig. 6. (a) Optical micrograph of polished sections of tested CG-C specimen after 6.2×105 s at 1373 K in argon. (b) Sche￾matic of damage zone showing parameters discussed in text and in Table 3. Contour of constant syy of 100 MPa of width 1.2 mm is superimposed on this schematic. terms of test time, test temperature, number of cracks in the damage zone, and crack lengths, are listed in Table 3. The cracks were hard to follow from the notch through the 0°-plies, but in several cases, the entire set of cracks (in that one plane) could be imaged and measured. Although the cracks were dif- ficult to image it appears that crack shedding is occur￾ring as the cracks propagate away from the notch, with the number of cracks decreasing by a factor of two. The crack length data for the CG-C specimens are more complete since only a single Hi-C specimen was sectioned. 4. DISCUSSION AND ANALYSIS OF RESULTS This paper describes time-dependent crack-growth experiments performed with two types of fibers and these results are used to explore the effects of fiber creep rates on the subcritical-crack-growth behavior of SiCf/SiC composites. The experimental findings support the hypothesis that fiber creep controls the rate of crack extension in these composite materials. Activation energies are calculated for cracking in argon and agree with fiber-creep activation energies for both fiber types. Optical microscopy reveals a multiply cracked damage zone, and crack lengths are obtained for comparison with model results. We esti￾mate the total plastic strain from the specimen dis￾placements and find that these values compare favor￾ably with estimates of single-fiber creep strains under similar conditions. Crack velocities are calculated from the optical measurements of crack length. 4.1. Fiber creep properties and activation energies Previous investigations of subcritical crack growth in CG-C composites by Henager and colleagues [2– 4, 13] hypothesized that fiber creep is the rate-con￾trolling deformation mechanism for crack extension when these composites are loaded above the matrix cracking stress in an inert environment. This is not the case for those materials tested in oxygen-containing environments [4, 33, 42, 44]. However, in inert environments at temperatures above about 1000 K, cracks bridged with fine-grained ceramic fibers extend under applied loads, apparently due to a decrease in crack shielding caused by fiber creep in the crack wake. Both the crack-tip stress intensities and crack-opening displacements are shown to be time dependent because of the relaxation of the crack￾wake bridging forces resulting from creep of the bridging fibers [2–4, 13]. Matching the measured temperature dependence of the cracking, using an activation-energy analysis, with the activation energy for creep in Nicalon-CG fibers was a significant step in identifying the mechanism that dominates the rate of cracking [4]. However, this hypothesis would be strengthened by including crack-growth rates in materials with other fibers, comparing the results in terms of published fiber-creep parameters as was done previously [4]. This is useful in understanding the dif￾ferences between crack growth controlled by fiber creep versus other time-dependent relaxation pro￾cesses, such as viscous sliding or oxidative interface removal mechanisms [4, 5, 45]. Previous SCG data were presented by computing crack velocities from displacement–time data using standard linear-elastic relationships between displace￾ments in bending and crack lengths. If our hypothesis is correct, the time-dependent displacement of the specimens is proportional to the creep rate of the fib￾ers and the displacement–time curves can be used to determine activation energies for crack growth. Therefore, measured displacement rates are adequate for this study. The shapes of the displacement–time curves are similar for the CG-C and Hi-C materials and exhibit a power-law form (Figs 1–4). DiCarlo et al. [34, 36, 37, 46–48] have performed the most sig￾nificant amount of single fiber-creep testing and have developed an extensive database of creep properties and empirical fitting parameters to a Sherby–Dorn transient creep expression as: