C Pergamon Acta mater.4902001)3727-3738 www.elsevier.com/locate/actamat SUBCRITICAL CRACK GROWTH IN CVI SIC!SIC COMPOSITES AT ELEVATED TEMPERATURES: EFFECT OF FIBER CREEP RATE C H. HENAGER Jr, C.A. LEWINSOHN+ and R. H. JONES Pacific Northwest National Laboratory,$ Materials Sciences/P8-15, 902 Battelle Blvd, PO box 999, Richland. WA 99352-0999 US Received 3 April 2000, received in revised form 9 July 2001: accepted 9 July 2001) Abstract-Subcritical crack-growth studies in SiC!SiC composites were conduc reinforced with Hi-Nicalon fibers over a broad temperature range materials reinforced with Nicalon-CG fibers. Composites with a 0/90 plain weave and carbon interphase were tested in argon from 1 173 to 1473 K. Crack growth data obtained ronments are onsistent with a proposed fiber-creep-controlled crack-growth mechanism Measured crack-growth activation energies and ti perature exponents in argon agree with fiber creep-activation energies and nonlinear reep equations for both fiber types. Estimates of local strains during crack growth are in reasonable agreement with estimated fiber creep strains for the given times and temperatures. The increased creep resistance of Hi-Nicalon fibers is reflected in reduced crack-growth rates for composites containing those fibers. 200/ Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Ceramics-structural; Composites; Mechanical properties-creep; High temperature; Fracture 1 INTRODUCTION stand critical time-dependent deformation mech- Continuous fiber ceramic matrix composites( CFCCs) anisms are more reliable than unreinforced ceramics [I], but Previous studies(this group and others)have are prone to time-dependent failures from stable crack shown that creep of bridging fibers at elevated tem- [2-71 or from mechanical embrittlement(unstable nation for the observed creep or slow crack growth crack growth)caused by environmental exposure 18, of these continuous-fiber composites where the Sic 9. In particular, we are interested in understanding matrix is more creep resistant than the fibers [2, 3 time-dependent properties for these materials in gas- 3-26]. These results support the hypothesis that cooled advanced fission and fusion reactor environ creeping fibers transfer stress back to the matrix ments [10-12). Such environments are pristine in causing further matrix cracking, a loss of matrix stiff terms of oxygen content, and composites with carbon- ness, and increased loading of the crack-bridging fib- based interphases appear attractive. Since life-predic- ers, ultimately leading to failure. Recent results rel- tion methodologies for these materials would neces- evant to this work include experimental creep or sarily include time-dependent crack growth as an crack-growth tests on SiC/SiC composites [18, 19, important failure mechanism, it is essential to under- 23-26] and models of time-dependent crack growth [27-31]A complete discussion of the models appears in a companion paper [32] and will not be I To whom all correspondence should be addressed E-mail address: chuck. henager(@pnl. gov(C. H. Hen- Evans and Weber [18] documented increased com- pliances due to matrix cracking and also observed supported by Associated Western Univer- fiber sliding stresses at 1473 K that were almost one west Division(AWU NW) under Grant order of magnitude smaller than at room temp rature DE-FC -75522, DE-FG07-93ER-75912, or DE- Wilshire et al. [23] compared composite creep rates AC06-76RLO1830 le to matrix cracking to account for observed com 1359-6454/01/S20.00@ 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. P:S1359-6454(01)00276-2Acta mater. 49 (2001) 3727–3738 www.elsevier.com/locate/actamat SUBCRITICAL CRACK GROWTH IN CVI SiCf/SiC COMPOSITES AT ELEVATED TEMPERATURES: EFFECT OF FIBER CREEP RATE C. H. HENAGER Jr†, C. A. LEWINSOHN‡ and R. H. JONES Pacific Northwest National Laboratory,§ Materials Sciences/P8-15, 902 Battelle Blvd, PO box 999, Richland, WA 99352-0999, USA ( Received 3 April 2000; received in revised form 9 July 2001; accepted 9 July 2001 ) Abstract—Subcritical crack-growth studies in SiCf/SiC composites were conducted with composites reinforced with Hi-Nicalon fibers over a broad temperature range for comparison to earlier studies on materials reinforced with Nicalon-CG fibers. Composites with a 0/90 plain weave architecture and carbon interphase were tested in argon from 1173 to 1473 K. Crack growth data obtained in inert environments are consistent with a proposed fiber-creep-controlled crack-growth mechanism. Measured crack-growth activation energies and time–temperature exponents in argon agree with fiber creep-activation energies and nonlinear creep equations for both fiber types. Estimates of local strains during crack growth are in reasonable agreement with estimated fiber creep strains for the given times and temperatures. The increased creep resistance of Hi-Nicalon fibers is reflected in reduced crack-growth rates for composites containing those fibers.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Ceramics—structural; Composites; Mechanical properties—creep; High temperature; Fracture & fracture toughness 1. INTRODUCTION Continuous fiber ceramic matrix composites (CFCCs) are more reliable than unreinforced ceramics [1], but are prone to time-dependent failures from stable crack growth occurring in inert and oxidizing environments [2–7] or from mechanical embrittlement (unstable crack growth) caused by environmental exposure [8, 9]. In particular, we are interested in understanding time-dependent properties for these materials in gas￾cooled advanced fission and fusion reactor environ￾ments [10–12]. Such environments are pristine in terms of oxygen content, and composites with carbon￾based interphases appear attractive. Since life-predic￾tion methodologies for these materials would neces￾sarily include time-dependent crack growth as an important failure mechanism, it is essential to under- † To whom all correspondence should be addressed. E-mail address: chuck.henager@pnl.gov (C. H. Hen￾ager Jr) ‡ Research supported by Associated Western Univer￾sities, Inc., Northwest Division (AWU NW) under Grant DE-FG06-89ER-75522, DE-FG07-93ER-75912, or DE￾FG06-92RL-12451 with the US Department of Energy. § Pacific Northwest National Laboratory is operated for the US Department of Energy by Battelle under Contract DE-AC06-76RLO 1830. 1359-6454/01/$20.00  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6454(01)00276-2 stand critical time-dependent deformation mech￾anisms. Previous studies (this group and others) have shown that creep of bridging fibers at elevated tem￾peratures in inert environments is one possible expla￾nation for the observed creep or slow crack growth of these continuous-fiber composites where the SiC matrix is more creep resistant than the fibers [2, 3, 13–26]. These results support the hypothesis that creeping fibers transfer stress back to the matrix, causing further matrix cracking, a loss of matrix stiff￾ness, and increased loading of the crack-bridging fib￾ers, ultimately leading to failure. Recent results rel￾evant to this work include experimental creep or crack-growth tests on SiCf/SiC composites [18, 19, 23–26] and models of time-dependent crack growth [27–31]. A more complete discussion of the models appears in a companion paper [32] and will not be included here. Evans and Weber [18] documented increased com￾pliances due to matrix cracking and also observed fiber sliding stresses at 1473 K that were almost one order of magnitude smaller than at room temperature. Wilshire et al. [23] compared composite creep rates at 1573 K to fiber creep rates to demonstrate the degree of load transfer to the fibers that must occur due to matrix cracking to account for observed com-