COMPOSITES SCIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 61(2001)1331-1338 www.elsevier.com/locate/compsci Notch sensitivity of fatigue life in a Sylramic M/Sic composite at elevated temperature J.C. McNulty, M.Y. He, F W. Zok Materials Department, University of California, Santa Barbara, CA 93106, US.A Received 16 March 2000: received in revised form 20 February 2001; accepted 7 March 2001 The effects of holes and notches on the fatigue life of an advanced SylramicTM/SiC composite at 815.C have been examined.At this temperature, fracture occurs by an oxidative embrittlement mechanism, common to most Sic-based composites In unnotched specimens, embrittlement is manifested at stresses above the matrix cracking limit, omc, leading to fracture following relatively short exposure times(100 h). As a consequence, a fatigue threshold is obtained at a stress, oth a Ome. This threshold is due to the absence of an easy path for oxygen ingress when matrix cracks are not present. In center-hole specimens, an analogous threshold is btained at a stress, oth S Omc/ke, where ke is the elastic stress concentration factor(=2.5). That is, once the cracking limit is exceeded at the hole edge, embrittlement and fracture ensue. The threshold stress for center-notch specimens with stress con- centration ke 7 is numerically similar to that of the center-hole specimens with ke 2.5, indicating some tolerance to local stress levels above the global matrix cracking limit in sharply notched geometries. Non-linear finite-element calculations of the stresses in the center-hole and center-notch specimens are used to infer the local conditions associated with the threshold a key result is that the damage tolerance and notch insensitivity normally associated with inelastic straining cannot be exploited at temperatures at which the embrittlement mechanism operates. The implication is that composite structures with holes and notches must be designed extremely conservatively to ensure long lifetimes(>100 h).@ 2001 Elsevier Science Ltd. All rights reserved Keywords: Notch sensitivity; Fatigue; SiC composite; Embrittlement 1. Introduction based CFCCs exhibit inelastic deformation associated with matrix cracking and interface debonding, provid Continuous-fiber ceramic composites(CFCCs)based ing damage tolerance and strength retention in the pre- on SiC constituents have been under development for sence of holes and notches [3-5] the past two decades, mainly for use in advanced gas The most significant problem hindering SiC--CFCCs is turbine engines. The motivation for this activity is the oxidation embrittlement. The embrittlement most com- desire to increase operating temperatures and hence monly occurs by oxygen ingress through matrix cracks improve engine efficiency and performance. Additional followed by reaction of the oxygen with the fibers and the benefits associated with the elimination of film cooling fiber coatings [6-22]. The problem is particularly severe of combustor liners and the ensuing reductions in No at intermediate temperatures(500-900oC)[6, 15, 23-34] emissions have also been identified [1, 2 It is expected to be exacerbated by cyclic loading, since The selection of Sic as the main constituent in high- under such conditions the reaction gases contained performance CFCCs is based on a number of attractive within the crack are expelled during unloading and the physical and mechanical characteristics. Notably, their oxidizing atmosphere drawn into the composite through ow diffusivity results in good creep resistance at high matrix cracks during reloading. Cyclic loading may also temperatures. Furthermore, their high thermal con- accelerate fiber fracture in regions where the fibers have ductivity and low thermal expansion coefficient lead to bonded to the surrounding matrix as a consequence of high thermal shock resistance. In addition, the Sic- the oxidation process, thereby limiting the extent of further sliding along the fiber/matrix interface Corresponding author. Tel: +1-805-893-8699: fax: 1-805-893 The present paper examines the embrittlement phe- nomenon in an advanced Sic-CFCC which is currently E-imail address: zok(@engineering. ucsb. edu(F w. Zok) of interest to both the aerospace and the land-base 0266-3538/01/ S.see front matter C 2001 Elsevier Science Ltd. All rights reserved. PII:S0266-3538(01)00032-XNotch sensitivity of fatigue life in a Sylramic TM/SiC composite at elevated temperature J.C. McNulty, M.Y. He, F.W. Zok* Materials Department, University of California, Santa Barbara, CA 93106, USA Received 16 March 2000; received in revised form 20 February 2001; accepted 7 March 2001 Abstract The effects of holes and notches on the fatigue life of an advanced SylramicTM/SiC composite at 815
C have been examined. At this temperature, fracture occurs by an oxidative embrittlement mechanism, common to most SiC-based composites. In unnotched specimens, embrittlement is manifested at stresses above the matrix cracking limit,
mc, leading to fracture following relatively short exposure times (100 h). As a consequence, a fatigue threshold is obtained at a stress,
th
mc. This threshold is due to the absence of an easy path for oxygen ingress when matrix cracks are not present. In center-hole specimens, an analogous threshold is obtained, at a stress,
th
mc=ke, where ke is the elastic stress concentration factor (= 2.5). That is, once the cracking limit is exceeded at the hole edge, embrittlement and fracture ensue. The threshold stress for center-notch specimens with stress con￾centration ke ¼ 7 is numerically similar to that of the center-hole specimens with ke ¼ 2:5, indicating some tolerance to local stress levels above the global matrix cracking limit in sharply notched geometries. Non-linear finite-element calculations of the stresses in the center-hole and center-notch specimens are used to infer the local conditions associated with the threshold. A key result is that the damage tolerance and notch insensitivity normally associated with inelastic straining cannot be exploited at temperatures at which the embrittlement mechanism operates. The implication is that composite structures with holes and notches must be designed extremely conservatively to ensure long lifetimes (> 100 h). # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Notch sensitivity; Fatigue; SiC composite; Embrittlement 1. Introduction Continuous-fiber ceramic composites (CFCCs) based on SiC constituents have been under development for the past two decades, mainly for use in advanced gas turbine engines. The motivation for this activity is the desire to increase operating temperatures and hence improve engine efficiency and performance. Additional benefits associated with the elimination of film cooling of combustor liners and the ensuing reductions in NOx emissions have also been identified [1,2]. The selection of SiC as the main constituent in high￾performance CFCCs is based on a number of attractive physical and mechanical characteristics. Notably, their low diffusivity results in good creep resistance at high temperatures. Furthermore, their high thermal con￾ductivity and low thermal expansion coefficient lead to high thermal shock resistance. In addition, the SiC￾based CFCCs exhibit inelastic deformation associated with matrix cracking and interface debonding, provid￾ing damage tolerance and strength retention in the pre￾sence of holes and notches [3–5]. The most significant problem hindering SiC–CFCCs is oxidation embrittlement. The embrittlement most com￾monly occurs by oxygen ingress through matrix cracks, followed by reaction of the oxygen with the fibers and the fiber coatings [6–22]. The problem is particularly severe at intermediate temperatures (500–900
C) [6,15,23–34]. It is expected to be exacerbated by cyclic loading, since under such conditions the reaction gases contained within the crack are expelled during unloading and the oxidizing atmosphere drawn into the composite through matrix cracks during reloading. Cyclic loading may also accelerate fiber fracture in regions where the fibers have bonded to the surrounding matrix as a consequence of the oxidation process, thereby limiting the extent of further sliding along the fiber/matrix interface. The present paper examines the embrittlement phe￾nomenon in an advanced SiC–CFCC which is currently of interest to both the aerospace and the land-base 0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(01)00032-X Composites Science and Technology 61 (2001) 1331–1338 www.elsevier.com/locate/compscitech * Corresponding author. Tel.: +1-805-893-8699; fax: +1-805-893- 8486. E-mail address: zok@engineering.ucsb.edu (F.W. Zok).