COMPOSITES SCIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 59(1999)801-811 Fibre strength parameters measured in situ for ceramic-matrix composites tested at elevated temperature in vacuum and in air lan J Davies Takashi Ishikawa, * Masaki Shibuya, Tetsuro Hirokawa rframe Division, National Aerospace Laboratory, 6-13-1 Ohsawa, Mitaka-Shi, Tokyo 181, Japan cOrporate Research and Development, Ube Industries Ltd, 1978-10 Kogushi, UbeShi, Yamaguchi 755, Japan Research and Development Department, Industrial Textile Division, Shikibo Ltd, 1500-5 Shibahara-Minami, Yokaichi-Shi, Shiga 527, Japan Received 3 July 1997; received in revised form 15 July 1998; accepted 7 January 1999 Abstract In situ fibre fracture characteristics have been investigated for Si-Ti-C-O fibres after tensile testing up to 1380"C in vacuum and in air. Specimens tested in air at 1 100 and 1200 C generally had flat fracture surfaces with less than 20% of fibres exhibiting fracture mirrors: this is attributed to oxygen ingress into the fibre bundles. Fibre strength characteristics normalised to a 10-3 m gauge length indicated that fibres tested in air at elevated temperature have significantly lower strengths and average Weibull parameter. m, compared to the room-temperature, 1200 and 1300 C/vacuum cases, and this is attributed to oxygen damage of the fibre toge- her with oxidation of the fibre/matrix interface. The fibre/matrix interface shear strength, t, was low for the room-temperature specimens and increased slightly with temperature when tested in vacuum, possibly as a result of a change in the thermal mismatch between fibres and matrix. Values of r for specimens tested at 1100 and 1200 C in air were an order of magnitude greater than those for room-temperature specimens, indicating a significant degree of oxidation damage at the fibre/ matrix interface to have occurred C 1999 Elsevier Science Ltd. All rights reserved 1. ntroduction shear strength, t, and fibre properties such as the radius r, and the Weibull strength parameters, So and m omposites(CMCs) that utilise con- Values for So and m may be obtained by measuring the tinuous fibres as reinforcement have many potential in situ strength of fibres and fitting a two-parameter high-temperature structural applications, particularly in Weibull curve [7] to the resulting data,i.e the area of space re-entry vehicles. Recent advances include production of 3-D woven composites based on F=1-c-(÷) the sic/Sic system that possess short-term tensile (1) strengths of nearly 400 MPa in vacuum at room tem- perature and 1200C with tensile strains to failure in where F is the cumulative failure probability of fibres at excess of 1%[1-3]. However, the mechanical properties a stress, S, and S, and m are empirical constants known of CMCs often degrade at elevated temperature in the as the Weibull strength parameters. Although the ex situ presence of oxygen which is known to attack the fibre/ strength of ceramic fibres is often well known, there is a matrix interface. Although studies have shown that relative dearth of data concerning in situ fibre strength sealing the surface of CMCs with a glass-based com- One method of estimating in situ fibre strength involves pound may allow similar mechanical properties to be the measurement of mirror radi-a feature often pre- achieved at elevated temperature in air and in vacuum sent on the fracture surface of ceramic fibres. Fig. I [4], improvement of the fibre/matrix interface is still a illustrates such a fracture mirror observed on the frac- ture surface of a Tyranno Si-Ti-C-O fibre It can be Recent work [5,6] has indicated that several impor seen that the fracture mirror is cent tant composite mechanical properties may be predicted to the initiating defect in the fibre and is surrounded by from a knowledge only of the fibre/matrix interface a region of multiple fracture planes. Past research has shown most fracture mirrors to 4 Corresponding author. initiate at flaws present on the surface of the fibre [8, 9 0266-3538/99/S- see front matter C 1999 Elsevier Science Ltd. All rights reserved. PlI:S0266-3538(99)00011-1Fibre strength parameters measured in situ for ceramic-matrix composites tested at elevated temperature in vacuum and in air Ian J. Davies a , Takashi Ishikawa a,*, Masaki Shibuya b, Tetsuro Hirokawa c a Airframe Division, National Aerospace Laboratory, 6-13-1 Ohsawa, Mitaka-Shi, Tokyo 181, Japan bCorporate Research and Development, Ube Industries Ltd, 1978-10 Kogushi, Ube-Shi, Yamaguchi 755, Japan c Research and Development Department, Industrial Textile Division, Shikibo Ltd, 1500-5 Shibahara-Minami, Yokaichi-Shi, Shiga 527, Japan Received 3 July 1997; received in revised form 15 July 1998; accepted 7 January 1999 Abstract In situ ®bre fracture characteristics have been investigated for Si±Ti±C±O ®bres after tensile testing up to 1380C in vacuum and in air. Specimens tested in air at 1100 and 1200C generally had ¯at fracture surfaces with less than 20% of ®bres exhibiting fracture mirrors: this is attributed to oxygen ingress into the ®bre bundles. Fibre strength characteristics normalised to a 10ÿ3 m gauge length indicated that ®bres tested in air at elevated temperature have signi®cantly lower strengths and average Weibull parameter, m, compared to the room-temperature, 1200 and 1300C/vacuum cases, and this is attributed to oxygen damage of the ®bre toge￾ther with oxidation of the ®bre/matrix interface. The ®bre/matrix interface shear strength, , was low for the room-temperature specimens and increased slightly with temperature when tested in vacuum, possibly as a result of a change in the thermal mismatch between ®bres and matrix. Values of  for specimens tested at 1100 and 1200C in air were an order of magnitude greater than those for room-temperature specimens, indicating a signi®cant degree of oxidation damage at the ®bre/matrix interface to have occurred. # 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction Ceramic-matrix composites (CMCs) that utilise con￾tinuous ®bres as reinforcement have many potential high-temperature structural applications, particularly in the area of space re-entry vehicles. Recent advances include production of 3-D woven composites based on the SiC/SiC system that possess short-term tensile strengths of nearly 400 MPa in vacuum at room tem￾perature and 1200C with tensile strains to failure in excess of 1% [1±3]. However, the mechanical properties of CMCs often degrade at elevated temperature in the presence of oxygen which is known to attack the ®bre/ matrix interface. Although studies have shown that sealing the surface of CMCs with a glass-based com￾pound may allow similar mechanical properties to be achieved at elevated temperature in air and in vacuum [4], improvement of the ®bre/matrix interface is still a major area of research. Recent work [5,6] has indicated that several impor￾tant composite mechanical properties may be predicted from a knowledge only of the ®bre/matrix interface shear strength, , and ®bre properties such as the radius, r, and the Weibull strength parameters, So and m. Values for So and m may be obtained by measuring the in situ strength of ®bres and ®tting a two-parameter Weibull curve [7] to the resulting data, i.e. F ˆ 1 ÿ eÿ S So … †m …1† where F is the cumulative failure probability of ®bres at a stress, S, and So and m are empirical constants known as the Weibull strength parameters. Although the ex situ strength of ceramic ®bres is often well known, there is a relative dearth of data concerning in situ ®bre strength. One method of estimating in situ ®bre strength involves the measurement of mirror radiiÐa feature often pre￾sent on the fracture surface of ceramic ®bres. Fig. 1 illustrates such a fracture mirror observed on the frac￾ture surface of a Tyranno1 Si±Ti±C±O ®bre. It can be seen that the fracture mirror is a smooth region adjacent to the initiating defect in the ®bre and is surrounded by a region of multiple fracture planes. Past research has shown most fracture mirrors to initiate at ¯aws present on the surface of the ®bre [8,9] Composites Science and Technology 59 (1999) 801±811 0266-3538/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(99)00011-1 * Corresponding author