S Ochiai et aL./Composites: Part A 35 (2004)33-40 the vacuum exposure at higher temperatures. The weight 3. 2. Degradation mechanism loss increased with increasing time and it became larger than 30% when the specimens were exposed at 1473 and The as-supplied composite showed fiber-pullout type 1673 K for more than 3.6X 10s. Such a large weight loss fracture mode. In the case of air-exposure, with progress of could be attributed to the decomposition of fiber and matrix oxidation, the fracture mode changed from the fiber-pullout involving generation of Sio and CO gases [6,7] type to the nonfiber-pullout, but the reduction in composite The average room temperature strength of the as- strength did not correspond to the change in fracture supplied specimens was 630 MPa. Fig. 2 shows the mode. Namely, most specimens for the strength range of measured room temperature-residual strength of the speci- 130-630 MPa showed fiber-pullout type, and only the mens exposed in vacuum at(a)973, ()1123, (c)1273, (d) severely degraded specimens with the strength range of 1473,(e)1573 and(f)1673 K, plotted against exposure time 60-130 MPa showed the nonfiber-pullout one On the other t. As shown later in Section 3.3, from the analysis and hand, in the vacuum exposure, all specimens showed the modeling, the parameters, which can describe the exper- fiber-pullout type. The fiber-pullout length increased with imental data, were estimated. The solid lines show the increasing temperature and time. The variation of the results calculated by such estimated values. Within the fracture morphology with increasing time in the case of range of the exposure time investigated (up to 1473 K-exposure is shown in Fig 3. In this example, the t= 3.6x 10 s), following features are read strength of the specimen exposed for 3. 6X 10-s was nearly the same as that of the as-supplied specimens(630 MPa)but (1)For 973, 1123 and 1273 K-exposure, no or only a slight it decreased with increasing exposure time, falling to around reduction in strength arose as in(a)-(c), respectively. 100 MPa for the exposure time of 3. 6x 10 s, as has been (i) For 1473 K-exposure, the strength for the 3.6X 10 shown in Fig. 2(d (shortest exposure time in this work) was comparable Fig. 4 shows the appearance of the matrix and fiber in the to that of the as-supplied (nonexposed) specimens, but composite specimens exposed at the same temperature of it decreased with increasing time as in(d) 1473 K in vacuum. (a)-(c) show the defects in the matrix, (iii) For 1573 and 1673 K-exposure, the strength decreased on the side surface of the pulled-out fiber and in the fracture with increasing time surface of the fiber, respectively. In this way, many defects 1000 1000 973K 1123K 1273K g100-10810105105要10-10-10410510 02103104105106 Exposure time, t(s) time, t(s) 1000 1000 n1000 1473K 1573K 画1098101-1:/重10 02103104105106 Exposure time, t(s) Exposure time, t (s) Exposure time, t(s) Fig. 2. Relation between the residual tensile strength and exposure time in vacuum. The solid lines refer to the analytical results based on proposed model whose details will be shown in Section 3.3the vacuum exposure at higher temperatures. The weight loss increased with increasing time and it became larger than 30% when the specimens were exposed at 1473 and 1673 K for more than 3.6 £ 104 s. Such a large weight loss could be attributed to the decomposition of fiber and matrix involving generation of SiO and CO gases [6,7]. The average room temperature strength of the as￾supplied specimens was 630 MPa. Fig. 2 shows the measured room temperature-residual strength of the speci￾mens exposed in vacuum at (a) 973, (b) 1123, (c) 1273, (d) 1473, (e) 1573 and (f) 1673 K, plotted against exposure time t. As shown later in Section 3.3, from the analysis and modeling, the parameters, which can describe the exper￾imental data, were estimated. The solid lines show the results calculated by such estimated values. Within the range of the exposure time investigated (up to t ¼ 3:6 £ 105 s), following features are read (i) For 973, 1123 and 1273 K-exposure, no or only a slight reduction in strength arose as in (a)–(c), respectively. (ii) For 1473 K-exposure, the strength for the 3.6 £ 102 s (shortest exposure time in this work) was comparable to that of the as-supplied (nonexposed) specimens, but it decreased with increasing time as in (d). (iii) For 1573 and 1673 K-exposure, the strength decreased with increasing time. 3.2. Degradation mechanism The as-supplied composite showed fiber-pullout type fracture mode. In the case of air-exposure, with progress of oxidation, the fracture mode changed from the fiber-pullout type to the nonfiber-pullout, but the reduction in composite strength did not correspond to the change in fracture mode. Namely, most specimens for the strength range of 130–630 MPa showed fiber-pullout type, and only the severely degraded specimens with the strength range of 60–130 MPa showed the nonfiber-pullout one. On the other hand, in the vacuum exposure, all specimens showed the fiber-pullout type. The fiber-pullout length increased with increasing temperature and time. The variation of the fracture morphology with increasing time in the case of 1473 K-exposure is shown in Fig. 3. In this example, the strength of the specimen exposed for 3.6 £ 102 s was nearly the same as that of the as-supplied specimens (630 MPa) but it decreased with increasing exposure time, falling to around 100 MPa for the exposure time of 3.6 £ 105 s, as has been shown in Fig. 2(d). Fig. 4 shows the appearance of the matrix and fiber in the composite specimens exposed at the same temperature of 1473 K in vacuum. (a)–(c) show the defects in the matrix, on the side surface of the pulled-out fiber and in the fracture surface of the fiber, respectively. In this way, many defects Fig. 2. Relation between the residual tensile strength and exposure time in vacuum. The solid lines refer to the analytical results based on proposed model, whose details will be shown in Section 3.3. S. Ochiai et al. / Composites: Part A 35 (2004) 33–40 35