S Ochiai et al./ Composites: Part A 35(2004)33-40 as shown by the slope of the solid lines in the streng reduced region in Fig. 2. From the fitting of the model to No reduction experimental data in Stage II in Fig. 2, the constant SI and (ocu=Oco) the activation energy g for n= 1/2 were estimated to be In(ocu) 0.35 MPa/s and 220 kJ/mol, respectively. The solid lines in Fig. I show the calculated In(ocu)-In(t)diagram by using thus estimated values, describing fairly well the experimental results At each temperature, the calculated lines in Fig. 2 do not Shift to right necessarily go through the center of the measured values, since the values of n, Sn and Q were estimated as to fit the all exposure data covering all temperatures investigated. The validity of the estimated values of n= 1/2, Sn=0.35 MPa/sand Critical time Q=220 kJ/mol can be demonstrated as follows. Eq. (3) In(t) with n= 1/2 can be re-written as In(ocu)=(-1/4)(In(t)-20/kT)+In(Sm) (4) Low Temp. Using Eq. (4), we can superimpose all data of Tcu in graph by plotting In(ocu) against In(t)-20/kT. Fig. 6 shows the results of such plotting for vacuum exposure, gether with those for air exposure [1] for reference. The Intermediate Temp. i solid lines show the calculated ones by using the foremen tioned oc=630 MPa, n= 1/2, S=0.35 MPa/s 4and 2=220 kJ/mol for the vacuum exposure and by using 1000 D In(t) Fig. 5. Schematic representation of variation of te strength with exposure time. (a) The principle variation of the In(oc) as a function of n(n) which would be realized if the tre time covers zero to infinite seconds. (b) The variation of the In(o)as a function of In(n) in the limited exposure time range as in the present experiment. The strengths of the composite exposed at low, intermediate and high temperature vary alon AB, ACD and Ef, respectively. 15 the time satisfying dc= c is noted as the critical time, n t-2O/RT below and beyond which Stages I and ll arise, respectively (a)Air variation shown in Fig. 5(a) would be realized if the exposure time covers zero to infinite seconds. As the exposure time is limited to 3.6 x 102-36x 10s in the 乏 present work, both Stages of I and Il do not appear necessarily at respective temperature, since the critical time, corresponding to the transition from Stage I to Il, becomes shorter and longer when the exposure temperature is high and low, respectively. Which of Stages I and Il arise in the In(cu-In() curve in the limited time range at different temperatures is schematically shown in Fig. 5(b). The trengths of the composite exposed at low, intermediate and s high temperature vary along AB (only Stage I arises), ACD (Stage I arises up to the time C(critical time), beyond which Stage II arises)and EF (only Stage II arises), respectively. n t-2/RT The slope of In(ocu)-In(t) curve in Stage Il is -n/2 While the data were scattered, n= 1/2 showed a good description of the slope at all temperatures investigated Fig. 6. Measured values of In(ocu) plotted against In(r)-2o/kT.the time satisfying s0 cu ¼ sp c is noted as the critical time, below and beyond which Stages I and II arise, respectively. The variation shown in Fig. 5(a) would be realized if the exposure time covers zero to infinite seconds. As the exposure time is limited to 3.6 £ 102 –3.6 £ 105 s in the present work, both Stages of I and II do not appear necessarily at respective temperature, since the critical time, corresponding to the transition from Stage I to II, becomes shorter and longer when the exposure temperature is high and low, respectively. Which of Stages I and II arise in the lnðscuÞ–lnðtÞ curve in the limited time range at different temperatures is schematically shown in Fig. 5(b). The strengths of the composite exposed at low, intermediate and high temperature vary along AB (only Stage I arises), ACD (Stage I arises up to the time C (critical time), beyond which Stage II arises) and EF (only Stage II arises), respectively. The slope of lnðscuÞ 2 lnðtÞ curve in Stage II is 2n=2: While the data were scattered, n ¼ 1=2 showed a good description of the slope at all temperatures investigated as shown by the slope of the solid lines in the strength￾reduced region in Fig. 2. From the fitting of the model to experimental data in Stage II in Fig. 2, the constant SII and the activation energy Q for n ¼ 1=2 were estimated to be 0.35 MPa/s1/4 and 220 kJ/mol, respectively. The solid lines in Fig. 1 show the calculated lnðscuÞ 2 lnðtÞ diagram by using thus estimated values, describing fairly well the experimental results. At each temperature, the calculated lines in Fig. 2 do not necessarily go through the center of the measured values, since the values of n; SII and Q were estimated as to fit the all data covering all temperatures investigated. The validity of the estimated values of n ¼ 1=2; SII ¼ 0:35 MPa=s 1=4 and Q ¼ 220 kJ/mol can be demonstrated as follows. Eq. (3) with n ¼ 1=2 can be re-written as lnðscuÞ¼ð21=4Þ{lnðtÞ 2 2Q=kT} þ lnðSIIÞ ð4Þ Using Eq. (4), we can superimpose all data of scu in one graph by plotting lnðscuÞ against lnðtÞ 2 2Q=kT: Fig. 6 shows the results of such plotting for vacuum exposure, together with those for air exposure [1] for reference. The solid lines show the calculated ones by using the aforemen￾tioned s0 cu ¼ 630 MPa; n ¼ 1=2; SII ¼ 0:35 MPa/s1/4 and Q ¼ 220 kJ/mol for the vacuum exposure and by using Fig. 5. Schematic representation of variation of composite strength with exposure time. (a) The principle variation of the lnðscuÞ as a function of lnðtÞ; which would be realized if the exposure time covers zero to infinite seconds. (b) The variation of the lnðscuÞ as a function of lnðtÞ in the limited exposure time range as in the present experiment. The strengths of the composite exposed at low, intermediate and high temperature vary along AB, ACD and EF, respectively. Fig. 6. Measured values of lnðscuÞ plotted against lnðtÞ 2 2Q=kT: 38 S. Ochiai et al. / Composites: Part A 35 (2004) 33–40