696 A Dalma- et al Table 3. Conditions of cyclic fatigue tests in tension-tension loading Room temperature 600C 1000°C Peak stress(MPa) Number of cycles (1)at failure(2)without failure 710)4(1) 07(2) 107(2) 2105(1) unload loops, or to the time needed to reach that num- 3.2.3 Change in fatigue behavior as a function of ber of cycles. In order to evaluate the effect of cycling temperature frequency fatigue tests have been performed (at room Under the same peak load, corresponding to 75%of temperature. 600 and 1000C)with a frequency jump ultimate stress for room temperature, 1000 and 1500C after the first 50 cycles from 0-05 to I Hz. Comparing test, and 60% of the ultimate stress at 600 C. the life the change of the tangent moduli. with number of and the fatigue limit increase both as the temperature cycles, no significant difference is observed. Thus. we rises. at least up to 1000 C. A shorter life is always can conclude that time-dependent phenomena are not obtained at room temperature preponderant in the fatigue mechanisms under an inert when the peak load is high enough to lead to fatigue atmosphere. This is also reported by Shulerfor tests failure. the residual elongation increases at room tem performed at room temperature at a frequency lower perature and very sharply at 1500 C. Below the fatigue than 10 Hz. We have. therefore. studied only the effect limit, the residual strain is virtually stable, but tends to of the number of cycles on the behavior of the material. decrease at room temperature and to increase at 600oC The fatigue tests are divided in two types(Table 3) and 1000C. Increasing test temperature leads to lower loop area: there is a factor of 4 between the area at 3.2.2.1 Tests conducted be ond /(r cycles withour room temperature and that at 1500C As shown in Fig. 3, the modulus increases sig- nificantly, especially at 600C. with the number of cycles. This stiffening changes with increase in the resi 2.210° dual strain(Fig. 5). The hysteresis(loop area)becomes ower at clavated temperature( Fig. 6)and decreases as the fatigue cycling proceeds 15() 3. 2.2.2 Tests leading to failure before 10 cycles In all cases, the tangent modulus decreases up to failure (Figs 2 and 4). At room temperature, the slope at the 50 end of loading decreases after 2000 cycles and the resid al strain decreases(Fig. 5). This phenomenon must be confirmed by more systematic tests. For the test at 1500C, the principal observation is a very sharp increase in the residual strain up to failure(Er-1.96%at STRAIN (%) failure). As before. the area of the loops decreases as the Fig 4. Cyclic fatigue tests at 15009 230 MPa. failure cycling proceeds at22×105c 500°C 20 14 z150 乙∽5 2 口100 4 600C 0.00.20.40.60.8 oy(NUMBER OF CYCLES STRAIN (%) Fig. 5. Residual strains during fatigue cycling at different Fig 3. Cyclic fatigue tests at 600C(omax =230 MPa, spec temperatures (omax=230 MPa (600, 1000 and 1500C) en not failed after 10 cycles) omax-220 MPa(room temperature)696 A. Lkdtw~ et al. Table 3. Conditions of cyclic fatigue tests in tension-tension loading Room temperature 600°C Peak stress (MPa) 220 I IO 230 Number of cycles (I) at failure (2) without failure 710J (I) IO” (2) IO’ (2) 1000”c i5OO”C 230 230 IO’ (2) 210’ (I) unload loops, or to the time needed to reach that num￾ber of cycles. In order to evaluate the effect of cycling frequency. fatigue tests have been performed (at room temperature. 600 and IOOO’C) with a frequency jump after the first 50 cycles from 0.05 to I Hz. Comparing the change of the tangent moduli, with number of cycles, no significant difference is observed. Thus. we can conclude that time-dependent phenomena are not preponderant in the fatigue mechanisms under an inert atmosphere. This is also reported by Shulerj for tests performed at room temperature at a frequency lower than 1OHz. We have, therefore, studied only the effect of the number of cycles on the behavior of the material. The fatigue tests are divided in two types (Table 3). 3.2.2. I Tests conductrd hqvnd IO” c~,~~~it~.s withour ,fhilurr As shown in Fig. 3. the modulus increases sig￾nificantly, especially at 6OO”C, with the number of cycles. This stiffening changes with increase in the resi￾dual strain (Fig. 5). The hysteresis (loop area) becomes lower at elavated temperature (Fig. 6) and decreases as the fatigue cycling proceeds. 3.2.2.2 Tests leuding to,fitiluw lwffore 10” c~~~ic~.s In all cases, the tangent modulus decreases up to failure (Figs 2 and 4). At room temperature, the slope at the end of loading decreases after 2000cycles and the resid￾ual strain decreases (Fig. 5). This phenomenon must be confirmed by more systematic tests. For the test at 15OO”C, the principal observation is a very sharp increase in the residual strain up to failure (or== 1.96% at failure). As before, the area of the loops decreases as the cycling proceeds. 0.0 0.2 0.4 0.6 0.8 1 .o STRAIN (%) Fig. 3. Cyclic fatigue tests at 600°C (urnax = 230 MPa, speci￾men not failed after IO7 cycles). 3.23 C’hungr in,fktigur hrhuvior as u,fimc.tion of’ tciqwruturc Under the same peak load, corresponding to 75% ot ultimate stress for room temperature, 1000 and 1500°C test. and 60% of the ultimate stress at 6OO”C, the life and the fatigue limit increase both as the temperature rises. at least up to 1000°C. A shorter life is always obtained at room temperature. When the peak load is high enough to lead to fatigue failure. the residual elongation increases at room tem￾perature and very sharply at 1500°C. Below the fatigue limit, the residual strain is virtually stable, but tends to decrease at room temperature and to increase at 600°C and 1000°C. Increasing test temperature leads to lower loop area; there is a factor of 4 between the area at room temperature and that at 1500°C. 0 0.0 0.5 1.0 1.5 2.0 2.5 STRAIN (%) Fig. 4. Cyclic fatigue tests at 1500°C (crmax = 230 MPa, failure at 2.2 x 105cycles). 2 1500 “C ; 1.X . + I IO I 0’ IO’ IO1 IO‘ IIf log(NtiMlEKOFCYCLES) Fig. 5. Residual strains during fatigue cycling at different temperatures ((J,,, = 230 MPa (600, 1000 and 1 SOOT), (T,,, = 220 MPa (room temperature))