1594 M B. Ruggles-Wrenn et al / Composites Science and Technology 68(2008)1588-1595 (a) (b)(c) 5/mm 5 mm mm Fig 10. Fracture surfaces obtained in tensile tests conducted on specimens subjected to 100 h of prior creep at 35 MPa at 1200C in: (a)air. (b) steam and a 10oum厂100m 100m Fig. Il. SEM micrographs of the fracture surfaces obtained in tensile tests conducted on specimens subjected to 100 h of prior creep at 35 MPa at 1200C in:(a) air, (b) steam and (c)argon. through matrix damage and interply delamination. The are observed. For a given applied stress creep strain accu- specimen pre-crept in air appears to have failed through a mulation is highest in argon, followed by that in steam combination of fiber fracture and matrix failure. It is possi- and in air ble that the sintering of the matrix is accelerated in the pres- Creep strain rates range from 4.2 x 10-6to 9.4 x 10-3s-1 ence of steam. It is also somewhat surprising that in air, from 58x 10 to 1. 4 x 10-s in steam, and from strengthening of the matrix also occurs in argon environ- 4.9x10 to 1.6x 10S in argon For creep stress levels ment, where no oxygen is present. <35 MPa environment has little effect on rate, all creep rates are less than 10-s-. At 45 MPa, creep rates in all environ- 4. Concluding remarks ments increase by at least three orders of magnitude. At 45 MPa, creep rate in argon is about one order of magnitude The tensile stress-strain behavior of the N720/A com- higher than the rates produced in air and in steam site with +45 fiber orientation was investigated and Creep run-out of 100 h was achieved at applied stress the tensile properties measured at 1200C. The elastic mod- levels <35 MPa in all environments. The run-out speci ulus was 46 GPa and the UTS was 55 MPa. These proper- mens exhibited an increase in stiffness and in strengt ties are significantly lower than the corresponding values Prior creep significantly diminished composite's capability for the 0/90 fiber orientation. The stress-strain behavior for inelastic straining. For applied stresses >40 MPa, creep departs from linearity at a low stress of 15 MPa. Once lifetimes were drastically reduced in the presence of steam the UTS= 55 MPa and the corresponding strain of 0.27% and especially in the presence of argon. are reached the softening commences. Considerable inelas- For all test environments investigated in tests of less than tic strains develop at stresses 50 MPa 100 h duration, the failure occurs primarily through matrix The creep-rupture behavior of the N720/A composite damage and interplay delamination, with minimal fiber frac with +45 fiber orientation was characterized for stress lev- ture. For test durations >100 h, the failure mechanism els ranging from 15 to 45 MPa at 1200C in air, steam and dominated by fiber fracture. It is possible that the matrix argon environments. For the stress levels <35 MPa the undergoes additional sintering during the long-term tests material exhibits primary and secondary creep regimes. Additional sintering and consequently strengthening of the At 45 MPa, primary, secondary and tertiary creep regimes matrix may be behind the change in failure mechanismsthrough matrix damage and interply delamination. The specimen pre-crept in air appears to have failed through a combination of fiber fracture and matrix failure. It is possi￾ble that the sintering of the matrix is accelerated in the pres￾ence of steam. It is also somewhat surprising that strengthening of the matrix also occurs in argon environ￾ment, where no oxygen is present. 4. Concluding remarks The tensile stress–strain behavior of the N720/A com￾posite with ±45 fiber orientation was investigated and the tensile properties measured at 1200 C. The elastic mod￾ulus was 46 GPa and the UTS was 55 MPa. These proper￾ties are significantly lower than the corresponding values for the 0/90 fiber orientation. The stress–strain behavior departs from linearity at a low stress of 15 MPa. Once the UTS = 55 MPa and the corresponding strain of 0.27% are reached, the softening commences. Considerable inelas￾tic strains develop at stresses 50 MPa. The creep-rupture behavior of the N720/A composite with ±45 fiber orientation was characterized for stress lev￾els ranging from 15 to 45 MPa at 1200 C in air, steam and argon environments. For the stress levels 635 MPa the material exhibits primary and secondary creep regimes. At 45 MPa, primary, secondary and tertiary creep regimes are observed. For a given applied stress creep strain accu￾mulation is highest in argon, followed by that in steam and in air. Creep strain rates range from 4.2 · 106 to 9.4 · 103 s 1 in air, from 5.8 · 106 to 1.4 · 102 s 1 in steam, and from 4.9 · 106 to 1.6 · 101 s 1 in argon. For creep stress levels 635 MPa environment has little effect on rate, all creep rates are less than 105 s 1 . At 45 MPa, creep rates in all environ￾ments increase by at least three orders of magnitude. At 45 MPa, creep rate in argon is about one order of magnitude higher than the rates produced in air and in steam. Creep run-out of 100 h was achieved at applied stress levels 635 MPa in all environments. The run-out speci￾mens exhibited an increase in stiffness and in strength. Prior creep significantly diminished composite’s capability for inelastic straining. For applied stresses P40 MPa, creep lifetimes were drastically reduced in the presence of steam and especially in the presence of argon. For all test environments investigated in tests of less than 100 h duration, the failure occurs primarily through matrix damage and interplay delamination, with minimal fiber frac￾ture. For test durations P100 h, the failure mechanism is dominated by fiber fracture. It is possible that the matrix undergoes additional sintering during the long-term tests. Additional sintering and consequently strengthening of the matrix may be behind the change in failure mechanisms. Fig. 10. Fracture surfaces obtained in tensile tests conducted on specimens subjected to 100 h of prior creep at 35 MPa at 1200 C in: (a) air, (b) steam and (c) argon. Fig. 11. SEM micrographs of the fracture surfaces obtained in tensile tests conducted on specimens subjected to 100 h of prior creep at 35 MPa at 1200 C in: (a) air, (b) steam and (c) argon. 1594 M.B. Ruggles-Wrenn et al. / Composites Science and Technology 68 (2008) 1588–1595