L.B. Ruggles-Wrenn, CL. Genelin/Composit 500 空(b 1.0mm c 2.0mm (a) 0 mm 500 f 10m Fig 8. SEM micrographs of the fracture surfaces of specimens tested in creep at 1200C: (a)at 73 MPa in air. ( b)at 73 MPa in argon, (c)at 73 MPa in steam, (d)at 136 MPa in air,(e)at 136 MPa in argon, and (f at 136 MPa in steam. ance could be correlated with the failure time, with a predomi- strain obtained in the tension test. Primary, secondary and tertiary nantly planar fracture surface corresponding to a short life and fi- creep regimes are observed in argon and in steam. Creep strains brous fracture indicating longer life. In the case of N720/A, the accumulated at stresses <91 MPa in argon and in steam are signif- near-planar fracture surfaces were attributed to matrix densifica- icantly larger than those produced tion and subsequent loss of matrix porosity, which resulted in de- Minimum creep rate was reached in all tests. Creep strain rates creased damage tolerance. In contrast, the SEM micrographs of the range from 9.2 x 10-9to 8.7 x 10-75-l in air. the presence of N720/ AM fracture surfaces obtained in creep tests at 1200C in air, steam or argon accelerates creep rates of n720/AM by nearly two argon and steam( Fig 8 )show that for N720/ AM, the fracture sur- orders of magnitude. Creep run- out of 100 h was achieved at ap face appearance cannot be directly correlated with the creep plied stress levels <91 MPa in air. The run-out specimens exhibited time. All fracture surfaces in Fig. 8 are dominated by pla an increase in strength, but stiffness loss of up to 9% was observed. regions of coordinated fiber failure. Compare the fracture surface The presence of steam or argon dramatically reduced creep life of the specimen tested at 73 MPa in air(Fig 8a) and that of the times. The reduction in creep lifetime due to steam was 63% at pecimen tested at 136 MPa in steam( Fig. 8d). The two fracture 73 MPa and 98% at 136 MPa. The reduction in creep lifetimes surfaces have essentially the same topography. Yet the specimen due to argon was at least 80% at stresses >91 MPa. in Fig. Sa achieved a 100-h creep run-out and failed in a subse- The n720 AM fracture surfaces obtained at 1200C are domi- quent tensile test, while the specimen in Fig. 8d failed after mere nated by regions of planar fracture. The near-planar fracture su 36 s of creep faces suggest the loss of matrix porosity and subsequent matrix densification due to additional sintering. The fracture surface 4. Concluding remarks appearance cannot be directly correlated with the creep lifetime The tensile stress-strain behavior of the N720/ AM composite Acknowledgement was investigated and the tensile properties measured at 1200C. The uts was 153 MPa. the elastic modulus was 74. 5 GPa and the The financial support of the Air Force Research Laboratory, Pro- failure strain was 0.34%. The stress-strain curve was nearly linear pulsion Directorate(Dr. R. Sikorski and Dr. J. Zelina)is highly The influence of loading rate was explored at 1200C in steam, in tests conducted with constant loading rates of 0.0025 and References 25 MPa/s. The tensile properties were stre influenced by the ding rate. As the loading rate decreases by four orders of magn [11 Zok F. Developments in oxide fiber composites. J Am Ceram Soc de, the uts drops by 39%. At 0.0025 MPa/ s, the tensile stress- [21 Zawada LP, Staehler J. Steel S. Consequence of intermittent exposure to strain behavior is distinctly nonlinear. Considerable inelastic strains develop as the stress exceeds 20 MPa. The creep-rupture behavior of the N720/ AM composite [3I Schmidt S, Beyer S, Knabe H, Immich H, Meistring R, Gessler A. Advanced rials for current and future propulsion characterized for stress levels ranging from 73 to 136 MPa at in air, argon and steam environments. In air N720/ AM 41 Szweda A Millard ML Harrison MG. Fiber-reinforced ceramic-matrix primary and secondary creep regimes. Creep strains accu- 5 Sim SM, Kerans R). Slurry infiltration and 3-D woven composites Ceram Eng at stresses <114 MPa considerably exceed the failure Sci Proc1992:13(9-10):632-41ance could be correlated with the failure time, with a predomi￾nantly planar fracture surface corresponding to a short life and fi- brous fracture indicating longer life. In the case of N720/A, the near-planar fracture surfaces were attributed to matrix densifica￾tion and subsequent loss of matrix porosity, which resulted in de￾creased damage tolerance. In contrast, the SEM micrographs of the N720/AM fracture surfaces obtained in creep tests at 1200 C in air, argon and steam (Fig. 8) show that for N720/AM, the fracture sur￾face appearance cannot be directly correlated with the creep life￾time. All fracture surfaces in Fig. 8 are dominated by planar regions of coordinated fiber failure. Compare the fracture surface of the specimen tested at 73 MPa in air (Fig. 8a) and that of the specimen tested at 136 MPa in steam (Fig. 8d). The two fracture surfaces have essentially the same topography. Yet the specimen in Fig. 8a achieved a 100-h creep run-out and failed in a subse￾quent tensile test, while the specimen in Fig. 8d failed after mere 36 s of creep. 4. Concluding remarks The tensile stress–strain behavior of the N720/AM composite was investigated and the tensile properties measured at 1200 C. The UTS was 153 MPa, the elastic modulus was 74.5 GPa, and the failure strain was 0.34%. The stress–strain curve was nearly linear to failure. The influence of loading rate was explored at 1200 C in steam, in tests conducted with constant loading rates of 0.0025 and 25 MPa/s. The tensile properties were strongly influenced by the loading rate. As the loading rate decreases by four orders of magni￾tude, the UTS drops by 39%. At 0.0025 MPa/s, the tensile stress– strain behavior is distinctly nonlinear. Considerable inelastic strains develop as the stress exceeds 20 MPa. The creep–rupture behavior of the N720/AM composite was characterized for stress levels ranging from 73 to 136 MPa at 1200 C in air, argon and steam environments. In air N720/AM exhibits primary and secondary creep regimes. Creep strains accu￾mulated at stresses 6114 MPa considerably exceed the failure strain obtained in the tension test. Primary, secondary and tertiary creep regimes are observed in argon and in steam. Creep strains accumulated at stresses 691 MPa in argon and in steam are signif￾icantly larger than those produced in air. Minimum creep rate was reached in all tests. Creep strain rates range from 9.2  109 to 8.7  107 s1 in air. The presence of steam or argon accelerates creep rates of N720/AM by nearly two orders of magnitude. Creep run-out of 100 h was achieved at ap￾plied stress levels 691 MPa in air. The run-out specimens exhibited an increase in strength, but stiffness loss of up to 9% was observed. The presence of steam or argon dramatically reduced creep life￾times. The reduction in creep lifetime due to steam was 63% at 73 MPa and 98% at 136 MPa. The reduction in creep lifetimes due to argon was at least 80% at stresses P91 MPa. The N720/AM fracture surfaces obtained at 1200 C are domi￾nated by regions of planar fracture. The near-planar fracture sur￾faces suggest the loss of matrix porosity and subsequent matrix densification due to additional sintering. The fracture surface appearance cannot be directly correlated with the creep lifetime. Acknowledgement The financial support of the Air Force Research Laboratory, Pro￾pulsion Directorate (Dr. R. Sikorski and Dr. J. Zelina) is highly appreciated. References [1] Zok F. Developments in oxide fiber composites. J Am Ceram Soc 2006;89(11):3309–24. [2] Zawada LP, Staehler J, Steel S. Consequence of intermittent exposure to moisture and salt fog on the high-temperature fatigue durability of several ceramic–matrix composites. J Am Ceram Soc 2003;86(8):1282–91. [3] Schmidt S, Beyer S, Knabe H, Immich H, Meistring R, Gessler A. Advanced ceramic matrix composite materials for current and future propulsion technology applications. Acta Astronaut 2004;55:409–20. [4] Szweda A, Millard ML, Harrison MG. Fiber-reinforced ceramic–matrix composite member and method for making. US Pat. No. 5 601674; 1997. [5] Sim SM, Kerans RJ. Slurry infiltration and 3-D woven composites. Ceram Eng Sci Proc 1992;13(9–10):632–41. Fig. 8. SEM micrographs of the fracture surfaces of specimens tested in creep at 1200 C: (a) at 73 MPa in air, (b) at 73 MPa in argon, (c) at 73 MPa in steam, (d) at 136 MPa in air, (e) at 136 MPa in argon, and (f) at 136 MPa in steam. 668 M.B. Ruggles-Wrenn, C.L. Genelin / Composites Science and Technology 69 (2009) 663–669