M.B. Ruggles-Wrenn, N.R. Szymczak/Composites: Part A 39(2008)1829-1837 18 mm 18 mm Fig 10. Fracture surfaces of N720/A specimens tested in creep at-60 MPa at 1200C:(a)in air, specimen failed in compression test following 100 h at-60 MPa and (b)in steam, specimen failed during creep tes 4. Concluding remarks For stresses >-60 MPa, compressive creep strains accumulated The compressive stress-strain behavior of N720/A composite in d c am are an order of magnitude higher than those accumulate was investigated and the compressive properties measured at 00C in air, compressive creep rate magnitudes(ranging 1200C in laboratory air and in steam. The influence of the from 1.3 x 10-to 3.5x 10-75-)are nearly an order of magnitude loading rate was explored in constant stress-rate tests conducted higher than the tensile creep rates. In steam, compressive creep at 25 and 0.0025 MPa/s In air at 25 MPa/s, the compressive strain rate magnitudes (ranging from 6.8x10-sto modulus was 74 GPa and the compressive strength was 1.9x 10-25-1)can be as high as 105 times the tensile creep rates 128 MPa. While the compressive modulus is similar to the Compressive creep run-out was achieved in all tests conducted in tensile modulus, compressive strength magnitude is significantly air. The presence of steam dramatically reduced compressive creep lower than the UTS. The compressive stress-strain curve consists lifetimes. Reductions in compressive creep life due to steam were of two nearly linear portions with a decrease in slope occur- at least 96% ring at the stress of -100 MPa Compressive strength and mod- A linear elastic crack growth model was used to predict the ulus are influenced by the loading rate. As the loading rate creep lifetimes from the constant loading rate test data decreases by 4 orders of magnitude, compressive modulus drops 1200C in steam. Good agreement between predicted creep life- by 45% and compressive strength, by 16% At 0.0025 MPa/s in air times and experimental results suggests that environmentally as- the compressive stress-strain behavior becomes distinctly sisted slow crack growth is the governing failure mechanism for the n720/A composite in compression at 1200C in steam. The compressive properties deteriorated in the presence of steam. At 25 MPa/s in steam the compressive modulus was Acknowledgement 64 GPa and the compressive strength was-115 MPa. In steam, the effect of loading rate on compressive properties and stress- The financial support of Dr. R Sikorski and Dr ]. Zelina, Pro strain behavior is greatly magnified At 0.0025 MPa/s,, the compres- sion Directorate, Air Force Research Laboratory is highly sive modulus is 22 GPa and the compressive strength is only 54 MPa At 0.0025 MPa/s in steam the compressive stress-strain behavior is strongly nonlinear Considerable inelastic strains devel- References op as the stress magnitude exceeds 10 MPa. The compressive creep-rupture behavior was evaluated for [1] Ohnabe H, Masaki S, Onozuka M, Miyahara K, Sasa T Potential application of stress levels ranging from -60 to-100 MPa at 1200C in air and engine components. Comp for a stress levels in the -40 to -100 MPa range at 1200C in steam. The N720/A composite exhibits primary and secondary [2] Zawada L, StaehlerJ Steel S Consequence of intermittent exposure to moisture durability of several ceram creep regimes in both air and steam environments. In air, compres- matrix composites. J Am Ceram Soc 2003: 86(8): 1282-9 reep strain accumulation increases with the magnitude of ap [3 Parlier M, Ritti MH. State of the art and perspectives for oxide/ oxide plied stress. At-60 and -80 MPa, compressive creep strains are [41 Mattoni MA. Yang JY, Levi CG, Zok Fw. Zawada LP. Effects of combustor rig lower than the failure strain obtained in compression test. At ure strain in compression test. In steam, compressive creep strain 5 Pxide-saxrade composites in a novel Combustor wali application Int j appt accumulation decreases with increasing creep stress magnitude Ceram Tech2005:2(2):122-324. Concluding remarks The compressive stress–strain behavior of N720/A composite was investigated and the compressive properties measured at 1200 C in laboratory air and in steam. The influence of the loading rate was explored in constant stress-rate tests conducted at 25 and 0.0025 MPa/s. In air at 25 MPa/s, the compressive modulus was 74 GPa and the compressive strength was 128 MPa. While the compressive modulus is similar to the tensile modulus, compressive strength magnitude is significantly lower than the UTS. The compressive stress–strain curve consists of two nearly linear portions with a decrease in slope occur￾ring at the stress of 100 MPa. Compressive strength and mod￾ulus are influenced by the loading rate. As the loading rate decreases by 4 orders of magnitude, compressive modulus drops by 45% and compressive strength, by 16%. At 0.0025 MPa/s in air the compressive stress–strain behavior becomes distinctly nonlinear. The compressive properties deteriorated in the presence of steam. At 25 MPa/s in steam the compressive modulus was 64 GPa and the compressive strength was 115 MPa. In steam, the effect of loading rate on compressive properties and stress– strain behavior is greatly magnified. At 0.0025 MPa/s, the compres￾sive modulus is 22 GPa and the compressive strength is only 54 MPa. At 0.0025 MPa/s in steam the compressive stress–strain behavior is strongly nonlinear. Considerable inelastic strains devel￾op as the stress magnitude exceeds 10 MPa. The compressive creep-rupture behavior was evaluated for stress levels ranging from 60 to 100 MPa at 1200 C in air and for a stress levels in the 40 to 100 MPa range at 1200 C in steam. The N720/A composite exhibits primary and secondary creep regimes in both air and steam environments. In air, compres￾sive creep strain accumulation increases with the magnitude of ap￾plied stress. At 60 and 80 MPa, compressive creep strains are lower than the failure strain obtained in compression test. At 100 MPa, compressive creep strain significantly exceeds the fail￾ure strain in compression test. In steam, compressive creep strain accumulation decreases with increasing creep stress magnitude. For stresses P 60 MPa, compressive creep strains accumulated in steam are an order of magnitude higher than those accumulated in air. At 1200 C in air, compressive creep rate magnitudes (ranging from 1.3 108 to 3.5 107 s1 ) are nearly an order of magnitude higher than the tensile creep rates. In steam, compressive creep strain rate magnitudes (ranging from 6.8 105 to 1.9 102 s 1 ) can be as high as 105 times the tensile creep rates. Compressive creep run-out was achieved in all tests conducted in air. The presence of steam dramatically reduced compressive creep lifetimes. Reductions in compressive creep life due to steam were at least 96%. A linear elastic crack growth model was used to predict the creep lifetimes from the constant loading rate test data at 1200 C in steam. Good agreement between predicted creep life￾times and experimental results suggests that environmentally as￾sisted slow crack growth is the governing failure mechanism for the N720/A composite in compression at 1200 C in steam. Acknowledgement The financial support of Dr. R. Sikorski and Dr. J. Zelina, Propul￾sion Directorate, Air Force Research Laboratory is highly appreciated. References [1] Ohnabe H, Masaki S, Onozuka M, Miyahara K, Sasa T. Potential application of ceramic matrix composites to aero-engine components. Comp A 1999;30:489–96. [2] Zawada L, 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] Parlier M, Ritti MH. State of the art and perspectives for oxide/oxide composites. Aerospace Sci Technol 2003;7:211–21. [4] Mattoni MA, Yang JY, Levi CG, Zok FW, Zawada LP. Effects of combustor rig exposure on a porous-matrix oxide composite. Int J Appl Ceram Tech 2005;2(2):133–40. [5] Parthasarathy TA, Zawada LP, John R, Cinibulk MK, Zelina J. Evaluation of oxide–oxide composites in a novel combustor wall application. Int J Appl Ceram Tech 2005;2(2):122–32. Fig. 10. Fracture surfaces of N720/A specimens tested in creep at 60 MPa at 1200 C: (a) in air, specimen failed in compression test following 100 h at 60 MPa and (b) in steam, specimen failed during creep test. 1836 M.B. Ruggles-Wrenn, N.R. Szymczak / Composites: Part A 39 (2008) 1829–1837