MB. Ruggles-Wrenn, P D. Laffey Composites Science and Technology 68(2008)2260-2266 as a function of test rate using double-notch shear specimens and consisting of a porous alumina matrix reinforced with Nextel M720 emonstrated that the interlaminar shear strength (ILSS)degraded fibers. There is no fiber coating. The composite was supplied in a with decreasing test rate. A phenomenological, power-law based form of 5.2-mm thick plates comprised of 24 00/90 woven layers crack growth model was proposed to account for the degradation with a density of 2.83 g/cm, a fiber volume of 46.6%, and of the IlSS of the composite at elevated temperatures. High-tem- matrix porosity of w%. The fiber fabric was infiltrated with the perature stress-rupture tests in interlaminar shear were employed matrix in a sol-gel process. the laminate was dried with a"vac- to validate the proposed model. These studies focused on the non- uum bag"technique under low pressure and low temperature, oxide CMcs with matrix-dense interlaminar regions, where the and then pressureless sintered [32]. Representative micrograph interlaminar failure is controlled by the fiber-matrix interface In of the untested material is presented in Fig. la, which shows 0o the case of the porous-matrix oxide-oxide CMCs, interlaminar and 90 fiber tows as well as numerous matrix cracks In the case shear failure is controlled by the exceptionally weak porous of the as-processed material, most are shrinkage cracks formed matrix. The objective of this effort is to evaluate the interlaminar during processing. Porous nature of the matrix is seen in Fig. 1b shear strength and to investigate the creep behavior in interlami The double-notch shear(DNS)test specimens measuring ar shear of an oxide-oxide Cmc consisting of a porous alumina 150 mm x 20 mm were cut from a single N720 A panel. The thick- matrix reinforced with the Nextel 720 fibers. Several previous ness of the specimens was the same as the nominal thickness of studies examined the in-plane mechanical behavior of this com- the composite panel, i.e. N5.2 mm. The notches of 0.5 mm width posite [28-31 at elevated temperature. This study investigates were extended to the middle of each test specimen within creep behavior of the Nextel 720/alumina(N720/A)composite in +0.05 mm so that shear failure occurred on the plane between interlaminar shear at 1200C in air and in steam environments. the notch tips. The distance between the notches was 13 mm. The composite microstructure, as well as damage and failure Schematic of the dNS specimen and notch details are shown in mechanisms are discussed Fig. 2a and b, respectively. Dimensions of the DNS specimens used in this effort were different from those recommended in the astm 2. Material and experimental arrangements Standard C1425. The 13-mm distance between the notches was chosen specifically to enable the measurement of compressive The material studied was Nextel M720/alumina(N720JA), an strain between the notch tips with an MTS high-temperature xide-oxide CMC(manufactured by COl Ceramics, San Diego, CA) extensometer of 125-mm gage length. The overall specimen 三200um Fig. 1. As-processed material: (a)overview and (b) porous nature of the matrix is evident. Load 20 150 13 5mm Load Fig. 2. Double-notch shear specimen: (a) configuration and dimensions and (b) notch detailsas a function of test rate using double-notch shear specimens and demonstrated that the interlaminar shear strength (ILSS) degraded with decreasing test rate. A phenomenological, power-law based crack growth model was proposed to account for the degradation of the ILSS of the composite at elevated temperatures. High-tem￾perature stress-rupture tests in interlaminar shear were employed to validate the proposed model. These studies focused on the non￾oxide CMCS with matrix-dense interlaminar regions, where the interlaminar failure is controlled by the fiber-matrix interface. In the case of the porous-matrix oxide–oxide CMCs, interlaminar shear failure is controlled by the exceptionally weak porous matrix. The objective of this effort is to evaluate the interlaminar shear strength and to investigate the creep behavior in interlami￾nar shear of an oxide–oxide CMC consisting of a porous alumina matrix reinforced with the NextelTM720 fibers. Several previous studies examined the in-plane mechanical behavior of this com￾posite [28–31] at elevated temperature. This study investigates creep behavior of the NextelTM720/alumina (N720/A) composite in interlaminar shear at 1200 C in air and in steam environments. The composite microstructure, as well as damage and failure mechanisms are discussed. 2. Material and experimental arrangements The material studied was NextelTM720/alumina (N720/A), an oxide–oxide CMC (manufactured by COI Ceramics, San Diego, CA) consisting of a porous alumina matrix reinforced with NextelTM720 fibers. There is no fiber coating. The composite was supplied in a form of 5.2-mm thick plates comprised of 24 0/90 woven layers, with a density of 2.83 g/cm3 , a fiber volume of 46.6%, and matrix porosity of 22%. The fiber fabric was infiltrated with the matrix in a sol–gel process. The laminate was dried with a ‘‘vac￾uum bag” technique under low pressure and low temperature, and then pressureless sintered [32]. Representative micrograph of the untested material is presented in Fig. 1a, which shows 0 and 90 fiber tows as well as numerous matrix cracks. In the case of the as-processed material, most are shrinkage cracks formed during processing. Porous nature of the matrix is seen in Fig. 1b. The double-notch shear (DNS) test specimens measuring 150 mm 20 mm were cut from a single N720/A panel. The thick￾ness of the specimens was the same as the nominal thickness of the composite panel, i.e. 5.2 mm. The notches of 0.5 mm width were extended to the middle of each test specimen within ±0.05 mm so that shear failure occurred on the plane between the notch tips. The distance between the notches was 13 mm. Schematic of the DNS specimen and notch details are shown in Fig. 2a and b, respectively. Dimensions of the DNS specimens used in this effort were different from those recommended in the ASTM Standard C1425. The 13-mm distance between the notches was chosen specifically to enable the measurement of compressive strain between the notch tips with an MTS high-temperature extensometer of 12.5-mm gage length. The overall specimen Fig. 1. As-processed material: (a) overview and (b) porous nature of the matrix is evident. Fig. 2. Double-notch shear specimen: (a) configuration and dimensions and (b) notch details. M.B. Ruggles-Wrenn, P.D. Laffey / Composites Science and Technology 68 (2008) 2260–2266 2261