R. Naslain et al/Composites: Part A 30(1999)537-547 10 120 25°C750°900C1050° 100 乙 1000 3000 Tensile failure stress(MPa) Fig. 8. Tensile failure stress distributions for Nicalon fibers as measured from: bare single filaments with a gauge length of 75 mm(O), bare fiber 0.0 3 bundle with a gauge length of 75 mm(e), mirror radii in Nicalon/PyC/SiC Strain(%0) (CVI)minicomposites with Vr=0.27(4),Vr=0.60(%)and Vr=0.70 Fig. 10. Stress-strain tensile curves for C(ex-PANVPyC/SiC O); according to Ref. [27] minicomposites, recorded in argon(PAr =65 kPa) and at incre temperatures, showing a rigidification effect related to crac preparation and test procedures are used [14, 34] and assum- according to Ref. 26 ing that the effects of the surrounding fibers are the same (minicomposites) or can be neglected(microcomposites) be used on a comparative basis in processing condition Stress-displacement push-out curves, recorded for Nica- lon/BN/SiC (CVi) real(2D)-and micro-composites, are shown in Fig 9. They obviously exhibit similar features. 3. 3. Mechanical behaviour at high temperature and efect of The data have been used to assess the FM-interfacial para meters according to available micromechanics-based models [32]. The interface properties determined using The effect of temperature on the mechanical behaviour of push-in tests on single filament microcomposites were in non-oxide composites has been assessed through test agreement with those obtained by analysis of tensile tests performed on ID-composites, under an atmosphere of [14]. As an example, the interfacial parameters calculated argon. As an example, Fig. 10 shows the tensile curves for Nicalon/BN/SiC (CVI) microcomposites were T,= recorded at temperature ranging from 25"C to 1050C, for 18 MPa and Gic=3. 1 J/m2 from push-in data[14] versus carbon(ex-PAN)PyC/Sic (CVI) minicomposites. Such T,=24 MPa and Gic= 2.3 J/m2'from tensile test data[6]. materials undergo microcracking during processing when Push-in or push-out tests on micro/minicomposites can cooled from the CVI-temperature (1000oC)to room temperature, due to coefficient of thermal expansion (CTE) mismatch. This feature explains the rigidification effect observed at high temperatures(T> 750C)and low applied stresses. Under such conditions, the matrix cracks 2.0 due to processing are actually closed and the composite Microc stiffness is equal le of mixture Conversely, at low temperatures(T<750C)and whatever 21.5 2D composite the applied stress, or at T>750C for high applied stresses the matrix cracks are open and hence the stifness is lower 1.0 Crack closing as temperature is raised has been observed posites, with a heated tensile stage set in the SEM, and modelled [26] The combined effects of temperature and oxidizing atmo- sphere, i.e. the ambient air, have been studied via static(or cyclic) fatigue tests run on both micro-and mini-model Displacement(um) composites. The potential of the micro/ mini composite approach in this field is shown through a few examples Fig 9. Stress-displacement push-out curves recorded for Nicalon/BN/SIC illustrating the effect of engineered interphases on the life- (CVD)real (2D)-and micro- composites, according to Refs. [14, 32]- time under load. First, a series of Nicalon/C(B)SIC (CVIpreparation and test procedures are used [14,34] and assum￾ing that the effects of the surrounding fibers are the same (minicomposites) or can be neglected (microcomposites). Stress–displacement push-out curves, recorded for Nica￾lon/BN/SiC (CVI) real (2D)- and micro-composites, are shown in Fig. 9. They obviously exhibit similar features. The data have been used to assess the FM-interfacial para￾meters according to available micromechanics-based models [32]. The interface properties determined using push-in tests on single filament microcomposites were in agreement with those obtained by analysis of tensile tests [14]. As an example, the interfacial parameters calculated for Nicalon/BN/SiC (CVI) microcomposites were ti ˆ 18 MPa and Gic ˆ 3.1 J/m2 from push-in data [14] versus ti ˆ 24 MPa and Gic ˆ 2.3 J/m2 from tensile test data [6]. Push-in or push-out tests on micro/minicomposites can be used on a comparative basis in processing condition screening. 3.3. Mechanical behaviour at high temperature and effect of the environment The effect of temperature on the mechanical behaviour of non-oxide composites has been assessed through test performed on 1D-composites, under an atmosphere of argon. As an example, Fig. 10 shows the tensile curves recorded at temperature ranging from 258C to 10508C, for carbon (ex-PAN)/PyC/SiC (CVI) minicomposites. Such materials undergo microcracking during processing when cooled from the CVI-temperature (<10008C) to room temperature, due to coefficient of thermal expansion (CTE) mismatch. This feature explains the rigidification effect observed at high temperatures (T . 7508C) and low applied stresses. Under such conditions, the matrix cracks due to processing are actually closed and the composite stiffness is equal to that predicted by the rule of mixture. Conversely, at low temperatures (T , 7508C) and whatever the applied stress, or at T . 7508C for high applied stresses, the matrix cracks are open and hence the stiffness is lower. Crack closing as temperature is raised has been observed and measured through tensile tests performed on minicom￾posites, with a heated tensile stage set in the SEM, and modelled [26]. The combined effects of temperature and oxidizing atmo￾sphere, i.e. the ambient air, have been studied via static (or cyclic) fatigue tests run on both micro-and mini-model composites. The potential of the micro/mini composite approach in this field is shown through a few examples illustrating the effect of engineered interphases on the life￾time under load. First, a series of Nicalon/C(B)/SiC (CVI) 544 R. Naslain et al. / Composites: Part A 30 (1999) 537–547 Fig. 8. Tensile failure stress distributions for Nicalon fibers as measured from: bare single filaments with a gauge length of 75 mm (B), bare fiber bundle with a gauge length of 75 mm (QR), mirror radii in Nicalon/PyC/SiC (CVI) minicomposites with Vf ˆ 0.27 (O); Vf ˆ 0.60 (V) and Vf ˆ 0.70 (X); according to Ref. [27]. Fig. 9. Stress–displacement push-out curves recorded for Nicalon/BN/SiC (CVI) real (2D) -and micro- composites, according to Refs. [14,32]. Fig. 10. Stress–strain tensile curves for C (ex-PAN)/PyC/SiC pre-craked minicomposites, recorded in argon (PAr ˆ 65 kPa) and at increasing test temperatures, showing a rigidification effect related to crack closing, according to Ref. [26]