H Mei/Composites Science and Technology 68(2008)3285-3292 and the ae counts enlarged with increasing inelastic strain at stress 80 mm higher than 50 MPa Theonset"strain(obtained from extrapola- ion of the initial high-rate aE energy to the abscissa for this strain in Fig. 5, see Ref [13])is round 0.05% Multiple matrix cracking and 8 mm interface debonding resulted in macroscopic nonlinear mechanical response leading to this aE activity. the slope of the tensile curve Fiber architectures continuously decreases as the stress increases between 50 an 150 MPa. Above 150 MPa, with a sudden reduction in AE energy the stress-strain relationship turns into apparent linearity up to 3 mm final fracture at stress of 250.60 MPa and strain of 0.65% g. 3. Geometry and dimensions of the tensile test pieces. 3.2. Reloading-unloading behaviors Fig 6 presents the hysteresis loop evolutions of the 2D C/Sic 3. Results and discussion composites during the loading-unloading-reloading cycle tests. It can be seen from this figure that the loading curve in each loop was mostly linear to the stress level of the preceding step and then became nonlinear, following the envelope which is basically iden- > Fig. 5 gives a typical stress-strain curve of 2D C/SiC composite tical to the monotonic tensile stress-strain curve shown in Fig 5. mple with corresponding acoustic emission(AE)signals during The nonlinear monotonic tensile testing. The AE energy below proportional limit schematically depicted in Fig. 7. The inelastic strain E upon loading stress of omc =50 MPa was small, that above the proportional limit includes the sliding strain, es. that arises from the matrix crack obviously became large, indicating that the onset of significant opening caused by interface debonding/sliding, and the thermal propagation and multiplication of the as-processed matrix micro- misfit relief strain er caused by relief of the misfit strain. The elastic cracks correlated closely to the proportional limit stress. After strain, ee, represents the reversible strain upon unloading. The total the initiation period, the accumulated AE energy increased rapidly strain a at each peak applied stress p is thus b SiC matrix Fig 4. SEM micrographs showing(a)the woven 0/90 fiber architectures and (b) the constituent micro res of Hi-Nicalon SiC fiber, Pyc interphase and CVI-SiC matrix Time(s) 0300600900120015001800 250 Linearity 6x105 195 175 AE energy 155 3155555 1x10° 00010203040.50.60.7 (%) Fig. 5. Typical monotonic tensile stress-strain curve of the 2D C/Sic composite with the associated acoustic emission response.3. Results and discussion 3.1. Monotonic tensile behaviors Fig. 5 gives a typical stress–strain curve of 2D C/SiC composite sample with corresponding acoustic emission (AE) signals during monotonic tensile testing. The AE energy below proportional limit stress of rmc = 50 MPa was small, that above the proportional limit obviously became large, indicating that the onset of significant propagation and multiplication of the as-processed matrix micro￾cracks correlated closely to the proportional limit stress. After the initiation period, the accumulated AE energy increased rapidly and the AE counts enlarged with increasing inelastic strain at stress higher than 50 MPa. The ‘‘onset” strain (obtained from extrapola￾tion of the initial high-rate AE energy to the abscissa for this strain in Fig. 5, see Ref. [13]) is round 0.05%. Multiple matrix cracking and interface debonding resulted in macroscopic nonlinear mechanical response leading to this AE activity. The slope of the tensile curve continuously decreases as the stress increases between 50 and 150 MPa. Above 150 MPa, with a sudden reduction in AE energy the stress–strain relationship turns into apparent linearity up to final fracture at stress of 250.60 MPa and strain of 0.65%. 3.2. Reloading–unloading behaviors Fig. 6 presents the hysteresis loop evolutions of the 2D C/SiC composites during the loading–unloading–reloading cycle tests. It can be seen from this figure that the loading curve in each loop was mostly linear to the stress level of the preceding step and then became nonlinear, following the envelope which is basically iden￾tical to the monotonic tensile stress–strain curve shown in Fig. 5. The nonlinear phenomena associated with matrix cracking are schematically depicted in Fig. 7. The inelastic strain ei upon loading includes the sliding strain, es, that arises from the matrix crack opening caused by interface debonding/sliding, and the thermal misfit relief strain eT caused by relief of the misfit strain. The elastic strain, ee, represents the reversible strain upon unloading. The total strain e* at each peak applied stress rp is thus 120 mm Fiber architectures Aluminum tab 3 mm 8 mm 80 mm Fig. 3. Geometry and dimensions of the tensile test pieces. Fig. 4. SEM micrographs showing (a) the woven 0/90 fiber architectures and (b) the constituent microstructures of Hi-Nicalon SiC fiber, PyC interphase and CVI-SiC matrix in the 2D SiC/SiC composite. Fig. 5. Typical monotonic tensile stress–strain curve of the 2D C/SiC composite with the associated acoustic emission response. H. Mei / Composites Science and Technology 68 (2008) 3285–3292 3287