lic society24(2004)56-578 materials approach each other. Most remarkable was (taken from a parallel study of load cycling)- which the large increase in strength of the +45 material, with indicates that the knee is associated partly with a corresponding increase in strain to failure when tested reduction in stifness and partly with an irreversible strain increment. Indication of shear strain damage Whereas the 0/90 samples fractured by fibre bundle occurring diagonally from the hole in the 0/90 compo- fracture and pull-out (Fig. 5 a), the samples with +4 sites prior to failure was provided by a parallel study fibre orientation fractured with little or no indication of using thermal emission- but also here by diagonal bundle fracture(Fig. 5 b). The fracture is presumably able stepped fracture paths in which individual fibre bundles to occur by matrix failure followed by ply separation failed in a tensile mode but away from the centre plane The 0/90 stress-strain curves exhibited a relatively of the sample(Fig 5a) distinct fall in slope (knee) situated in the room-tem- In Fig. 7 the effect of the heat-treatments on strength perature tests at a net stress level of between 30 and 50 are presented in the form of a Larson-Miller plot. For MPa and a strain of between 0.03 and 0.06%. This net treatments above 500 oc the results(for stress level corresponded to a theoretical intensified follow a uniform trend which makes it possible to pre- stress at the hole of 70-135 MPa Interestingly, the knee dict strength values for heat-treatments at other times stress and strain were somewhat higher at 1000 and and/ or temperatures. The values from Jurf and But 1 100C(60-90 MPa and 0.07-0.15%, respectively). The ner, also included in the figure show good agreement knee is associated with damage in the composite Fig. 6 with the present results. A minor difference is seen for shows the stress-strain curve of a load-cycled sample higher temperatures and or times, where the material in 02 20 l00 hrs at 1000°C 00050,10150,20 Strain o Strain[%] Fig 4. Representative stress-strain curves of +45 samples. Fig. 6. Stress-strain curve of a load-cycled sample Fig. 5. As-received samples after tensile testing, showing extensive fibre bundle pull-out. (a)0/90 fibre orientation; (b)+45 fibre orientation.materials approach each other. Most remarkable was the large increase in strength of the 45 material, with a corresponding increase in strain to failure when tested at 1000
C. Whereas the 0/90 samples fractured by fibre bundle fracture and pull-out (Fig. 5 a), the samples with 45
fibre orientation fractured with little or no indication of bundle fracture (Fig. 5 b). The fracture is presumably able to occur by matrix failure followed by ply separation. The 0/90 stress–strain curves exhibited a relatively distinct fall in slope (knee) situated in the room-tem￾perature tests at a net stress level of between 30 and 50 MPa and a strain of between 0.03 and 0.06%. This net stress level corresponded to a theoretical intensified stress at the hole of 70–135 MPa. Interestingly, the knee stress and strain were somewhat higher at 1000 and 1100
C (60–90 MPa and 0.07–0.15%, respectively). The knee is associated with damage in the composite. Fig. 6 shows the stress–strain curve of a load-cycled sample (taken from a parallel study of load cycling)25 which indicates that the knee is associated partly with a reduction in stiffness and partly with an irreversible strain increment. Indication of shear strain damage occurring diagonally from the hole in the 0/90 compo￾sites prior to failure was provided by a parallel study using thermal emission25 but also here by diagonal, stepped fracture paths in which individual fibre bundles failed in a tensile mode but away from the centre plane of the sample (Fig. 5a). In Fig. 7 the effect of the heat-treatments on strength are presented in the form of a Larson–Miller plot. For treatments above 500
C the results (for a given a/w) follow a uniform trend which makes it possible to pre￾dict strength values for heat-treatments at other times and/or temperatures. The values from Jurf and But￾ner,13 also included in the figure show good agreement with the present results. A minor difference is seen for higher temperatures and/or times, where the material in Fig. 5. As-received samples after tensile testing, showing extensive fibre bundle pull-out. (a) 0/90 fibre orientation; (b) 45
fibre orientation. Fig. 4. Representative stress–strain curves of 45
samples. Fig. 6. Stress–strain curve of a load-cycled sample. 570 M.-L. Antti et al. / Journal of the European Ceramic Society 24 (2004) 565–578