G N Morscher et al /Composites Science and Technology 68(2008 )3305-3313 surfaces (creep-formed cracks usually emanated from or to a sur face)and the average value was taken as the matrix crack density 165 MPa. 100h DF 192 MPa. 250h DF E=230 GPa 3. Results All of the creep and fatigue data are shown in Fig. 2 in terms of As-Produced maximum applied stress during the test versus exposure time, that E=259 GPa is, time for either test interruption or specimen rupture. Note that the open symbols indicate that the specimen did not fail and thus as esidual testing at room temperature. Df was only carried out to 250 h and there were no failures for this condi tion over the entire stress range. Creep-rupture occurred at the high stresses(220 MPa)for shorter times(<100 h)and for longer 50 times(up to 2000 h) at the lower stresses(110 and 165 MPa). Fa- tigue failure occurred for all of the 30 Hz HCf tests over a wide 020.30.40.50.60.7 stress range, 179-220 MPa, which indicates this was a more severe Strain. % condition for failure compared to the creep and dwell fatigue con- ditions. The 30 Hz HCF specimen tested for the peak stress condi b 220 MPa. 168h DF tion of 165 MPa exceeded the run-out condition of 10 cycles (42,000,000 cycles survived) and was tested at room temperature for determination of residual properties. For comparison, 0.33 Hz 165 MPa 100h DF fatigue data for the same type of composite system from Kalluri et al. [12] is plotted on Fig. 2 showing similar behavion 3. 1. Residual properties of specimens that did not fail 8 More than twenty five different specimens were tested at room 20.41 110 MPa, 100h DF temperature after being subject to tensile creep or fatigue, as well s two as-produced specimens for comparison. The stress-strain AS-Produced behavior and aE behavior is shown in Fig 3 for some representa- tive specimens. The 220 MPa DF specimens was the only specimen that survived the 220 MPa applied stress condition for any signifi 0 500 cant time. The degradation in ultimate strength after the 220 MPa. AE onset stresses Stress. MPa For the specimens that had experienced dwell fatigue or creep, Fig 3. Room temperature ously dwell fatigue specimens. Note that the stress-strain curve there was an increase in the stress above which non-linearity oc- in(a) are offset for clarity and the specimens were tested to failure. The modulus curs, i.e. the proportional limit stress. There was also an increase values refer to the initial linear(10-55 MPa) portion of the stress-strain curve in the stress at which significant high energy AE occurs ("AE Onset"in Fig. 3b). AE onset stress has been shown to be a good parameter for the onset of significant matrix cracking in this com- osite system[11]. Fig. 4 shows the increase in AE matrix cracking stress(Ae onset stress) with time-dependent strain for some of 280 the specimens. This can be explained by the increase in residual mpressive stress in the matri amount of residual compressive stress in the matrix can be deter- mined from the intersection of the top part of the hysteresis loop 220 6200 Creep specimens 5160 193 MPa (28ks 0.05 0.1 0.15 Fig 4. Room temperature AE onset stress versus time dependent strain for DF and creep(where noted) tests. Data points encircled refer to creep data as noted. x:10 +0.33 Hz(tailed)-Ref 12 with the original loading curve(Fig. 3a)[13]. For as-produced specimens, the residual compressive stress is about 50 MPa 0.1 1000 was observed in Ref [11. However, after tensile creep, the matrix residual compressive stress increases to over 100 MPa. Similar Fig. 2. Creep and fatigue data versus exposure time at 1204 C for the different behavior was reported in Ref. [7] which was attributed to matrix levated temperature tests. Open symbols indicate test specimens that did not fail. relaxation during creep. Upon unloading the fibers put the matrixsurfaces (creep-formed cracks usually emanated from or to a sur￾face) and the average value was taken as the matrix crack density. 3. Results All of the creep and fatigue data are shown in Fig. 2 in terms of maximum applied stress during the test versus exposure time, that is, time for either test interruption or specimen rupture. Note that the open symbols indicate that the specimen did not fail and thus was removed for residual testing at room temperature. DF was only carried out to 250 h and there were no failures for this condi￾tion over the entire stress range. Creep-rupture occurred at the high stresses (220 MPa) for shorter times (6100 h) and for longer times (up to 2000 h) at the lower stresses (110 and 165 MPa). Fa￾tigue failure occurred for all of the 30 Hz HCF tests over a wide stress range, 179–220 MPa, which indicates this was a more severe condition for failure compared to the creep and dwell fatigue con￾ditions. The 30 Hz HCF specimen tested for the peak stress condi￾tion of 165 MPa exceeded the run-out condition of 107 cycles (42,000,000 cycles survived) and was tested at room temperature for determination of residual properties. For comparison, 0.33 Hz fatigue data for the same type of composite system from Kalluri et al. [12] is plotted on Fig. 2 showing similar behavior. 3.1. Residual properties of specimens that did not fail More than twenty five different specimens were tested at room temperature after being subject to tensile creep or fatigue, as well as two as-produced specimens for comparison. The stress-strain behavior and AE behavior is shown in Fig. 3 for some representa￾tive specimens. The 220 MPa DF specimens was the only specimen that survived the 220 MPa applied stress condition for any signifi- cant time. The degradation in ultimate strength after the 220 MPa, 168 h, DF condition is evident. For the specimens that had experienced dwell fatigue or creep, there was an increase in the stress above which non-linearity oc￾curs, i.e., the proportional limit stress. There was also an increase in the stress at which significant high energy AE occurs (‘‘AE Onset” in Fig. 3b). AE onset stress has been shown to be a good parameter for the onset of significant matrix cracking in this com￾posite system [11]. Fig. 4 shows the increase in AE matrix cracking stress (AE onset stress) with time-dependent strain for some of the specimens. This can be explained by the increase in residual compressive stress in the matrix with creep or fatigue. The amount of residual compressive stress in the matrix can be deter￾mined from the intersection of the top part of the hysteresis loop with the original loading curve (Fig. 3a) [13]. For as-produced specimens, the residual compressive stress is about 50 MPa as was observed in Ref. [11]. However, after tensile creep, the matrix residual compressive stress increases to over 100 MPa. Similar behavior was reported in Ref. [7] which was attributed to matrix relaxation during creep. Upon unloading, the fibers put the matrix 0 50 100 150 200 250 300 0.1 1 10 100 1000 10000 Exposure Time (hrs) Max Applied Stress (MPa) Dwell Fatigue 1204ºC (did not fail) Creep 1204ºC (did not fail) Creep 1204ºC Failure 30 Hz (failed) 30 Hz (did not fail) 1 Hz (failed) 0.33 Hz (failed) - Ref 12 Fig. 2. Creep and fatigue data versus exposure time at 1204 C for the different elevated temperature tests. Open symbols indicate test specimens that did not fail. 0 50 100 150 200 250 300 350 400 450 500 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Strain, % Stress, MPa residual compressive stress As-Produced E = 259 GPa 110 MPa, 100h DF E = 250 GPa 165 MPa, 100h DF E = 230 GPa 192 MPa, 250h DF E = 230 GPa 220 MPa, 168h DF E = 221 GPa 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 100 200 300 400 500 Stress, MPa Norm Cum AE As-Produced 110 MPa, 100h DF 165 MPa, 100h DF 192 MPa, 250h DF 220 MPa, 168h DF AE onset stresses a b Fig. 3. Room temperature stress strain behavior (a) and AE behavior (b) for an as￾produced and previously dwell fatigue specimens. Note that the stress-strain curves in (a) are offset for clarity and the specimens were tested to failure. The modulus values refer to the initial linear (10–55 MPa) portion of the stress–strain curve. 100 120 140 160 180 200 220 240 260 280 300 0 Time-dependent DF or Creep Strain, % AE Onset Stress, MPa 193 MPa (28ksi) 165 MPa (24ksi) 110 MPa (16ksi) As-Produced creep specimens 0.05 0.1 0.15 0.2 Fig. 4. Room temperature AE onset stress versus time dependent strain for DF and creep (where noted) tests. Data points encircled refer to creep data as noted. G.N. Morscher et al. / Composites Science and Technology 68 (2008) 3305–3313 3307