G.N. Morscher et al/ Composites Science and Technology 68 (2008)3305-3313 in greater compression which must be overcome in order to pro- uce and propagate matrix cracks. It should be noted that oxida tion within surface cracks could also place the composite in esidual compression. However, multiple creep experiments have err performed on this same composite system at 1204C in in- 台 environment which showed the same magnitude of residual stress(150 MPa) for tensile creep at 220 MPa after hundreds of hours[14. DF, HCF or Creep Stress: Fig 5 shows the effect of residual compressive stress on AE set stress. There is nearly a direct relationship between the two. It 200x220MPa(32ks) 193 MPa(28ksi should be noted that for the higher stress conditions, 192 and in 2 100 4110 MPa(16ksi) 日165MPa(24ksi particular 220 MPa, significant matrix cracking had occurred the specimens tested at room temperature as will be discussed be- ●As- Produced low. Therefore the ae onset stresses for 192 and 220 MPa mask the fact that there already were some large matrix cracks formed dur ing the elevated temperature stressed-oxidation condition How- Time of test, hr ever, for specimens exposed to 165 MPa and lower stresses at elevated temperature, matrix cracking was minor(see below) Fig. 6. Ultimate tensile strength(UTS)at and the increase in aE onset stress corresponds to the stresses re- fter creep or fatigue specimens. Straight line indicates average of 193 MPa-tested pecimens indicating a loss in UTS at stresses above the onset stress for matrix quired to form and propagate initial matrix cracks of significant cracking. size, i.e., the matrix cracking stress increased with low stress creep he ultimate tensile strength properties are shown in Fig. 6 for he different specimens. Note that for applied creep/fatigue stres- and polished (usually about 1 to 1.5 mm from the edge surtace). ses of 165 MPa or lower, there is very little if any degradation in A few specimens were aligned along the face of the specimen. ultimate strength, even for specimens subjected to nearly 250h. For specimens polished along the length edge, no cracks were There appears to be a 10% drop in strength of specimens tested observed for the specimens subjected to 110 MPa For 165 MPa ap- at 193 MPa up to 250 h (straight line in Fig. 6)and about a 20% plied stresses and above, surface 90 minicomposite micro cracks drop for specimens tested at 220 MPa for 250 h. Specimens that (Fig. 7a)and inner back-to-back 90 minicomposite cracks were radation in room temperature ultimate strength. The room tem- For 193 MPa applied stresses and above, cracks which penetrated perature strength of the specimen that survived 1239 h at up to two plies in from the face were common( Fig 8).With 110 MPa was degraded about 20% compared to as-produced mate increasing time and stress, unbridged cracks were observed along rial. Whereas the room temperature strength of the two specimens (Fig. 7b) However, even for the specimens subjected to the highest the length for 165 MP that survived greater than 2000 h of creep at 110 and 165 graded almost 50% of the as-produced composite stre stresses(220 MPa), these surface cracks containing unbridged por- should also be noted that the retained room-temperature tions typically extended only two plies and sometimes three into odulus was only reduced slightly, as much as 10% after the cree the thickness of the specimen. The presence of unbridged regions or fatigue exposure [10] of cracks filled with a glass indicates that the fibers had failed at some significant period of time prior to ultimate failure of the 3. 2. Optical microscopy along the length composite In order to quantify matrix cracking, the crack densi age secte o f the tntey the extent of damage development in the可p,ap贴 acks were recorded tion).Most specimens were aligned so that the edge was ground cracks increase. However, for most cracks, even for specimens tested at 220 MPa, very few appear to propagate through-the- thickness of the specimen. When polished along the face of the specimen so that cracks can be observed along the 8.2 mm width, atrix cracks rarely appear to traverse the width, even for the 220 MPa stress-condition experiments, although they always ema- △110MPa(16ks 250-165MPa(24ksi) nate either from or to an edge(corner) ·192MPa(28ks) At room temperature, matrix cracks do appear to go through the 230x220MPa cross-section of the composite at the higher stre ≥200MP [11 In Ref [11. it was shown that the normalized cumulative AE energy was nearly directly proportional to measured matrix crack density when plotted versus stress. The matrix crack density versus stress measured in this study for elevated temperature tested specimens is compared to the room temperature data in Fig. 10. Note that at the lower stresses, low energy AE events pre 170 dominate and are caused by tunnel cracks which propagate An unbridged crack is defined as a crack which had originally propagate Residual Compressive Stress, MPa temperature/stress, some of those fibers in the crack wake, typically near the Fig. 5. Matrix cracking stress as determined from AE versus the residual compres- or edge of the specimen fail leaving a formerly bridged portion of the matrix crack sive stress in the matrix. Most of the data is for DF tests except where noted.in greater compression which must be overcome in order to pro￾duce and propagate matrix cracks. It should be noted that oxida￾tion within surface cracks could also place the composite in residual compression. However, multiple creep experiments have been performed on this same composite system at 1204 C in in￾ert environment which showed the same magnitude of residual stress (150 MPa) for tensile creep at 220 MPa after hundreds of hours [14]. Fig. 5 shows the effect of residual compressive stress on AE on￾set stress. There is nearly a direct relationship between the two. It should be noted that for the higher stress conditions, 192 and in particular 220 MPa, significant matrix cracking had occurred in the specimens tested at room temperature as will be discussed be￾low. Therefore, the AE onset stresses for 192 and 220 MPa mask the fact that there already were some large matrix cracks formed dur￾ing the elevated temperature stressed-oxidation condition. How￾ever, for specimens exposed to 165 MPa and lower stresses at elevated temperature, matrix cracking was minor (see below) and the increase in AE onset stress corresponds to the stresses re￾quired to form and propagate initial matrix cracks of significant size, i.e., the matrix cracking stress increased with low stress creep. The ultimate tensile strength properties are shown in Fig. 6 for the different specimens. Note that for applied creep/fatigue stres￾ses of 165 MPa or lower, there is very little if any degradation in ultimate strength, even for specimens subjected to nearly 250 h. There appears to be a 10% drop in strength of specimens tested at 193 MPa up to 250 h (straight line in Fig. 6) and about a 20% drop for specimens tested at 220 MPa for 250 h. Specimens that survived greater than 1000 h at lower stresses showed greater deg￾radation in room temperature ultimate strength. The room tem￾perature strength of the specimen that survived 1239 h at 110 MPa was degraded about 20% compared to as-produced mate￾rial. Whereas the room temperature strength of the two specimens that survived greater than 2000 h of creep at 110 and 165 MPa de￾graded almost 50% of the as-produced composite strength. It should also be noted that the retained room-temperature elastic modulus was only reduced slightly, as much as 10% after the creep or fatigue exposure [10]. 3.2. Optical microscopy along the length In order to quantify the extent of damage development in the gage section of the specimens subjected to creep and fatigue, over a dozen specimens were polished along the length (loading direc￾tion). Most specimens were aligned so that the edge was ground and polished (usually about 1 to 1.5 mm from the edge surface). A few specimens were aligned along the face of the specimen. For specimens polished along the length edge, no cracks were observed for the specimens subjected to 110 MPa. For 165 MPa ap￾plied stresses and above, surface 90 minicomposite micro cracks (Fig. 7a) and inner back-to-back 90 minicomposite cracks were evident for stress-temperature conditions up to 250 h (Fig. 7b). For 193 MPa applied stresses and above, cracks which penetrated up to two plies in from the face were common (Fig. 8). With increasing time and stress, unbridged cracks1 were observed along the length for 165 MPa and greater applied stress conditions (Fig. 7b). However, even for the specimens subjected to the highest stresses (220 MPa), these surface cracks containing unbridged por￾tions typically extended only two plies and sometimes three into the thickness of the specimen. The presence of unbridged regions of cracks filled with a glass indicates that the fibers had failed at some significant period of time prior to ultimate failure of the composite. In order to quantify matrix cracking, the crack density, number of cracks per mm, and the depth of cracks were recorded and are plotted in Fig. 9. It is evident that with increasing stress and time that the number of cracks increases and the depth of the matrix cracks increase. However, for most cracks, even for specimens tested at 220 MPa, very few appear to propagate through-the￾thickness of the specimen. When polished along the face of the specimen so that cracks can be observed along the 8.2 mm width, matrix cracks rarely appear to traverse the width, even for the 220 MPa stress-condition experiments, although they always ema￾nate either from or to an edge (corner). At room temperature, matrix cracks do appear to go through the cross-section of the composite at the higher stresses (P200 MPa) [11]. In Ref. [11], it was shown that the normalized cumulative AE energy was nearly directly proportional to measured matrix crack density when plotted versus stress. The matrix crack density versus stress measured in this study for elevated temperature tested specimens is compared to the room temperature data in Fig. 10. Note that at the lower stresses, low energy AE events pre￾dominate and are caused by tunnel cracks which propagate 150 170 190 210 230 250 270 0 Residual Compressive Stress, MPa AE Onset Stress, MPa As-produced 110 MPa (16ksi) 165 MPa (24ksi) 192 MPa (28ksi) 220 MPa (32ksi) 30 Hz Creep 50 100 150 200 Fig. 5. Matrix cracking stress as determined from AE versus the residual compres￾sive stress in the matrix. Most of the data is for DF tests except where noted. 0 100 200 300 400 500 600 1 10 100 1000 10000 Time of test, hr RT Residual Strength, MPa 220 MPa (32ksi) 193 MPa (28ksi) 165 MPa (24ksi) 110 MPa (16ksi) As-Produced DF, HCF, or Creep Stress: Fig. 6. Ultimate tensile strength (UTS) at room temperature for as-produced and after creep or fatigue specimens. Straight line indicates average of 193 MPa-tested specimens indicating a loss in UTS at stresses above the onset stress for matrix cracking. 1 An unbridged crack is defined as a crack which had originally propagated some distance so that load-bearing fibers bridge the crack. However, after some time/ temperature/stress, some of those fibers in the crack wake, typically near the surface or edge of the specimen fail leaving a formerly bridged portion of the matrix crack unbridged. 3308 G.N. Morscher et al. / Composites Science and Technology 68 (2008) 3305–3313