G N Morscher et al /Composites Science and Technology 68(2008 )3305-3313 a 0.1mm 0.1mr Stress direction Fig. 7. Typical creep-formed cracks: (a) surface 90 minicomposite(110 MPa, 2036 h, did not fail in rupture)and (b)inner back-to-back 90 minicomposite cracks which extended to the surface through a 0minicomposite(165 MPa; 1508 h creep rupture). mm several unbridged cracks emanating from both surfaces 0.2mm g 8. Matrix cracks that extend at least two plies from the surface and in some cases have fractured fibers in the matrix crack wake. This specimen had undergone 220 MPa 30 Hz fatigue and lasted approximately 1.2 h at 1204 C. ■220MPa(32ks)HcF through the 90 minicomposites The formation and propagation of bridged-matrix cracks at room temperature relates to the high en- ▲193MPa(28ks)HcF 08■ ergy ae events. If the low energy event data are removed from the 193 MPa(28 ksi) DF ae and only the high energy event data are used, there is a very °165MPa(24ksi) Creep good correlation between the room temperature and elevated tem- perature stress-dependent matrix crack density with the exception that the elevated temperature matrix crack density was not through-the-cross-section. The elevated temperature matrix crack 193 MPa(28 ksi) Dwell Fatigue densities tend to fall below the room temperature distribution, i.e less cracking at high temperature compared to room temperature. mostly surface cracks, a few tw brid em vated temperature with time(Fig. 9). the crack density tends to in- 165 MPa(24 ksi) Creep crease towards the room temperature derived matrix crack density value with time. Therefore, the room temperature stress-depen- 1000 0000 dent matrix crack is a reasonable, at least conservative, representa- tion for modeling matrix crack density at elevated temperature as Fig 9. Crack density along the length of the specimen for different stress, time, and long as the depth of matrix cracks at elevated temperatures at a given stress is taken into considerationthrough the 90 minicomposites. The formation and propagation of bridged-matrix cracks at room temperature relates to the high en￾ergy AE events. If the low energy event data are removed from the AE and only the high energy event data are used, there is a very good correlation between the room temperature and elevated tem￾perature stress-dependent matrix crack density with the exception that the elevated temperature matrix crack density was not through-the-cross-section. The elevated temperature matrix crack densities tend to fall below the room temperature distribution, i.e., less cracking at high temperature compared to room temperature. Even though some increase in crack density was observed at ele￾vated temperature with time (Fig. 9), the crack density tends to in￾crease towards the room temperature derived matrix crack density value with time. Therefore, the room temperature stress-depen￾dent matrix crack is a reasonable, at least conservative, representa￾tion for modeling matrix crack density at elevated temperature as long as the depth of matrix cracks at elevated temperatures at a given stress is taken into consideration. Fig. 7. Typical creep-formed cracks: (a) surface 90 minicomposite (110 MPa, 2036 h, did not fail in rupture) and (b) inner back-to-back 90 minicomposite cracks which extended to the surface through a 0 minicomposite (165 MPa; 1508 h creep rupture). Fig. 8. Matrix cracks that extend at least two plies from the surface and in some cases have fractured fibers in the matrix crack wake. This specimen had undergone 220 MPa 30 Hz fatigue and lasted approximately 1.2 h at 1204 C. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1 10 100 1000 10000 Time, hr Crack density, mm-1 220 MPa (32 ksi) HCF 220 MPa (32 ksi) DF 193 MPa (28 ksi) HCF 193 MPa (28 ksi) DF 165 MPa (24 ksi) Creep most cracks go through two plies or more, ~ 1/3 of the cracks are unbridged most cracks go through one to two plies, a few unbridged mostly surface cracks, a few through one ply 165 MPa (24 ksi) Creep 193 MPa (28 ksi) Dwell Fatigue two plies, some unbridged Fig. 9. Crack density along the length of the specimen for different stress, time, and loading conditions. G.N. Morscher et al. / Composites Science and Technology 68 (2008) 3305–3313 3309