13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- However, an empirical approach has the disadvantage that the effects of changing processing conditions cannot be predicted. Such predictions are the province of mechanical models. An attractive future development will link mechanical modeling of preform deformation to empirical statistics, using the rich data content of detailed 3D measurements to calibrate and validate the mechanical model. The Markov Chain method may remain a useful tool within the linked super-model. (a) (b) Figure 3. The textile reinforcement of one virtual specimen generated with 3D tow representations using the Markov Chain method [25], showing (a) warp and weft tows together and (b) warp tows only. Tow shapes possess a combination of non-stochastic periodic variations (crimp features, etc.) and non-periodic stochastic deviations. 4. Observations of damage The properties (strength, etc.) of a composite’s constituent materials and their interfaces are generally unknown at high temperature. Phase properties cannot be calibrated by simple material tests, because the strength of different phases when they are juxtaposed at nm and μm scales is not represented by tests on large specimens of each phase isolated as a monolithic material. Tests are required on the composite materials themselves, executed at expected use conditions (i.e., temperatures of 1500ºC and higher), with sufficient resolution of mechanisms to enable the deduction of local material properties that control the mechanism. Direct observation of mechanisms is also critical to choosing correct formulations for simulations. Measurements made at high temperature are the only faithful source of the details of failure. If a test specimen is cooled to an experimentally convenient 25°C for examination, the cooling itself introduces thermal strains of the order of at least 0.1–0.5% depending on composition and cooling rate, which can change the cracking patterns present before such cracks can be measured. An apparatus was recently reported for acquiring 3D images via μCT of a specimen loaded in tension or compression at temperatures of 1500 – 1700°C in inert or oxidizing atmospheres (Fig. 4) [27]. Current maximum spatial resolution is 0.65 μm/voxel, yielding rich data on microstructure down to the fiber scale and μm-scale local failure mechanisms. Key data include variations of the opening displacements of fiber breaks and matrix microcracks as a function of load. Data for a monotonic tension test of an angle interlock carbon-SiC woven composite specimens reveal different mechanisms of failure operating at 25°C and 1700°C (Fig. 5). At the lower resolution (1.3μm/voxel) used in these images, individual carbon fibers were not resolved. Nevertheless, the fiber tows are clearly distinguished from the matrix, which consists of a thin brighter layer of CVI SiC coating each tow and a polymer-derived SiC filling the remaining
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