1347 Remaining Useful Life

Many efforts have been put forth to develop analytical models for life prediction of ceramic composites [60, 61]. However, these models are based on micromechanics and require the constitutive properties of each component in the material. That is, properties of the individual fibers, the matrix, and the coating on the fibers and are necessary for each condition (e.g., temperature, time at load, etc.) that the material will be expected to encounter. An alternative approach is to try to utilize data from NDE methods that could be coupled to a more

Fiber bundle 90° to axial fiber bundle

Matrix

Axial fiber bundle

Fiber bundle 90° to axial fiber bundle

Matrix

Axial fiber bundle

Region 2 Region 3 (b)

FIGURE 13.42 Diagram of stress-strain for typical SiC/SiC: (a) definition of regions and (b) schematics of damage expected in each region on stress-strain diagram.

Region 2 Region 3 (b)

FIGURE 13.42 Diagram of stress-strain for typical SiC/SiC: (a) definition of regions and (b) schematics of damage expected in each region on stress-strain diagram.

macromechanic model to predict remaining useful life with the proviso that the component would not be subjected to conditions too far outside certain bounds. One such approach is to measure elastic modulus of the material. Figure 13.42 shows a typical stress-strain diagram for an SiC/SiC material as well as diagrams depicting damage in each of the "damage zones." Clearly there is a change in the elastic modulus with the detail of region 2 defining the upper limit of the elastic region of region 1 and the second elastic region of region 3.

Recent work has shown that guided plate waves [62] can be used to measure in-plane elastic modulus and that this can track damage states. The experimental setup used for initial measurements is shown in Fig. 13.43. Three acoustic-emission transducers are used each with a 400-kHz center frequency. One transducer is used to generate an acoustic pulse and two other transducers, with well-defined separation distance, are used to measure the time of flight (TOF) of the elastic wave. This TOF measurement can be used to determine the elastic modulus, which in turn can be related to a damage parameter [63] as

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