E

Eq where D = damage parameter

E = measured elastic modulus after the material has been subjected to various road conditions E0 = elastic modulus of the new material prior to any exposure to loads

Two types of damage have been used for initial studies: impact and cyclic fatigue. In the first example, an SiCf /barium-magnesium aluminosilicate matrix material was studied. Repeated pendulum-type impacts with 235 J per impact were induced in a 1-cm-wide specimen. The damage was assessed using the

Digital Oscilloscope

Digital Oscilloscope

Transducers

FIGURE 13.43 Schematic diagram of the guided plate wave data acquisition system.

Transducers

FIGURE 13.43 Schematic diagram of the guided plate wave data acquisition system.

Number or Impacts

FIGURE 13.44 Measured accumulated damage as function of number of impacts for 8-ply SiC/SiC.

Number or Impacts

FIGURE 13.44 Measured accumulated damage as function of number of impacts for 8-ply SiC/SiC.

guided plate-wave method. The assessed damage after each impact is shown in Fig. 13.44. Two regions on the sample were studied. The high region, which included the impact-damaged region, and a low region away from the impact. Clearly the damage region is detected.

For the second set of tests, a specimen of the same material was used in tension-tension cyclic fatigue to induce damage. Again the experimental setup was used. However, data were obtained for regions under two load conditions: while under tensile load and with no load. Figure 13.45 shows the measured

FIGURE 13.45 Accumulated damage induced through load/unload/reload tensile tests corresponding to three regions of the stress-strain curve shown in Fig. 13.41.

Stress (MPa)

FIGURE 13.45 Accumulated damage induced through load/unload/reload tensile tests corresponding to three regions of the stress-strain curve shown in Fig. 13.41.

damage as a function of cyclic load stress. To be noted are two observations: (a) that the damage parameter clearly detects the three regions of the stress-strain curve and (b) that when the specimen is under load, the damage parameter is significantly higher. This is because the cracks are kept open at load and this impacts the elastic wave speed.

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