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FIGURE 6.24 Typical sources of initial crack like defects for SCS-6/Ti-15-3 uMMC: (a) Fiber warts, (b) voids at interface, (c) broken fibers at edges, (d) touching fibers. (Photos taken from [68]).

FIGURE 6.24 Typical sources of initial crack like defects for SCS-6/Ti-15-3 uMMC: (a) Fiber warts, (b) voids at interface, (c) broken fibers at edges, (d) touching fibers. (Photos taken from [68]).

From a structural point of view, those favorable to initiate fatigue cracks are: (a) broken fibers at the edges (damaged during machining), (b) voids in the interface, (c) broken reaction layers, (d) warts on the fibers, and (e) fiber touching (Fig. 6.24).

In general, manufacturing defects in uMMCs are divided into two categories, the surface defects and the through-thickness defects. Undoubtedly, surface defects (broken fibers and touching fibers) can be easily identified by microscopic inspection after minimum surface preparation. The number of broken fibers per surface unit (the surface unit consists of a rectangle with fiber centers at the corners) can provide useful information regarding the uniformity and concentration of such defects, (Fig. 6.24d). On the other hand, to reveal through-thickness defects (fiber warts, voids, etc.) careful surface etching until the fiber layer should be employed. The quantification of such defects, due to the lack of a specific recommendation, is subjected to personal assessment. In [69], however, it was suggested that the number of voids per critical fiber length could provide some severity indication of these defects.

Undoubtedly, surface defects are in direct connection to the fatigue endurance of the material. This is because broken fibers at edges, touching fibers, and most importantly their combinations could host significant stress concentrations. Such concentrations are represented by circular, semicircular, or quarter-elliptical shape notches [109, 110].

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