6424 Micromechanisms of Fatigue Crack Growth

A vital step to an efficient and safe damage design is the understanding of the fatigue characteristics of uMMCs. Most of the fatigue fracture micromechanisms, especially in Ti-based MMCs, include fiber failure ahead of the crack tip [74, 78], the propagation of the matrix crack under conditions of crack deflection at fibers [8], matrix crack shielding by the fibers [7], crack bridging by unbroken fibers [79-81], and fiber pull-out produced by broken fibers [82].

Fiber Failure Ahead of Crack Tip The issue of broken fibers ahead of the crack tip in uMMCs has a significant engineering interest since it incorporates fatigue damage with the residual strength of the material. In general fiber failure ahead of the crack tip has to be initially distinguished into two categories: (a) fiber failure prior to debonding and (b) fiber failure after debonding. To understand the idea of fiber failure prior to debonding, let us define two possible damage scenarios. The first scenario is that the normal stress sustained by a fiber close to the crack tip is significantly increased by a single overload. In this case, depending on the amount of the overload and the ability of the material to redistribute stress through the interfacial shear stress, a number of successive fibers close to crack tip may fail while others will experience severe debonding [69]. Both events will provide a significant loss of strength. The second scenario is based on the presence of fiber warts. Generally fiber warts play the role of stress raisers at the fiber without promoting interfacial debonding as in the case of broken interfaces [68]. On the other hand, postdebonding fiber failure is likely to happen due to stress concentration at the transition point between debonded and intact interface. It should be noted that such an event is likely to happen at long crack lengths where the operational life of the material is almost consumed [10]. This event may exhibit notable fiber pull-out. All the above are graphically shown in Fig. 6.14.

In general fiber fracture ahead of the crack tip could take place when the local stresses exceed one of the following three failure criteria [8]:

1. The normal stress oyy acting along the fiber exceeds the fracture strength of the fiber. This case is of particular interest when the fiber matrix interface is very strong and thus fiber failure is to be expected instead of interfacial debonding. This is not the case for the SCS-6/Ti composite systems in the as-fabricated condition (since the fiber attains a high value of Weibull modulus and no stress concentrations are developed due to changes on the reaction layer thickness). However, this mechanism may occur when the material is highly heat treated [72]. The stress oyy is usually computed using elastic crack tip stress fields:

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