K

V2nr

FIGURE 6.14 SEM micrograph showing fiber pull-out for 32% SCS-6/Ti-15-3 [0]8 tested at amax = 800 MPa, R = 0.1. The black line represents final crack length. (Photo taken from [69]).

2. The normal stress oyy exceeds the axial debonding strength of the fiber-matrix interface. In this case, the load carried by the interface is transferred to the fiber before debonding is achieved, and so fiber failure could take place.

3. Interface shear debonding occurs when the interface shear stress Txy exceeds the interfacial shear strength. In this case, the shear load sustained by the interface is transferred to the fiber. The change of the stress carried by the fiber due to debonding is given as [83]:

K ld

where t is the average shear strength of the interface, ld is the debond length, and D is the fiber diameter.

It should be noted that Eqs. 6.8 and 6.9 can be effectively used only in cases where small-scale yielding conditions prevail.

Crack Deflection of Main Crack to Fiber-Matrix Interface The joining of the main matrix crack with a preexisting interface crack leads to a kinked crack deflected to propagate along the fiber-matrix interface. Crack deflection generally reduces the mode I crack driving force, since the crack path is now deviated from the direction of maximum tensile stress [8]. The mixed-mode solution proposed by Cotterell and Rice [84] for a simply kinked crack subjected to a nominal Kl stress intensity factor in a monolithic material can also be used in the case of MMCs [8]. Clearly, the corresponding stress intensity factor of a mixed-mode crack is computed in terms of the individual local K tensors using the expression

0 0

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