Initial debond stress

In light of the foregoing discussion concerning the functional partitioning of the partial debond stress, the characteristic debond stresses can be evaluated. The initial debond stress, ao, is obtained for an infinitesimal debond length where the frictional stress component is zero, i.e.,... 

Similarly, the initial debond stress, oq, is obtained for the infinitesimal debond length, the maximum debond stress, debond length becomes = L - Zmax and the post-debond initial friction pull-out stress, [Pg.153]

Fig. 4.39. Comparisons of initial debond stress,

Details of the instability conditions of the debond process and Zmax are discussed in Section 4.3.4. Further, the solution for the initial frictional pull-out stress, (T, upon complete debonding is determined when the debond length, , reaches the embedded length, L, and the crack tip debond stress, ai, is zero ... 

Fig, 4.27. Comparisons between experiments and theory of (a) maximum debond stress, oj, and (b) initial frictional pull-out stress for steel fiber-epoxy matrix composites (O) uncoated fibers ( ) release agent... 

It is clear that ab will be extremely large for inclusions of small radius r, even if the level of adhesion, represented by Ga, is relatively small, only of the order of magnitude of van der Waals attractions. For example, when E is assumed to be 2 MPa, representative of soft elastomers, and Ga is given the relatively low value of 10 J/m, then the critical applied stress for detachment is predicted to reach a magnitude similar to E when the radius of the inclusion is reduced to about 20 ixm, even if the initially debonded zone is as large as feasible, 6 — 45°. These considerations appear to account for some features of reinforcement of elastomers by particulate fillers Fillers (140). 

It should be noted that these methods work well for inferring the interfacial crack propagation behaviour, but they cannot measure the conditions under which a crack will initiate. Estimates of the interfacial shear stress to initiate debonding can be obtained from indentation tests using an approximate stress analysis in which the coating is assumed very thin, so that stresses are approximately constant through the thickness [34]. In this model [58], the plastic zone below the indentation is modelled as a cylinder, which is replaced conceptually by a hole under an internal pressure (see Fig. 4), p, which is related to the indentation hardness of the material ... 

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