Maximum debond stress

The instability condition requires that the derivative of the partial debond stress with respect to the remaining bond length z = L — j k equal to or less than zero, i.e., dtr /dz O (Kim et al., 1991). Therefore, the fiber debond process becomes unstable if L - t) is smaller than a critical bond length, Zmax. where the slopes of the curves become zero in Figs. 4.23 and 4.24. At these bond lengths, the partial debond stress, [Pg.135]

Fig. 4.26. Comparisons between experiments and theory of (a) maximum debond stress, trj, and (b) initial frictional pull-out stress for carbon fiber-epoxy matrix composites. After Kim et al. (1992).

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]

In the case of a debonding involving the first layers of the masonry and with anchor lengths equal to or larger than the optimum length, the factored tensile stress within the reinforcement, that is, the value of the maximum tensile stress at which the reinforcement can work on the ends of the anchor section, once the transfer of stresses from the masonry to the FRP reinforcement has taken place, is as follows ... 

Failure may occur when the maximum shear stress reaches the interface strength (tiu), which has a maximum absolute value at jt = 0, that is, at the surface where the fiber leaves the block of polymer. The debonding force is then Fi = nr Ofe. From Eq. 4 is obtained ... 

Comparing the failure mechanisms predicted, matrix shearing and fiber/matrix debonding seem to be the most dominant ones at the beginning of the wear process. They are followed by fiber cracking, as a further wear phenomenon, because the maximum compression stress at the top of the fibers is close to the compression strength limit. 

An important consideration is the effect of filler and its degree of interaction with the polymer matrix. Under strain, a weak bond at the binder-filler interface often leads to dewetting of the binder from the solid particles to formation of voids and deterioration of mechanical properties. The primary objective is, therefore, to enhance the particle-matrix interaction or increase debond fracture energy. A most desirable property is a narrow gap between the maximum (e ) and ultimate elongation ch) on the stress-strain curve. The ratio, e , eh, may be considered as the interface efficiency, a ratio of unity implying perfect efficiency at the interfacial Junction. 

One of the major differences between the results obtained from the micromechanics and FE analyses is the relative magnitude of the stress concentrations. In particular, the maximum IFSS values at the loaded and embedded fiber ends tend to be higher for the micromechanics analysis than for the FEA for a large Vf. This gives a slightly lower critical Vf required for the transition of debond initiation in the micromechanics model than in the FE model of single fiber composites. All these... 

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