Debond length

The interface debond criterion used in this analysis is based on the concept of fracture mechanics where the strain energy release rate against the incremental debond length is equated to the interface fracture toughness, Gk, which is considered to be a material constant... 

Debond length and mean fiber fragment length... 

Fig. 4.8. Variations of debond length, t, as a function of applied stress, for different coefficients of friction, p, for a XAIOO carbon fiber-epoxy matrix composite. After Zhou et al. (1995a, b).

Determination of the crack tip debond stress, ae, at a debond length, , is contingent to the condition that the fiber axial strain is equivalent to the matrix axial strain at the boundary between the bonded and debonded regions (i.e. duf z)/dz — diP a,z)/dz at z = (L — )). Within the debonded region, the matrix axial strain at the interface is greater than the fiber axial strain due to the relative slip between fiber and matrix. Therefore, combining Eqs. (4.8), (4.9) and (4.61) at the boundary, is obtained from... 

The basic requirement necessary to satisfy the partially debonded interface is that the crack tip debond stress, cr, (and the debond length, ) must be greater than zero. From the debond criterion given by Eq. (4.68)... 

Fig. 4.14. Plots of applied stresses required for interface debonding, <7od (solid lines), and for fiber fragmentation,

Fig. 4.15. Plots of interface shear bond strength, tb, as a function of normalized debond length, H/a, illustrating the areas corresponding to debonding only (region A), fiber fragmentation without further...

Tf is the crack tip debond stress at the boundary between the bonded and debonded regions at z = f, as defined in Section 4.2.3. It should be noted, however, that the actual values of (Tf are different for different specimen geometry even for an identical debond length, t Therefore, the solutions of FAS, and the corresponding MAS, MSS and IFSS are obtained for the bonded region ( [Pg.129]

Partial debond stress, is the applied fiber stress during the progressive debonding process that may be written as a function of the debond length, f, and the crack tip debond stress, <7f, from Eq. (4.89)... 

It follows that consists of two stress components a crack tip debond stress, at, and a friction stress component, at is not only a function of the interfacial fracture toughness, G c, but is also dependent on the debond length, t, relative to the total... 

Fig. 4.23. Plot of parlial debond stress, as a function of debond length, (. for a carbon fiber-epoxy matrix composite. After Kim ct al, (1992).

Fig. 4.24. Plot of partial debond stress, oJJ, as a function of debond length, f, for untreated SiC fiber-glass matrix composite. After Kim et al. (1991).

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.,... 

The maximal debond stress, is determined immediately before the load instability (Karbhari and Wilkins, 1990 Kim et al., 1991) of the partial debond stress, (Tj, when the debond length becomes = L — z a ... 

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 ... 

Similarly, the initial debond stress, oq, is obtained for the infinitesimal debond length, the maximum debond stress, initial friction pull-out stress, [Pg.153]

Fig. 4.43. Growth of debond length, (, with increasing number of cycles, N, for (a) fiber pull-out and (b) fiber push-out. Initial debond length t = lOmm. After Zhou el al. (1993).

Figs. 4.44 and 4.45 show the increase in the debond length, f, and displacement, as a result of the reduction of p (from Po = 0.22 to p = 0.07) under cyclic loading. It is interesting to note that both I and 5 remain constant until the coefficient of friction, p, is reduced to a critical value p. (= 0.144 and 0.166, respectively for fiber pull-out and fiber push-out). The implication is that the debond crack does not grow... 

Fig. 6.12. Toughness maps depicting contours of predicted fracture toughness (solid lines in kJ/m ) for (a) glass-epoxy composites as a function of fiber strength, Uf, and frictional shear stress, tf and (b) Kevlar-cpoxy composites as a function of at and clastic modulus of fiber, Ef. The dashed line and arrows in (a) indicate a change in dominant failure mechanisms from post-debonding friction, Rif, to interfacial debonding, Sj, and the effect of moisture on the changes of Of and Tf, respectively. Bundle debond length...

Fig. 7,6. (a) Transverse impact fracture toughness and (b) interface debond length versus testing temperature for carbon fiber-epoxy matrix composites with and without PVAL coalings on fibers. After... 

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