Partially debonded interface

Based on the same average fiber tensile strength model as that employed in Section 4.2.3, the fiber fragmentation criterion is derived in terms of the external stress, ffa(= (h = o er, for the partially debonded interface ... 

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. 4.16. Variation of mean fiber fragmentation length, 2L, versus applied strain, , in the partially debonded interface model for Tb = 50 MPa. After Kim et al. (1993b).

Wu W, Jacobs E, Verpoest I and Vania J (1999) Variational approach to the stress-transfer problem through partially debonded interfaces in a three-phase composite, Compos Sci Technol 59 519-535. 

Interface debond criterion and partial debond stress... 

To show clearly how and to what extent the parameter, Zmax. varies with the properties of the interface and the composite constituents, a simple fiber pull-out model by Karbhari and Wilkins (1990) is chosen here. This model is developed based on the assumption of a constant friction shear stress, Tfr, in the context of the shear strength criterion for interface debonding. In this model, the partial debond stress may be written as... 

Equation 1 assumes that the shear stress at the interface is constant as a result of complete interfacial debonding. With good adhesion, only partial debonding or other micro-mechanical events such as transverse matrix cracking are observed, which invalidate the assumption of a constant interfacial shear stress. As a result, alternative data reduction techniques have been developed. For example, Tripathi and Jones developed the cumulative stress-transfer function, which deals with the limitations given above. This has been further refined by Lopattananon et al into the stress-transfer efficiency from which an ineffective length of that fibre in that resin can be determined. In this model, the matrix properties and frictional adhesion at debonds can be included in the analysis. It is also possible to use the three-phase stress-transfer model of Wu et al to include the properties of an interphase. 

Figure 10.24. Schematic description of possible fracture mechanisuis of CNTs (a) - ideal state of the CNT (b) - pullout caused by CNT/matrix deboudiug in case of weak interfadal adhesion (c) - rupture of CNT- strong interfacial adhesion in combination with extensive and fast local deformation (d) - telescopic pullout-fracture of the outer layer due to strong interfacial bonding and pullout of the inner tube and (e) - bridging and partial debonding of the interface - local bonding to the matrix enables crack bridging and interfricial failure in the non-bonded regions [67]...

From the discussion presented above, it is clear that the stability of the debond process can be evaluated by a single parameter, Zmax, which is the shortest (remaining) bond length needed to maintain the debond process stable, and is a constant for a given composite system. Therefore, three different interface debond processes are identified in the following totally unstable, partially stable and totally stable debond processes. The schematic plots of the applied stress versus displacement curves are illustrated in Fig. 4.25 for these debond processes. 

Figure 7.37 Schematic time-position dia- (Adapted from Ecault et al. (2013)). (a) Intact grams showing ID-shock wave propagation adhesion, (b) Partial interface debonding in a Ti6AI4V substrate coated with a Ti02 triggered by dynamic tensile stress induced bond coat and a hydroxyapatite top coat. by a laser-generated shock wave.

If the fiber-matrix interfacial adhesion is poor, debonding at the fiber-matrix interface occurs, followed by pulling out of fibers from the matrix see case (a) in Fig. 7.2. If partial interfacial adhesion is present, fiber gliding occurs (case (b) in Fig. 7.2), and if interfacial adhesion is strong enough, fiber breakdown results see case (c) in Fig. 7.2 [1]. 

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