Crack trajectory prediction

In this section, discussion focuses on the interface fracture mechanics and the details of crack trajectory predictions that are possible with numerical implementation of these concepts. According to the interface fracture mechanics theory discussed in Chapter 2, a crack at the interface between the adherend and adhesive can be represented by a sub-interface crack lying a small distance (St) below the interface and the complex stress intensity factors K and K2 for the interface crack are related to the conventional stress intensity factors and Ku for the sub-interface crack as... 

Fig. 33. The crack trajectory predicted by the finite element analysis using Franc2dl for the adhesive C system studied. The result reflects the overall characteristics of the actual crack trajectory such as the characteristic length of the crack as shown in the SEM micrograph.

Fig. 29 shows that due to the material mismatch, when the distance between the sub-interface crack and the interface 8t approaches zero, the corresponding component at the crack tip is veiy high and is acting in such a direction that the crack tends to deviate away from the interface toward the centerline of the bond. As the distance ht increases, the corresponding K value drops drastically, which suggests that the sub-interface crack will deviate from the interface in a rather gradual fashion. This prediction is consistent with crack trajectory shown in the SEM micrograph of the DCB specimens with directionally unstable cracks in Fig. 10. Since differences in the material mismatch will result in variations in the stress distribution. Fig. 29 also indicates that the crack propagation behavior will also be different for different materials systems.

The final locus of failure and fracture trajectory of an adhesive bond is the result of the interactions among the material properties such as the tensile strength of all the components, quality of adhesion at the interface, fracture toughness of the bonds, and the stress state at the crack tip of existing flaws or debonds [4]. To predict the locus of failure in an adhesively bonded joint loaded in an arbitrary manner such as illustrated in Fig. 1, the mechanisms controlling the... 

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