Directional stability of cracks

Fleck et al. [15] and Akisanya and Fleck [2,3] investigated this directional stability issue in adhesive bonds and concluded that, similar to the situation in homogeneous materials, the directional stability of cracks in adhesive bonds also depends on the magnitude of the T-stress when the bonds are under predominantly mode I loading. The crack propagation in an adhesive bond is directionally stable if the T-stress is negative and is directionally unstable if the T-stress is positive. This argument revealed the threshold of the transition of the directional stability of cracks in adhesively bonded joints and provided an important foundation for later studies. 

To eonfirm the theoretical predictions that the 7-stress affects the directional stability of cracks, DCB specimens with adhesive C and adherend thicknesses of 4.8 mm were prepared. The surface preparation for the adherends was simply an acetone wipe, and various levels of residual stress were achieved among the specimens using the stretching method. Specimens were then tested quasi-statically according to the procedure discussed earlier. The resulting fracture surfaces of the specimens were carefully examined and three representative specimens were selected as shown in Fig. 9, from which, the effect of the 7-stress on the directional stability of cracks can be inferred. The initial residual stress... 

Fig. 12. The effect of adherend thickness on the 7-stress level and the directional stability of cracks. The crack tends to be more directionally unstable when the thickness of adherend decreases.

Fig. 13. The effect of adhesive thickness on the directional stability of cracks in DCB specimens.

As pointed out by Pocius [60], the directional stability of cracks is significantly affected by the fracture toughness of adhesive bonds. An energy balance model used to analyze crack propagation predicted that directionally unstable cracks are more unlikely to occur as the fracture toughness of the adhesive bonds increase [32]. This energy balance model will be discussed in this chapter later. 

Energy balance and directional stability of crack propagation... 

All curves in Fig. 28 are monotonically decreasing with increasing adherend thickness, indicating an effect of adherend bending on the directional stability of cracks. For instance, the curve with gp = 1.3% and dashed line when H is less than 4 mm and is lower than the dashed line when H is greater than 4 mm. Since the dashed line represents the threshold of the directional stability of crack propagation, this curve indicates that for specimens with gp = 1.3% and oq = 13 MPa, the directional stability of cracks varies with the thickness of the adherends. This prediction is also consistent with the experimental results discussed in Section 2.3.1 where DCB specimens with adherend thicknesses H of mm and 3.2 mm, respectively, were tested to... 

In addition, Fig. 28 also indicates an effect of the toughness of the adhesive bonds on the directional stability of cracks. When the toughness of the bond increases, all the curves will shift down vertically, indicating that the transition between the directionally stable and unstable crack is less likely to occur. This result is again consistent with the experimental observations discussed in Section 2.3.8, which showed that as the rubber concentration in the adhesive increases, the fracture toughness of the bonds increases and consequently, the transition from directionally stable cracks to directionally unstable cracks is more unlikely to occur. 

These different failure locations lead to two important issues closely related to the fundamental mechanism of crack path selection in adhesive bonds locus of failure (either within the adhesive layer or at/near the interface) and directional stability of cracks (directionally stable or unstable). Fleck et al. concluded that, similar to the situation in homogeneous solid media, the directional... 

Failures modes in adhesives joints, attending the locus of failure and the directional stability of crack propagation, have been considered. Special attention has been paid to the microscopical features left on the fractured surfaces. Influence of the strain rate and the loading mode on the joint has been discussed. Fatigue and creep failure mechanisms have been briefly introduced. 

Finnic, I. and Saith, A., A note on the angled crack problem and the directional stability of cracks, Int. Journ. of Fracture, 1973, 9, 484-486. 

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