13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- a) b) c) Figure 4. Effect of the geometrical parameters on the failure load of the joint =0.3 and a=0.4. The strength and the fracture toughness are: 45MPa, .45N/mm c c . In Fig. 4a the effect of the overlap length L on the failure load is shown. It can be seen, that the failure load increases with higher overlap lengths. As it is well known from experiments a less pronounced increase of the failure load is observed for larger overlap lengths. The basic mechanism is the increased bondline area with more equally distributed stresses. But as the peak stresses at the end of the overlap do not become arbitrarily smaller with larger overlap lengths, the increase is less pronounced for higher overlap lengths. The effect of the adherend height on the failure load is shown in Fig. 4b. An increase of the failure loads is observed with higher adherend heights as it is for larger overlap lengths. The basic mechanism behind this effect is the improved bending stiffness of the adherends. As the bending stiffness increases the peel stresses are distributed more equally. Fig. 4c shows the dependence of the failure load on the adhesive layer thickness. A decrease of the bearable loads is predicted for thicker adhesive layers. This is in very good accordance with knowledge on adhesive joints obtained from practice and experimental results. But it is likewise contrary to failure load predictions from failure models that base on strength of material approaches [6]. The key point is the consideration of energetic criteria. The adhesive layer stores, due its relatively low stiffness, the main part of the elastic strain energy. Hence, it gives the largest contribution to the energy release rate. If now the adhesive layer is increased, the incremental energy release rate increases as well. In this model both criteria, the stress criterion and the energy criterion must be fulfilled simultaneously and hence an increased energy release rate leads to reduced failure loads. This work shows that it is possible to explain the effect of adhesive layer thickness on the failure load by means of a simple linear elastic analysis by consideration of the energy release.
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