Influence of surface roughness on the shear strength of direct injection molded plastic-aluminum hybrid-parts

The strength of hybrid metal and plastic joints is strongly influenced by the surface roughness of metal. Although many investigations on the change in shear strength of adhesively bonded joints due to roughening have been published, it is not completely understood how different mean roughness indexes Ra of directly joined plastic-metal-parts correlate to the resulting shear strength. This paper describes a schematic roughness - shear strength curve for adhesively bonded specimens and an experimental one for direct injection molded hybrid specimens which is reconciled with the state of the art. Roughening the surface of the metal is realized by grit blasting using fused alumina. After that, the metal is coated by direct injection molded PA 6.6 with 30% short glass fiber and shear strength tests are carried out. It can be concluded that with increasing Ra the shear strength of adhesively bonded specimens increases to a first maximum, then decreases and finally increases again. Direct injection molded samples exhibit a similar trend. The difference is that the second increase is significantly more pronounced, leading to a second maximum which is nearly twice as high as the first one. This difference is explained by the lower strength of the adhesive compared to the plastic and the different pronounced so called surface area effects, notch effects and thickness effects. By increasing Ra they promote cohesive fracture which is detrimental for adhesively bonded but more beneficial for direct injection molded samples. To further explain the experimental results, a FEM model to predict the probability of failure, the expected stress and the failure mode for direct injection molded samples is developed. The FE-analyses confirm the explanatory approach on the joint strength in dependence of the Ra value.