Effects of the bond line thickness on the fracture mechanical behaviour of structural adhesive joints
The influence of the adhesive layer thickness on the fracture energy of the crash-optimized high-strength adhesive SikaPower498TM is evaluated at two independent laboratories using the same batch of adhesive. Three different methods (DCB and two different TDCB tests) are used to measure the mode I fracture energy. All methods give virtually the same fracture energy for small layer thicknesses. Here, the fracture energy increases with increasing layer thickness. This is in accordance with the results of other studies. For larger thicknesses, the results from the different methods deviate. It is argued that the larger flexibility of the adhesive layer is the major reason for this as the TDCB methods are based on the assumptions of a rigid adhesive layer. This assumption is more correct for a thin layer. In mode II, the fracture energy is measured also with several methods. The comparison of the results shows a good agreement among all applied methods. No severe deviations could be detected as it was the case in the mode I investigations. Also in mode II, the fracture energy increases with increasing thickness, which is in accordance with previous studies. In summary, an increase of the fracture energy with increasing adhesive layer thickness is detected for the adhesive SikaPower498TM at quasi-static loading conditions in both mode I and II. The results show that further studies are necessary to improve the methods to measure mode I fracture energy for thick layers. For thin layer, all methods studied here give consistent results. For thick layers, further studies are necessary to gain confidence in the methods. Finally, there is still a lack of knowledge concerning the mixed-mode failure of structural adhesive joints. The theory of a controlled mixed-mode test has been shortly presented here. First experimental results with those recently developed tests are expected in the near future.