
The aim of the research project was to analyse and compare different test concepts in accordance with the state of the art and to use the results to develop a practice-relevant test methodology for the safe evaluation …
The aim of the research project was to analyse and compare different test concepts in accordance with the state of the art and to use the results to develop a practice-relevant test methodology for the safe evaluation of the crash behaviour of bonded and hybrid-joined sheet steel joints. This test methodology was then to be used to determine the crash behaviour of sheet steel joints, taking into account different adhesive-specific boundary conditions. As a basis for the development of a simple test methodology, extensive tests were carried out on different specimen shapes in order to determine their suitability for determining crash parameters for predicting component behaviour.
The KSII peel tensile test specimen proved to be suitable for determining characteristic values under impact load with significance for the component. This applies in particular to hybrid joints, where adhesive bonding is combined with a mechanical joining process or spot welding. A single overlapped tensile shear specimen was selected to determine the characteristic values for the case of an impact-type tensile shear load. Based on the test methodology, several test and adhesive-specific parameters were systematically varied in a comprehensive test programme in order to investigate their influence on the behaviour of the bonded and hybrid-joined joints at high loading speeds and to obtain information for the structural design. With regard to the influence of the test speed, it was found that higher strain rates lead to an increase in the material, adhesive and joint strengths. The material of the parts to be joined only has a major influence on the load-bearing behaviour under peel tensile load if plastic deformation of the parts to be joined occurs. In contrast to the quasi-static tests, the peel tensile tests at high speed showed an increase in the maximum force with the sheet thickness. The influence of temperature is even more pronounced under impact loading than in the quasi-static case and must therefore be taken into account in the design. A possible reduction in joint strength due to ageing could be demonstrated to the same extent under impact loading as under quasi-static loading. With regard to the influence of the joint zone geometry, it was determined, among other things, that an increase in the joint filling leads to an increased peel tensile force under both quasi-static and impact loading. In contrast, the influence of a small variation in the adhesive layer thickness on the tensile shear strength can be neglected. The transferability of the investigations was comprehensively verified by accompanying experiments on Z-profiles and real components. This also demonstrated the performance of the adhesives used in comparison to conventional spot welding. The combination of spot welding and adhesive bonding, for example, reduced the permanent deformation of a B-pillar by more than 25% compared to the purely spot-welded reference samples. Even when the number of spot welds was reduced to approx. 45% of the number used in the series, the permanent deformation of the combined joined samples was still 20% lower than that of the purely spot-welded B-pillars. The characteristic values determined with the KSII peel tensile test showed a uniform order of evaluation of the adhesives regardless of the variation in test speed, joining part materials, sheet thickness and joining part material coating. This sequence is consistent with the results of the component tests. The cost-effective laboratory test on the peel tensile test, which was further developed in the project, therefore represents an efficient method for This means that the low-cost laboratory test on the peel tensile specimen developed in the project is an efficient method for assessing the crash suitability of adhesives when the adhesive layer failure in the component is determined by peeling stress. The test methodology developed in this project created the basis for a standardised assessment of high-strength adhesives for use in crash-loaded sheet steel structures. The results thus obtained already form a comprehensive basis for the design of such structures and the future integration of FE simulation of bonded joints under crash load into the development process.

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