Browsing by Author "Ramsay, Gareth Allan"

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    The Influence of High Strength Steel on the Fatigue Life of Welded Joints in the Automotive Industry.
    (Stellenbosch : Stellenbosch University, 2022-04) Ramsay, Gareth Allan; Venter, Gerhard; Bredell, JR; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH SUMMARY: The ability to predict and protect against fatigue failure of structurally critical components, while still being able to produce high performance and costefficient designs is of great importance to the automotive industry. This study investigates the influence of high strength steels on the fatigue life of welded joints commonly used in the automotive industry, and compares the experimental fatigue data to commonly used fatigue design approaches, namely the BS 7608 approach and Shigley’s approach. Two joint details of interest are considered, namely a non-load bearing fillet welded T-joint and a load bearing fillet welded cruciform joint. Each joint geometry has three different base and filler material combinations, with varying material strengths, i.e. a total of six different specimen configurations. Two material combinations have a high strength steel (Strenx® 700 MC D) for the base material, with one combination having a matched filler material and the other having an undermatched filler material. The third material combination has a lower strength steel (S 355 JR AR) for the base material, with a matched filler material. Tensile tests were performed to confirm the base material mechanical properties and weld quality of the manufactured specimens. The welded specimens of both joint geometries were fatigue tested and the obtained data was used to generate experimental S-N curves. The experimental S-N curves for each joint geometry and material combination were compared with each other, as well as with the two fatigue design approaches. The investigation showed that there is no significant benefit to using high strength steel as the base material for fatigue loaded welded joints. Moreover, in some cases the use of high strength steel actually proved to be detrimental to the fatigue performance of the joint, compared to the use of a lower strength steel (e.g. T-joint with non-load bearing weld attachment). The results suggest that the fatigue performance of a non-load bearing weld attachment (T-joint) is highly dependent on material combination and strength, with lower strength base materials offering better fatigue performance than higher strength base materials. In this case the material combination and strength seems to be the dominating factors in fatigue performance over joint geometry. In contrast, the load bearing welded joint (cruciform joint) was shown to be less dependent on material combination and strength, with the joint geometry potentially being the more dominating factor on fatigue performance, as shown by the similarity in fatigue performance with the varying material combinations. The BS 7608 design curves generally tended to be quite conservative depending on the joint geometry and material combination considered. The BS 7608 does not account for material strengths and this could be why its fatigue performance predictions are quite conservative in certain cases. In general, Shigley’s material strength dependent approach overestimated the fatigue performance of the investigated welded joint details and is therefore not recommended for use in the fatigue design of these joints.