Experimental and numerical investigations on the mean stress effect on the fatigue of thin-walled tubular joints

Abstract Welded hollow section joints play a key role in lightweight automotive design, particularly in applications involving buses, trucks, and special-purpose vehicles. High-strength steels enable significant efficiency gains, especially when used in thin-walled constructions that fully exploit their load-bearing potential. In such applications, rectangular hollow section joints with a chord to brace ratio of 1 are widely used and exhibit favorable fatigue performance. However, accurately assessing the fatigue strength of these joints remains a challenge, particularly due to mean stress effects arising from service loads and welding-induced residual stresses. Current design codes address these effects inconsistently: the nominal stress approach often neglects them or provides conservative estimates, while the structural stress concept defined in ISO 14347 is not suitable for wall thicknesses below 4 mm. The effective notch stress approach, although more refined, assumes R-ratio-independent behavior for non-stress-relieved joints and does not differentiate between steel grades. To address these limitations, fatigue tests were conducted within the FOSTA P1603 research project at Munich University of Applied Sciences on X-joints under varying load ratios. The results were supplemented with numerical analyses to refine the assessment methods used in the nominal and the effective notch stress approach. This study contributes to a more accurate fatigue evaluation of thin-walled, high-strength steel joints and aims to extend the applicability of current design guidelines.