Influence of mean stress on the fatigue of welded tubular structures under uniaxial fatigue loading

Abstract Welded hollow section joints are essential for weight reduction in load-bearing structures of agricultural machinery and cranes while maintaining high load capacity. In mobile and gantry cranes, their favorable structural properties enhance durability and significantly reduce overall weight, thereby improving maneuverability and energy efficiency. In these applications, primary loads induce cyclic stresses with either tensile or compressive mean components, which interact with residual stresses from the welding process. In the nominal stress approach of existing codes and guidelines, mean stress effects are typically disregarded due to the assumption of high residual tensile stresses. The structural stress method outlined in the CIDECT Design Guide also does not account for residual or mean stresses, nor does it differentiate between material grades. In contrast, the effective notch stress concept considers mean stress effects via enhancement factors but assumes stress ratio independence for non-stress-relieved and complex welded structures. To address these limitations, fatigue tests with wall thicknesses below 10 mm were conducted within the FOSTA P 1603 research project at Munich University of Applied Sciences. The tests focused on X-joints made of rectangular and circular hollow section tubes using S355J2H and high-strength S620QH steels. The joints were tested under up to four different load ratios. The results are evaluated using the nominal stress and local stress fatigue approaches to identify discrepancies in current guidelines and to develop more accurate, less conservative recommendations. This paper presents the experimental findings and highlights the need for improved fatigue assessment methods for welded hollow section joints.