Fatigue of structures and secondary bending in structural elements

Secondary bending occurs in structural elements with geometric eccentricities when the element is loaded in tension. Eccentricities are present in lap joints, but also in plates with a locally increased thickness. Due to the eccentricities out-of-plane displacements occur with local bending as a result. This secondary bending phenomenon is unfavourable for the fatigue properties of a structure. In the paper secondary bending is analyzed with FE calculations and also with a neutral line model. A bending factor is defined as the ratio of the bending stress and the nominally applied tensile stress. Numerical data are presented for riveted joints and for an open hole with a reinforced edge around the hole. Stress concentrations are significantly enlarged by the bending stress. This problem should be considered for designing against fatigue, but secondary bending has received limited attention so far.