An investigation of distortion-induced fatigue cracking under variable amplitude loading using 3D crack propagation analysis

• The crack growth rates are different in the three directions in the detail.
• The distortional cracking is without question a common problem in bridges.
• The behaviour of the cracks is totally controlled by the deformation.
• The studied crack direction criteria yielded the same crack formation.
• The results show the importance of accurately defining the bridge load.

The distortion-induced fatigue cracks in the welded details of the Söderström Bridge are analytically and numerically investigated by performing 3D crack propagation analyses with variable amplitude fatigue loading. In the crack propagation analyses, the effects of bridge loading are defined on the basis of the field measurements in order to simulate crack growth and predict the residual fatigue life of the studied detail as accurately as possible. The effect of crack closure and crack direction while considering the most common criteria is also studied. The results are compared with those obtained from the crack propagation analyses with constant amplitude fatigue loading presented in Aygül et al. [1].The results of the crack growth simulations with variable amplitude fatigue loading have generally shown good agreement with the real crack formation and reveal that the crack growth rates are different in different directions. The crack behaviour in the damaged detail is mainly controlled by the loading and geometrical arrangement of the detail components. There is generally a significant difference between constant and variable amplitude fatigue crack growth analyses and the variable amplitude fatigue crack growth analyses yield more conservative results. The main reason for this difference is the bridge loading and the number of stress cycles defined in the analyses. The crack direction criteria studied in this investigation showed basically the same crack formation and crack growth rate.