Low-cycle fatigue of fillet-welded steel plate connections

The present study focuses on the mechanical behavior of fillet-welded steel plated joints and the development of a strain-based fatigue assessment procedure, motivated by the seismic response of unanchored steel tanks that exhibit repeated uplifting, leading to low-cycle fatigue failure of the base plate connection with the tank shell, due to cyclic inelastic deformation. Low-cycle fatigue experiments on small-scale fillet-welded joints are performed, representing the welded connection the tank base plate with the tank shell, aimed at determining the relationship between the strain range developed at the welded vicinity and the corresponding number of cycles to failure. Prior to fatigue experiments, material tests have been conducted, to determine the mechanical properties of the base plate material, and the weld has been examined with stereo optical microscopy and micro-hardness measurements. In addition to experiments, numerical simulations are also performed, to elucidate special features of joint behavior, and calculate accurately the local strain at the weld region, complementing the experimental results and observations. The numerical simulations employ an advanced cyclic-plasticity model, based on the bounding-surface concept, implemented into the finite element model using an in-house material user subroutine. The experimental and numerical result enable the development of a strain-based fatigue curve, which can be used for the seismic design and assessment of liquid storage tanks. It is further concluded that the fillet-welded connections under consideration are capable of sustaining substantial strain levels for a significant number of cycles, until low-cycle fatigue failure occurs.