Fatigue life enhancement of welded stiffened S355 steel plates with noncircular openings

• The object is welded stiffened plate containing T-shape girders and stiffeners.
• A critical place is the roundings of the non-circular openings made in the girders.
• Beneficial residual stresses are introduced around the roundings.
• Residual stress redistribution is studied experimentally and numerically.
• A multi-objective optimization task is set and solved.

The paper is devoted to fatigue life enhancement of S355 steel welded stiffened plate containing T-shaped stiffeners and girders with non-circular openings. Its rounded corners are natural stress concentrators and potential places for initiation and growth of first-mode fatigue cracks. To slow down this process, beneficial residual compressive stresses are introduced around the non-circular opening corners through mandrel cold working of preliminarily drilled round holes in the corners' zone. The creation through cutting of the non-circular openings and subsequent welding of the T-shaped girders generates residual stress redistribution. The latter is studied both experimentally and numerically. X-ray diffraction method has been employed for residual stress analysis. The mandrel cold working process and subsequent cutting to form the non-circular opening have been simulated through a nonlinear finite element method (FEM) analysis. In order to simulate the residual stress redistribution due to welding of the T-shaped girders with non-circular openings, a sequentially coupled thermal-stress FEM analysis has been carried out. In order to identify the optimal value of the mandrel cold working interference fit (the difference between the mandrel diameter and the initial hole diameter) and welding sequence, a multi-objective optimization task is set and solved. The optimization has been based on a planned numerical experiment. The optimal value of the interference fit and the welding sequence have been found, which ensure high intensity and homogeneity of the residual compressive field around the opening roundings.