Research Field

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 One of the main concerns in the structural integrity of offshore structures is mechanical damage from external loads. Offshore structures are exposed to fatigue failure in welded joints due to geometric discontinuity. In addition, fatigue loads such as currents, waves, and platform motions may cause significant plastic deformation and fracture within low-cycle regime. The 2007 ASME Div.2 Code adopts the master S-N curve for the fatigue evaluation of welded joints based on the mesh-insensitive structural stress. An extension to the master S-N curve was introduced to evaluate the low-cycle fatigue strength. In this research, we have extended various material behavior to structural strain method. Also, the improved prediction method was validated using fatigue test data.

 To ensure the safety and effectiveness of welded structures, it is important to know their fatigue strength. Large welded structures such as offshore structures and ships are subject to cyclic loading, which may cause fatigue damage. However, the fatigue strength is significantly affected by the welding procedure during the fabrication process. To relieve this effect, high-frequency mechanical impact (HFMI) post-treatment is widely used in steel bridge and power plant construction. This study introduces a numerical procedure for the fatigue strength estimation of HFMI-treated welded joints considering the weld toe magnification factor Mk and the stress ratio model. The target is a butt-welded joint, and Mk is obtained from the geometric profile at the weld toe in HFMI-treated conditions. The proposed method improves the accuracy of fatigue strength prediction and was validated with experiment data.

 Lately structural bonding technology uses adhesives to bond and hold parts through chemical/mechanical surface adhesion forces. It has the advantage of being lightweight and preventing corrosion of materials. The single lap joint with the dimensions and the material shown in the Figure above. It was modelled in ABAQUS standard finite element code. As shown in figure the overlap length was 30mm, the free length was 75mm, the substrate thickness was 4.73mm, the substrate width was 12.5mm and the thickness was 0.2mm.

 A non-linear finite element model was developed to examine the SLJ under static tensile loading and predict the static strength. It was found that the results of three-dimensional model were more consistent with the plane stress results than the plane stain results. Consequently, four-noded plane stress elements(CPS4) were employed for the aluminium substrates and four-noded cohesive elements(COH2D4) with the traction-separation description were used to study the progressive damage in the adhesive. Moreover, in this static simulation, process zones were small and once a crack appeared at the end of overlap, it propagated very quickly leading to a catastrophic rupture.

 Welding structures have various flaws during construction and operation due to weld defects, fatigue loads, corrosion, etc. It is very important to analyze the fracture properties of materials for the evaluation of structural integrity of weld structures where defects exist. In particular, the fracture behavior of materials is an important parameter necessary for predicting the lifetime and repair timing of structures. From this perspective, various studies have been conducted on the prediction of fracture behavior of materials, among which a representative study is the master curve approach. Master curve represents fracture behavior in which the brittle and brittle-ductile transition of the steel corresponds to the region and enables stochastic evaluation of structural integrity according to statistical approaches.