The authors have already established a new CTOD calculation formula, and have been revised a CTOD testing standard of WES1108 in the Japanese Welding Engineering Society by using this formula. Because the fatigue pre-crack tip should be placed in the specified microstructure such as HAZ for the critical CTOD evaluation of welded joints, the next step is the application of the formula to various crack length-to-width ratio, a0/W, conditions, especially for shallow crack specimens. In the previous work, a rotational deformation was apparently observed, and the plastic rotational factor, rp, was described as a function of wide a0/W range. However, a simple use of rp did not result in a better CTOD estimation for shallow crack specimens. In the present work, another way of calculating CTOD was developed by the numerical investigation of single edge notch bend, SE(B), specimens with various a0/W. Non-linear crack opening profiles were dominant for smaller a0/W specimens than 0.2 due to plastic deformation behind their crack tips, or a straight crack opening profile deviated from a triangle side assumed by the plastic hinge model for a0/W =0.7. In this paper, a correction factor of the plastic component of clip gauge displacement, CVp, was proposed considering non-linear crack opening and crack profile deviation, and it was demonstrated that CVp gave a better CTOD estimation than current CTOD formulae such as BS7448 and ASTM E1820 for the wide range of a0/W.
Hydrogen-induced cold cracking of welds needs to be solved to promote the application of high-strength steel. Cold cracking has been investigated through slit-type weld cracking tests. These tests afford appropriate conditions to avoid cold cracking in terms of factors such as chemical composition, hydrogen concentration, welding conditions, plate thickness, and groove geometry. However, the process of cold cracking of welds should be based on more microscopic criteria, because stress concentration and diffusible hydrogen accumulation are the governing processes of cold cracking on a microscopic scale. In this study, cold cracking of a high-strength steel weld metal was evaluated by considering local stress and locally accumulated hydrogen concentration. First, a limit condition for the high-strength steel weld metal was determined through a series of slow strain rate tensile tests. Finite element simulation of stress distribution and hydrogen concentration distribution were performed to determine the limit condition of cold cracking using local stress and locally accumulated hydrogen concentration. Second, finite element simulations of weld-induced stress and hydrogen diffusion in the Y-groove weld cracking test specimen were performed. The histories of local stress and locally accumulated hydrogen concentration at the weld root were calculated under various initial diffusible hydrogen concentrations. When the initial diffusible hydrogen concentration was higher than a certain value, the local stress and locally accumulated hydrogen concentration at the weld root could exceed the cold cracking limit. Whether cold cracking occurred or not was agreed with the experimental results.
The static and fatigue strength of crush durable structural adhesive bonded lap joints of steel sheets for automobiles were evaluated by tensile shear tests. The steel sheets used in this study were uncoated and galvannealed (GA) with tensile strength ranging from 270MPa-grade to 980MPa-grade and the thickness ranging from 0.7mm to 1.8mm. Also, the effects of the adhesive types were evaluated. The results are as follows: In the static tensile shear tests, when the steel sheets deformed during the tensile test, the tensile shear strength increased with the increase in the base metal properties, such as the yield strength and thickness, however, when the base metal properties were sufficiently high not to undergo plastic deformation, the tensile shear strength exhibited a constant value. On the other hand, the effect of base metal properties on the fatigue joint strength was relatively small. The static joint strength of the GA steel joints was slightly lower than that of the uncoated steel sheets, however, the fatigue strength of the GA steel joints was higher than that of the uncoated steel sheets. The coating failure of the GA was affected by the type of adhesive, base metal properties and type of test. Choosing the proper adhesive can reduce the failure of the GA coating, and the high strength steel showed fewer coating failures than the mild steel.
This paper describes the effect of friction welding condition on joint properties of austenitic stainless steel (SUS304) joints, which was made by friction stud welding. When the joint was made at a friction pressure of 30MPa with a forge pressure of 30MPa, the joint efficiency of 100% was not successfully achieved. The joint with a forge pressure of 270MPa was not obtained the fracture in the base metal, although the joint efficiency increased. The cause of the joint with the fracture between the weld interface and the base metal was that the peripheral portion of the weld interface of the stud side was not completely joined at this friction pressure. On the other hand, the fracture on the base metal and the joint efficiency of 100% were successfully achieved when the joint was made at a friction pressure of 90MPa and a friction time of 0.3s (just after the initial peak) with a forge pressure of 240MPa or higher. This joint had the bend ductility of over 90 degrees with no crack at the weld interface by three-point bending test, and it also had that of over 45 degrees with no crack at the weld interface by impact shock bending test. In conclusion, to obtain the joint efficiency of 100% and the fracture in the base metal with no cracking at the weld interface, the joint must be made with high friction pressure, opportune friction time such as the friction torque reached to just after the initial peak, and with adding high forge pressure.
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