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.
In order to realize low cost productions with the high quality, various flux cored wires have been developed corresponding to needs of industrial fields. In this study, the improvement of the toughness in the weld metal by Titania-based flux cored wires is described. We have tried various alloy-designed flux-cored wires, and have estimated their microstructures. It has been found that decreasing the oxygen content and lowering impurity contents of Nb and V are extremely effective to improve the toughness in the weld metal by the Titania-based flux cored wire.
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.
This study focuses on the opening mode of induction bends; this mode represents the deformation outside a bend. Bending experiments on induction bends are shown and the manner of failure of these bends was investigated. Ruptures occur at the intrados of the bends, which undergo tensile stress, and accompany the local reduction of wall thickness, i.e., necking that indicates strain localization. By implementing finite element analysis (FEA), it was shown that the rupture is dominated not by the fracture criterion of material but by the initiation of strain localization that is a deformation characteristic of the material. These ruptures are due to the rapid increase of local strain after the initiation of strain localization and suddenly reach the fracture criterion. For the evaluation of the deformability of the bends, a method based on FEA that can predict the displacement at the rupture is proposed. We show that the yield surface shape and the true stress-strain relationship after uniform elongation have to be defined on the basis of the actual properties of the bend material. The von Mises yield criterion, which is commonly used in cases of elastic-plastic FEA, could not predict the rupture and overestimated the deformability. In contrast, a yield surface obtained by performing tensile tests on a biaxial specimen could predict the rupture. The prediction of the rupture was accomplished by an inverse calibration method that determined the true stress-strain relationship after uniform elongation.
The lap joints of upper Al alloy sheets (1.0-mm-thick A5052) and lower Zn-coated steel sheets (1.2-mm-thick GI steel or GA steel) were welded using insert steel sheets (0.6-mm-thick SPCC) by a spot welding process with a tool having a spherical ceramic tip, i.e., "Friction Anchor Welding." As a result, straight (not-rugged) steel projections were formed in the Al alloy sheets for both the GI and GA, while steel projections were not formed for the GI, rugged steel projections were formed for the GA without the insert steel sheets. In addition, the tensile shear strength for the GI was greater than that for the GA. In other words, the tensile shear strengths reached about 3.9 kN/point for the GI and about 3.2 kN/point for the GA, which were greater than those of the welds without the insert steel sheets. On the other hand, the cross tensile strengths for the GI and GA were almost the same, which reached about 2.6 kN/point. Additionally, for the GI, the Zn layer on the GI steel sheet melted and was totally removed due to the pressure and heat caused by the rotating tool, which facilitated the welding between the SPCC and GI steel sheets. For the GA, however, the Zn-Fe layer on the GA steel sheet changed to a solid-liquid mixture and was not completely removed, which prevented the welding between the SPCC and GA steel sheets. Therefore, the thickness of the steel-steel welded region (i.e., the SPCC-GI or the SPCC-GA welded region) for the GI was greater than that for the GA. We estimated that the difference in this thickness is significantly related to the fracture mechanism during the tensile shear test and the cross tensile test.