Hiroki Fujimoto

Fatigue Strength Prediction of Spot-Welded Joints Using Small Specimen Testing

It is well known that fatigue strength of spot weld of high strength steel sheet is not improved, compared with that of mild steel sheet. In this study, the governing factors and the effects of steel grade on fatigue strength of spot weld is investigated. Firstly, small specimens with total length of less than 3mm are taken from the spot weld of mild steel sheet (270MPa-grade) and high strength steel sheet (590MPa-grade). And then, tensile and high cycle fatigue properties are individually evaluated by newly-developed testing technique. Secondly, finite element analyses of tensile-shear specimen of spot-welded joints under cyclic loading are carried out and fatigue limit of the joints are predicted, using the above-mentioned local material strength properties and considering welding residual stresses around spot weld. Predicted results are nearly equal in both steels, which coincides with experimental results. It is found that fatigue strength of HAZ, which is the crack initiation site in joint, of 590MPa-grade steel is higher than that of 270MPa-grade steel. However, residual stress in 590MPa-grade steel is also higher and this may be one of the reasons why 590MPa-grade steel exhibits little improvement in fatigue strength of the joint over 270MPa-grade steel.

Fatigue strength of hot-stamped spot welded joints*—study on spot welding tailored blank technology

ABSTRACT Hot stamping spot welding tailored blank (TB) technology is a process to produce spot welded automotive body parts by the following process: Spot welding steel sheets in lap configuration → Hot stamping (Heating to about 900°C → Quenching and forming in water-cooled die → Shot blasting to remove scale). This process has the advantage of producing high strength lap welded automotive body parts without increasing the number of forming dies. In this study, the tensile shear fatigue strength of the spot welding TB joints (Spot welding → Hot stamping) and conventional spot welded joints (Hot stamping → Spot welding) of the 1500MPa class uncoated boron steel sheets are compared. The obtained results are as follows. The fatigue life of the spot welding TB joints was more than two times longer than that of the conventional spot welded joints. The long fatigue life of the spot welding TB joints was not caused by the heating and quenching process but by the shot blasting process after heat treatment. Shot blasting on the outer sheet surface caused the high compressive residual stress on the outer surface and did not affect the residual stress on the lapped surface. Shot blasting on the outer sheet surface increased the initiation life of fatigue crack which occurred on the lapped surface and also reduced the crack propagation speed which propagates from the lapped surface to the outer surface. FE-analysis suggested that compressive residual stress on the outer surface reduce the opening of sheet separation of joints in fatigue tests and reduce the maximum principal stress around the edge of corona bond.

Study on static and fatigue strength of structural adhesive-bonded joints of steel sheets for automotive application

ABSTRACT The static and fatigue strength of crush durable structural adhesive-bonded lap joints of steel sheets for automobiles was evaluated by tensile shear tests. The steel sheets used in this study were uncoated and galvannealed (GA) with tensile strength ranging from 270 MPa-grade to 980 MPa-grade and the thickness ranging from 0.7 to 1.8 mm. 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.