Method for Evaluating the Ductile Crack Propagation Properties of Steel Sheet under Tensile Stress Conditions

Asato Hatamoto･Hiroshi Shimanuki (NIPPON STEEL)

During collisions, base metals may incur ductile fracture without evident defects under complex stress conditions. Herein, we propose a test method for evaluating the propagation behavior of internal ductile cracks in metals under tensile stress conditions. The applicability of the developed specimen to the evaluation of ductile crack propagation properties was discussed based on the results of tensile fracture tests and finite element analyses.

Fatigue Strength Evaluation of Lap Fillet Welded Joints using Infrared Thermoelastic Method

Hideki Ueda･Taizo Makino･Hiroshi Shiromizu (Nippon Steel)･Reiji Tanaka (Nippon Steel Technology)

We investigated the effect of load frequency using thermoelastic finite element (FE) analysis on Lap fillet arc weld joint commonly used in automobile parts. To simulate the thermoelastic effect, we developed a stress field-temperature field thermoelastic FE analysis technique that calculates the heat transfer of heat generation and heat absorption according to compression and tensile stress. The calculated stress distribution showed a good agreement with thermoelastic stress distribution measured using an infrared ray method. Moreover, we verified the accuracy of estimating the fatigue limit obtained from the point of sudden increase in dissipated energy by infrared measurement.

Effects of Tension and Bending Loads on the Fatigue Threshold in a Stainless Steel Sheet

Gyoko Oh (Tokyo Roki)

By conducting fatigue tests with different loading modes using specimens with different notch lengths, we clarify and propose the criteria for fatigue thresholds. Cyclic plastic strain occurred near the notch, and was larger under tensile load than under bending load. The fatigue limit under tensile load was lower than under bending load, and there was a knee point on the relationship line with notch length. A calculation model for the critical stress intensity range ∆Kthc was presented. It was revealed that ∆Kthc varies greatly depending on the load type, but remains constant regardless of the length of the notch or crack.

Improving the Fatigue Limit of Spot-Welded Tensile Shear Joints Made of High-Tensile Steel Sheet (First Report)
-Improvement of Fatigue Limit due to Single Overload-

Akifumi Okabe･Tatsuya Ito･Noboru Tomioka (Nihon University)

A single overload was applied to spot-welded tensile shear joints using 590 MPa class high-strength steel as the base metal, and the effect on fatigue life was investigated. A large bending deformation occurred near the nugget edge of the tensile shear joint due to a single overload, resulting in large compressive residual strain on the inner surface. This is the reason for the improvement of the fatigue limit. In the case of a constant amplitude load range, a single overload increased the crack initiation life by 5 times and the crack propagation life by 2 times.

Improving the Fatigue Limit of Spot-Welded Tensile Shear Joints Made of High-Tensile Steel Sheet (Second Report)
-Fatigue Life Assessment based on Effective Strain Range-

Tatsuya Ito･Noboru Tomioka･Akifumi Okabe (Nihon University)

In this study, fatigue tests were conducted on spot-welded tensile-shear joints using 590 MPa-class high-tensile steel sheet as the base material, with various load ratios in the constant amplitude load range applied following a single overload, and the threshold load ratio at which the effect of the single overload disappears was experimentally determined. This threshold gives the effective strain range for fatigue damage, and it was shown that this effective strain range can be used to uniformly evaluate the fatigue life with and without a single overload.