Yoshihiro Sakino

Application of laser peening without coating on steel welded joints

Purpose – Laser peening without coating (LPwC) is an innovative surface enhancement technology for introducing compressive residual stress in metallic materials. The purpose of this study is to examine the characteristic at the laser‐peened welded zone and the fatigue lives of the welding joints.Design/methodology/approach – LPwC conditions for 490 MPa grades of structural steels were selected. By using the conditions, the characteristic at the laser‐peened welded zone, residual stresses, hardness and roughness of welding toes were examined. Moreover, the fatigue lives of the toes of box‐welded joints and butt welded joints pre‐treated by LPwC were compared to the fatigue lives of those that were not pre‐treated by LPwC.Findings – The main results are: LPwC conditions for 490 MPa grade steels were established; residual stresses, Vickers hardness and roughness at the laser‐peened welded zone were revealed; and LPwC can dramatically extend the fatigue life of welded joint.Originality/value – The effects of ...

Laser Peening without Coating to Mitigate Stress Corrosion Cracking and Fatigue Failure of Welded Components

The authors have applied laser peening without coating (LPwC) to metallic materials. Compressive residual stress nearly equal to the yield strength of the materials was imparted on the surface. Accelerating stress corrosion cracking (SCC) tests showed that LPwC had a significant effect to prevent the SCC initiation of sensitized materials of SUS304, Alloy 600 and the weld metal, Alloy 182. Push-pull type fatigue testing demonstrated that LPwC drastically enhanced the fatigue strength of fillet-welded rib-plates of SM490A.

Effects of strain rate on tensile strength of steel specimens of HAZs with stress concentrations

It is well known that the strain rate affects tensile behaviour; for example, it affects the tensile strength and fracture elongation of metals. When examining the tensile behaviour of steel, it is important to pay attention to the heat-affected zone (HAZ) as well as the base metal. The effects of the strain rate on the tensile strength were investigated by using specimens with stress concentrations; in particular, the tensile strength of SM400, SM490 and HT780 were obtained from HAZs by performing tensile tests at strain rates of 0.085%/s, 85%/s and 1000%/s. The HAZs were simulated by using a thermal/mechanical simulator. The main results are summarized as follows. (1) Regardless of the grade of steel, the tensile strength of specimens that underwent ductile fracture increase at high strain rates, whereas the tensile strength of specimens that underwent brittle fracture did not increase. (2) Regardless of the grade of steel and regardless of whether welding heat was supplied or not, the rate of increase in the tensile strength as a result of the high strain rate can be predicted from the tensile strength, which can be determined by static testing.

A basic study for application of laser peening to large-scale steel structure

Laser peening can introduce compressive residual stress to the surface and therefore is effective in enhancing fatigue strength. In this study, we conducted laser peening in air with a water film formed by a nozzle and examined whether the distribution of residual stress along the thickness was different from that achieved by laser peening in water in our previous studies. We also assessed the resultant residual stress and fatigue life when the pulse energy was reduced for developing a simple method to conduct laser peening on large structures. Compressive residual stress equivalent to that observed after laser peening in water was obtained in nozzle-type laser peening in terms of magnitude and depth. With the reduction of pulse energy, it was observed that the depth of the compressive residual stress tended to decrease significantly and the fatigue life also tended to reduce. The results indicate that the depth of the compressive residual stress has a considerable effect on the fatigue life of welded structures as well as the magnitude of the surface residual stress.

Damages to steel structures caused by the 1995 Kobe Earthquake

The Kobe Earthquake, with a magnitude of 7.2 on the Richter scale, that struck at 5:46 in the morning of January 17, 1995, caused considerable and various types of damage in the Hanshin area around...

4.4 – Estimation of the Applicability of Thin Steel Plate as Sacrificial Test Piece

Thin steel plates, which have initial cracks at the center, are used as the Sacrificial Test Pieces in this study. “The Sacrificial Test Piece” is attached to the member of a main structure in order to evaluate the damage before the appearance of a crack in the member of the main structure. The purpose is to show the practical applicability of “the thin steel plate with crack as the Sacrificial Test Piece” for monitoring the fatigue damage parameters on bridge members. In this research, it is decided that the applicable range of crack length and the crack propagation properties of the Sacrificial Test Pieces to evaluate the fatigue damage parameter are obtained. And it is suggested that the fatigue damage parameter can be estimated by the Sacrificial Test Pieces with practical accuracy by the test under constant amplitude loading.

12.1 – Application of Laser Peening on Welded Rib-plate of Mild Steel

Laser peening is an innovative surface enhancement technology to introduce a compressive residual stress on metallic materials. Experimental results showed that laser peening was effective to prevent SCC and enhance fatigue strength. However, effects of laser peening to steel for structure and its welded zone are not clarified. In this paper, residual stress of welded toes in rib-plates and fatigue life with laser peening was investigated by comparing to that without laser peening. X-ray diffraction was used to measure the residual stress. The main results are summarized as follows. 1) Laser peening can change the tensile residual stress to large compressive residual stress in welded toes. The nearer from the welded toe, the larger the effect by laser peening was. 2) Fatigue crack was not initiated from peened parts, even though stress amplitude of the peened part is twice stress amplitude of crack-initiated parts. 3) Fatigue life of the rib-plates with laser peening was prolonged at least three times or more compared to that without laser peening.