Hitoshi Soyama

Enhancing the aggressive intensity of hydrodynamic cavitation through a Venturi tube by increasing the pressure in the region where the bubbles collapse

In this paper, we used a Venturi tube for generating hydrodynamiccavitation, and in order to obtain the optimum conditions for this to be used in chemical processes, the relationship between the aggressive intensity of the cavitation and the downstream pressure where the cavitationbubbles collapse was investigated. The acoustic power and the luminescence induced by the bubbles collapsing were investigated under various cavitating conditions, and the relationships between these and the cavitation number, which depends on the upstream pressure, the downstream pressure at the throat of the tube and the vapor pressure of the test water, was found. It was shown that the optimum downstream pressure, i.e., the pressure in the region where the bubbles collapse, increased the aggressive intensity by a factor of about 100 compared to atmospheric pressure without the need to increase the input power. Although the optimum downstream pressure varied with the upstream pressure, the cavitation number giving the optimum conditions was constant for all upstream pressures.

Mechanical Surface Treatment of Duralumin Plate by Bubble Induced by Pulse Laser

Surface of duralumin plate can be treated by not only laser abrasion but also bubble induced by pulse laser. The pulse laser produced two kinds of shock waves related to laser abrasion and bubble collapse. In the case of laser peening, the shock wave induced by abrasion was normally used. In the present paper, the behaviour of bubble was observed and noise was detected by the hydrophone. It was revealed that the impact induced by the bubble collapse produced by the pulse laser was stronger than that of the laser abrasion. A numerical simulation was also carried out to investigate phenomenon of bubble collapse.

Removal of oral biofilm on an implant fixture by a cavitating jet

Purpose: To demonstrate the effectiveness of the cavitating jet in removing biofilms from the rough surface of 3-dimensional structures. Materials and Methods: The optimal nozzle dimensions and injection conditions were identified by cavitation impact measurements. Biofilm was grown intraorally for 72 hours by 4 volunteers. The stained fixtures were assigned to different experimental groups. One comparison was performed between the cavitating jet and the water jet at 60 seconds. Additional comparisons were conducted among the time course experiments at 30, 60, and 180 seconds. After injection, the residual plaque biofilm (RPB) area was measured using a digital microscope. Results: The total RPB of the cavitating jet was significantly lower than that of the water jet. Although there were no significant differences between the total RPB at 30 and 60 seconds, a significant difference was detected between 60 and 180 seconds. The RPB on the root sector was significantly lower than that on the crest sector at 60 and 180 seconds. Conclusion: The cavitating jet can effectively clean the biofilm formed on the rough surface of the implant screw, especially on the root sector.

Reduction of hydrogen embrittlement cracking of stainless steel SUS316L by cavitation peening

In order to demonstrate reduction of hydrogen embrittlement cracking by improving residual stress using cavitation peening, crack growth of stainless steel SUS316L exposed to hydrogen was investigated comparing with and without cavitation peening by a plate bending fatigue test with a notched specimen. The tensile residual stress was introduced into specimen surface by an angle grinder. It was revealed that the residual stress of the specimen surface was improved from tension to compression by cavitation peening using a cavitating jet in air, and the crack growth rate of the specimen exposed to hydrogen was supressed by cavitation peening.

Effect of Hydrogen on the Micro- and Macro-Strain near the Surface of Austenitic Stainless Steel

The objective of this study is to evaluate the effect of hydrogen on the micro-and macro-strain of austenitic stainless steel using X-ray diffraction. When hydrogen is trapped in lattice sites, it can affect both the micro-and macro-strain. The micro-strain was evaluated through fitting profiles to measured X-ray diffraction profile using a fundamental parameter method. The macro-strain, i.e., the residual stress, was evaluated by a 2D method using a two-dimensional PSPC. The experimental samples were charged with hydrogen by a cathodic charging method. The results revealed that the induced residual stress was equi-biaxial and compressive, and that the micro-strain increased. Both of these varied rapidly with increasing hydrogen charging time. Saturation occurred at a compressive stress of around 130 MPa. On reaching saturation, the hydrogen charging was terminated and desorption of hydrogen began at room temperature. Then, the strains decreased and the compressive stress reverted, ultimately, to a tensile stress of 180 MPa. Martensitic transformation occurred due to hydrogen charging and this had a significant effect on the X-ray diffraction profile.

Effect of Indentation Load on Vickers Hardness of Austenitic Stainless Steel After Hydrogen Charging

In order to reveal the effect of indentation load on Vickers hardness of austenitic stainless steel after hydrogen charging, the Vickers hardness measurements have been conducted with three different indentation load of 0.49, 1.96 and 9.80 N on the surface of type 316L austenitic stainless steel after hydrogen charging. Relationship between plastic deformation behavior during indentation process and hydrogen absorption behavior was revealed. In the Vickers hardness test, Vickers hardness keeps same value though the indentation load varies. Needless to say, the value did not depend on magnitude of the indentation load before hydrogen charging in the present study. However, the Vickers hardness increased along with hydrogen charging time and, interestingly, the increase in the Vickers hardness due to the presence of hydrogen depends on magnitude of the indentation load. In the load of 0.49 N and 9.80 N, the Vickers hardness has a maximum value of 3.04 and 2.04 GPa which is 1.58 and 1.15 times larger than value of 1.73 and 1.70 GPa before hydrogen charging, respectively. The hydrogen-induced hardening behavior observed by the Vickers hardness tests employing different indentation load would be evaluated by the relationship between the plastic deformation depth and the hydrogen absorption depth.Copyright