Showing papers in "Transactions of JWRI in 2003"

Fundamental Studies on Biomachining of Carbon Steel by Iron Oxidizing Bacteria(Materials, Metallurgy & Weldability, INTERNATIONAL SYMPOSIUM OF JWRI 30TH ANNIVERSARY)

Abstract: Bacteria living in natural environments usually attack metals leaving unacceptable corrosion and causing critical deterioration. This negative phenomena is studied in this paper in a positive sense by means of control of these bacteria for the purpose of biomachining of metals. The main objective of this research is the achievement of a low cost machining process at normal temperature with less energy involvement and more involvement of bacteria in biomachining without straining them. The main focus in this paper is on the effect of Thiobacillus ferrooxidans, iron oxidizing bacteria and their ability to oxidize iron. It has been found that the extent of corrosion of carbon steel in the presence of Thiobacillus ferrooxidans was remarkably large. A tendency towards preferential corrosion offerrite was also observed. The results of our experiments show that biomachining using Thiobacillus ferrooxidans is possible by controlling the structure of steel.

Elucidation of Laser Welding Phenomena and Porosity Formation Mechanism(Physics, Processes, Instruments & Measurements, INTERNATIONAL SYMPOSIUM OF JWRI 30TH ANNIVERSARY)

Abstract: Imperfections, especially porosity, are easily formed in deeply penetrating weld fusion zones produced with high power lasers. To investigate the mechanism of porosity formation, welding phenomena, such as laser-induced plume, keyhole behavior, bubble formation and melt flow were observed during CO 2 or YAG laser welding of aluminum alloy and stainless steel plate by using high speed video camera and X-ray transmission real-time imaging systems. It was revealed that the keyhole fluctuated violently in depth and size at slow speeds during continuous wave welding and that the liquid chiefly flowed backwards near the bottom part of a molten pool from the tip of the keyhole. Such keyhole instability and liquid flow induced by evaporation enhanced the generation of bubbles and pores including the shielding gas. On the other hand, at high welding speeds, different main melt flows toward the upper keyhole inlet were induced by metallic vapor jets from the front wall of the keyhole, leading to the reduction in porosity. Consequently, porosity formation mechanism and laser welding phenomena were clarified in terms of keyhole dynamics, vapor jet directions and liquid flows.

Simulation of Resistance Welding for Selection of Optimum Welding Conditions and Process Control(Mechanics, Strength & Structure Design, INTERNATIONAL SYMPOSIUM OF JWRI 30TH ANNIVERSARY)

Abstract: Weldability, or the formation of the sound nugget is influenced by various factors, such as electric current, electrode force, welding speed, shape of electrode tip and initial gap between work pieces. This makes it difficult to select the appropriate welding conditions and to control them dynamically during welding. To solve such problems, finite element methods are developed for numerical simulations of resistance spot welding and seam welding. The usefulness of the proposed methods is demonstrated through examples.

Target Safety Levels for Design and Evaluation of Bridges(Mechanics, Strength & Structure Design, INTERNATIONAL SYMPOSIUM OF JWRI 30TH ANNIVERSARY)

Abstract: The development of limit state codes and/or load and resistance factor design (LRFD) codes for the design and evaluation of bridges requires the knowledge of the target reliability level. The optimum safety level depends on the consequences of failure and cost of safety. Selection of the target reliability can be based on consideration of these two parameters. In this paper; the analysis is performed for ultimate limit states (ULS) and serviceability limit states (SLS). Primary and secondary components are considered Serviceability limit states have a lower level of consequences of failure. Therefore, lower values of the target reliability index are selected for SLS than ULS.

Monitoring of Accumulative Traffic Loads on Bridge Members by Sacrificial Test Pieces(Mechanics, Strength & Structure Design, INTERNATIONAL SYMPOSIUM OF JWRI 30TH ANNIVERSARY)

Abstract: The purpose of this study is to show the practical applicability of \"the Sacrificial Test Piece\" for monitoring the accumulative traffic loads on bridge members. In this Report, the applicable range of notch length and crack propagation properties of the Sacrificial Test Pieces are obtained from experiments. And we veriy that the accumulative traffic loads under constant amplitude loading can be carried out by the crack growth of the Sacrificial Test Piece.

Time Dependent Numerical Analysis of Stationary GTA Welding Process(Physics, Processes, Instruments & Measurements)

Abstract: A development of a numerical model of GTA welding process is useful for understanding the heat transfer phenomena in the process and clarifying those effects on the weld penetration geometry. This paper presents results for the stationary argon GTA welding process at the atmospheric pressure. The whole region of the welding process, namely, tungsten cathode, arc plasma and stainless steel anode is treated in a unified numerical model to take into account the close interaction between the arc plasma and the molten anode. The time dependent two-dimensional distributions of temperature and velocity in the whole region of the welding process are predicted at a current of 150 A. The weld penetration geometry as a function of time is thus predicted. It is shown also that calculated convective flow in the weld pool is mainly dominated by the drag force of the cathode jet and the Marangoni force as compared with the other two driving forces, namely, the buoyancy force and the electromagnetic force.

Influence of MgO Crystal Surface Orientation on Its Wettability by Molten Al(Materials, Metallurgy & Weldability)

Abstract: The wettability of three different oriented MgO single crystals, (100), (110) and (111), by molten Al was measured by an improved sessile drop method in a purified Ar-3%H 2 atmosphere at temperatures between 1073 and 1473K in order to determine the effect of the MgO substrate crystallographic orientation. Although the reaction products at the Al-MgO interfaces consist of somewhat different forms of Al 2 O 3 phases, the wettability of all these substrates by liquid Al is similar. As a consequence, this system displays almost no anisotropy. Some plausible explanations based on several hypotheses are provided to account for this result.

Development of New Interface Potential for Simulating Mode I & II Fracture Behavior(Mechanics, Strength & Structural Design)

Abstract: In order to describe both the opening and shear deformation by one interface element, a new type interface potential was developed, which included an interaction between two modes. This proposed method was applied to the analyses of the fracture strength of co-cured joint between steel and resin, where the stress in the joint was applied by a simple tensile load or a thermal strain. From these computations, the predicted joint strength was found to decrease by increasing the order of stress singularity at the joint interface regardless of the type of applied stress.

Numerical Simulation of Fuzzy Control for Resistance Spot Welding by Monitoring Electrode Temperature(Physics, Processes, Instruments & Measurements)

Abstract: It is well known that resistance spot welding is a complex process in which coupled interactions exist between electrical, thermal, mechanical, and metallurgical phenomena. Because of this complexity, it is very difficultto obtain insightful information of welding process through the most ambitious experiments alone. In this paper, the thermo-elastic-plastic FEM is used to simulate the process of resistance spot welding. The effect of the welding current and squeezing force on the properties of spot welding is examined, in particular, from the aspect of the relation between the electrode temperature and the nugget formation in spot welding. Through this study, it is found that a sound nugget can be achieved when the electrode temperature is controlled within an acceptable range. Based on this knowledge, a fuzzy control method in which the welding current is modified in real time by monitoring electrode temperature is proposed. It is demonstrated that sound spot welding joints with large enough nuggets can be achieved by using the proposed fuzzy control method It is also shown that the weldability lobe can be expanded by employing the fuzzy control.

Rupture Locations of Friction Stir Welded Joints of AA2017-T351 and AA6061-T6 Aluminum Alloys(Materials, Metallurgy & Weldability, INTERNATIONAL SYMPOSIUM OF JWRI 30TH ANNIVERSARY)

Abstract: The tensile rupture locations of friction stir welded joints of AA2017-T351 and AA6061-T6 aluminum alloys were examined. The experiments show that the rupture locations of the joints are different for the two aluminum alloys, which are influenced by the welding parameters. When the joints are free of welding defects, the AA2017-T351 joints are ruptured in the weld nugget adjacent to the thermo-mechanically affected zone on the advancing side and the rupture surfaces appear as oval contours of the weld nugget, while the AA6061-T6 joints are ruptured in the heat affected zone on the retreating side and the rupture surfaces are inclined at a certain degree to the bottom surfaces of the joints. When welding defects are present in the joints, the AA2017-T351 joints are ruptured in the weld center, while the AA6061-T6 joints are ruptured on the retreating side near the weld center. The rupture locations of the joints are dependent on the internal structures of the joints and can be explained through them.