An anti-fatigue smart paste, which consists of fine alumina particles and silicone grease with low viscosity, was applied to a bolt hole (and its periphery in some cases) in a steel plate specimen, and the effects of the smart paste on automatic restraint and visual detection of fatigue crack growth were experimentally investigated through fatigue tests. Fractographic observations using a SEM (Scanning Electron Microscope) were also carried out. As a result, approximately 20∼410% increase in failure life was produced by the wedge effect of the alumina particles in the smart paste. When the smart paste was applied, remarkable black color developed in the white paste along the paths of crack growth, exceedingly facilitating the visual detection of the crack growth.
A series of fatigue experiments and elastic analysis were carried out for investigating fatigue characteristics of patch plate joints assembled by fillet welding assisted with bonding. In the case that fatigue cracks occurred at the weld toe by the 4-points bending fatigue experiment, the fatigue life of joints assembled by welding and bonding (WB specimens) were almost the same as those by only welding (W specimens). The elastic analysis simulating the 4-point bending loaded situation on W and WB specimens was performed for elucidating its reason. The stress concentration at the weld toe was even high in the WB specimens. Therefore, the fatigue life of WB specimens was not longer than that of W specimens. On the other hand, the stress around the weld root of WB specimen was around 30% of that of W specimen. The possibility of stress reduction effect by bonding was indicated around the weld root rather than around the weld toe. In order to verify this possibility, the 4-point bending fatigue experiment was performed by setting the specimens so that the tensile stress was applied on the weld root. It was confirmed that the fatigue cracks occurred from the weld root in both of W and WB specimens. The fatigue life defined in this study of WB specimens was from 4 to 8 times longer than that of W specimens when the applied nominal stress range was under 175 MPa. The fatigue life defined in this study of WB specimens was from 2 to 3 times longer than that of W specimens when the applied nominal stress range was over 200 MPa. The results indicated the fatigue life improvement of patch plate joints by fillet welding assisted with bonding when the fatigue cracks occurred at the weld root.
Spectroscopic measurements for GMA phenomena have been recently performed. The studies have reported that the metal vapor behavior greatly affects the arc properties. However, they can be applied only to axially symmetric phenomena because of the assumption used for the measurement. GMA welding are normally performed with a travel speed and most of the phenomena become axially asymmetric. This study constructed the simultaneous and multidirectional measurement system by 12 CCD cameras which can capture such axially asymmetric GMA phenomena. We measured the MIG welding process with use of two types of narrowband interference filters for Ar I and Fe I during the one measurement, and observed axially asymmetrical intensity distributions in the globular and the spray transfer mode. We found that the globular transfer mode that is seemingly chaotic distribution can be regarded as the distribution where the deviation of Ar I is larger than Fe I from the axially symmetric double ring distribution consist of Ar I and Fe I.
Since Crack tip opening displacement (CTOD) has been proposed as a fracture mechanics parameter CTOD in 1960s CTOD is widely used in steel structure industries for toughness evaluation of the materials and/or defect assessment of the welded structures based on the fitness for service standard. National standard in each country and also ISO standard have been established which is a similar procedure to the firstly standardized DD19 in British Standard Institution in 1972. The CTOD calculation is composed of plastic hinge model which assumes rotational deformation around the specific point. Standard range of crack length to width ratio (a0/W) is limited to be from 0.45 to 0.5. In the last 15 years, the discussion of CTOD has revived at a chance of the J based CTOD proposal in ASTM. The J based CTOD is capable of CTOD calculation in the wide a0/W range. CTOD evaluation using single edge notched tension (SENT) specimen with a sallow crack is also desired by industries at the same time. However, the capability of rotational CTOD evaluation in the wide a0/W range has not completely been concluded, yet. In this study, aiming to propose the new calculation method of CTOD, the rotational deformation is examined in the a0/W range from 0.05 to 0.70, and the applicability of rotational CTOD calculation for a shallow cracked specimen is discussed.
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 were 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.
This study was undertaken in order to investigate the effect of reduced ambient pressure from an atmospheric pressure (101 kPa) to 0.1 kPa on one-pass full penetration welding of thick high tensile strength steel plate of 23 mm thickness. A 16 kW disk laser of 1030 nm in wavelength was employed to weld HT980 grade plates at the speed of 5 to 25 mm/s. In partial penetration welding, it was revealed that humping phenomena occurred easily. Full penetration welding of the high tensile strength steel plates could not be achieved at 101 kPa. On the other hand, full penetration welding was obtained at the welding speed of less than 20 mm/s at the pressure of less than 10 kPa. Especially, at 0.1 kPa and 17 and 20 mm/s, sound weld joints without defects were obtained. According to the observation results of a keyhole inlet and a surface molten pool during welding with a high-speed video camera, the melt in front of a keyhole was smaller and the behavior of a keyhole and a plume was much more stable at 0.1 kPa than at 101 kPa. Moreover, in the full penetration welding, spattering was suppressed under the proper conditions. Such phenomena became more stable in fast welding. It was revealed in laser welding of thick high tensile strength steel plates that the formation of narrow I-shaped weld beads by achieving full-penetration welding in low vacuum was essential for the production of sound welds without defect.
Interactions between elasto-plastic contact deformation, electric current and heat conduction produces complicated triply coupled effect in the resistant spot welding. Since the coupled effect affects weld strength, the coupled finite element analysis is required to determine welding parameters such as welding current, electrode force, welding time and electrode shape. This paper discusses in detail the elasto-plastic contact, electric current and thermal conduction triply coupled analysis method. The coupled analysis method uses modified electrical contact resistance model, modified thermal contact resistance model and iterative partitioned coupled algorithm suitable for strong coupled problem. The validity of the method are discussed under the different condition such as welding current, electrode force, welding time, material and number of the steel sheets. The analysis result of nugget growth curve agrees very well with experimental result for two sheet welding. The validity is confirmed by not only nugget growth curve but also surface temperature of steel sheet and electrode for three sheet welding with high tension steel. The solution convergence is discussed for the iterative partitioned coupled algorithm and the conventional coupled algorithm. Since the coupled effect is strong for between electric current and thermal conduction, the choice of the coupled algorithm is important for the resistance spot welding analysis. Effect of elasto-plastic contact deformation, change of electric current distribution and thermal conduction on resistance spot welding are discussed from the viewpoint of coupled effect. The coupled model with ability to simulate the chain of coupled effect is required to improve the analysis reliability. The importance of temperature dependency of material property, electrical contact resistance, thermal contact resistance and latent heat effect are also discussed.
In this paper, we gave a definition based on thermodynamics and continuum mechanics of surface energy of a homogeneous isotropic linear elastic body. Next, we got a new condition of fracture occurrence of a homogeneous isotropic linear elastic body by using above mentioned definition of surface energy and by thermodynamical study according with Griffith's theory. By using the result of the mathematical solution for stress-strain field of the clack tip which exsists in a homogeneous isotropic linear elastic body to the new fracture condition in the reach of infinite small strain, we got the same fracture occurrence condition which was obtained from conventional theory.
Cast irons have some excellent characteristics in wear resistance and vibrational damping etc., and widely used in many industrial machinery applications. But the weldability has been much inferior originated from its high carbon content. Several researches have reported that some of the important factors affecting the weldability were formation of blowhole and chill layer during fusion welding and subsequent rapid cooling. In this study, the formation behaviors of those were investigated in both flake and nodular graphite cast irons. Tungsten inert gas arc welding was performed using several kinds of filler metal varied nickel and chromium content. The study was made by paying attention to carbon behavior and influence of nickel and chromium in filler metal on it. Experimental results obtained were as follows. (1) The amount of blowhole increased with nickel content and decreased with chromium content. (2) On the other hand, the amount of chill layer increased with chromium content and decreased with nickel content. (3) It was estimated that both blowhole and chill layer formation were related to activity and diffusivity changes of carbon affected by nickel and chromium content. Then consequently they have a correlation relationship each other.
Solidification cracking susceptibility of extra high-purity type 304 and 316 stainless steels was evaluated by the transverse-Varestraint test using arc and laser welding, in order to clarify the feasibility of "ferrite-free welding". The solidification brittle temperature ranges (BTRs) of 304EHP and 316EHP steels were slightly increased with a decrease in the amount of δ-ferrite in the weld metal, while they were considerably lower than those of commercial-purity steels with FA solidification mode during arc as well as laser welding. It follows that the ferrite-free welding of austenitic stainless steels could be accomplished by applying the extra high-purity steels. Theoretical approach to the BTR was made to elucidate the depression mechanism of solidification cracking susceptibility in extra high-purity steels. From the fact that solidification segregation of minor and impurity elements such as C, P and S was considerably depressed in the extra high-purity weld metal, the solidification cracking susceptibility would be reduced even when the amount of ferrite was extremely small or free, due to the inhibition of solidification segregation. Numerical analysis of the BTR suggested that the ferrite-free welding of type 304 and 316 stainless steels could be achieved at the compositional range of P+S+0.1C<80ppm in the weld metal.