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.