High strain-rate compressive stress-strain characteristics of AA2024-T3 and its welds as produced by the friction stir welding (FSW) process are investigated using the conventional split Hopkinson pressure bar. Friction stir welded AA2024-T3 joints are made under a fixed set of appropriate welding conditions. Cylindrical specimens machined along the thickness direction of both the base material and the friction stir weld nugget region are used in the static and impact compression tests. It is shown that the compressive flow stress of the weld nugget is reduced by 7 to 11 % compared with that of the base material, and that both the base material and the weld nugget exhibit almost no strain rate effect up to nearly =103/s. The Ramberg-Osgood equation can be used to describe the compressive stress-strain behavior at low and high rates of strain.

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The impact tensile strength of structural epoxy adhesive butt joints is determined with the split Hopkinson pressure bar (SHPB) using a hat-shaped joint specimen. A two-component structural epoxy adhesive (3M Scotch weld: DP-460) and two adherends (or 7075-T6 Al alloy and 99% pure titanium) are used in the adhesion tests. The impact tensile strengths of similar (Al-to-Al and Ti-to-Ti) adhesive butt joints are evaluated from the applied maximum tensile loads. The corresponding static tensile strengths are determined on an Instron testing machine. An axisymmetric finite element analysis is performed to investigate the stress distributions in the adhesive layer of the joint specimens. The effects of adherend and adhesive layer thickness on the tensile strength of the adhesive butt joints are examined. It is shown that the joint tensile strength increases greatly with increasing loading rate, decreases with increasing adhesive layer thickness, and varies, depending on the adherend materials. Microscopic examinations are conducted to identify the dominant failure modes occurring on the joint specimen surfaces.

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An accelerating laminar air flow initially at rest is generated in a circular pipe. Analytical solutions for the axial velocity component and wall shear stress of the flow are derived. Information on these quantities is helpful for a better understanding of the transition to turbulence in accelerating laminar pipe flows. A hot-wire anemometer is used for the measurement of these quantities. Both the measured values of the axial velocity component and wall shear stress before transition to turbulence are found to be favorably predicted by the analytical solutions.

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Mechanism of bubble coalescence is not clear. It is one of the unknown problems in multiphase flow phenomena. Recently, the utilization of microbubble becomes a key technology in both mechanical and chemical engineering fields. In general, it is believed that the microbubble is hard to coalesce with each other unlike bubbles of millimeter size. However, we found many microbubble coalescences in the microbubbly flow. Until today, the microbubble coalescence has not been reported via the experimental observations, and there is no theory that can explain the mechanism of the bubble coalescence in all bubble scale. Our observation reported here overrides the conventional conception and this fact is important for both engineering and academic fields. The purpose of this paper is to report our findings by using the high-speed camera, and to try to elucidate the mechanism of the microbubble coalescence.

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In this paper, ductile fracture loci formulated in the space of the effective plastic strain to fracture and the stress triaxiality for a cast aluminum alloy A356 and a low pressure die cast aluminum alloy were obtained using a combined experimental-numerical approach. A total of 12 tests were conducted including 6 tensile tests on notched and unnotched round bars and 6 biaxial loading tests on flat butterfly specimens in each material. Corresponding finite element analysis was performed to determine the evolution of stress and strain states. It was found that the material ductility strongly depends on the stress triaxiality for both present cast alloys. In addition, the material ductility of A356 was larger than that of low pressure die cast alloy. Meanwhile, the fractographic study reveals that many small voids nucleated uniformly in A356 although the low pressure die cast alloy showed the randomly distributed large voids at the high stress triaxiality. The smooth shear fracture surface was obtained in A356 while both many voids and small shear fracture surfaces were observed in the low pressure die cast alloy.

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In this study, the formability of a developed asymmetrically rolled(ASR)aluminum alloy sheet and the conventional symmetrically rolled(SR)one are examined experimentally by employing limiting dome height(LDH)test. The micro crystalline morphological changes, such as the texture evolution under the various proportional straining paths, are observed by using SEM-EBSD measurement. Simultaneously, the failures of the sheets are detected to establish the forming limits in the in-plane principal strains space, such as forming limit diagram. SEM-EBSD measurement results show a significant effect of the strain path on the texture evolutions. It reveals that the ASR sheet evolutes the shear texture more than SR one, and it can be related to improve the formability by employing this asymmetrical rolling. Consequently, ASR sheet shows the better forming limit for the particular strain path, such as the strain path ε1/ε2=tan60°(proportional rolling ratio) and the bi-axial stretching.

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This paper describes a technique to produce a natural crack in the heat resisting glass plates and the verification of the optical interferometry to measure crack opening displacement. First, it is confirmed through the experiment using Newton rings that crack opening displacement can be evaluated by employing the white light source and it is shown that there are some advantages in employing the white light source in the optical interferometric technique. Next, a technique to produce a natural crack with various shapes as single edge, quarter-elliptical and semi-elliptical cracks in the heat resisting glass plates is proposed by applying thermal stress which is caused through the repeated heating and cooling processes, and by observing the interferometric fringe pattern. In the single edge crack, the crack growth velocity is about 0.6-32mm/min and this shows that it is very easy to control the crack length. The technique proposed in this study confirmed the possibility of producing the natural cracks with the desired dimensions and shapes in the glass plates.

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In this study, usefulness of lamb waves generated by chirp signals for NDI was examined. The target for the inspection is wall parts of composite propellant tanks. We first studied characteristics of lamb waves propagated around delamination zone by computer simulation. Then, the damage inspection using lamb waves generated by chirp signals was conducted on the composite plates with artificial damaged zone. When the sensor is on the damaged zone, maximum amplitude became 17% of those at sound zone. On the other hand, when the damage zone was between two sensors, scattered lamb waves were monitored. Finally damage inspection system was developed by utilizing these characteristics of lamb waves.

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In this study polylactic acid(PLA) was used as a biodegradable plastic. Plastic sheets of PLA were rolled with the reduction ratio of 25 to 75% and at the temperature of 70 to 100°C. Tensile and bending test were carried out for rolled sheets. Both of tensile and bending strength in rolling direction were increased with increase of rolling reduction. The tensile strength and Young’s modulus of the rolled biodegradable plastics were 1.6 times and 1.7 times higher as compared with the non-rolled plastics, respectively. Effects of rolling on microstructure were observed with X-ray diffraction, polarization microscope, and differential scanning calorimeter(DSC). Consequently, relationships of mechanical properties with orientation of molecular chains and crystallization of the polymer were made clear.

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