Papers by Author: Minoru Yamashita

Abstract: Three point bending test of aluminum tubular structure with hat cross-section was carried out under impact condition. The structures which were strengthened with carbon fiber reinforced thermoplastic sheet attached to hat-top or hat-side were also tested. When the structure made with only aluminum was bent, one-lobe deformation mode arose in most cases, where the buckling lobe was formed at center exhibiting very low deformation resistance. This mode was found to be attributed to the low friction at central anvil by conducting the numerical simulation. The energy absorption performance was evaluated for a variety of structures. When the sheet attachment was applied for the structure with one-lobe deformation mode, the absorbed energy was improved drastically.

Abstract: Impact joining device was developed to achieve the joining at plate edge. The sheared faces obtained by high-speed shear are availed in the joining method. The temperature remarkably elevates and the material softens in the thin severely deformed layer. The cut faces are contacted each other with sliding motion immediately after shearing process. Slight overlap for the opposed cut faces is set. In the similar material combination of mild steel (SPC), the effect of overlap length on the average joining efficiency was not clear. The data spread became smaller for the shorter overlap length. Microscopic observation discovered that there is a narrow band where the grain flow was not visible due to the remarkable temperature rise. In the dissimilar material combination of titanium (TP340C) and SPC, generation of a certain metallic compound was implied by energy dispersive X-ray analysis.

Abstract: A novel method for joining of sheet edges was investigated. It makes use of the sheared faces under high-speed. The temperature remarkably elevates and the material softens in the thin severely deformed layer. The cut faces are contacted each other with sliding motion immediately after shearing. Sheet materials are a pure aluminum A1050 and its alloy A5052. Two similar sets of circular shearing punch, die and sheet are concentrically stacked in the device for performing the simultaneous shearing. The upper sheared circular sheet slides into the hole of the lower one for joining shortly after the shearing. Strain-rate order was about 10⁴ /s in shearing, where the punches were driven by a drop-weight. Quasi-static test was also conducted for comparison. Joining was achieved only in the high-speed test. Joining boundary was not visible at the central portion in thickness, though the gap openings were seen near both sheet surfaces. Joining efficiency was evaluated as the relative strength to the tensile strength of the material. It was improved as the shearing clearance becomes smaller. The maximum efficiency was about 30 % for A1050 material. It is about 25 % for A5052. It was at most 10 % in the joining of A1050 and A5052.

Abstract: In order to accomplish the circular cup drawing of the sheet material whose ductility is extremely poor under the cold forming condition, the Maslennikov's technique was applied. A deformable rubber ring was used instead of the hard punch. Test material was magnesium alloy AZ31-O sheet. Small die profile radius was applied, which was twice or 4 times of the sheet thickness. A semisolid lubricant was used for the lubrication of the blank - die interface, on the other hand, the rubber - blank interface was degreased to increase friction. The limiting drawing ratio of 1.31 was attained. A peculiar fracture mode arises, where the material suddenly fractured with crack evolution emanating from the flange periphery. The fracture strain is found approximately equal to the work hardening exponent n-value in plastic property. Another kind of crack arose in the circumferential direction of the cup during unbending process with smaller die profile radius. To decrease the scratched lines of the cup surface caused by very high sliding contact pressure to the die, high-speed drawing was tested using a drop-weight. The surface was improved, because the material - tool contact was successfully prevented by the hydrodynamic lubrication film.

Abstract: Generallythe critical distance stress theory was applied for the fatigue limitestimation of general structures. In thismethod, it needs only two parameters, fatiguelimit of smooth specimen (σ_w0), and threshold stress intensityfactor range (∆K_th). In this paper we extended this method for theestimation of low cycle fatigue lifetoo. In this improvement wedefine the critical distance (r_c’) on static strength conditions,which is calculated using ultimate tensile strength (σ_B) andfracture toughness (K_IC), in addition to the critical distance onfatigue limit condition (r_c). Then the critical distances of any lowcycle fatigue conditions can be calculated by interpolation of criticaldistance on fatigue limit (r_c) with critical distance on staticstrength (r_c’). By unifying these low cycle fatigue life estimationmethod with high cycle fatigue limit estimation method we can estimate the fullrange fatigue life easily. And to confirm the availabilityof this estimation method we perform the fatigue test for circlehole specimens, sharp V notch specimens andfretting fatigue specimens.

Abstract: For the improvement of the deformation characteristic and the energy absorption efficiency of the tubular structure at impact event, a method generating the first buckling lobe using the inertia force was investigated. The solid block was attached to the wall so that its inertia force causes the partial plastic deformation that plays a role of the trigger of progressive buckling at the beginning of impact. Drop-weight impact experiment revealed that the onset of progressive buckling was achieved at the desired portion by the method. To increase the variety of tube shapes, numerical examination was conducted with the dynamic explicit finite element method. Long straight and S-curved tubes, which have square cross-section, were numerically modeled with shell elements. They were assumed to be impacted to the rigid wall to estimate the dynamic collapse behavior. The first buckling lobe generated by inertia force was demonstrated for the straight tube. The S-curved tube exhibited a bending collapse mode without the method. However, such mode was avoidable by applying the method. Then the energy absorption efficiency of the tube was drastically increased.

Abstract: The drop-hammer compression testing method where the stroke is calculated by solving the equation of motions of the drop-hammer and anvil was improved in order to determine the strain-rate dependence of the elastic modulus of rubber material. An additional tool for interrupting the compression was embedded in the apparatus. The tool was a thick washer which informed the time when the prescribed compressive strain was achieved by the sharp increase of compression force. The oscillatory stress-time curve obtained by the load cell was appropriately smoothed employing the method of moving average. The effect of the friction at the interface between the rubber specimen and the tools was properly eliminated by the extrapolating method using the specimens with several variations in heights. The stress-strain relationship was obtained under the dynamic condition. The numerical simulation of impact compression disclosed that the effect of inertia on the deformation pattern was practically small under the experimental condition adopted. Conducting the low-speed compression tests, the dependence in elastic modulus of rubber material on the strain-rate was appropriately determined.

Abstract: The dynamic axial compression of adhesive-bonded tubular structures with hat-shaped cross-section was numerically investigated using dynamic explicit finite element method. The numerically modeled Type I tubular structure consisted of a hat-shaped part and a flat plate. Type II consisted of two hat-shaped parts. The hat flange portion was assumed to be joined by adhesive bonding. The impact velocity of 10 m/s was given. Parametric computation was performed, where thicknesses of the plate and adhesive layer, and mechanical properties of the plate material were varied. As the result of computation, separation behavior of the hat flange portion was almost avoided for the case where the strength of plate material was lower. However, it was clearly observed for the higher strength material, though the plate was thin. The separation of the flange portion in Type I structure was more remarkable in comparison with that in Type II. The crush strength increased as the thickness of the adhesive layer thickened, when the plate thickness was thin and the strength was low. The Type II structure exhibited larger crush strength than that of Type I.

Abstract: The wood bar with square cross-section was biaxially compressed with respect to the cross-sectional plane and the impact 4-point bending test was performed using the compressed wood. The wood material used was sap wood of Sugi, which is a kind of Japanese cedar. The biaxial compression apparatus which enabled to prevent the gap generation between the material and the tool was used. The fixation of wood specimen was also processed. The bar length was 100 mm and the edge length in cross-section was 15 ~ 25 mm. In the 4-point bending test, the distance between the supports and the stress points were 80 mm and 40 mm. The stress points were impacted by a light-weight drop-hammer with 3.0 kg mass. The impact velocity was 2 m/s. The surface strain was measured by a strain gage. When the Young’s modulus was estimated by assuming a linear elastic property, it attained about 32 GPa, which was more than 4 times of the uncompressed wood. The stress – strain hysteresis loop was more remarkable under the impact condition. It was found that the compressed wood provided not only a high rigidity, but also a good property in dissipation of vibration energy.

Abstract: Stress-strain relationships of polycarbonate (PC) were determined over a very wide range of strain rates including shock wave regime. High-velocity plate impact tests, drop-weight tests, and quasi-static tests using universal and Instron testing machines were used for the high strain rate (107 s-1), medium strain rate (102 s-1) and low strain rate (10-4 s-1) tests, respectively. The revised unsteady wave sensing system (UWSS) for plate impact tests was newly developed to determine the stress-strain relationships and Hugoniot linear relation of PC. The system consists of a powder gun for plate impact tests and three polyvenylidene fluoride (PVDF) gauges embedded in the PC utilizing a newly developed nanosecond UWSS. As originally proposed, UWSS is aimed in obtaining experimental inputs for the Lagrangian analysis used to determine the dynamic behavior of materials. The new method to determine also the shock Hugoniot stress-strain curves is proposed for PC at medium particle velocities up to about 1 km/s. The revised, unsteady wave sensing system (M-UWSS, which we proposed before) using plate impact experiment with three PVDF gauges embedded is applied to construct stress-strain curves under shock loading up to Hugoniot stress σH and Hugoniot strain εH. Linear relationship between shock velocity Us and particle velocity Up: Us = C0 + S x Up, where C0 and S are material constants, is used to determine the constant S, since the constant C0 is determined as bulk sound velocity at ambient pressure. By using the momentum conservation and the mass conservation relations, S = (1 - C0 / CH) /εH, is derived from the linear relationship described above, where , ρ is density and CH ≈ Us.