Papers by Author: Toshio Hattori

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: Generally the critical distance stress theory was applied for the fatigue limit estimation of general structures using fatigue limit of smooth specimen (σ_w0), and threshold stress intensity factor range (K_th). In this paper we extended this method for the estimation of low cycle fatigue life too. In this method we define the critical distance (r_c) on static strength conditions, which is calculated using ultimate tensile strength (σ_B) and fracture toughness (K_IC), in addition to the critical distance on fatigue limit condition (r_c). Then the critical distances of any low cycle fatigue conditions can be calculated by interpolation of critical distance on fatigue limit (r_c) with critical distance on static strength (r_c). By unifying these low cycle fatigue life estimation method with high cycle fatigue limit estimation method we can estimate the full range fatigue life easily. And to confirm the availability of this estimation method we perform the fatigue test for any stress concentration specimens.s

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 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: The stress and displacement fields near the bonding edge, sharp notch, and contact edge show singularity behaviours, so methods of evaluating the strength of these points using maximum stresses calculated by a numerical stress analysis, such as the finite element method, are generally not valid. We have previously presented a new method of evaluating the strength of these singular points using two stress singularity parameters H and λ. In this paper we have developed a method of formularizing critical stress-singularity parameter Hth for each order of stress singularity λ by utilizing critical distance stress theories (point method and line method), which can be derived from two typical strength parameters, namely, fatigue limit σw0 and threshold stress-intensity factor range ΔKth. These estimated critical Hth (λ) value agreed well with the experimentally measured value. Using these simple critical distance stress approach we estimated the fretting-fatigue-crack initiation criteria for any contact edge angle and optimized the contact-edge geometry. Moreover, we apply this new strength criteria to general stress concentration structures.

Abstract: The method for prescribing the site of the first buckling lobe in the axial impact of the tubular structure is proposed. The inertia force induced by the solid mass attached to the tube during the impact is exploited to trigger the first buckling lobe. When the tube with the solid mass undergoes a large acceleration, the inertia force of the solid mass is expected to bend on the tube wall. In the experiment, the rectangular solid mass was attached to the aluminum alloy square tube. The tube fixed to the drop-hammer was impacted against the stationary rigid plate at the velocity of 5 or 7.7 m/s. For the case of the tube without the solid mass, the site of the first buckling lobe varied and the slight wavy plastic deformation remained further than the buckling lobes. On the other hand, when the tube with the solid mass was impacted, the onset of the first buckling lobe was observed at the portion where the solid mass was attached and the wavy deformation stated above was suppressed. The corresponding computation was also conducted using the dynamic explicit finite element method. The result showed a good agreement with the experimental one.

Abstract: In this paper, we have evaluated the sliding and the loosening behavior of thread joints under transverse loading. Firstly, the critical relative slippage (Scr), less than which the thread joints can keep the fastening, is obtained by the cyclic loading tests. Then, this critical relative slippage is estimated according to the theoretically obtained equation considering the bending deformation of bolt and the geometrical constraint condition. The inclination compliance (kw) of the bolt head used in this equation is evaluated by comparing the experimental result with the corresponding analytical one. In consideration of the nonlinearity of kw with respect to the bolt axial tension, the Scr is well estimated by this equation.