Hisashi Nishimura

Effective thermal conductivity of compressed woods

Compression is one solution to improve the strength of softwoods. The effective thermal conductivities of compressed Japanese cedars (cryptomeria japonica), which were compressed in the radial direction of the wood, were measured. Both the effective thermal conductivities in the tangential and fiber directions increase proportionally to the density increment due to the compression. However, the thermal conductivity in the radial direction (compression direction) increases slightly with the density increment. Numerical computations were conducted to explain the characteristics of thermal conductivity in the radial direction by using a microscopic heat conduction model for the compressed wood. The numerical results were compared with the measured values. And the physical mechanism of the heat conduction in the compressed woods is discussed.

The influence of surface defects on the forming-limit diagram of sheet metal

Abstract In order to study the effects of surface defects, which are one of the causes of initial inhomogeneity of sheet thickness, on the forming-limit diagram (FLD), uniaxial tensile, in-plane stretching and hydraulic bulging tests were performed using soft and hard pure aluminum sheets, by introducing an artificial machined notch. The results of these experiments showed that the existence of a critical depth of surface defects α e was found, up to which fractures do not develop and the forming-limit does not decrease. Specifically, the forming-limit strain did not decrease for defect depths of up to 2 and 0.5% of the thickness for soft and hard sheets, respectively. Numerical calculations were performed also using the Marciniak–Kuczynski (M–K) model to evaluate the effects of defect depth theoretically and it was clarified that the rate of decrease of limit strain with respect to the increase of defect depth can be expressed qualitatively. However, quantitatively, only the calculated results for the effects of the defect depth on the limit strain of the soft sheets for the uniaxial tension case and those from the equi-biaxial tension case agreed with the results obtained by experiment. Moreover, a method for the estimation of α e was developed using the M–K model together with forming-limit strain data obtained by experiment. From these results, it is concluded that it is possible to decrease the rejection rate and reduce the material cost by applying less strict standards when inspecting the surface conditions of sheets in production shops, if the presence of defects is not a serious problem in terms of external appearance.

Development of real-time process control system for precision and flexible V-bending with an on-line database

Abstract An advanced process control system based on expertise is developed for the V-bending process. An experimental database and a fuzzy inference model are employed as the expertise for real-time process control. The punch force-stroke curves and other process parameters stored in the database are retrieved in-process and evaluated by the fuzzy model for precise prediction control. Also, a learning process, which is similar to the way craftsmen acquire knowledge, is applied for database and fuzzy model improvements in order to cope with new materials. Several experiments were carried out to confirm the validity of the developed control system. The results show that the process control is capable of coping with differences in the material properties and other forming parameters automatically even if the operator has no information on the material properties and the process.

A finite-element model for the superplastic bulging deformation of Ti-alloy pipe

Abstract The superplastic bulging deformation process of Ti-alloy pipe under constant strain rate at the apex has been analyzed using a finite-element model (FEM). For the simulation of the process and the prediction of the final thickness distribution of the deformed part, MARC-MENTAT was used as a computational tool. An incremental approach based on a rigid-plastic flow formulation was employed. Special sub-routines were developed and linked to the main program for the superplastic material properties and the results of pressure calculations of constant apex-strain-rate deformation for each step. By FE modelling it is confirmed that the constant apex-strain-rate deformation process is superior to the constant pressure deformation process with respect to wall-thickness bulging. The results of calculation are compared to experimental results.

Effects of the surface micro-geometry of steel sheets on galling behavior

Abstract Steel sheets with the same manufacturing history of melting, rolling and annealing are used for this investigation. The effects of various surface micro-geometries, which depend on skin pass conditions such as the surface micro-geometry of the work rolls and the rolling distance, on the development of galling are studied. A strip-draw-type device is used as the galling simulator. A rectangular sheet specimen slides between flat dies, the contact length with the sheet of which is 18 mm, under a contact pressure of less than 60 MPa. After the consecutive sliding of ten specimens, the occurrence of galling on the last specimen is checked visually. The sliding distance of each specimen is 170 mm. It is found that the critical contact pressure at which galling occurs increases as the surface waviness of the steel sheets increases. However, the surface-roughness quantities R a (center-line average height), R z (ten-point average height) and PPI (peaks per inch) do not show significant correlations with the critical contact pressure. Increasing waviness represses the increase in contact between the specimen and the dies area at the die exit, presumably resulting in promoting the long-time preservation of the lubricating oil.

Development of an intelligent tool system for flexible L-bending process of metal sheets

A new intelligent tool system for sheet metal forming is proposed, and a flexible L-bending control system using several sensors incorporated in the tools is developed. This system is autonomous and capable of changing the shape of the tools and the pressure acting on the workpiece to optimize the forming process and to improve the forming limit and accuracy. In this study, the springback characteristic in L-bending was assessed on-line using the intelligent tool system. Closed-loop control of flexible, high-precision bending was used to compensate for variations in the dimensions and material properties of the workpiece. The experimental results show that bend angles formed with very high accuracy have been obtained even with only limited knowledge of the material properties or thickness of the workpiece. This intelligent tool system significantly improves the bend accuracy and flexibility of the L-bending process.

Critical Stress of Microcracking in Alumina Evaluated by Acoustic Emission

Microfracture process during the bending tests of alumina ceramics were evaluated by acoustic emission technique. Different size specimens were used for the bending tests in order to investigate the dependence of microfracture process on the specimen size. A remarkable point in the AE generation pattern of each specimen, at which both AE events and energy increased rapidly, was observed before the final unstable fracture. It is important that the apparent stress at those points were independent of the AE threshold level and specimen size. Using the Fluorescent dye penetrant method, the fracture process on the surface was observed, then it has been understood that the stress corresponds to the critical stress for the maincrack formation due to the coalescence of microcracks and/or pores. Considering the microfracture process, statistical treatment for the strength of brittle materials has been discussed. Consequently, it was concluded that the critical stress can be the advanced evaluation parameter, which is equivalent to yield strength in metals, for ceramic materials.

Nosing of thin-walled tubes by circular curved dies

Abstract Experiments into the nosing of thin-walled aluminum tubes have been carried out in order to clarify the characteristics of nosing by a circular curved die. Comparing the results with those for a conical die, the advantages of a circular curved die are discussed in detail. It is established that a circular curved die is effective in reducing the nosing load, restraining unfavourable deformation, and improving the nosing limit for tubular materials, which buckle at the cylindrical part in conical nosing. It is suggested that it is of advantage to introduce a circular curved part into the inlet of a conical die. Further to the above, numerical analysis is performed on the optimum die radius and die contact pressure, upon which the effect of the die profile is examined.

Experimental analysis of the flattening of the cross-section, the springback and the bending moment of clad tubes in uniform bending

Abstract In order to clarify the fundamental bending-deformation characteristics and the load property of clad tubes, the flattening behavior, the springback and the bending moment of adhesive-bonded two-layer clad tubes are investigated experimentally. The experiments were carried out on clad tubes consisting of soft aluminum, hard aluminum and copper by means of a 4-point bending test, a new measuring instrument having been developed for the continuous measurement of the flattening and the bending curvature during deformation. A unique negative flattening deformation behavior in the bending direction was observed in the case of the clad tubes (Al/Al) with hard aluminum on the outside. It is found that the springback on the bending curvature and the bending moment of a clad tube having a material with large flow stress on the outside are greater than those of a clad tube having the reverse configuration.

The role of strain hardening and strain-rate sensitivity in uniaxial tension and hemispherical-punch stretching

Abstract In this paper, the expression δ = K ϵ . m ϵ n has been adopted for the plastic flow of the sheet metal, for which expression Swifts criterion for diffuse necking has then been developed. The analytical prediction emphasizes the significance of the rate of change of the strain rate during stretching (γ = dln ϵ dot / d ϵ) , and assesses the role of the m value in combined form with the n value and the strain-rate path γ. For some non-ferrous metals — including monolithic clad metals — tensile testing and hemispherical-punch stretching were carried out at room and elevated temperature. The presently available clad metal composed of aluminum and superplastic alloy exhibited a rise in ductility with increase in the deformation rate in both tests, which must be a very advantageous feature for practical press forming. It should be appreciated as a new observation, that one can explain the deformation behavior in terms of the experimentally measured strain-rate path.