Hiroshi Hamasaki

Elasto‐Plasticity Behavior of Type 5000 and 6000 Aluminum Alloy Sheets and Its Constitutive Modeling

To examine the deformation characteristic of type 5000 and 6000 aluminum alloy sheets, uniaxial tension, biaxial stretching and in‐plane cyclic tension‐compression experiments were performed, and from these, r‐values (r0, r45 and r90), yield loci and cyclic stress‐strain responses were obtained. For the accurate description of anisotropies of the materials, high‐ordered anisotropic yield functions, such as Gotoh’s biquadratic yield function and Barlat’s Yld2000‐2d, are necessary. Furthermore, for the simulation of cyclic behavior, an advanced kinematic hardening model, such as Yoshida‐Uemori model (Y‐U model), should be employed. The effect of the selection of material models on the accuracy of the springback prediction was discussed by performing hat bending FE simulation using several yield functions and two types of hardening laws (the isotropic hardening model and Y‐U model).

Cyclic Stress-Strain Response and Martensitic Transformation Behavior for Type 304 Stainless Steel

Stress-strain responses of type SUS304 stainless steel at large strain under uniaxial tension and cyclic loading were investigated with special reference to plastic strain induced martensitic transformation. From the experiment it was found that the martensitic transformation plays an important role for the workhardening of the material at large strain, and a new finding from the cyclic experiment is that the stagnation of martensitic transformation appears at an early stage of reverse deformation. The evolutions of martensite volume fraction during monotonic and cyclic deformations were calculated by Stringfellow model, and it was found that the model is accurate enough for predicting the martensite volume fraction vs. plastic strain curve under monotonic loading case. On the other hand, it significantly overestimates the evolution of martensite volume fraction in a cyclic deformation.

A Study of High Temperature Viscoplastic Deformation of Beta Titanium Alloy Considering Yield-Point Phenomena

In this paper, the high temperature, deformation behaviour of beta titanium alloy Ti-20V-4Al-1Sn sheet is studied by performing uniaxial tension experiments at three different strain rates at high temperatures of 700°C, 750°C and 800°C. The stress-strain curves for these temperatures show strain rate sensitivity, yield point phenomena and continuous flow, softening patterns. Microstructures of deformed specimens at several representative deformation stages and different strain rates are studied using an optical microscope. Dynamic recovery does not occur at the early stage of deformation including the yield-point and the subsequent yield drop regime, but it is activated at a large deformation stage, where it is affected by both strain rate and strain. A viscoplastic, constitutive model, based on the assumption of rapid dislocation multiplication, is proposed to describe such high temperature, yield-point phenomena. In this modelling, the softening effect due to dynamic recovery is also considered. The stress-strain responses, predicted by the constitutive model, well capture the yield-point phenomena, strain rate sensitivity and subsequent continuous flow, softening behaviour of the beta titanium alloy.

Modeling of anisotropic hardening of sheet metals

To describe the evolution of anisotropy of sheet metals, in terms of both r-values and stresses, the present paper proposes anisotropic hardening models, where the shape of yield surface changes with increasing plastic strain. In this framework of modeling, any types of yield functions are able to be used. The evolution of anisotropy is expressed by updating the yield function as an interpolation between two yield functions defined at two different effective plastic strains. In this paper, two types of interpolation models, i.e., nonlinear interpolation model and piecewise interpolation model are presented. These models are validated by comparing the experimental data on 3003-O aluminum sheet (after Hu, Int J Plasticity 23, 620-639, 2007). To describe the Bauschinger effect, the combined anisotropic-kinematic hardening model is formulated based on Yoshida-Uemori kinematic hardening model.

Mechanical Behavior of 980MPa NANOHITENTM at Elevated Temperatures and its Effect on Springback in Warm Forming

High tensile strength steel sheets have large springback after being formend at room temperature. Warm forming can be a solution to reduce springback of high tensile strength steel parts. NANOHITENTM is a high strength ferritic steel precipitation-strengthened by nanometer-sized carbides developed by JFE Steel Corporation. Tensile strength of the steel at room temperature does not change before and after deformation at elevated temperatures up to 873K since the carbides in the steel are stable at high temperatures less than 973K. Therefore, the steel is suitable for warm forming. Springback of 980MPa NANOHITENTM parts warm formed at 873K is the same level of that of cold formed conventional 590MPa steel parts. In this study, two kinds of material testing at room temperature and at elevated temperatures between 573K and 937K were performed to understand the mechanical behavior of 980MPa NANOHITENTM: uniaxial tensile tests and bending tests. The steels flow stress depends on not only material temperature but also strain rate in uniaxial tensile tests. After a bending test, the specimen shows springback measured by the change of an angle between the two sides. Stress relaxation happens while a test specimen is held at the bottom dead point after bending. And the stress relaxation could be used to reduce springback of warm formed parts.

Experimental and Simulated Springback after Stamping of Copper-Based Spring Materials

The reliability of connector is depending on the contact force, generated by the spring in terminals of connectors. The springs are formed by stamping from a strip of spring material. Therefore, the prediction of its springback after stamping and force-displacement relation by the finite element (FE) simulation is important for the design of terminals. The mathematical model of stress-strain (s-s) responses of the materials is required for the simulation. When the materials are deformed in forward and the following reverse directions, almost all the spring materials show different s-s responses between the two directions, due to the Bauschinger effect. This phenomenon makes the simulation difficult because the s-s response depends on the prior deformation. The authors reported that s-s responses of copper-based spring materials under cyclic tension and compression deformations could be expressed accurately by the Yoshida-Uemori model, which is a constitutive model describing the Bauschinger effect. In this paper, experimental and simulated springback after stamping will be presented. The simulation results using Yoshida-Uemori model were good agreements with the experimental. The use of this model for the FE simulation would be recommended for the more accurate prediction of springback.

Plastic-Bending of Adhesively Bonded Dissimilar Sheet Metals

In this paper, the effect of material characteristics on plastic forming of adhesively bonded dissimilar sheet metals was investigated by experiments and finite element method (FEM). The acrylic adhesive employed in the experiments has visco-plasticity characteristics with high ductility and strong strain-rate and temperature sensitivity in strength. Major results obtained are summarized as follows: (1) In the adhesively bonded dissimilar sheet metals, the gull-wing bend and the shear deformation of the adhesive layer are suppressed by the combination of the sheet metals when a bending inside sheet has high-tensile strength. (2) The gull-wing bend is suppressed by high-speed forming at a lower temperature as well as the same kind of sheet metals. (3) The calculated results using MSC Marc2010 are relatively good agreement with the experimental results.

Influences of Materials’ Workhardening and Stretching Force on Onset of Necking under In-plane Stretch-Bending

The necking limit of a sheet metal under stretch bending is much larger than that in uniaxial tension because of the existence of a strain gradient. This paper discusses the influences of materials’ workhardening and stretching force on the onset of necking under in-plane stretch-bending of sheet metals based on the experiment and its corresponding finite element (FE) simulation. To represent the workhardening of the material in the simulation, a combined Swift-Voce hardening model with a weighting coefficient μ was used. The anisotropy of sheets was considered using a sixth-order polynomial yield function. The results of experiment and simulation on three types of high-strength steel sheets are presented. From these results, it was found that the necking is more likely to occur for low workhardening materials under large stretching force, although the stretching can prevent the buckling of the sheet.

Equi-Plastic Work Locus of 5000 Series Aluminum Alloy Sheet at Warm Temperature

Warm temperature biaxial stretching equipment was developed in order to observe equi-plastic work locus at elevated temperature. A thermostatic bath is installed in the conventional biaxial stretching machine, and a heat gun and an electric heater in the bath enable to heat the specimen up to 260°C. Since flow stress is affected by strain rate at warm temperature, the equipment enables stress ratio and strain rate controls at the same time. Equi-plastic work loci of AA5182-O was obtained by the developed apparatus at R.T., 170 and 260°C, and temperature dependence on biaxial deformation was discussed.

Description of Closure of Cyclic Stress-Strain Loop and Ratcheting Based on Y-U Model

This paper proposes a cyclic plasticity model to describe the closure of a cyclic stress-strain hysteresis loop based on the Y-U model. In this model, the backstress moves in a cyclic memory surface following a newly proposed kinematic hardening law. For this model just the same Y-U parameters can be used, and no additional material parameters are needed. By using a supplementary rule, this model is also able to describe ratcheting.