Key Engineering Materials Vol. 794

Abstract: TRIP steel shows excellent mechanical properties such as greatly high strength, ductility and toughness by means of the appropriate combination of the strain-induced martensitic transformation (SIMT) behavior and the deformation behavior of each phase at crystal scale. In the past, the effect of grain size in the austenite on the deformation behavior of TRIP steel is investigated by introducing the grain size into a generalized model for the kinetics of SIMT. In order to validate the size-dependent kinetics modelling, it is necessary to simulate the deformation and SIMT behavior of the polycrystalline for the different grain size at the crystal scale. This study focuses on an investigation of SIMT behavior in polycrystalline TRIP steel by finite element simulation. The constitutive formula for monocrystalline TRIP steel including transformation strain in each variant system derived on the basis of the continuum crystal plasticity theory is applied. For the polycrystalline model, Voronoi tessellation is employed. The deformation behavior with a patterning process of martensitic phase in two different numbers of grains with initial crystal orientations for describing the deformation-related length scale is simulated under plane strain condition with two planar slip systems by a cellular automata approach.

Abstract: In this study, the analysis method for thermomechanical properties of plain-woven composites is developed, and applied to thermoelastoviscoplastic analysis of plain-woven glass fiber-reinforced plastic (GFRP) composites. For this, a time-dependent constitutive equation depending on temperature for matrix materials is incorporated into the micro/meso/macro-scale thermo-elastic homogenization method for plain-woven composites developed by our research group. This method enables us to analyze thermoelastoviscoplastic properties in not only fiber bundles but also fibers and matrix materials in fiber bundles, as well as macroscopic thermal properties. This method is then applied to the thermal expansion analysis of a plain-woven GFRP composite subjected to a macroscopic temperature change from 25°C to 80°C before it is cooled to 25°C. Comparing the analysis results with experimental data, we validate the present method. It is also shown that the present method can evaluate themal residual stress and strain in the composite.

Abstract: In this study, development of a decoupled multiscale analysis method for woven composites is conducted. To this end, an elastic-viscoplastic macroscopic constitutive model which is able to express strong anisotropy of composites is introduced, and the material parameters in the constitutive model are determined based on the results of triple-scale homogenization analysis. Moreover, the constitutive model is implemented in the finite element analysis code LS-DYNA. The developed method is applied to 3-point bending analysis of plain-woven carbon fiber-reinforced plastic (CFRP) composites with various types of laminates configurations. It is shown that the present method can analyze their different behavior depending on the laminate configuration with greatly reduced computational costs.

Abstract: Size effects extremely exist in the metal micro-forming process. When a deformation process scales down to micro scale, the appearances of geometry size and single grain size start to play a major role in deformation. Generally, the size effects are unavoidable in the experimental work and cannot be neglect in the optimization of micro-forming processes. In this paper, size effect on flow stress is investigated in the form of the coupled effect of workpiece geometry (sample thickness) and grain size, (T/D) by the micro tensile test of pure copper foil. Following the previous approaches, a new hybrid material model is projected to describe the hardening behavior of grains in polycrystalline material. Tensile tests performed on the copper foil with constant thickness and width, while to get dissimilar grain sizes, the foil annealed for different times. The ratio of thickness to grain size (T/D) is limited to larger than 1 (T/D˃1). A hybrid material model is proposed and established based on grain heterogeneity and sample thickness. The hybrid material model builds a relationship between the surface layer and sheet interior. The hybrid material model developed by the strain gradient theory in which the dislocation cell structure, cell densities (interior and wall) engaged to define the polycrystalline aggregate and calculated the dislocations in a grain (grain interior and grain wall). The results show that flow stress varies with the different values of T/D, but with an increase of the share of the grains flow stress start to decreases. After applying the hybrid material model of flow stress, the micro-tensile test of copper foil is simulated by finite element method. The simulation outcomes well matched with experimental results.

Abstract: The paper presents the results of forming rifts in five-layer metal polymer laminates by electro-magnetic forming. During the experimental studies the discharge energy of the electro-magnetic machine was varied in such a way as to achieve different depths of the rift. Samples obtained by electro-magnetic forming were compared with control samples obtained by forming using a rubber pad under static loading. The strain state of the samples was analyzed using an digital image correlation system Vic-3D.

Abstract: Magnesium alloys have been widely used in automotive, bicycle, aerospace industries. Improving the mechanical properties of the materials by various forming processes has been an important issue. Grain refinement is an effective method to improve material properties. In this study, single-pass and multi-pass hot rolling processes are carried out to generate dynamic recrystallization (DRX) and fine grains and to obtain better mechanical properties on the rolled products. A mathematical model linked with FEM software DEFORM-2D is proposed to predict the dynamic recrystallization ratio and the grain size distributions during various multi-pass hot rolling processes. The effects of different thickness reductions on the grain size distributions inside the sheet are discussed. Multi-pass hot rolling experiments of magnesium alloy ZK60 sheets are carried out and the metallographic microstructures are observed. The measured grain sizes are compared with the simulation results to verify the validity of the proposed model. The effects of different thickness reductions on the mechanical properties of the rolled magnesium alloy ZK60 sheets are also investigated.

Abstract: This study used finite element simulations and experiments to investigate the forming limit in the nosing process of micro copper cups, and to establish the limit nosing curves in terms of nosing ratio, die angle, and friction factor. Two-stage processes, including backward extrusion and nosing processes, were considered in simulations and experiments at micro scale. The copper billets with 1 mm diameter and length were backwards extruded to produce the 1 mm diameter cups with 0.1 mm wall thickness. The cups were later used in the nosing processes under different forming conditions. By analysing the results of the nosed cups from the simulations, it is possible to identify the safe and failure forming conditions and establish the limit nosing curves for the nosing process. The simulation results show that the limit nosing ratio increases as the die angle or friction factor decreases. Two predicted results for poor and well lubricated conditions have been examined and are in good agreement with those from experiments. The study not only explores the characteristics of the noise process of copper cups at micro scale but also establishes the limit nosing curves which could be the guidelines for the design of micro metal components.

Abstract: In recent years, warm forming of magnesium sheets has been investigated by many researchers since the ductility of the sheets becomes considerably higher due to low CRSS (critical resolved shear stress) at high temperatures (e.g., [1]-[3]). In the present research, the springback of AZ31 magnesium sheet was investigated by performing a draw-bending experiment at several temperatures from 20 °C (room temperature) to 200 °C at drawing speeds ranging from 0.01 to 1.0 mm/s. From the experiment, it was found that the springback was remarkably reduced at 200 °C, especially at a low forming speed, since the flow stress was very low under such a forming condition, and furthermore, the stress relaxation effect was dominant. The effects of temperature and forming speed on springback were discussed.

Abstract: The mechanical responses of Ti-5Al-2.5Sn alloy at low temperatures were investigated under quasi-static and dynamic tensile loads using MTS system and SHTB system, respectively. Tensile stress-strain curves were obtained over the temperature range of 153 to 298K and the rate range of 0.001 to 1050 s^-1. Experimental results indicate that the tensile behavior of Ti-5Al-2.5Sn alloy is dependent on strain rate and temperature. Yield stress and flow stress increase with increasing strain rate and decrease with increasing temperature. Results also indicate that strain hardening rate of Ti-5Al-2.5Sn alloy is lower at high strain rate, while strain hardening rate varies little with testing temperature. The Khan-Huang-Liang constitutive model was chosen to characterize the tensile responses of Ti-5Al-2.5Sn alloy at low temperatures and different strain rates. The model results coincide well with the experimental results within the tested temperature and rate ranges.

Abstract: The aim of present wor is, therfore, to investigated the effect of the damage value prediction equation on the formability of compression specimen and find the optimize forming condition.Although Inconel 625 alloys are excellent materials, Ni-base alloy cannot be formed at room temperature owing to limitation of formability. To improve the formability of Inconel 625, it is necessary to investigate the formability at a high temperature range.A high temperature compression test is performed with a Gleeble 3500 testing machine at various temperatures (approximately 900 1200°C) and strain rates (10/s and 30/s) to obtain high temperature deformation characteristics of Inconel 625. Furthermore, high temperature tensile tests results are used to measure elongations and reductions in the area of Inconel 625.A rigid-plastic finite element simulation is applied to the high temperature compression process to obtain the damage valueThe results of the hot deformation experiment and analysis are presented for various conditions of temperatures and strain rates, and it is expected that damage value will be used in hot forming processes such as hot extrusion and rolling process.