Ninshu Ma 교수

In the present work, duplex stainless steels (DSSs) SAF2507 were friction stir processed with flowing cryogenic medium mixed with liquid nitrogen and industrial ethanol. Homogeneous ultrafine grained (UFG) dual-phase structure with ferrite (α) and austenite (γ) grains with average size of around 1 μm was obtained due to the significantly lower peak temperature and shorter high-temperature duration. The developed UFG dual-phase structure enabled a dramatic enhancement of yield strength by about 2

Keywords: explicit FEMvelocity scalingmass scalingwelding deformationlarge-scale modelheat conductionthermal elastic plastic theory

Authors programmed a subroutine of Yoshida-Uemori model to commercial software LS- DYNA considering Young's modulus change with plastic strain, and then carried out a springback simulation for two stamping parts using solid element and shell element respectively. To make a designed shape and size of a stamping part, a new function for springback reason research and an algorithm for tool surface compensation were proposed and programmed to simulation system JSTAMP. Lastly, a research for springba

Friction has a significant effect on the formability of stamping parts. A constant friction coefficient between stamping tools and blank is often employed in the metal forming simulating. In this presented work, several non‐linear friction models which considers of the change of friction coefficient with contact pressure, sliding velocity, sliding distance, frictional work, plastic strain and temperature were developed using LS‐DYNA customized friction subroutine. The validity was verified by nu

Resistance spot welding is the most popular joining process in automobile body assembly production lines. There have been many researches concerned with spot welding for two pieces of sheets. In order to reduce the cost and improve the strength of the lightweight automobile bodies, resistance spot welding for multi-pieces of sheets has to be investigated before production. In this paper, resistance spot welding for three pieces of high strength steel sheets is mainly taken as an object and the n

Abstract To estimate the strength of automotive bodies under impact loading and to predict the crack occurrence of high strength steel sheets, the local fracture strain and the ductile damage limit must be measured by experiments to provide data for metal forming simulation by using FEM. In the present study, the local strains at the cracking position and its nonlinear strain path during a simple uniaxial tensile test were measured using a digital image grid method developed by authors. Then, a

In this study, Al joint systems without heat-affected zones (HAZ) were developed on Al/Fe alloy lap joints fabricated by resistance element welding (REW). The robustness of the joints with and without HAZs was quantitatively evaluated to consider another side effect in REW Al/Fe alloy systems. Compared to conventional REW, removing the Al HAZ not only alters the steel properties, but also reduces the work hardening of the Al alloy due to the difference in the distinct heat transfer paths. Furthe

Parts created by Tube Hydro‐Forming (THF) are increasingly used in automobile bodies because of their high stiffness. THF parts have complex shape with varying cross‐sections along its length, therefore, a multi‐stage forming process such as bending, pre‐forming and hydro‐forming is generally necessary to produce them. To understand and to determine the forming conditions before production, simulation technology becomes indispensable. In this paper, firstly, the sheet metal simulation system JST

The generated residual stresses (RS) in wire-arc additively manufactured components might harm their capability and lifetime. This work aims to systematically elucidate the generation and distribution mechanism of RS in wire-arc additively manufactured components made of the increasingly applied ultra-high strength steel (UHSS). Here, numerical sensitivity analyses as well as experimental measurements were executed for investigation. The results indicate that, apart from the thermal effect, aust

An accelerated explicit method with two-stage computation scheme is developed for simulating transient thermasl stress and deformation occurring in a long time welding heating and cooling process. The first computation stage is based on a dynamic explicit method considering the characteristics of the welding mechanical process by controlling both the temperature increment and time scaling parameter. In the second computation stage, a static equilibrium computation scheme is implemented after dyn

To investigate the local strain and stress at the crack initiation position in shear fracture test pieces of ultra-high strength steels, a butterfly shear fracture specimen was employed. The crack initiation position and propagation direction were observed during shear fracture tests by high speed cameras and investigated through analysing the fracture surface by scanning electron microscope. Further, the finite element method was employed and the stress-triaxiality at the crack initiation posit