Papers by Author: Yu Liu

Abstract: Generally, the forming limit diagram is widely applied to predict the sheet necking and fracture in the conventional sheet forming process. In recent years, the fact that the forming limit is much higher using Incremental sheet forming (ISF) than that obtained in conventional sheet forming, has become a research hotspot in forming mechanism of local forming process. In this paper, the geometrical imperfection in the thickness was presumed to represent local weakening zone and the BAMMAN_DAMAGE material model which using void to describe non homogenization caused by geometric imperfection, were used respectively to investigate the effect of geometry imperfection on the sheet forming limit. Based on the W.C. Emmens’ experiment, the reason and mechanism of enhancement of forming limit during incremental forming was studied through the numerical simulation method. And the variations of stress and strain during the forming process were also studied.

Abstract: With the application of ultra high strength steel in vehicle manufacturing field, the hot forming method which is used to form the high strength sheet at high temperature and cool rapidly in dies to obtain the final part has been studied extensively. In this paper, The FE model that contained thermal mechanical-microstructure and strain model represented by a mixture law for hot forming was discussed, and further been applied to hot forming and quenching process simulation of 22MnB5 metal sheet of U-shape constant section part. The computed results of final part shape and hardness after cooled in air, in water without dies and cooled within dies were compared respectively.

Abstract: During the cold rolling forming of non-symmetric Z-section steel, the twist will appear because two sides of the blank suffering bending moments of the same direction. In this paper, simplified FE model was used to simulate the closing process of the rolls. The contact forces of the rollers on blank, and the stress and strain of the blank was obtained by FE analysis, and relations between section torque and the position angle θ as studied and the result that when θ=10⁰ the torque is minimum was discovered. The further detailed calculations showed that angle θ was only correlative with the geometry of the product section and didn’t change with the altering of material properties.

Abstract: In this paper, the hemming process, which widely used in automobile industry, was studied by a method of constructing mechanical models combined with FEM simulation results. The warp producing mechanism in the straight edge- plane hemming process was analyzed in detail using this mechanical model. The reason why the warp produced at the position of near the contact point between the out panel and the position surface of hemming die during the pre-hemming process, and the phenomena of warp reducing firstly and then increasing before the position point T and the warp reducing again after point T during the final hemming process were interpreted. The work finished in this paper provides a theoretical basis to understand hemming process and a tool to analyze the reasons why warp produced in the hemming processes.

Abstract: This paper presents a kind of all-digital integrated hydraulic actuator (IHA) unit to drive heavy load object without centralized oil tank. In order to improve the control quality of the actuation system while eliminating or reducing the disturbance, and also to solve the problem of flow rate mismatch existed in IHA with single-rod cylinder actuator, a fuzzy PID PWM controller is suggested. Simulations on the integrated hydraulic actuator unit are carried out to evaluate the effectiveness of the proposed control method when applied to hydraulic systems with various external disturbances encountered in real working conditions. Simulation results are discussed and some conclusions are given.

Abstract: Welding process of automobile panels is a key process in the manufacturing of automobile body and its quality directly impact on the appearance and quality of automobile. The causes of dimensional deviation of welded assembly could be workpieces variation, assembly tooling variation, and welding distortion. As a major source of assembly deviation, dimensional variations of workpieces have important effects on the assembly quality of automobile body. In this paper, pre-stressing was used to reflect the workpieces’ variation and the node coupling method was used in the numerical simulation to predict the deformation of assembly caused by the welding process of automobile panels. And further the computation results were listed and compared with the measured ones.