Acquisition Method Of Forming Limit Diagram And Its Application In Sheet Metal Forming

Forming limit diagram is a very important criterion in sheet metal forming for determination success or failure of a forming part. It is a good tool in FEM simulation and in material selection for the automotive parts. However due to the complexity of the forming process, many factors which effect the test and application has never been clearly understood.With some new material and forming technology are applied in industry, the traditional FLD study is not good enough. There are few study focus on material selection use FLD in China now. In this paper, experimental and theoretical methods for the development of FLD of the traditional softsteel and aluminum sheet were studied. Many factors that affect the FLD were investigated through experiment. The results and main contributions of the dissertation are as following:(1) Experimental investigation of FLD test method and factors which affect results. From the concept of forming limit diagram and its application. The process of conventional experiment method is analysised and factors in which affect the test results were find out. Realization of different strain path and estimation of forming limit condition are investigate and optimized. The effect of tools, specimens and strain measurement method was investigated through experiment. Application of the new developed automatic strain measurement in the experiment of forming limit diagram is introduced and analyzed.(2) Forming limit diagram prediction method based on the conventional plastic theory. A detail introduction of Swift diffuse necking theory, Hill localized necking theory and M-K theory is given. In addition, a numerical forming limit diagram prediction method based on these three theories is developed. A detail failure process of a soft steel and aluminum alloy is investigated through experiment. Results of theory prediction and experiment are compared with each other. Results show that the new developed FLD prediction method is convenient and can give a reasonable accurate prediction.(3) Numerical prediction method of the forming limit diagram based on Fem software. The forming limit diagram prediction method based on the FEM software is introduced. The failure criterion and theirs realization in the FEM software prediction are detailed. The effect of the material model and failure criterion to the results of the FEM predicted forming limit diagram are investigated through simulation experiment. A good model for the FEM forming limit diagram prediction method is developed. (4) Investigation on the path dependency of the stress and strain based forming limit diagram. The definition and develop method of the forming limit stress diagram are introduced. Path dependent of the stress and strain based forming limit diagram were investigated through experiment. An effective strain based path independent forming limit diagram is developed. A good guide for application of forming limit diagram in multistage forming process is given out.(5) Forming limit diagram for the forming process at elevated temperature. Failure criterion and forming limit of aluminum alloy AA5182,AA7075T6 and AZ31B at elevated temperature are investigated through experiment. Flow behavior of aluminum alloy at warm forming condition is investigated through tensile test and forming limit is got through experiment. Effect of the temperature and strain rate of the aluminum alloy are investigated through experiment. A fem based forming limit prediction method for aluminum at warm forming condition is developed.(6) Development of the material selection system for automotive sheet metal forming. A material selecting system based on forming limit diagram for the automotive company is developed. The system include a material database which can give a detail formability of different material, and a spare parts database which gives the information of different spare parts of automobile. This system can help the process and tools design department and the material supply department to select best material for certain part and reduce cost.