فهرست مطالب h. moslemi naeini

Flexible roll forming process is a novel method for producing profiles with variable cross-section. In this process, to produce the profiles with variable cross-section, the position of the rolls is controlled at any time and moved in accordance with their geometry. In this paper, using finite element simulations and experimental tests, the deformation length is investigated as one of the important parameters in the flexible roll forming process. For this purpose, a finite element model was created in Abaqus software, and then the accuracy of the model was confirmed by comparing the simulation results and experimental data for longitudinal strain in the compression and stretching zones of a channel profile with variable cross-section. The results showed that the deformation length in the flexible roll forming is changed not only in the different zones of the profile, but also in different areas of each zone. The highest deformation length was obtained at the middle of the stretching zone, which is 38% longer than it at the middle of the narrow and wide zones and 48% longer than it at the middle of the compression zone. It was also found that with increasing the forming angle and flange length of the profile, the deformation length increases in all zones of the profile with variable cross-section.

springback is one of the challenges in the final shape of the steel in the cold roll forming process of the steel plate.The precision of the simulation of metal sheet forming processes for prediction of the springback depends on the stress-strain relationship, the yield function and the selected hardening model and the anisotropy of the material. In this paper, using alternating tension-compression experiment and using simulation in software LS-OPT, the variables of the Yoshida-Uemori hardening model have been obtained for St37 steel. Springback in the final shape of steel plate deformation is a challenge of the roll forming process. Since consideration of the hardening law is important in the springback, various models of hardening law are proposed for more accurate prediction of material behavior. One of the most accurate models in predicting metal behavior is the Yoshida-Uemori hardening model. The experimental tests were also carried out as well as the simulations in the Ls-dyna simulation software with and without Yoshida-Umari hardening. It was shown that the prediction of springback with using the Yoshida-Umari model was more accurate.

In this paper, determination of the fracture onset by the ductile fracture criteria was studied and the effect of damage function, calibration method, and calibration tests were investigated on the fracture prediction accuracy. Based on the stress state analysis, three different tension tests including the uniaxial, plane strain, and notched tension tests were conducted to determine the critical value of the ductile fracture criteria. In order to investigate the fracture behavior of AA6061-T6 aluminum alloy sheets, The Ayada, Rice-Tracey, and normalized Cockroft-Latham phenomenological ductile fracture criteria were calibrated through a hybrid experimental-numerical method. The finite element (FE) method were used to simulate the process and calibrated fracture criteria were implemented using an appropriate user subroutine. Experimental force-displacement curve and the fracture displacement were used to validated the numerical simulation and investigate the fracture prediction accuracy. According to the result, calibration method based on the history of the stress state lead to higher fracture prediction accuracy compared to average value method (3.4% decrease in average value of the fracture prediction error). Among all the possible condition, the normalized Cockroft-Latham fracture criteria with the plane strain tension test are the most suitable fracture criterion and calibration test to predict the fracture in the positive value of the stress triaxiality.