Multi-square Punch Forming And Springback Prediction Of Strips

The Three-dimensional Computer Numerical Control Hull Plate Forming Machine,which improves the intelligence of the hull plate forming prcess largely,is a great breakthrough in the field of naval architecture and ocean engineering.The height and the rotation of the punches of the forming machine can be adjusted flexibly,cutting down the cost of the moulds for forming the hull plates that are not in mass production.In addition,the staggered arrangement of the punches of the forming machine overcomes the denting and the wrinkling problems,which improves the forming quality of the plate.However,the springback is still a predominant problem in the plate forming on the machine.To solve this problem,the thesis experimentally investigates the forming and the springback of the strips and the plates made from different materials on one type of the forming machine,i.e.SKWB-400,and reproduces the forming process of the strip by a simplified theoretical model to reveal the forming mechanism and to predict the springback.Besides,the springback compensation method applicable for multi-square punch forming is proposed and embedded into the springback prediction model,achieving the one-step or few-steps forming of the strip with high accuracy.The main research contents and results of the thesis are as following:(1)The springback database of the strips in multi-square punch forming is set up,which can provide experimental validations for the theoretical analysis of the forming and springback of strips under multi-square punch follower load.The forming and springback of different strips in multi-square punch forming are investigated experimentally,and the effects of different parameters on the strip’s springback are discussed qualitatively.The results indicate that the springback decreases with the increase of the forming curvature and the thickness,and increases with the increase of the yielding stress and the strain hardening.For anisotropic materials,cutting directions affect the material characteristics,and thus have effects on the springback of the strip.The forming approach(convexly or concavely)is also an important factor on the springback.Compare with the convexly forming experiment with the same forming conditions,the forming curvature in the concavely forming is large,and the springback is smaller after unloading.(2)A simplified theoretical model is established to simulate the convexly forming and springback of strips under multi-square punch follower load.Based on the mechanical properties of srips made from different materials,the convexly forming and springback formulae of elastic-perfectly-plastic strips,elastic-Swift hardening strips and elastic-linearly hardening are derived,revealing the convex forming and springback mechanism of strips in multi-square punch forming.Besides,the effects of the material properties,the cutting direction,the strip’s thickness,the forming curvature and the critical length on the convexly forming and springback of the strip are explained theoretically,providing guidance for the springback control and compensation of the strips under multi-squre punch convexly forming.(3)A simplified theoretical model is established to simulate the concavely forming and springback of strips under multi-square punch follower load.The concave forming and springback formulae of elastic-perfectly-plastic strips are derived,revealing the concavely forming and springback mechanism of strips in multisquare punch forming.Besides,the effects of the forming approach(concavely or convexly),the friction coefficiency and the boundary constraints on the concavely forming and springback of the strip are explained theoretically,providing guidance for the springback control and compensation of the strips under multisqure punch concavely forming.(4)The double-curvature forming experiments of the square plates made from three different materials are conducted on SKWB-400,which can provide experimental validations for the theoretical analysis of the forming and springback of plates under multi-square punch follower load.The forming and the springback of the plates are investigated and compared with the ones of the same strips in the same forming condition.The results show that the plate’s springback in the doublecurvature forming is significantly smaller than the strip’s,and the more advanced the part of the plate contact the lower dies closely,the smaller the springback is.Besides,the material anisotropy also leads to a great discrepancy of the springback laws btween the plate and the strip.(5)A springback compensation method is proposed for the strips in multi-square punch forming,and it is embedded into the springback prediction model of the strips under multi-squre punch follower loads.The mould shape can be predicted by the embedded model with the given target shape of the strip,facilitating the one-step or few-steps forming of the strip with high accuracy.The thesis revealed the forming and springback mechanism of the strips under multi-square punch follower loads,providing theoretical foundations for the strip’s springback prediction and compensation,which has important academic significance.In addition,a series C++ codes are developed based on the theoretical models in the thesis,and they can be embedded into the commercial software and existing multisquare punch forming program,which has considerable engineering application prospects.