本文採用Prandtl-Reuss 之塑流法則和Hill 的異向性降伏準則，結合有限變形理論，及Updated Lagrangian Formulation (ULF)的觀念，建立一增量型彈塑性大變形的三維有限元素分析程式，以探討方板對角線彎曲成形之沖頭負荷與衝程關係、應力分佈、應變分佈、變形歷程、前彈現象及工件厚度變化等，並設計一組V 型90馬勿s曲成形模具加以實驗，以驗證程式的可信度。經數值分析與實驗結果比較得知，成形工件之最終外形於對角線方向兩側之角隅處產生向下彎曲現象，且工件於除荷後發生前彈現象，其數值分析之前彈角度為86.8陛A與實驗中所量得之前彈角度86.64鬲蛪穜答鞢A故可驗證本三維有限元素分析程式的可信度及正確性。 A methodology for formulating an elasto-plastic finite element model, based on the Updated Lagrangian Formulation, Prandtl-Reuss flow rule, and Hill's yield criterion is developed to analyze diagonal bend ing process of the square metal sheet. Simulation results include the relationship between punch load and punch stroke, the distribution of the stress, the distribution of the strain, deformation diagrams in different forming stages, phenomenon of springforward, and variation of the workpiece thickness. A set of 90.degree. V-shaped tools are designed for experiments to verify the reliability of the program. According to the simulation and experiment results, the sharp end of the workpiece along diagonal direction was bent down and springforward after unloading in the final process of bending. The simulated angle of springforward of the workpiece was 86.80.degree. and the experimental angle of springforward was 86.64.degree.. Simulation results show good agreement with experiments that can be used to verify the reliability and accuracy of the three-dimensional finite element program in this study.