先進高強度鋼板反覆拉壓與雙軸拉伸變形特性之研究

Deformation characterization of advanced high strength steel sheets under cyclic tension-compression and biaxial stretching.

10.6342/NTU.2012.02883

蔡恒光

包辛格效應 ； 先進高強度鋼板 ； 材料模型 ； 反覆拉壓實驗設計 ； 有限元素法 ； 雙軸拉伸實驗設計 ； 回彈分析 ； Bauschinger effect ； advanced high strength steel sheets ； material model ； tension-compression test ； finite element analysis ； biaxial test ； springback analysis.

國立臺灣大學機械工程學系學位論文

2012年

博士

陳復國

繁體中文

先進高強度鋼板已經被廣泛應用於汽車結構件，由於強度較高，其沖壓成形問題較傳統低強度鋼複雜，尤其是回彈、側壁捲曲及扭曲等缺陷更為嚴重，即使是使用最佳之模擬參數進行有限元素法分析，其對先進高強度鋼板沖壓回彈模擬預測之準確性仍嫌不足。因此為探討先進高強度鋼板之成形特性以及提升沖壓回彈模擬之準確性，目前產學研究各界均轉從材料模型進行研究。 本論文所探討之材料模型主要包含加工硬化準則及降伏準則。在加工硬化準則方面，由於先進高強度鋼板之包辛格效應較明顯，且對於塑流應力與彈性係數均有影響，因此為探討包辛格效應，特針對板材反覆拉伸與壓縮試驗，設計專屬之限制治具，以防止板件於壓縮試驗時產生挫曲。而為解決限制治具與試片間以及限制治具外側與機台夾頭間兩處之挫曲問題，本論文除使用有限元素分析進行限制治具設計之定性模擬觀察外，也從有限元素模擬分析結果中推導出限制治具各項功能尺寸之經驗設計公式，以使本設計適用於不同板厚之實驗。從針對不同鋼種之反覆拉壓變形實驗結果中，本論文證實先進高強度鋼之包辛格效應的確較為顯著，且其循環硬化之特性與低強度鋼有所不同，且在反覆拉壓變形過程中，板材之彈性係數呈現出非線性之變化。為瞭解產生包辛格效應之機制，本論文亦從材料微觀組織觀察到在反覆變形過程中，差排密度之變化係與流應力之大小有關且變化趨勢相同。 為將拉壓實驗結果應用於有限元素模擬分析，本論文針對商用軟體泛用之Yoshida-Uemori模型，建立該模型所需之各項材料參數，除藉以模擬本論文所進行之拉壓實驗以驗證該材料參數之適用性與正確性外，亦針對實際板件沖壓成形進行模擬分析與驗證，結果發現在 沖壓成形過程中，若板件有承受明顯拉壓之變形歷程，則採用具包辛格效應之材料模型，將可大幅提高有限元素模擬回彈分析預測之準確性。 在降伏準則方面，本論文為探討最能描述先進高強度鋼板塑性變形之降伏準則，以及降伏準則與材料異向性的關係，特針對單軸拉伸試驗機台，設計一組具雙軸拉伸機制之夾治具。在雙軸拉伸機構設計中，本研究利用有限元素法模擬分析，成功完成機構驅動與兩軸拉伸應力比值的設計，並計算出該機構於滑動時之摩擦係數。同時利用有限元素法，本研究亦設計出試片中央為雙軸主應力之最佳試片幾何形狀，且該區域範圍足供貼黏兩片應變規以供應變之量測。從雙軸拉伸試驗結果發現Barlat降伏準則較為適合描述先進高強度鋼(590Y)在雙軸受力下之塑性變形行為。此外本研究也使用基礎載具及業界載具(車側門檻構件)進行有限元素回彈模擬分析與實驗驗證，從模擬分析與實驗驗證結果確認，採用Barlat降伏準則並結合考慮包辛格效應之加工硬化準則，可有效提高有限元素模擬回彈預測之準確性。

Advanced high strength steel sheets have been widely used in automobile structural parts due to their high strength. However, the technical difficulties are experienced in the stamping of the advanced high strength steel sheets, such as the presence of springback, sidewall curl, and distortion. The finite element analysis also could not render satisfactory results in the springback prediction for the advanced high strength steel sheets even with the optimum simulation parameters adopted. Therefore, in order to examine the forming characteristics of advanced high strength steel sheets and enhance the accuracy of the finite element analysis in springback prediction, the study of material model becomes necessary for the forming of advanced high strength steel sheets. The material model discussed in this thesis included work hardening rule and yield criterion. In the aspects of work hardening rule, the Bauschinger effect exhibited in the advanced high strength steel sheets was examined by conducting cyclic tension-compression tests with a novel constraint jig developed in the present study. The constraint jig could prevent the sheet specimen from being buckled during the compression test. The finite element analysis was also performed to derive the various characteristic dimensions of the constraint jig to make it applicable to cyclic tension-compression tests with different sheet thicknesses. From the results of repeated tension-compression deformation tests on sheet specimens with different strengths, the Bauschinger effect of advanced high strength steel sheets was confirmed to be indeed significant, and their cyclic hardening characteristics were difference from those of low strength steel sheets. It is also found from the test results that the elastic modulus of steel sheets shows a nonlinear change during the reverse loading path that may affect the springback of sheet metal forming. The microstructure observation made in the present study reveals that the coefficient of dislocation density change during the cyclic tension-compression deformation had the same changing trend as that of flow stress of the sheet metal. In order to apply the tension-compression test results to the finite element simulations, the material parameters adopted in the Yoshida,-Uemori model that takes the Bauschinger effect into the work hardening rule were constructed. The simulation results showed that during the forming process, if the sheet metals were subjected to cyclic tension-compression deformation, the material model with Bauschinger effect considered was found capable of improving the accuracy of springback prediction significantly. In yield criteria aspects, in order to explore the best description on yield criteria of plastic deformation of advanced high strength steel sheets, as well as the relationship between yield criteria and material anisotropy, a set of biaxial tension mechanism was especially designed on the clamping jig for the uniaxial tensile testing machine by this thesis. In the design of biaxial tensile mechanism, a finite element analysis was used by this study to conduct a simulation analysis and was successful to design the mechanical drive and two-axis tensile stress ratio, as well as calculated the coefficient of friction during the mechanism slide motion. Meanwhile, the finite element analysis was used by this study to design the central point of test specimen that found to be the best geometrical shape of the specimen in biaxial principal stress, and the range of such region was sufficient to laminate with two strain gauges to measure the supply changes. From biaxial tensile test results, the Barlat yield criteria was discovered to be more appropriate to describe the plastic deformation behavior of advanced high strength steel sheet (590Y) under the biaxial stress. In addition, the basic carrier and industrial carrier (door beam component of vehicle) were also used by the study to conduct finite element springback simulation analysis and experimental verification. From the simulation analysis and experimental verification results, the Barlat yield criteria used by this study plus the consideration of work hardening criteria of Bauschinger effect had proved to be able to improve the predictive accuracy of simulation of springback of the finite element effectively.

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