發(fā)布時間：2018-04-22 20:23

  本文選題：非線性應(yīng)變路徑 + 成形極限��； 參考：《湖南大學(xué)》2015年博士論文

【摘要】：金屬薄板沖壓成形是一種廣泛用于汽車、航空航天、裝備制造業(yè)零部件生產(chǎn)的塑性成形工藝,成形極限是評價成形零件質(zhì)量好壞和工藝設(shè)計優(yōu)劣的重要指標(biāo),準(zhǔn)確與否對成形工藝設(shè)計至關(guān)重要。傳統(tǒng)的成形極限圖是基于線性應(yīng)變路徑假設(shè)進(jìn)行理論計算或試驗測試獲取的,然而,實際沖壓成形過程中的材料所經(jīng)歷的多為非線性應(yīng)變路徑狀態(tài),在復(fù)雜的非線性加載路徑條件下,由晶體塑性變形理論,一個滑移方向的塑性變形會影響晶體其它方向的后續(xù)變形,應(yīng)變路徑變化對材料行為尤其是強化特性和成形極限有著顯著的影響。因此,對非線性應(yīng)變路徑條件下的材料本構(gòu)模型和成形極限進(jìn)行研究,探索預(yù)應(yīng)變加載條件下的成形極限理論,分析熱沖壓工藝條件下的成形極限,對于提高成形性評價準(zhǔn)確度及其在金屬塑性成形工藝中的工程應(yīng)用具有重要意義。本文圍繞非線性應(yīng)變路徑變化條件下金屬薄板的成形極限,開展了預(yù)應(yīng)變加載條件下的成形極限理論與計算方法、連續(xù)應(yīng)變路徑變化條件下的成形極限試驗測試方法和數(shù)值模擬分析,熱沖壓工藝條件下的成形極限及其在熱成形工藝中的應(yīng)用等方面的研究。本論文完成的主要研究內(nèi)容和所取得的研究成果如下:(1)針對Swift分散性失穩(wěn)理論和Hill集中性失穩(wěn)理論受到線性應(yīng)變路徑假設(shè)條件約束的問題,建立了變應(yīng)變路徑條件下材料失穩(wěn)的約束方程,并基于該約束方程,提出了適用于含預(yù)應(yīng)變加載條件的雙段線性應(yīng)變路徑成形極限理論與計算方法。在理論計算中,當(dāng)采用復(fù)雜的材料本構(gòu)模型時,往往難以得到直接的解析結(jié)果,此時,采用基于溝槽缺陷假設(shè)的M-K模型成為計算非比例加載條件下的板料成形極限的合理選擇,然而,材料本構(gòu)模型中高次屈服函數(shù)的引入易使得M-K模型中力平衡條件和幾何相容條件組成的非線性方程組的求解計算產(chǎn)生振蕩,造成塑性應(yīng)變率比達(dá)不到臨界收斂值的問題,為了克服這一問題,在非線性方程組的迭代求解中引入了一個回溯算子,對迭代步長進(jìn)行相應(yīng)的縮放,有效的抑制和消除了迭代計算過程中的振蕩和不收斂問題,使得基于M-K模型的成形極限計算更易于得到收斂的結(jié)果。同時,采用文中的方法對鋁合金板Al 2008-T4在雙段線性應(yīng)變路徑條件下的成形極限進(jìn)行了計算,結(jié)果表明,計算結(jié)果與成形極限試驗曲線吻合良好,較為準(zhǔn)確的預(yù)測了預(yù)應(yīng)變對材料成形極限曲線的影響。(2)提出了包含應(yīng)變路徑加載歷史的全新成形極限評價指標(biāo)Fsp,為準(zhǔn)確判斷連續(xù)應(yīng)變路徑變化條件下的金屬薄板成形性提供了科學(xué)的依據(jù),為驗證該成形性評價指標(biāo)的準(zhǔn)確性,基于拉延成形工藝中材料的變形模式,設(shè)計了拉延-反拉延成形極限試驗裝置,通過試驗研究了采用DIC技術(shù)獲取板料成形過程中實時應(yīng)變路徑的方法和成形極限應(yīng)變狀態(tài)的判斷方法。測量得到的板料初始失效點的應(yīng)變路徑數(shù)據(jù)表明,拉延-反拉延成形極限試驗裝置成功的實現(xiàn)了薄板成形過程中應(yīng)變路徑的典型變化,得到的連續(xù)應(yīng)變路徑變化模式包括:單軸拉伸-平面應(yīng)變轉(zhuǎn)變模式、等雙軸拉伸-平面應(yīng)變轉(zhuǎn)變模式、平面應(yīng)變-等雙軸拉伸-平面應(yīng)變轉(zhuǎn)變模式�；诶�-反拉延成形極限試驗中板料的變形特點,修正了極限拱頂高評價指標(biāo),定量地分析了試樣形狀和工藝條件對拉延-反拉延成形極限試驗過程中應(yīng)變路徑和修正的極限拱頂高評價指標(biāo)的影響,依據(jù)測量得到的試樣初始失效點應(yīng)變路徑試驗數(shù)據(jù)驗證了所提出的連續(xù)應(yīng)變路徑變化條件下的薄板成形性評價指標(biāo)的準(zhǔn)確性。(3)建立了考慮非線性應(yīng)變路徑引起的材料潛在硬化(Latent Hardening)效應(yīng)的各向異性本構(gòu)模型,該模型通過引入反映材料變形過程加載路徑變化的內(nèi)變量,分別考慮了非線性應(yīng)變路徑加載中材料微結(jié)構(gòu)變化對各向同性硬化和隨動硬化模量的影響,反映了材料特性的應(yīng)變路徑相關(guān)性,能夠表征材料在反向加載時的包辛格效應(yīng)和正交應(yīng)變路徑變化時的交叉硬化效應(yīng),彌補了標(biāo)準(zhǔn)的各向同性-隨動強化組合模型存在沒有考慮因非比例加載路徑變化引起的潛在硬化效應(yīng)的缺陷。編寫了二次開發(fā)材料子程序UMAT將該模型集成到薄板成形有限元數(shù)值模擬軟件LS-DYNA中。采用文中建立的各向異性本構(gòu)模型對拉延-反拉延成形極限試驗和先進(jìn)高強度鋼的扭曲回彈進(jìn)行了數(shù)值模擬仿真,得到的結(jié)果和相應(yīng)的試驗數(shù)據(jù)比較表明,考慮了潛在硬化效應(yīng)的強化模型結(jié)合先進(jìn)的各向異性屈服準(zhǔn)則提高了數(shù)值模擬的精度。(4)分析了溫度和應(yīng)變率對22Mn B5超高強度鋼的材料特性和力學(xué)行為的影響,引入Logistic方程對熱成形板料塑性各向異性特性的溫度相關(guān)性進(jìn)行描述,構(gòu)建了熱成形工藝條件下的塑形各向異性材料本構(gòu)模型,研究結(jié)果表明,所建立的本構(gòu)方程對板料在高溫奧氏體條件下的流動行為描述與試驗結(jié)果一致;基于M-K模型,計算得到了恒溫常應(yīng)變率穩(wěn)態(tài)條件下的成形極限曲線,結(jié)果表明,在考慮的工藝條件范圍內(nèi),當(dāng)溫度升高時,板料的成形性提高,當(dāng)應(yīng)變率增加時,成形極限曲線有所提高;構(gòu)建了適用于熱沖壓成形工藝過程的三維成形極限曲面,建立了B立柱的熱-力耦合有限元模型,B立柱熱成形數(shù)值模擬結(jié)果和試驗結(jié)果的對比表明,對成形后零件截面的厚度分布二者吻合良好,有限元仿真準(zhǔn)確的預(yù)測了工件的破裂失效位置,驗證了所建立的材料本構(gòu)模型和熱-力耦合模型的準(zhǔn)確性以及三維成形極限曲面在熱成形工藝中應(yīng)用的有效性,提出的熱沖壓成形極限的模擬預(yù)測方法為溫?zé)釥顟B(tài)下薄板的成形性分析提供了重要的依據(jù)。
[Abstract]:Sheet metal forming is a plastic forming process widely used in automobile, aerospace, and equipment manufacturing parts production. The forming limit is an important index to evaluate the quality of forming parts and the advantages and disadvantages of the process design. The accuracy is very important to the design of forming process. The traditional forming limit diagram is based on the linear strain path false. It is obtained by theoretical calculation or test test. However, the material in the actual stamping process is mostly nonlinear strain path state. Under the condition of complex nonlinear loading path, the plastic deformation theory of crystal, the plastic deformation of a slip direction will affect the subsequent deformation of the other direction of the crystal and the strain path change. Therefore, the constitutive model and forming limit of materials under the condition of nonlinear strain path are studied. The forming limit theory under pre strain loading conditions is explored and the forming limit is analyzed under the conditions of hot stamping, and the accuracy of formability evaluation is improved. It is of great significance for the engineering application of metal plastic forming process. In this paper, the forming limit theory and calculation method under the condition of pre strain loading are carried out around the forming limit of the metal sheet under the condition of nonlinear strain path change, and the test method and numerical simulation of the forming limit test under the condition of continuous strain path change are carried out. Analysis, research on the forming limit of hot stamping process and its application in hot forming process. The main contents and achievements of this paper are as follows: (1) according to the problem of the Swift dispersion instability theory and the Hill centralized instability theory being constrained by the linear strain path hypothesis, the changes are established. The constraint equation of material instability under the strain path condition, and based on the constraint equation, the forming limit theory and calculation method suitable for the double segment linear strain path with pre strain loading conditions are proposed. In the theoretical calculation, when the complex material constitutive model is used, it is often difficult to get the direct analytical results. At this time, the trench based on the ditch is adopted. The M-K model of the hypothesis of slot defect is a reasonable choice for calculating the forming limit of sheet metal under non proportional loading. However, the introduction of high yield function in the material constitutive model makes it easy to make the calculation of nonlinear equations composed of force equilibrium and geometric compatibility conditions in the M-K model, resulting in the ratio of plastic strain rate to the ratio. To overcome this problem, in order to overcome this problem, a backtracking operator is introduced in the iterative solution of nonlinear equations, and the iteration step is scaling accordingly. The oscillation and non convergence of the iterative calculation are effectively suppressed and eliminated, which makes the calculation of the forming limit based on the M-K model more easily convergent. At the same time, the forming limit of the aluminum alloy plate Al 2008-T4 under the condition of two segment linear strain path is calculated by the method. The results show that the calculation results are in good agreement with the forming limit test curve, and the effect of the pre strain on the forming limit curve of the material is accurately predicted. (2) the strain path loading is put forward. The new forming limit evaluation index Fsp of history provides a scientific basis for accurately judging the formability of metal sheet under the condition of continuous strain path change. In order to verify the accuracy of the formability evaluation index, based on the deformation mode of the material in the drawing process, a tensile and reverse drawing limit test device is designed, and the test is carried out through the test. The method of obtaining the real time strain path in the process of sheet metal forming with DIC technology and the method of determining the forming limit strain state are studied. The strain path data of the initial failure point of the sheet material measured shows that the typical change of the strain path in the Bo Bancheng shape process is successfully realized by the drawing limit test device. The model of continuous strain path change includes: uniaxial tension plane strain transition mode, double axis tensile plane strain transition mode, plane strain and equal biaxial tensile and plane strain transition mode. Based on the deformation characteristics of plate material in the limit test of drawing and reverse drawing, the evaluation index of limit vault height is corrected and the sample is analyzed quantitatively The influence of the shape and process conditions on the strain path and the revised limit vault height evaluation index during the drawing limit test of drawing and drawing, and the accuracy of the evaluation index of the Sheet Formability under the condition of the continuous strain path change were verified by the test data of the initial failure point strain path test obtained. (3) An anisotropic constitutive model for the potential hardening (Latent Hardening) effect of material caused by nonlinear strain paths is established. By introducing an internal variable reflecting the variation of the loading path in the process of material deformation, the model considers the isotropic hardening and the dynamic hardening modulus of the material microstructure change in the nonlinear strain path loading. The effect, reflecting the strain path correlation of material properties, can characterize the cross hardening effect of the Basinger effect and the change of the orthogonal strain path in the reverse loading, and make up for the defects of the standard isotropic and servo intensifying combination model without considering the potential hardening effect caused by the change of non proportional loading path. The two development material subprogram UMAT was compiled to integrate the model into the finite element numerical simulation software LS-DYNA of thin sheet forming. By using the anisotropic constitutive model established in this paper, the numerical simulation of the stretch back drawing forming limit test and the distorted springback of advanced high strength steel was simulated, and the results obtained and the corresponding test data were compared. It is shown that the enhancement model considering the potential hardening effect and the advanced anisotropic yield criterion improve the accuracy of numerical simulation. (4) the effect of temperature and strain rate on the material properties and mechanical behavior of 22Mn B5 super high strength steel is analyzed, and the temperature dependence of the plastic anisotropy of the thermally formed sheet is introduced by the Logistic equation. The constitutive model of plastic anisotropic material under the condition of hot forming is described. The results show that the flow behavior of the plate under the condition of high temperature austenite is consistent with the experimental results. Based on the M-K model, the forming limit curve of the constant temperature constant strain rate steady condition is calculated. The results show that the formability of the sheet is increased when the temperature is raised, and the forming limit curve is improved when the strain rate is increased. A three-dimensional forming limit surface suitable for the hot stamping process is constructed. The thermal force coupling finite element model of the B column is established, and the numerical simulation results of the B column hot forming are obtained. The comparison of the experimental results shows that the thickness distribution of the sections of the parts after forming is in good agreement. The finite element simulation can accurately predict the fracture failure position of the workpiece. The accuracy of the constitutive model and the thermal force coupling model of the material and the validity of the application of the three-dimensional forming limit curve in the hot forming process are verified. The simulation and prediction method of hot stamping forming limit provides an important basis for the formability analysis of thin sheet under warm and hot condition.

【學(xué)位授予單位】：湖南大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：TG306

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本文編號：1788766