為探討引發鍛粗製程( upsetting )之自由表面破壞的原因，本文從五種不同的角度觀點來評估五種延性破壞準則之製程適用性，並選用其中製程適用性最佳之準則來做為自由表面破壞之判斷依據，此五種觀點包括等效應力、最大拉張應力、最大拉張應力之正規化 ( normalize )、微觀空孔成長角度以及靜液壓應力等五種觀點。
文中吾人引用可成形性實驗所量測到的應變量，以適用性分析之方法，評估出最適合鍛粗製程的延性破壞準則後，再結合有限元素套裝軟體DEFORM-3D，針對高強度鋁合金Al-2017F進行圓柱壓縮模擬分析，預測Al-2017F在不同潤滑條件與胚料高度直徑比的成形極限。
本文成功的評估出Cockcroft and Latham延性破壞準則最適用於鍛粗製程之破壞預測，並利用該準則預測出Al-2017F在不同潤滑情況及胚料高度直徑比的成形極限。由模擬的結果顯示，摩擦係數與胚料高度直徑比並不會改變材料的破壞軌跡線，而實驗中途摩擦係數的變異，會導致應變路徑的改變，進而影響胚料的可成形性。透過本文的研究，未來僅需依製程特性選用適當的延性破壞準則，再透過有限元素法的模擬分析，最後只需做少數的實驗驗證，便可快速建立出材料完整的成形極限圖 ( FLD )，以提供製程或設計等相關人員在開發新產品或參數最佳化時，判斷胚料何時、何處破壞的依據。

To investigate the fracture on the free surface in upsetting process, five different ductile fracture criteria are evaluated in this research. The evaluated criteria include equivalent stress, maximum tensile stress, normalized maximum tensile stress, void growth and coalescence, and hydrostatic stress. After the most adoptable fracture criterion being obtained, we can identify the billet fractured or not by this criterion.
In this research, we used the limit strain data measured in the literature to evaluate the most adoptable fracture criterion. After the most adoptable fracture criterion being obtained, it is then used to create simulation model by the DEFORM-3D software to predict the forming limits under different process conditions. The simulation of cylinder compression for the Al-2017F alloy was performed in different friction conditions and height/diameter ratios.
The evaluated results show that the Cockcroft and Latham ductile fracture criterion is the most suitable for the fracture prediction in upsetting process. From the simulated results, friction coefficient and the height/diameter ratio will not change the fracture locus. The suddenly change of friction coefficient during experiments will affect the strain path, thus affecting the formability of the billet. With the proposed method, the construction of the forming limit diagram (FLD) can be simplified by selecting proper ductile fracture criteria during finite element simulation and comparing the simulated result with a few experimental results. Thus, the identification of fracture during process parameter optimization can be made.

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