損傷模型之建立及工程應用

Abstract

本研究在能量空間建立彈塑性損傷模型，根據不可逆連體熱力學框架建立材料組成律，採用微觀力學均質法估算機械性質的衰退歷程，採用Gurson塑性勢描述損傷材料的塑性行為。作者提出新的分析方法研究延性材料的破壞機制，由單軸拉伸狀態出發，在能量空間建立物理意義是極限塑性耗散的破壞判準，推廣到多軸應力狀態時，在主軸應力空間建立「損傷功」與「破壞能」以描述各種應力狀態的損傷演化與破壞起始。 本研究結合損傷模型與有限元素法建立成形極限預測法，以同樣的方法論預測板金成形、塊體成形與超塑性成形的成形極限，得到良好的結果，並且成功地將此方法應用於建立成形極限圖、成形程序評估與模具設計等。為了提升超塑性製程效率，作者結合損傷模型與超塑性失穩參數建立「定失穩度吹製法」，控制超塑性變形在特定的失穩程度以縮短成形時間並成功地完成吹製目標。

This research develops an elastoplastic damage model on the energy domain. The constitutive equations are derived from the continuum Thermodynamics of irreversible processes and theory of internal variables. The residual mechanical properties are evaluated by the homogenization method of micromechanics. The plastic deformation of damaged materials is described by Gurson’s plastic potential. A new analysis methodology for the fracture mechanism of ductile materials, based on the plastic energy dissipation, is proposed in this thesis. The fracture criterion starts from uniaxial tension and says that crack will initiate when plastic dissipation reaches a critical value. Generalized to a general stresses state, the new concepts of the damaging work and the fracture energy are proposed for the quantitative description of damage evolution and crack initiation. Combing the proposed damage model and finite element method, we develop a methodology for fracture prediction of metal forming processes, by which the forming limits of sheet forming, bulk forming and superplastic blow forming can be predicted by the universally applicable approach. Furthermore, the methodology is successfully applied to the development of forming limit diagram, the evaluation of forming processes, and mold design. In order to improve the efficiency of superplastic forming processes, we develop a new forming pressure design guideline, constant instability degree control method, based on the manipulation of superplastic instability. The forming time can be reduced significantly by controlling the deformation at a certain degree of instability.