| Summary: | 博士 === 國立成功大學 === 土木工程學系碩博士班 === 101 === This research developed constitutive laws as well as an FE model for the purpose of simulating the strain-rate dependent stress-strain characteristics of superplastic FSS materials. The materials used for FE case studies were codoped 3Y-TZP ceramics at 1400°C as well as an AZ31B-H24 mg alloy at 400°C. Two kinds of analyzed materials were assumed to be homogeneous and isotropic for FE simulations, and the incremental stress-strain relationships were formulated using a 3-D elastic-plastic model, which simulated the elastic response using Hooke’s law and the work hardening response using the flow rule associated with the von-Mises yield criterion combined with the isotropic hardening rule. The FE simulations on the uniaxial tensile tests of two kinds of materials were performed for the purpose of verifying the reliability of proposed constitutive laws of each material.
The methods used to develop the constitutive laws of two kinds of materials are described as follows: (1). The simplified constitutive laws of codoped 3Y-TZP ceramics were developed based on piecewise linear connections at the turning points of different deformation stages on the experimental stress-strain curves. (2). The constitutive law developed by curve fitting the tensile tests data of an AZ31B-H24 mg alloy was expressed as a flow stress function of strain and strain rate.
Both kinds of constitutive laws were embedded into the above mentioned elastic-plastic FE model to simulate both tensile tests. The results show that the stress-strain characteristics and the final deformed shapes in the FEA agree well with the experiments for both kinds of materials. These results show that the proposed FE model is suitable to simulate the mechanical behavior of superplastic materials, and the presented constitutive law of AZ31B-H24 mg alloy is reliable to describe the strain-rate dependent stress-strain characteristics, which combines advantages of viscoplastic and elastic-plastic constitutive models for superplastic materials.
In addition, the FE verification on a free bulge forming experiment of the AZ31B-H24 mg alloy show that the proposed FE model is practicable for mechanical analysis on superplastic forming problems. This research offers a selective numerical method for advanced analyses on superplastic materials.
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