Abstract
Tensile testing is widely used for the mechanical characterization of materials subjected to superplastic deformation. At the same time, it is known that the obtained flow data are affected by specimen geometry. Thus, they characterize the specimen rather than the material. This work provides the numerical analysis aimed to study how the material flow behavior affects the results of tensile tests. The simulations were performed by the finite element method in Abaqus software, utilizing user-defined procedures for calculation of forces acting on the crossheads. The accuracy of tensile testing is evaluated by the difference between the input material flow behavior specified in the simulations and the output one, obtained by the standard ASTM E2448 procedure based on the predicted forces. The results revealed that the accuracy of the superplastic tensile test is affected by the material properties. Even if the material flow behavior follows the Backofen power law, which is invariant for the effective strain, the output stress–strain curves demonstrate significant strain hardening and softening effects. The relation between the basic superplastic characteristics and the tensile test errors is described and analyzed.
Highlights
Superplastic sheet forming is a promising technology utilizing the ability of certain materials to reach extremely high deformations while forming at a specific temperature and strain-rate regimes [1,2].The accurate mechanical characterization is critically important in the determination of such regimes and subsequent design of forming technologies
The results presented are obtained for the ideal viscous material with the flow behavior described by the Backofen equation
The results presented in this paper show that even if the material behavior does not demonstrate any strain hardening or softening effects, they will appear in the stress–strain curve obtained by the standard tensile testing procedure
Summary
Superplastic sheet forming is a promising technology utilizing the ability of certain materials to reach extremely high deformations while forming at a specific temperature and strain-rate regimes [1,2].The accurate mechanical characterization is critically important in the determination of such regimes and subsequent design of forming technologies. Superplastic sheet forming is a promising technology utilizing the ability of certain materials to reach extremely high deformations while forming at a specific temperature and strain-rate regimes [1,2]. The experimental methods of mechanical testing, providing information about the flow behavior of superplastic materials, deserve extensive and thorough analysis. Tensile testing at constant strain rate remains the most widely used experimental way providing information about the deformation behavior of superplastic materials. Tensile testing is utilized for constitutive modeling of superplastic alloys [9,10,11,12,13], determination of optimal forming conditions, and the construction of material models for the design of forming technologies [14,15,16]. The stress–strain curves obtained by superplastic tensile tests at constant strain rates affect the microstructure changes during the deformation. Strain hardening at the lower strain rates is usually associated with grain growth; strain softening at higher strain rates is associated with dynamic recrystallization or void fraction growth [9]
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