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Tham khảo tài liệu 'modeling and simulation for material selection and mechanical design part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | A. Flow Curves In order to characterize the strain hardening behavior of metallic materials during plastic deformation one has to determine experimentally the relation SY sY eeq ẽeq T that defines the dependence of the flow stress SY on the plastic parts of the equivalent strain eeq the equivalent strain rate eeq and on the temperature T. Flow curves are defined as the relation SY sY eeq determined for eeq const at a constant temperature. They are often determined in compression test taking into consideration the influence of friction. They are also to be determined in tension test up to the ultimate force assuming uniform deformation. 1. Empirical Relations The flow curves are almost described by power laws. The oldest of these relations introduced in 1909 by Ludwik 1 is given by SY K Ken 5 This relation allows a good description of the flow curves of materials having a finite elastic limit. For a plastic strain e 0 the flow stress equals K0. It leads however to an infinite value for the slope of the curve dơỴ de at the yield point. A simplified form of this equation SY Ken 6 was suggested by Hollomon 2 . Because of its simplicity it is till now the most common relation applied for the description of the flow curve. However no yield point is considered by this relation as SY 0 for e 0. Especially for materials with a high yield point or materials previously deformed the flow stress cannot be described well by this relation in the region of small strains. A more adequate description is achieved by the Swift relation 3 SY K B e n 7 For e 0 a yield point is considered with a value of SY KBn. An alternative description SY a b 1 exp ce 8 was introduced by Voce 4 and is well applicable for the range of small strains. Figure 1 shows the optimum fit achieved by the four equations 5-8 for the flow curves of an austenitic steel at different temperatures in the range of relatively small strain up to 0.2. The figure shows that the Swift relation and the Voce-relation describe
