tailieunhanh - Materials Selection and Design (2010) Part 17

Tham khảo tài liệu 'materials selection and design (2010) part 17', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | In the second case P is increased from 0 to 534 kN 120 kips and then brought back to zero. The entire history of stresses in the three bars is shown in Fig. 1 b . When P exceeds 400 kN 90 kips the two outer bars deform plastically and because of the reduced modulus begin to share less load. The stress in the two outer bars follows the path ABCD whereas that in the middle bar follows the path ABEF. It can be seen that when P is again zero unloading the stresses in the three bars do not go back to zero. Instead the middle bar has a residual tensile stress of MPa ksi and each of the two outer bars has a residual compressive stress of MPa ksi . Because there is no external load on the assembly the residual stresses in the three bars are in self-equilibrium. A comparison of the two loading histories indicates that the presence of inhomogeneous plastic deformation in the three bars is responsible for the generation of residual stresses. Similarly mechanical residual stresses occur in any component when the distribution of plastic deformation in the material is inhomogeneous such as the surface deformation in shot-peening operation. Thermal Loads. A similar three-bar model explaining the generation of residual stresses due to inhomogeneous plastic deformation caused by thermal loads is discussed by Masubuchi Ref 6 presumably adopted from Ref 7 . In this model three carbon-steel bars of equal length and cross-sectional area are connected to two rigid blocks at their ends. The middle bar is heated to 593 C 1100 F and then cooled to room temperature while the two outer bars are kept at room temperature. Some of the details are not clearly explained in Ref 6 but the problem is very similar to the previous example. When the temperature in the middle bar is raised the requirements of compatibility and equilibrium imply that a compressive stress be generated in the middle bar and tensile stresses in the two outer bars the stress in each of the two outer bars .

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