| 其他摘要 | Indentation-strain method is a new non-destructive method used for residual stress measurement with some virtues, such as convenience, wide application, highly precision, ect. This method has got more and more attention. In order to solve some existing problems, for example, the size of plastic zone under different biaxial residual stresses, the regularities of distribution of strain increment around indentation in different directions and different zone, the difference between dynamic and static indention, a finite element analysis was conducted. The beneficial results obtained in the paper are as follows:
1) Under axial stress, it has been found that the size of plastic zone decreases in the direction parallel to maximum principle stress and increases in the direction normal to maximum principle stress, when the residual stress changed from maximum compress stress to maximum tensile stress. Under biaxial stress, the shape of plastic zone is rather complicated, but the maximum size of plastic zone is less than that in axial stress at all time. The change of plastic zone size was not significant before and after unloaded under any stress field.
2) The change of relation between strain increment and residual strain was not significant before and after unloaded under any stress field. In elastic zone around indentation, strain increment increased when the measure zone get near to the center of indentation. And the residual stress obtained get more exactly under this situation. The change of strain increment caused by eccentric indentation has relation to the size of measured zone (the length of gage lines). The maximal error is 10.6% when the eccentricity of indentation was 0.1mm and gage lines’ length is 0.5-0.9mm. And the error decreased with the increase of measuring zone’s area.
3) The relation between strain increment and effective strain (named the ratio of external strain to the strain when material yield) was different when the inclination with x axial is larger or smaller than 62o. And the relation was changed with this inclination. When the strain increment was measured in the zone where the distance from the center of indentation is 4~5mm, it varied with tensile effective strain corresponding to a square function, and varied with compressive effective strain corresponding to a linear law and square function respectively when the inclination with x axial is larger or smaller than 62o. When the strain increment was measured in the zone where the distance from the center of indentation is 3~4mm, the basic relation between the strain increment and effective strain was unchanged. But when the inclination with x axial is smaller than 45o, the square function which was used to describe the relation of the strain increment and tensile strain can be replaced by linear law. And the slop of the linear law varied with the inclination corresponding to a square function. Under equal biaxial stress field, the regularities of distribution of strain increment in all directions were the same. In any biaxial stress field, strain increment in the direction parallel to maximum principle stress was affected by the stress in this direction and normal to this direction. If plus the product of Poisson’s ratio and the strain in the direction normal to maximum principle stress to the strain in the direction parallel to maximum principle stress, then the relation was the same with that in axial stress field.
4) When the stress field and load are constant, indentation depth, size of plastic zone and the absolute value of strain increment will decrease with the raising of yield strength.
5) Compared with the situation of static indention with the same penetration depth and stress field applied, the size of dynamic indentation plastic zone changed a little, but the strain increment of dynamic indentation was smaller.
Key words: indentation-strain method, FEM, biaxial residual stress, strain increments, static load indentation，dynamic load indentation |
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