Research On Residual Stress Measurement Methods And Experiments Based On Pile-up Of Indentation

In the process of machining, the plastic deformation caused by external force and temperature is the main reason of the residual stress, and the residual stress has a great influence on the mechanical behavior and service life of the material. Indentation method has been widely used in the measurement of material mechanical properties and residual stress as its simple, fast and non-destructive characteristics. However, the indentation measurement method can only obtain the average stress of a measure point so far, the stress component and direction of residual stress cannot be obtained. Therefore, in order to measure the component and direction of residual stress in the general stress state, this paper intends to explore new detection and analysis technology of indentation method from experimental and simulation aspects.In the indentation test, because of the plastic deformation of the material, the material accumulation and subsidence occurs around the indentation. Through analysis, it is found that the deformation amount of the indentation, especially the maximum pile-up around the indentation after unloading, is related to the magnitude and direction of the residual stress. In order to obtain the quantitative relationship between the maximum pile-up and residual stress, the finite element analysis method was used in this paper. Spherical indentation processes under the conditions of different combinations of residual stress were computed and analyzed and the quantitative bilinear relationship between pile-up and residual stress is obtained. It was found that the tensile residual stress increase the pile-up and the compressive residual stress decrease it after unloading. In the aspect of the research of the residual stress direction, the direction of maximum pile-up is the same as the direction of the maximum stress, and its vertical direction is the same as the direction of the minimum stress. In addition, by extracting the displacement data from the surface of the indentation after simulation, the interference fringes are formed according to the interference principle in this paper, and the experimental interference fringes are similar to those in relevant literature.In order to verify the correctness of the simulation results, this paper designed an experimental apparatus which can provide uniaxial and biaxial residual stress. After applying the known stress to the specimen, the static indentation test was carried out on the middle part of the specimen. The diameter of the indenter is 2.5mm and the pressure on the indenter is constant. After unloading, the non-contact measurement of the surface profile of the indentation was carried out by using a laser scanning confocal method with high precision. The results of theory and simulation are verified by the experiment, the experiment shows the bilinear relationship between the residual stress and the amount of the pile-up. It can be confirmed that the residual stress can be characterized by the pile-up of the indentation after unloading.In addition, the impact indentation tests under different stress states were carried out by using the impact indenter of KJS-3 type, and the profiles after unloading were measured by optic method. The results showed that there is a similar law between the pile-up and the residual stress under impact indentation.According to the results of theoretical simulation and experimental analysis, this paper presents a general empirical formula of the indentation pile-up and biaxial residual stress. The direction and components of biaxial residual stress can be directly calculated from the pile up around the indentation based on this formula. The method proposed in this paper is also applicable to impact indentation.The accuracy of the method proposed in this paper is largely determined by the accuracy of 3D profile measurement. Therefore, the paper researched three kinds of non-contact measurement methods: laser triangulation method, the method of tomography and the method of projection. These optic methods are used to obtain accurate pile-up around the indentation after unloading.In this paper, a new method for measuring the residual stress of the two dimensional measurement is presented, which is based on the spherical indentation test. This method can quickly and effectively measure the residual stress in the object, which can meet the needs of engineering application, and provides a new method for measuring and analyzing the surface properties and the remaining life of components.