Measurement Of Residual Stress By Indentation Testing And Effect On Micro-mechanical Behaviors Of A Duplex Stainless Steel

Residual stress usually arises in forming,processing or service environment.Residual stress has great influence on the performance of materials and structural components.Especially such as fatigue life,deformation,dimensional stability,corrosion resistance and brittle fracture.Although the technologies of residual stress control have been greatly improved in recent years,considerable efforts are still needed to develop economic and effective methods for residual stress measurement.For different situations,there are many different ways to measure residual stress.In actual measurement,when we select the experimental scheme,full consideration should be given to the specific conditions,such as the feasibility,adaptability,cost and efficiency.With the development of science and technology,many residual stress problems can not be solved or solved well by the existing methods.Therefore,many times we should take corresponding measurement methods according to specific problems,which lead to new methods,combining the existing measurement methods,proposed according to the specific problems.In recent years,many attentions have been paid to the new residual stress measurement methods combining optical measurements with destructive methods.This combination can integrate the advantages of two methods to a certain extent and help to solve some new problems.This paper focuses on the measurement of equal-biaxial residual stress on the surfaces of components/specimens by combining indentation testing and digital image correlation(DIC).This method combines the advantages of DIC and indentation.First of all,relative to other non-contact measurement methods,the advantages of DIC lie in the potential of fast measurement,simple surface preparation,applicability of rough or curved surface,ability to modify the rigid body movement,and simple requirement of the equipments.At the same time,the damage of indentation technique is very small compared to destructive methods,which have a large damage to the components or specimens.It is of great practical significance for the engineering problems,which are not allowed to use destructive methods because of safety problems.This method not only expands the application scope of DIC,but also complements and perfects the residual stress measurement system to a certain extent.Then,the influence of residual stress on the micro-mechanical behaviors of duplex stainless steel is discussed in detail by means of scanning electron microscope(SEM)and digital image correlation.First,based on cavity model,the relationship between deformation field around the indentation and residual stress was derived.According to the relationship,the experimental process and residual stress simulation device were designed.In the experiments,the CCD camera was used to capture the speckle images.The displacement field around the indentation was calculated using DIC,and then residual stress was derived according to the relationship between displacement field andresidual stress.The final results showed that this residual stress measurement method is feasible and reliable.Secondly,the deformation of plastic region around the indentation using simulation method was studied.Then,the influence and role of plasticity on the residual stress measurement was discussed,which was introduced into the displacement fitting function.The modified displacement function solved the problem that the fitting effect on the displacement field around the indentation was not good using the displacement function based on cavity model.Finally,the accuracy and range of this method was increased and expanded.Thirdly,combining with SEM and DIC,taking 2205 duplex stainless steel as the object of study,the effect of residual stress on the micro-mechanical behaviors of materials was studied.And we measured the residual stress in the 2205 duplex stainless steel using the residual stress measurement method studied in this paper.Residual stress existing in the microstructure was simulated by thermal loading.In the study of initial deformation response,it was found that residual compressive stress in ferrite can restrain its deformation to a certain extent.Meanwhile,residual tensile stress in austenite can promote the deformation of austenite.By comparing the true stress-strain curves,we found that residual stress can make the plastic deformation and failure of materials faster.In other words,residual stress will speed up the failure of materials or components in service.In the study of cyclic-loading response,it was found that positive strain concentration was mainly in austenite,which was directly related to the existence of residual stress to a certain extent.Residual tensile stress can increase the deformation of austenite in cyclic-loading test,and further aggravated the strain concentration in austenite.In addition,compared with the research results before,it was found that residual stress had a certain inhibitory effect on the hardening phenomenon of duplex stainless steel in the cyclic-loading experiment.The result of the inhibition is to accelerate the failure of specimens/components,thereby reducing the fatigue life of materials.Finally,by using stress relief annealing,the influence of residual stress on micro-tensile and relaxation behavior of duplex stainless steel was studied from a point of view mean strain.In the study of micro-tensile,it was found that residual stress had little effect on the deformation of duplex stainless steel in TD(Transverse direction),but can enhance the deformation in RD(Rolling direction).In the study of relaxation behavior,it was found that residual stress made the duplex stainless steel have more rapid and larger relaxation behavior in TD,and had a certain inhibitory effect on the hardening of relaxation behavior.The two findings indicate that residual stress will have a negative effect on the dimensional stability and service life of materials,and accelerate their failure.