Research On The Determination Of The Fracture Toughness Of Nuclear Steels By Automated Ball Indentation Testing Method

Nuclear energy has been developed and used by more and more countries in the world as a kind of non-pollution clean energy.However,because of the danger of the nuclear energy,it’s very important for the structural integrity assessment and safe use of the nuclear reactor pressure vessels.Fracture toughness,as the ability to resist crack extension,is a very important index for nuclear reactor pressure vessels in service.How to measure the fracture toughness nondestructively for nuclear reactor pressure vessels in service has been focused on in recent years.Automated ball indentation(ABI)method can be used to assess the fracture toughness of nuclear vessels in service.The author traced and analyzed the principle of material fracture toughness tested by ABI method.The author simulated the automated ball indentation process and conventional fracture tests process by finite element software ABAQUS.The stress and strain field beneath the indenter and the crack tip are analyzed respectively.It is found that the stress beneath the indenter is compressive and the stress of the crack tip region is mainly tensile,but there are still some similarities between the two regions.The shear stresses at the crack tip and at the rigion 45 degree to the loading direction are similar.The shear stress can promote the dimple fracture.Therefore,there is a characteristic region beneath the indenter where the shear stress is similar to that at the crack tip,which provides some support for the determination of the fracture toughness by ABI tests.The author used common nuclear pressure vessel steels SA508-3,SA516Gr70 and SA533B as the study materials.Conventional tensile tests and fracture tests were carried out to get the fracture toughness of the materials respectively at room temperature.At the same time,ABI tests were performed and the fracture toughness of the corresponding materials were got by the HFTM(Haggag Fracture Toughness Method)model and CIE(Critical Indentation Energy)model.The compared results show that the fracture toughness got from ABI tests has a certain degree of deviation with the conventional fracture test results.Therefore the principle of the ABI method to determine the fracture toughness still needs to be corrected.SEM observations on ABI tested specimens were carried out across the cross sectional view of the indentation to study the evolution of the holes beneath the indenter.The observation results show that ABI tests can cause damage beneath the indenter.The holes increase with the increasing of the depth of the indenter,and the holes are most located in the rigion 45 degree to the loading direction.Cyclic loading and unloading tensile tests were conducted on the materials at room temperature and the critical void volume fraction f*corresponding to the critical fracture in CIE model of ABI tests were calculated based on continuum damage mechanics.The calculation results show that the critical void fraction f*of the three materials is 0.221,0.247 and 0.229 respectively.Optical and SEM observations were conducted around the indentation.The observation results show that there is obvious plastic deformation.The phenomenon of pile-up and sink-in surrounding the indentation were studied by ABAQUS.The influences of the depth h,strain hardening index n and yield strain on the height of the pile-up were discussed combined with the dimension analysis theory.An empirical expression was established between the pile-up coefficient c~2 and the three factors above,which can provide support for the calculation of the projected area of the indentation.Based on the above research,the author has made some corrections to the CIE model by using the critical void fraction f*=0.221,0.247 and 0.229 and the average/=0.232 to determine the critical depth of the indenter,and using the empirical function of the pile-up coefficient c~2 to determine the true contact area between the indenter and material.The results of the corrected CIE model are compared with the results obtained by the conventional fracture test.It is found that the calculated results of the corrected CIE model are significantly close to those of the conventional fracture test.The overall deviation can be less than 14%and 22%by using the critical void fraction of each material and the average respectively.The former results are more accurate but the later results are more universal,especially for those nuclear steels without names.Using the average critical void fraction f*=0.232 to determine the critical depth of the indenter is more convenient to do with the practical engineering problems.