Research On The Determination Of The Fracture Toughness Of Pressure Vessel Steels By The Continuous Spherical Indentation

In order to ensure the security of the large-scale process industrial equipment,whose process conditions tend to be more severe,the structural integrity assessment and the risk-analysis technique are increasingly applied.And their analysis is based on the determination of mechanical properties of in-service equipment.Fracture toughness,a property that indicates the ability of a material to resist crack propagation,is one of the most important mechanical properties that deserves our special attention.Therefore,determination of the fracture toughness for in-service equipment by means of no-sampling approaches can not only ensure their safety but also yield considerable economic benefits.In response to the research hotspot,the author studied the mechanism for determining the fracture toughness by the continuous spherical indentation tests,modified the Critical Indentation Energy(CIE)model,and verified the validity of the modified CIE model at room temperature and elevated temperatures.The specific works are mainly as follows:Micro fracture and damage morphology of CT specimens,indentation specimens,tensile specimens and pure sheared specimens were observed though Scanning Electron Microscope(SEM).The results indicated that the fractures of CT specimens and tensile specimens are dimple fractures formed by void nucleation,growth and linkage,and the micro damage morphology was manifested as spherical(ellipsoidal)micro-voids.They have the same damage mechanism,and the main reason for micro-voids forming around second-phase particles or inclusions is that the tensile stress is applied to break the interfacial bonds between the particle and the matrix.The fracture of the indentation specimens and the pure sheared specimens showed shear fracture(The indentation specimens tend to fracture under shear stress,however there are no macroscopic cracks as a result of compressive field).The micro damage morphology of both showed the wedge-shaped micro-voids.And thus they also have the same damage mechanism.Micro-voids nucleation can be explained by the dislocation pile-up in obstacles under the shear stress.The stress state of all these specimens was analyzed by Finite Element Methods(FEM)or analytic methods.It is proved that the stress triaxiality of the fatigue pre-crack tip of CT specimens and neck section of the tensile specimens exceeded the high stress triaxiality threshold.And void nucleation,growth,and linkage dominating the damage mechanism at high stress triaxiality.While the stress triaxiality of the damaged region of indentation specimens and the sheared section of pure sheared specimens is lower than the threshold value of the low-stress state."Shear fracture" dominating the damage mechanism at negative stress triaxiality.All the results once again proved the correctness of the results of microscopic observations.The damage degree of materials under shear stress was defined as shear damage value.Based on the fact that critical damage strain energy release rate of material is an independent value,the relationship between the critical tensile damage value and the critical shear damage value was deduced.By this way,the damage degree under tensile stress was associated with the damage degree under shear stress.This leads us to the conclusion that the tensile tests and the pure shear tests could be used to correlate the continuous spherical indentation tests with the conventional fracture toughness tests.Based on the mechanism for determining the fracture toughness by continuous spherical indentation tests and actual deformation of indentation specimens during tests,the two parameters which determine the accuracy of the fracture toughness measurement of the CIE model,the critical damage degree of the material and the effective elastic modulus,were modified from the following three aspects:First of all,the critical indentation depth corresponding to the characteristic fracture initiation point should be determined from the critical shear damage value.The critical tensile damage value could be obtained by the loading-unloading tensile tests,then it was converted to the critical shear damage value.The results indicated that the critical shear damage value(Mi*)is 0.25 for materials investigated in this study.Secondly,considering the plastic deformation of the actual materials(elastic-plastic materials)before unloading,the modified model based on Hertz contact theory was proposed to obtain the effective elastic modulus through the unloading slope of the indentation tests.Finally,the formula for calculating the pile-up factor of the materials in accord with the Johnson-Cook model(four-parameter constitutive model)was derived using a combination of dimensional analysis and FEM,which provided a basis for the accurate calculation of the indentation projection area in the formula of the effective elastic modulus.The conventional mechanical property tests and the continuous spherical indentation tests were carried out on five low-alloy steels commonly used pressure vessel,Q345R(C-Mn steel),15CrMoR(Cr-Mo steel),SA508-3(Mn-Ni-Mo steel),18MnMoNbR(Mn-Mo-Nb steel)and S30408(Cr-Ni steel).Then,the fracture toughness of all these steels were determined using the original and the modified CIE model.The results indicated that there is a considerable deviation between these results measured by CIE model and their conventional counterparts(from Compact Tension(CT)tests),while the modified CIE model has higher accuracy(with a macimum error within 15%)at room temperature and pretty well stability.Besides,the modified model could stably describe the fracture toughness changes with temperature below 250?.At this point,the validity of the modified CIE model was verified at room temperature and elevated temperatures.It can be conveniently applied in engineering.To sum up,the mechanism for determining the fracture toughness by continuous spherical indentation tests was investigated in this study through SEM observations on the fracture and damage micro-morphology for materials under the tensile/shear stress.A modified CIE model was then proposed,which provides theoretical support for determining the fracture toughness of in-service equipment without sampling and has significant importance on structural integrity assessment for those in-service equipment.