Research On Fracture Toughness Of G20Mn5QT Cast Steel Considering In-plane And Out-of-plane Constraints

Fracture toughness is a measure of a material’s ability to resist fracture,which is significantly affected by crack tip constraints.The constrained fracture theory has been developed for decades,single parameter,two parameter,three parameter and unified parameter theories have been proposed to characterize in-plane constraints,out-of-plane constraints and their composite constraints.However,these theories can only characterize the stress or strain field at the crack tip and their calculation process is complicated.Therefore,how to put forward a unified constraint parameter that can characterize the in-plane and out-of-plane constraints and accurately describe the stress-strain field at the crack tip is the key of structural safety design and integrity evaluation.In this paper,the effects of in-plane and out-of-plane constraints on the fracture toughness of G20Mn5QT cast steel were investigated by means of theoretical analyses,tests and numerical simulations.A new unified parameter that can characterize in-plane and out-of-plane constraints was proposed,and the correlation line between this parameter and ductile fracture toughness values was established.Through introducing image processing and acoustic emission technique,the stage analyses of fracture toughness test process were carried out,and the fracture toughness value at the boundary point of each stage was calculated,so as to clarify the crack evolution process and establish the fracture criterion.The main research contents and conclusions are as follows:(1)J-R curves and ductile fracture toughness JIC of compact tensile(CT)specimens with different in-plane constraints of G20Mn5QT were determined by the Normalization Method.With the increase of in-plane constraints,J-R curves and JIC decrease.The fracture morphology was analyzed by Scanning Electron Microscope(SEM)to clarify the micro fracture mechanism.With the increase of in-plane constraints,the fracture mechanism changes from ductile fracture through mixed ductile and brittle fracture to brittle fracture.(2)Combining the image processing and acoustic emission technique,the stage analyses of fracture toughness test process of CT specimens with different in-plane constraints were carried out.The crack evolution process changes from prefabricated fatigue crack through crack initiation,and then slow,stable and rapid propagation to finally instability.The fracture toughness corresponding to the stable crack propagation time JIPAE is taken as the critical value of a specimen,and the fracture criterion J≥JIPAE is established and compared with the classical fracture criterion J≥JIC.The results show that the fracture criterion J≥JIPAE is more conservative in engineering.(3)The GTN model embedded in ABAQUS finite element software is used to predict J-R curve and ductile fracture toughness JIC of CT specimens with different in-plane and out-of-plane constraints of G20Mn5QT.The enhancement of in-plane or out-of-plane constraints will reduce the fracture toughness performance of materials.There exists interaction between the in-plane and out-of-plane constraints.The lower in-plane constraints will strengthen the out-of-plane constraints effects,while the higher in-plane constraints are not sensitive to the out-of-plane constraints.(4)Based on the area under the equivalent stress-strain curve in the yield range of the constraint control zone on the crack growth path,a new in-plane and out-of-plane unified constraint parameter AESS is defined.There exists a sole linear relation between the normalized fracture toughness JIC/Jref and AESS regardless of in-plane or out-of-plane constraints or both.AEES-JIC/Jref can be used to characterize the effect of in-plane and out-of plane constraints on the fracture toughness of G20Mn5QT.It provides calculation parameters and theoretical guidance for the safety design and integrity evaluation of practical engineering components or structures.