The Effect Of Thickness And Loading Configuration On Crack-tip Constraint

The thickness and loading configuration of the test specimens will change the constraint of crack-tip fields,and then influence the fracture toughness.According to BSI 7448 Part I,this paper designed three types of CTOD specimens with different thickness B of E36 steel,and CTOD tests were carried out at various temperatures to determine their ductile-brittle transition temperature cures.The test results revealed that E36 steel shows typical ductile-brittle transition characteristic and its ductile-brittle transition cure can be fitted well by Boltzmann function,and all the parameters has its own physical meaning.It is suggested that the Boltzmann function is used to fit the ductile-brittle transition cure.With the increase of the test specimen thickness?TST?,the ductile-brittle transition cure shifts to higher temperature.What is more,the TST only change the position of the ductile-to-brittle transition cure,but almost not change its shape.In addition,we use the Master Curve method to verify the Boltzmann curve,finding that the difference between characteristic temperatures obtained by the two methods is less than 10?.Therefore,it is considered that the ductile brittle transition curve fitted by Boltzmann function is effective.In order to investigate the relationship between the crack tip restraint and the fracture toughness,an elastic-plastic finite element method?FEM?is used to analyze the stress distributions ahead of the different thickness specimens crack tips.The application of the triaxiality parameter h and T_Z factor to the 3D crack front stress field is investigated,and the impact of B on the constraint parameters is characterized.We found that T_z become larger with the increase of specimens thickness.This phenomenon is consistent with the change of the temperature range of the ductile to brittle transition.Therefore,we can get the conclusion that the out-of-plane constraint parameter T_z can demonstrate the TST effect on the ductile-brittle transition temperature.In addition,using the elastic-plastic finite element method,this paper obtained the in-plane constraint parameter Q and A₂ of three different loading methods?CT?SENB and SENT?.The results indicate that loading modes will change the in-plane constraint.A2 is a distance-independent parameter which can quantitatively characterize the constraint in the crack tip fields of different loading modes.