Research On The Creep Deformation Of Three-point Bending Specimen With Fixed Constraint

Small specimen creep test is an important method of measuring creep properties of in-service equipment.Three-point bending specimen with fixed constraint(TPBSF)has a considerate research value because of its simple stress state and easily achievement of rupture data.However,the TPBSF is limited in many applications,for its basic deformation theory was not unified.In this study,based on the error function,the critical displacement of small creep deformation and large deformation was established.On the basis of beam bending theory,the creep deformation formula of TPBSF under small deformation was modified.In the stage of large deformation,general deformation theory was introduced to establish the creep deformation constitutive equation of TPBSF.A set of methods for measuring the creep properties of TPBSF was proposed.The main conclusions are as following:(1)the critical displacement was used to divide the creep displacement-time curve of TPBSF into small deformation zone,transition zone and large deformation zone.Different from the traditional creep three-phase division,the small deformation zone and transition zone correspond to the primary stage of creep,while the large deformation zone corresponds to the second and third stage of creep.The existence of the critical displacement and the accuracy of the critical displacement formula proposed this paper were verified by experiments.For the creep curves of steady-state creep displacement in different deformation stages,the corresponding theoretical analysis methods should be used.(2)In the small deformation stage,compared with the effective span method,the modified creep constitutive equation based on the beam bending theory reduces the average error between TPBSF of A7N01 at 350? and uniaxial creep from 92.2%to 12.4%.The internal forces caused by the compatibility of deformation tend to equalize the Mises stress in the cross-section with creep.The internal forces violate the assumption of beam bending theory.When the creep displacement increases to the critical displacement,the beam bending theory will fail,and the effect of large deformation needs to be considered.(3)The general deformation theory was introduced,and formulas of equivalent strain and stress of TPBSF in the large deformation stage was established.Based on the large deformation stage test data of A7N01,Norton creep parameters were obtained by inversion.Compared with uniaxial creep,the exponential parameters are extremely close,with an error of only 0.87%.It is found that the first stage of creep displacement-time curve of TPBSF,whose steady creep happens at large deformation stage,doesn't correspond to the first stage creep properties of material but satisfies the Norton creep equation of material which describes the second stage of creep.The first stage of creep displacement-time curve at high displacement rate is the result of beam bending mechanics of the specimen under high stress level.As the creep progresses,the mechanical model of TPBSF changes from beam bending stage into the rod stretching stage with lower stress level.Thus,a second stage of creep displacement-time curve with a lower displacement rate is formed.