多結晶金属材料の高温クリープにおける塑性法則

静水圧応力およびひずみ履歴の影響に対する一考察

抄録

It was the aim of the present study to elucidate the influences of both hydrostatic stress and strain history on creep deformation of polycrystalline metallic materials at elevated temperatures. In the present paper, the special interest was taken in studying the correlation between the effect of hydrostatic stress and that of strain history through the study on plastic potential for metallic creep.
From the flow rule based on the yield function f=D(J₁)+H(J₂', ∫dε_ij), the following conclusion was made, J₁ being the first invariant of stress σ_ij, J₂' the second invariant of deviatric stress S_ij, and ∫dε_ij the strain of the material experienced during creep. First regarding the influence of hydrostatic component of stress on the creep, the comparison between both the test data under simple tension and simple torsion was cited as the basic examination. The ratio r of simple tensile creep rate ε_z to shearing creep rate γ at the same equivalent strain ε was given as
γ≡(γ/√3)/ε_z=1-1/4Aε²+3/4A²ε²/3D+1+Aε+3/4A²ε²,
where A is a scalar anisotropic parameter and D a scalar parameter of the volumetric stress. Therefore. it was found that r took the numerical value of r_??_1 under D_??_0 and A_??_0. D≈0 and A>0 was examined in the present types of tests of 0.15per cent carbon steel at 450°C. It was also ascertained that the decrease in the value of r in progress of creep resulted from the development of anisotropy of the material during the creep.
Secondly, under general loadings of rotated principal stress axes and fixed equivalent stress, the development of anisotropy of 0.15per cent carbon steel in the course of creep was remarkable than that under the fixed principal stress axes. It was also much dependent on strain history. As a matter of fact, the numerical values of anisotropic parameter and the parameter of Bauschinger effect of the material varied discontinuously after changing the stress components. Therefore, the translation of location and the form of the yielding curve were also ascertained. This resulted in the discrepancy between the principal direction of stress and that of creep rate, and caused disagreement in creep curves of tests which were performed under different load paths.
Thirdly, regarding the effect of combined loading of hydrostatic pressure p on the creep, the ratio of axial strain rate under the confining pressure ε_z to that at atomosphere ε_zo of the material which experienced the strain history of ε was given as
ε_z/ε_zo=1-9D/3D+1+Aε+3/4A²ε²·p/σ_z,
where σ_z is a fixed axial tensile stress. Therefore, the ratio took the numerical value of ε_z/ε_zo_??_1 under D_??_0 independent of the sign and the numerical value of A. It was also found that the ratio was not only dependent on D but also on the development of anisotropy of the material, that is, Aε.