単軸クリープ試験片・ツバ部へのスリット導入による標点間内のクリープひずみ分布と破断寿命

Abstract

Constant-load creep testing machines have mainly been employed to obtain constitutive equations of creep. Although constant-stress ones would be more preferable to create an accurate constitutive equation of creep, they have not been employed yet because of their complex structures. Furthermore even if constant-stress creep machines were employed, constraint of limbs attached on a gauge length in the uniaxial creep specimen would have been another inevitable issue. That is, extensometer limbs in the specimen usually prevent uniform deformation along the gauge length. In order to release the constraint of circumferential deformation around the extensometer limb, 24 slits were introduced into each limb in this study. Firstly, Finite Element analyses using Norton’s creep law were performed to examine the validity of introducing these slits for the uniform deformation. The analytic results showed that the creep specimen with slits could release the circumferential constraint and raise the uniform deformation along the gauge length. It was also found that there was an optimum depth of the cutting slit to deform uniformly. Subsequently, employing SUS304 austenitic stainless steel, uniaxial creep tests were conducted at two initial stress levels. Rupture lives using specimens with the slit were shorter than those using the conventional specimen when the constant-stress creep machine was employed. Each creep test was interrupted four times to measure the distribution of deformation in the specimen, and then it was found that uniform deformation continued up to the middle of the tertiary stage of creep. The distribution of creep strain depended on an amount of deformation regardless of the type of specimen and testing machine. FE analyses showed that the distribution of strain was also influenced by a small variation of diameter in the specimen and a small superimposed bending load.