Method for description of creep and long-term strength with pure elongation

Abstract

The present article gives the results of creep tests with pure elongation, carried out in the Institute of Mechanics of Moscow State University, and proposes a model for the description of monoaxial creep of a material right up to fracture. Tests were made of 21 samples of one melt of stainless steel Khl8N10T. The samples were tubes with an outside diameter of 12 mm, a wall thickness of 0.5 ram, and a working length of 70-100 ram. The temperature during the tests was constant and equal to 850~ The experiments were carried out with the action of a constant elongational load on the sample. The axial deformations were determined using strain gauges, which were attached with an adhesi')e to elastic elements, connected with the sample and led out of the furnace. In Figs. 14 the continuous lines give curves of the monoaxial creep p(t) with initial stresses % equal, respectively, to 4; 5, 6, and 8 kg/mm 2 . The mean values of the fracture time t* and the corresponding deformation time p* for each stress % are given in Table 1. If it is assumed that the dependence of the rate i~ of fully established creep on the stress a is expressed by the power function i0 = Ao n with constants of the material A and n, and if these constants are determined by the method of least squares, we obtain the following values for them: n = 3.2; A = 10-s (kg/mm2) -3~ h -x. Following [ 1], we shall describe the process of fracture with creep by the introduction of the parameter co(t), characterizing the degree of damage to the material. As usual, it is assumed that co(0) = 0 and co(t*) = 1. We take the relationships of monoaxial creep for a material with reinforcement in the form ~=A[~/(l--o'l)]" (t+Cp-~), p(0) = 0,