Study Of Small Specimen Creep Test Based On The Beam Bending Theory

In process and power generating industry fields, due to the high efficiency and high temperature process, many components and structures have served at high temperature. The material of the components deteriorates with time and the structural integrity confronts challenges. Evaluation of creep properties of materials is essential to ensure the safe operation of in-service components at high temperature.Creep properties are generally determined by using standard tensile creep testing, which requires bulk materials. But it is difficult to manufacture large specimens from the in-service components due to the limited materials available. Small specimen creep testing technologies have therefore been developed. In this paper, three types of small specimens, including three-point bending, cantilever beam and beam specimen with fixed constraint, are proposed to determine the creep properties of materials. The main conclusions have been drawn as following:(1) Based on the beam theory and continuum damage mechanics, the analytical solution of the steady-state displacement rate and failure time are established for the three types of specimens. The load and displacement rate of the small specimen can be converted into equivalent uniaxial stress and strain rate. On the basis of membrane mechanics, the semi-analytical correlation between the load and equivalent uniaxial stress is derived for the beam specimen with fixed constraint. It is benefit of this correlation to determine the applied load before testing.(2) Combined with Kachanov-Rabotnov creep damage constitutive equations, finite element(FE) models are established and applied to analyze the creep behavior of the three types of specimens. The simulated load-point displacement-time curves are similar to the uniaxial creep curves. Based on the numerical results, creep parameters of the constitutive equation are regressed and used to calculate the uniaxial data. The calculated strain rates correspond well with the uniaxial experimental values. There is a bit of difference between the calculated failure times and uniaxial ones, but the tendency correspond well with each other. The analytical solutions for the three types of specimens are thus theoretically verified to be reasonable.(3) The effect of large deformation on three typical specimens, including circular ring, three-point bending and cantilever-beam specimens, are investigated based on the beam bending theory. The factors affecting the evaluation accuracy are analyzed. Based on the limit load approach, a critical load method is proposed to control the large deformation effect. Numerical and experimental results show that the critical load hence provides a promising tool to guarantee the accuracy of creep properties evaluation by bending tests with small specimens. The effect of friction on small punch, impression, three-point bending and cantilever beam specimen is also investigated by FE method. Friction can reduce the displacement rates of small specimen. The higher the friction, the lower the displacement rate. It is related to the ratio of characteristic punch size to gauge length, constraint types and contact state between the punch and specimen. Beam specimen with fixed constraint gives minimal errors in a wide range of friction coefficients and is thus recommended. FE analysis also indicates that the beam specimen with fixed constraint is non-sensitive to the variation of the punch dimension, which has an obvious advantage over the small punch specimen.(4) A multi-specimens creep experimental system has been invented and custom-designed. Creep tests containing multi-specimens can be performed at a time. It can be used for many kinds of small specimens, such as small punch, impression or three-point bending specimens. With the aid of this machine, the consumed time and expense of creep tests can be largely reduced. It provides a promising tool for the experimental studies of miniaturized specimens.(5) The creep tests with three-point bending, cantilever beam and beam specimen with fixed constraint are performed on the custom-designed platform. It shows that the creep data obtained by the small specimens can be well repeated. The creep curves of the three-point bending and cantilever beam specimen can be divided into two stages, i.e. primary and secondary stages. Creep parameters of Norton’s law can be obtained by the three-point bending and cantilever beam specimens, which correspond reasonably well with those of uniaxial values. For the beam specimen with fixed constraint, whole creep curves can be obtained. Combined with experiment and numerical simulation, creep strain and rupture data can be obtained, which agree reasonably well with the uniaxial data. Meanwhile, according to the uniaxial data, the semi-analytical correlation between the load and equivalent stress of the beam specimen with fixed constraint are modified. The empirical coefficient KBSFC for the aluminum alloy A7N01is to be2.18at350℃and1.88at380℃.