No abstract

The review is concerned with the use of miniature specimens to identify the mechanical/creep properties of metals and alloys. It is not intended to consider the nanoscales, which have been reviewed elsewhere, but focus on the size ranges and forms that are of use to areas such as alloy development, surveillance monitoring, effects of irradiation, properties of weld structures and remaining life. For many of these cases, there are technical advantages in the use of small specimens. Where possible, available reviews in this area are used and identified. The needs for small specimen sizes are considered, and the wide range of sizes of both conventional and more unusual specimen designs is reviewed. The potential effects of specimen size are considered: the microstructural features of the material, the effect of constraint and the actual region of the specimen that is undergoing deformation, and finally statistical approaches. The results of published studies are considered under the general groupings of yield/deformation behaviour, creep properties, toughness and fatigue, with an examination of the current state of interpretation of data from mini-specimens because interpretation is one of the key areas of interest. Finally, the current status of each area is considered, as certain designs of specimen will be better suited to answering specific technical questions. The need for standards for specific designs is also examined.

This paper describes a novel, high-sensitivity, ring-type of small specimen creep test method, which can be used to obtain accurate creep strain data. A full theoretical description of the test technique is given; this is based on the complementary strain energy approach, which leads to an analytical solution for the load line deformation of an elliptical ring. Using the analytical solution, a reference stress approach is used to establish the conversion relationships between the applied load and the equivalent uniaxial stress and between the experimentally measured creep deformations and the equivalent uniaxial creep strains. The main features of the test method are: (a) the ring specimen has a significantly larger equivalent gauge length (EGL), when compared with that of other small specimen types; (b) the method is suitable for testing at lower stresses compared with commonly used current smallspecimen test methods (this is because relatively low strains can be obtained from relatively large deformations); (c) specimens have simple geometries and the tests are easy to perform, and (d) the conversion relationships are material independent and practically insensitive to geometry change due to deformation. Experimental validation of the test method is made using the results obtained from creep tests for a P91 steel at 650 uC. Practical applications and future exploitation of the technique are addressed.

No abstract

An experimental programme of cyclic mechanical testing of a 316 stainless steel, at temperatures up to 600• C, under isothermal conditions, for the identification of material constitutive constants, has been carried out using a thermo-mechanical fatigue (TMF) test machine with induction coil heating. The constitutive model adopted is a modified Chaboche unified viscoplasticity model, which can deal with both cyclic effects, such as combined isotropic and kinematic hardening, and rate-dependent effects, associated with viscoplasticity. The characterization of 316 stainless steel is presented and compared with results from cyclic isothermal tests. A least-squares optimization algorithm has been developed and implemented for determining the material constants in order to further improve the general fit of the model to experimental data, using the initially obtained material constants as the starting point in this optimization process. The model predictions using both the initial and optimized material constants are compared to experimental data.