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.
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.
The small punch creep testing method is highly complex and involves interactions between a number of nonlinear processes. The deformed shapes which are produced from such tests are related to the punch and specimen dimensions and to the elastic, plastic and creep behaviour of the test material, under contact and large deformation conditions, at elevated temperature. Due to its complex nature, it is difficult to interpret the small punch test creep data in relation to the corresponding uniaxial creep behaviour of the material. One of the aims of this paper is to identify the important characteristics of the creep deformation resulting from "localised" deformations and from the "overall" deformation of the specimen. Following this, the results of approximate analytical and detailed finite element analyses of small punch tests are investigated. It is shown that the regions of the uniaxial creep test curves dominated by primary, secondary and tertiary creep, are not those which are immediately apparent from the displacement versus time records produced during a small punch test. On the basis of the interpretation of the finite element results presented, a method based on a reference stress approach is proposed for interpreting the results of small punch test experimental data. Future work planned for the interpretation of small punch tests data is briefly addressed.
Residual macrostresses in a multipass circumferentially butt-welded P91 ferritic steel pipe have been determined numerically and experimentally. The welded joint in a pipe with an outer diameter of 290 mm and a wall thickness of 55 mm is typical of power generation plant components. An axisymmetric thermomechanical finite element model has been used to predict the resulting residual hoop and axial stresses in the welded pipe. The effects of the austenite to martensite phase transformation have been incorporated into the simulation. Residual stresses have been measured using the X-ray diffraction technique along the outer surface of the pipe and using the deep-hole drilling technique through the wall thickness at the center of the weld. Good correlation has been demonstrated between the residual hoop and the axial stresses obtained numerically and experimentally. The paper demonstrates the importance of using a mixed experimental and numerical approach to determine accurately the residual macrostress distribution in welded components.
The finite element (FE) method has been applied to simulate residual axial and hoop stresses generated in the weld region and heat-affected zone of an axisymmetric 50-bead circumferentially butt-welded P91 steel pipe, with an outer diameter of 145 mm and wall thickness of 50 mm. The FE simulation consists of a thermal analysis and a sequentially coupled structural analysis. Solid-state phase transformation (SSPT), which is characteristic of P91 steel during welding thermal cycles, has been modelled in the FE analysis by allowing for volumetric changes in steel and associated changes in yield stress due to austenitic and martensitic transformations. Phase transformation plasticity has also been taken into account. The effects of post-weld heat treatment (PWHT) have been investigated, including those of heat treatment holding time. Residual axial and hoop stresses have been depicted through the pipe wall thickness as well as along the outer surface of the pipe. The results indicate the importance of including SSPT in the simulation of residual stresses during the welding of P91 steel as well as the significance of PWHT on stress relaxation.
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