The evaluation of actual mechanical properties of the in-service structures after some time of operation or determination of local properties for detailed FEM simulation yields the necessity to obtain relevant material data with high accuracy from small volume of the experimental material. Therefore, non-destructive or semi-destructive techniques using small size samples are being developed. The use of small-scale samples also enables the evaluation of material properties in various locations of tested component; for example, the mechanical properties of the individual regions of welds, local properties determination for properties anisotropy assessment and properties determination in cases when small volume of the experimental material is available e.g. residual service life assessment of in service components, bulk nanostructured materials… There are shown results of small size tensile tests (M-TT) and small sized fatigue tests (SFT). In the case of small size specimens testing a machining becomes more pronounced that in the case of standard sized specimens. The current study brings information on the machining influence on the expected results obtained by small size specimens in the case of quasi-static tensile tests and fatigue test.
The fatigue phenomenon is said to be responsible for more than 80% of structural failures and operational accidents. One of the most important branches of industry, where unexpected outages can cause extremely expensive costs is the power producing industry. The steam turbine rotors represent large, expensive and, in the case of failure, potentially dangerous components. Their local properties generally differ from one forging to another, or if we compare head and bottom parts of the original ingot, or central and circumferential locations of one rotor body respectively, or if we compare the properties of separate discs, e.g. in the case of welded rotors. These differences stem from both even slight changes in the chemical composition (of separate heats or even within one ingot), heat treatment and in the differences in technology with respect to the real shape and size of the forgings in question.At present, questions on quantitative evaluation of remaining lifetime (to avoid failure) become urgent and, because fatigue is the most frequent cause of material degradation and resulting structural components failures, the remaining lifetime assessment is based on the evaluation of mechanical properties before and after some time of service. The original material data are usually tested by means of classic test specimens, which can hardly be used in the case of components in service, where there is no possibility to withdraw a sufficient volume of representative material for the manufacturing of a classic test specimen. In such cases today, methods of semi-destructive removal of a small volume of test material are utilized. This makes it possible to produce miniature test specimens. This paper describes the results of fatigue tests performed on miniature test specimens in comparison with the classic fatigue tests for several alloys applied in the power producing industry. The miniature test specimens were produced by water-jet cutting from the large test specimens. With respect to the specimen shape, stress concentration had to be taken into consideration for the purposes of comparison with the classic test specimens.
Medium manganese steels fall into the category of modern third-generation high-strength steels. Thanks to their alloying, they use a number of strengthening mechanisms, such as the TRIP and TWIP effects, to achieve their mechanical properties. The excellent combination of strength and ductility also makes them suitable for safety components in car shells, such as side reinforcements. Medium manganese steel with 0.2% C, 5% Mn, and 3% Al was used for the experimental program. Sheets with a thickness of 1.8 mm without surface treatment were formed in a press hardening tool. Side reinforcements require various mechanical properties in different parts. The change in mechanical properties was tested on the produced profiles. The changes in the tested regions were produced by local heating to an intercritical region. These results were compared with classically annealed specimens in a furnace. In the case of tool hardening, strength limits were over 1450 MPa with a ductility of about 15%.
Assessment of remaining lifetime represents a very complicated problem, which needs the knowledge of degradation processes in the material of a component, and also the service conditions of the components, e.g. way of loading and the influence of the surrounding environment. There is a common interest to operate the produced components as effectively as possible and thus as long as possible without reducing their safety and reliability, what could cause economic and human losses. This is a problem of safe operation and its prolongation in justifiable cases. As a result of new modern and more resistant materials development, the general interest is to be able to evaluate the extent and rate of degradation processes at various service conditions, mainly to prevent the components from brittle fracture. At present, the assessment of component material microstructure is one of the methods that makes it possible to evaluate its remaining lifetime. It is thus important to be able to evaluate the extent of material mechanical properties degradation as a result of various service factors and the elaboration of methods for its assessment. Nowadays, the evaluation of component material microstructure represents one of the possible methods for remaining lifetime assessment.
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