Papers by Keyword: Small Punch Test

Abstract: Within the nuclear power plant operational life management, components lifetime extension requires information on structural material degradation. Innovative testing methods of Small punch testing and Automated ball indentation test are based on the determination of material properties from sub-sized specimens. Present paper is focused on the employment of these techniques in the NPP irradiated materials testing and evaluation at the accredited hot cell testing laboratory of ÚJV Řež, a. s., Mechanical testing department. Comparison with the testing results from the conventional methods is depicted.

Abstract: Small punch test (SPT) is a miniature sample test technique which can evaluate in-service material properties with an almost non-destructive method. Since the deformation behavior of the small punch specimen is complicated, finite element simulation embedded with Gurson-Tvergaard (GTN) model is adopted to simulate elastic-plastic behaviour until fracture. Choosing the proper GTN parameters is crucial for the small punch simulation, which directly influence the precision of load-displacement curve obtained from simulation. In this paper, load-displacement curve is divided into five stages and the parameters identification process is done by adjusting the simulation curve with experimental load-displacement curve in different stages which controlled by separately parameters. The results show that the parameters determined based on this criterion are not unique. In order to validate the reliability of this method, specimen’s minimum thickness of cross-section after fracture was introduced as an extra criterion which turned out to be feasible. Load-displacement curves cannot serve as the only criterion to verify the GTN parameters.

Abstract: This paper describes an approach to identify the influence of mechanical properties of the materials under the condition of containing residual stress. The numerical method of simulating small punch test (SPT) is used to determine the material response under loading. The simulated material behavior of the specimen is based on the ductile elastoplastic damage theory of Gurson, Tvergaard and Needleman (GTN). The residual stress can be prefabricated on the specimen by loading and unloading. By comparing the original specimen with the specimen contains residual stress, the change of the mechanical properties of the materials can be studied. The results of simulation indicate that the material properties decrease with the increase of the residual stress.

Abstract: Very high plastic strain zones with equivalent plastic strain above 0.2, PZ_0.2 and above 0.5, PZ_0.5 in 304 stainless steel small punch specimens loaded at RT to various level were observed and measured by martensite formation and recrystallization technique, respectively. It is found that both the very high plastic zones are formed ,at middle stage of the small punch test, at first near the outer surface region of the specimen where the loading ball is contacted to the specimen. The zones extend with increasing load toward the inner surface. Thus the contact area part of the specimen with the ball causes a significant strain gradient through thickness. This will be due to the constraint of the plastic deformation near the contact region by the friction force.

Abstract: Four levels of load were performed on different small punch creep specimens in order to investigate the creep properties of the P91 by small punch creep test at 600°C. A practical method was proposed to identification of creep parameters of P91 by comparison of experimental curves and simulate curves from only one experimental curve. The other three experimental curves was performed to verify the practical method.

Abstract: Mechanical properties of the electron beam weld zone, which are different from the base metal, of Hastelloy C-276 were studied by small punch test (SPT) technology. Load–displacement curves and material properties were obtained by using miniaturized plate-type specimens (3mm diameter×(0.15-0.4) mm thick) from the weld and base metal respectively. The result shows that the maximum load of weld is larger than that of the base metal under the same thickness, meaning the higher strength of the weld. Furthermore, finite element method analysis was performed to investigate the effects of specimen thickness, the diameter of loading ball and the diameter of center hole in holder on the plastic damage of Hastelloy C-276.

Abstract: The T91 steel is considered a perspective structural material for Generation IV (GEN IV) nuclear reactors components. The operating conditions of GEN IV nuclear reactor, which include higher operating temperatures, higher neutron fluencies and using liquid metals as coolants, constitute new challenges for structural materials validation. To properly assess the impact of radiation, thermal, load and environmental damage the screening test procedures for initial overview of the induced changes of mechanical properties need to be developed and validated. One of the test procedures with potential to be used for the mechanical properties screening is Small Punch test. Small Punch test receive considerable attention since their development in the 1980's. The use of miniaturized test specimen constitutes minimal requirements for the test material and the option of obtaining the test material directly from the component. They have been successfully used on industrial scale for design life extension of components for energy industry in the past. In this paper we address the issue of the Fracture Appearance Transition Temperature (FATT) determination by the means of Small Punch test for the T91 steel, manufactured according to ASTM standard A387-Ed99. The test specimen were manufactured and used for the experiments in a wide range of test temperatures. Multiple levels of deformation energy were used to provide more representative interpretation of the test data. A data fit is applied on the deformation energy – test temperature dependence to obtain the value of FATT from Small Punch test, which is subsequently correlated to the values obtained from testing sub-size Charpy specimens.

Abstract: The specific desired properties for structures and components working in critical environments (e.g. different structure parts of power plants) require current information about degradation processes coming out in materials. Obtaining of this information by the help of the classical tests of mechanical properties (tensile test, Charpy test, fracture toughness test, creep test etc.) is very limited namely in the case of nuclear power plants pressure vessel. Hence, the new innovative techniques based on miniaturized specimens have been developed for evaluation of mechanical properties and their changes. One of very promising techniques is Small Punch Test. Present paper deals with characterization of three different steels (15Ch2MFA, 10GN2MFA with different heat treatment and steel O8Ch18NT10 with various degree of deformation).

Abstract: The sensitivity to liquid sodium embrittlement (LME) of T91 martensitic steel, one of the selected structural materials for future sodium fast reactors has been investigated. The study took into account the role of microstructure. Small punch tests and three points bending tests were carried out in a purified and controlled atmosphere. Precipitation state and dislocations structure resulting from a tempering at 550° C provoked LME of the T91 steel between 200 and 550 °C. Secondary Ions Mass Spectroscopy investigation suggested that sodium penetration at prior austenitic grains boundaries promoted by plastic deformation occurred and caused brittle crack initiation. Brittle cracks propagated in sodium preferentially along martensitic laths-boundaries. J integral calculations confirmed a drop in toughness of T91 tempered at 550°C by liquid sodium up to 80 %.

Abstract: Several possible routes are available for estimation of fracture behaviour from the results of small punch tests performed at constant rate of deflection. The routes include: (i) measurement of relevant dimensions directly on ruptured specimens, (ii) determination of critical deflections on the load vs. deflection curves and (iii) integration of these curves up to specific points. Equivalent fracture strain, fracture energy or fracture toughness are then evaluated from the obtained quantities. The mutual relations among the quantities are demonstrated by the results of small punch tests performed on a Fe-Al-based alloy in the temperature range extending from brittle up to ductile fracture appearance.