Petr Dymáček

Influence of Friction on Stress and Strain Distributions in Small Punch Creep Test Models

The FEM modeling of small punch tests on miniaturized thin discs (SPT) of two heat resistant steels was performed. The FE models did represent the creep SPT, i.e. tests with constant acting force. It was shown that different values of the surface friction coefficient used in the calculations have significant impact on the calculated stress and strain state and consequently on the deformed shape of the disc. Thus, the surface friction coefficient should be considered one of the key factors for any correct correlation of SPT and uniaxial creep test results. Proper attention must be paid to the friction conditions during the long term creep deformation. An attempt to define simple approach how to relate the SPT with uniaxial tests is suggested. Some of the calculated results are compared with experiment.

Small Punch Testing of Sanicro 25 Steel and its Correlation with Uniaxial Tests

Small punch testing under constant deflection rate, constant force and constant deflection (i.e. force relaxation) were performed on the new austenitic steel Sanicro 25. Constant deflection rate experiments were correlated to uniaxial tensile tests at room temperature and 700°C with the help of several empirical relationships. Small punch creep testing was performed in as received state. Correlation of the small punch results with uniaxial creep test results was done and the force/stress ratio Ψ and kSP parameter were determined. The constant deflection small punch test was correlated with the uniaxial stress relaxation test and good agreement was reached.

Short Term Creep Small Punch Testing of P91 and P92 Steels, Observations and Correlations with the Numerical Results

The contribution is presenting experimental results of conventional creep and small punch testing performed on miniaturized discs prepared from P91 and P92 steels. Some of the test cases were also numerically modeled by Finite Element Method (FEM) with help of Nortons constitutive model and the results were compared with the experiment. Presented work complements the existing empirical investigations on small punch and conventional creep tests relations.

Tensile and Creep Testing of Sanicro 25 Using Miniature Specimens

Tensile and creep properties of new austenitic steel Sanicro 25 at room temperature and operating temperature 700 °C were investigated by testing on miniature specimens. The results were correlated with testing on conventional specimens. Very good agreement of results was obtained, namely in yield and ultimate strength, as well as short-term creep properties. Although the creep rupture time was found to be systematically shorter and creep ductility lower in the miniature test, the minimum creep rates were comparable. The analysis of the fracture surfaces revealed similar ductile fracture morphology for both specimen geometries. One exception was found in a small area near the miniature specimen edge that was cut by electro discharge machining, where an influence of the steel fracture behavior at elevated temperature was identified.

Small Punch Testing of Exposed High-Pressure Steam Pipelines

Non-disruptive material sample scooping is frequently applied to a range of operating components since it is supposed that removal of small amount of material from the surface has no effect on components integrity. As-removed material is then subjected to mechanical testing by small punch test to estimate tensile, creep and fracture properties necessary for assessment of residual life of the component. The question arises whether the assumption that measurements on surface specimens are representative for the characterization of whole component is justified. For the verification of this assumption, four steam pipes after either plant- or simulated-exposure up to 104 000 hours were used. Small punch specimens were prepared from different positions within pipes. The dimensions of specimens enable to estimate mechanical properties for several dozens of specimens along available power-plant pipes’ walls. The paper summarizes the results of small punch tests at both constant deflection rate and constant force and at room and elevated temperature, respectively. In the walls of the investigated pipes, the results obtained on surface specimens do not differ substantially from the results on specimens prepared from central parts.Copyright

Small Punch Testing and Its Modeling for the Evaluation of Mechanical Properties of Heat-Resistant Steels

Small punch tests on miniaturized thin discs (SPT) can be considered as one of the promising methods predominantly for an assessment of the residual life of parts in service of power plants and thermal facilities. These tests can be used for determining a number of material properties. Two variations of the test seem to have a good potential for use in wide range of temperatures. The CF test (constant force) is a test in which the puncher penetrates under constant load and the time dependence of the deflection is measured. This test is similar to a conventional creep test. The CDR test (with constant deflection rate conditions), in which the puncher penetrates through the disc at a given constant rate of deflection (i.e., central deflection measured in a direction perpendicular to the disc) and the necessary force is measured. This mode of the tests can serve similarly as conventional tensile tests for determining of the static material properties, the estimation of fracture toughness and transitions in material behavior. The article summarizes the capabilities of the small punch technique and presents results from testing of two heat resistant steels, i.e., CSN 15313 and P91 used in the Czech power industry. Experimental results are compared with finite element modeling.Copyright

New approach to determine uniaxial creep properties from small punch creep curves

ABSTRACT A key issue in the small punch creep test (SPC) is to determine the equivalent stress that results in the same time to rupture in a uniaxial creep test (UAC). A new approach is proposed based on formulas between the ratio of force in SPC to stress in UAC and the deflection at the minimum deflection rate. Another formula is defined between the minimum deflection rate and the minimum strain rate. In both cases, they were created from a large experimental data pool of high temperature creep-resistant steels. The predicted Norton law and the rupture time dependence on stress are in good agreement with the experimental UAC results. In all cases, the predictions using the present approach are much better than those based on Chakrabarty membrane stretch model.

Determination of Mechanical Properties of Metallic Materials from Very Small Volumes by Means of Small Punch Test

The paper summarizes capabilities of the small punch tests on miniature disc specimens from metallic materials. Results obtained by small punch tests can be correlated with mechanical properties determined by conventional or other miniature tests. Three basic types of small punch test setup: i) static/fracture, ii) creep and iii) stress relaxation are described and the basic quantities that can be obtained are shown. Relations for an assessment of conventional mechanical properties from quantities obtained by small punch test are presented.

Estimation of Anisotropy of Creep Properties in Al and Mg Alloys by Means of Small Punch Test

Small punch test was used to evaluate the properties of light alloys in various directions. Three different materials were studied: (i) magnesium alloy WE54 prepared by a powder metallurgical route with final hot extrusion, (ii) aluminium alloy reinforced with 20 vol. % of Saffil fibres with planar orientation, and (iii) Al-Al4C3 composite prepared by mechanical alloying and subjected to equal channel angular pressing as a final step. Tests were performed under constant force at elevated temperatures. The observed orientation dependence of creep properties is strongly material dependent. The results confirm the feasibility of the small punch test for determination of anisotropy of mechanical properties.

Determination of Mechanical Properties of Magnesium Alloys and Composites by Small Punch Testing

Tests on miniature samples are increasingly used for the determination of mechanical properties of materials available in small volumes (non-destructive or semi-destructive approach). Small punch testing at constant deflection rate (SPT-CDR) of selected magnesium alloys and composites was performed at room temperature. Mechanical properties (yield strength, ultimate strength) were evaluated from SPT and correlated with results of uniaxial tensile tests (UTT). SPT characteristics were converted to uniaxial tensile properties by empirical formulas available in the literature. New formulas more appropriate for magnesium alloys were suggested.