Small Sample Test Technique

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Authors: Karel Matocha, Roger Hurst
Abstract: The development and practical use of the Small Punch test technique for the evaluation of mechanical properties of metallic materials in Europe resulted in the publication of the CEN document CWA 15627 Small Punch Test Method for Metallic Materials in 2007. The evaluation of the actual mechanical properties of metallic materials from the results of such tests according to CWA 15627 were presented at the first three International conferences SSTT "Determination of Mechanical Properties by Small Punch and other Miniature Testing Techniques" held in Ostrava (2010, 2012) and Castle Seggau in Austria (2014). In the present paper the motivation and the steps taken to lead to the setting up of a working group and programme facilitating the transformation of CWA 15627 Small Punch Test Method for Metallic Materials into an EN standard are summarized along with early results of the programme.
Authors: Martin Abendroth
Abstract: In recent years the small punch test method has become an attractive alternative compared to traditional material testing procedures, especially in cases where only small amounts of material are available. In contrast to standard test methods, the relevant material parameters can not be as simply obtained from the experimental measurements of SPTs because of its non-uniform stress and deformation state. However this can be achieved by comparing the experimental SPT results with those obtained by finite element simulations of SPT using advanced material models. Then the task is to determine the parameters of the material models using special optimization techniques. This paper gives an overview about the common techniques used to simulate SPT experiments. It should give the reader answer to the questions: Why are FEM simulations useful? How should such simulations be performed? Which material laws can be used? What are the limitations of finite element simulations? How to determine material parameters from SPT-experiments and the corresponding simulations?
Authors: G.A. Jackson, Hao Chen, Wei Sun, D. Graham McCartney
Abstract: Thermal barrier coatings (TBC’s) protect superalloy components from excessively high temperatures in gas turbines. TBC’s comprise of a ceramic top coat, a metallic bond coat and a thermally grown oxide (TGO). The creep behaviour of the MCrAlY bond coat, which is sensitive to the composition and the method of deposition, has a significant effect on the life of the TBC. High velocity oxy-fuel (HVOF) thermal spraying is a popular deposition method for MCrAlY bond coats however the creep properties of HVOF MCrAlY coatings are not well documented. The creep behaviour of a HVOF thermally sprayed CoNiCrAlY coating has been determined by small punch creep (SPC) testing. Tests were conducted between an equivalent uniaxial stress range of 37-80 MPa at 750 °C on two different SPC rigs and between 30-49 MPa at 850 °C on a single SPC rig. The measured steady-state creep deformation rates at 750 °C were consistent across the two rigs. A comparison with previous work demonstrated that the creep behaviour of HVOF CoNiCrAlY coatings is not sensitive to the manufacturing variability associated with HVOF thermal spraying. The CoNiCrAlY coating exhibited typical SPC deformation at 750 °C. At 850 °C the CoNiCrAlY coating showed significantly different creep behaviour which could be attributed to the onset of superplasticity.
Authors: Henry Illsley, Robert Lancaster, Roger Hurst, Spencer Jeffs, Gavin Baxter
Abstract: Small punch (SP) tensile testing provides several advantages over conventional test techniques for mechanical property characterisation of components produced using novel manufacturing processes. Additive layer manufacturing (ALM) is becoming more widespread, particularly in high value manufacturing sectors such as the gas turbine industry as it allows near net shape manufacture of near fully dense components with complex geometries. One such ALM process which is receiving attention from the gas turbine industry is electron beam melting (EBM), a powder bed process which uses an electron beam energy source. The additive nature of ALM processes including EBM results in the microstructures produced differing significantly to those produced by conventional processing techniques. As well as being influenced by the input parameters, the microstructure and hence mechanical properties are also affected by the geometry of the component being manufactured, primarily due to the effect this has on the cooling characteristics. SP testing of material manufactured by EBM allows the mechanical property characterisation of local component representative geometries which wouldn’t be possible using conventional uniaxial testing techniques. This work is aimed towards developing and validating the SP tensile technique for this application; different Ti-6Al-4V material variants manufactured using EBM as well as conventional methods have been characterised with a range of test conditions.
Authors: Robert Lancaster, Henry Illsley, Roger Hurst, Spencer Jeffs, Gavin Baxter
Abstract: Miniaturised mechanical test approaches are now widely recognised as an established means of obtaining useful mechanical property information from limited material quantities. To date these methods have largely been adopted to characterise the creep, tensile and fracture characteristics of numerous material systems from a range of industrial applications. One method developed for miniaturised testing is the small punch test. Many international institutions and research faculties have now made a significant investment in realising the potential that small punch testing has to offer. However, limited success has been made in replicating a miniaturised test approach for determining the cyclic fatigue properties of a small punch disc due to the complex biaxial stress field that typically occurs in any small punch test. Therefore, to realise such an approach and to interpret the fatigue behaviour of small scale components, the mechanical test arrangement must clearly be of a highly bespoke nature. This paper will discuss the ongoing research and progress in developing a novel small punch fatigue testing facility at the Institute of Structural Materials in Swansea University. Several experiments have been performed on the titanium alloy Ti-6Al-4V at ambient room temperature and effort has been made to understand the complex damage mechanism.
Authors: Petr Dymáček, Ferdinand Dobeš, Luboš Kloc
Abstract: 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.
Authors: Roberto Lacalle, David Andrés, José Alberto Álvarez, Federico Gutiérrez-Solana
Abstract: The behaviour of the materials in ductile-brittle transition region must be known when performing structural integrity assessments of nuclear reactor vessels working under the effects of neutron irradiation. The characterisation of this region has been usually carried out by means of Charpy impact tests. Just during last few years new approaches based on direct fracture mechanics tests have begun to be used. In most of these cases, the Master Curve methodology, which allows the transition region to be characterised using only one parameter (T0 reference temperature), has been employed. In this paper the transition region of two materials –one vessel steel and one common structural steel-has been characterised by means of Small Punch Tests. First of all, this zone has been characterised using conventional specimens and the results were compared with those of Charpy impact tests. Finally a new approach based on the use of notched Small Punch samples together with Master Curve methodology has been proposed.
Authors: Karel Matocha, Ondrej Dorazil, Roger Hurst
Abstract: The principal difference between the SP testing technique and standardized Impact testing lies in the fact that the SP tests carried out in accordance with CWA 15627 use disc-shaped test specimens without a notch. Especially in tough materials, the temperature dependence of fracture energy in the transition area is ​​very steep and lies close to the temperature of liquid nitrogen. In this case, t he determination of T SP can lead to significant errors in its determination . Efforts to move the transition area of penetration testing closer to the transition area of standardized impact tests led to the proposal of the notched disc specimen 8 mm in diameter and 0.5 mm in thickness with a “U" shaped notch 0.2mm deep in the axis plane of the disc.The paper summarizes the results obtained to date when determining the transition temperature of Small Punch tests, TSP, determined according to CWA 15627 for material of pipes made of P92 and P22 steels in the as delivered state and a heat treated 14MoV6-3 steel. Although the results obtained confirmed the results of other workers in that the presence of a notch in a SP disc is insufficient to increase the transition temperature significantly and certainly not to the values obtained by Charpy testing, comparison of the different behaviours of the alloys tested reveals some evidence that the notch reduces the energy for initiation. This implies that the test on a notched disc is more a test of crack growth and would be a useful instrument if included in the forthcoming EU standard for SP testing.
Authors: Sean Davies, Robert Lancaster, Spencer Jeffs, Gavin Baxter
Abstract: Additive Layer Manufacturing (ALM) technologies, such as Powder Bed Direct Laser Deposition (PB-DLD), have gained increasing popularity within the aerospace industry due to the advantages they hold over conventional processing routes. Among the advantages are the ability to produce more sophisticated cross-sectional geometries, a decrease in production lead times and an improvement to the buy-to-fly ratio. However, build quality and microstructural characteristics have a dependency on the process variables such as build direction. In order to understand the influence of grain size and build orientation on tensile behaviour, the Small Punch Tensile (SPT) testing technique has been applied to variants of the nickel based superalloy C263, manufactured using the PB-DLD method. The test technique utilises miniaturised samples, requiring only small volumes of material and is therefore a desirable test method to employ. SPT testing has characterised the mechanical properties between vertically and horizontally built PB-DLD C263 in comparison with the cast material derivative. Differences in mechanical performance between each variant have been revealed and found to be associated with microstructural variations. The deformation behaviour across each material variant have been exposed by interrupted tests. SPT results have also been accompanied by fractography, fracture energy calculations along with comparisons with uniaxial data.

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