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Performing mechanical tests at high temperatures is a nontrivial issue: Compared to room temperature testing, additional phenomena like time-dependent Deformation processes and oxidation effects raise the complexity of the material’s response, while more sophisticated test setups and additional control parameters increase the number of potential sources of error. To a large extent, these complications can be overcome by carefully following all recommendations given in the respective high temperature testing standards, but more comprehensive background information helps to identify points of specific importance in particular test campaigns. In this chapter, an overview is given on general high temperature testing issues like the appropriate choice of experimental equipment and key aspects of temperature measurement. In subsequent sections, the major static and dynamic high temperature test methods are reviewed and their Special features, as compared to testing at room temperature, are highlighted based on example data sets. Influences of specimen size and environmental effects are shortly outlined in a concluding section. In the whole chapter, a focus is set on testing of “classical” metallic high temperature materials, but many considerations are equally valid for testing of intermetallics, composites, and high temperature ceramics.
Experimental and analytical investigation of the TMF-HCF lifetime behavior of two cast iron alloys
(2017)
The combined loading Thermomechanical Fatigue (TMF) with High Cycle Fatigue (HCF) has been experimentally investigated for two cast iron alloys. Both alloys contain globular graphite nodules but the first one has a ferritic structure while the second one has an austenitic crystal structure. In particular, the influences of the HCF frequency, of the HCF loading amplitude and of the location of the superposed HCF cycles have been investigated. It was observed that the HCF frequency has a limited impact on the fatigue life. On the other side, the HCF-strain amplitude has a highly non-linear influence on the fatigue life. The experimental results can be understood in terms of a fracture mechanics based damage mechanism: Cracks quickly initiate due to the TMF loading and the growth of the cracks up to a few mm controls the fatigue life. If HCF-loading cycles are superposed, cyclic crack propagation dramatically accelerates after a threshold has been reached. This threshold is regarded as controlling the fatigue life reduction. The previous ideas have been expressed in a model that can be very simply applied to estimate the fatigue life reduction ratio due to the superposed HCF cycles. It only contains two adjustable parameters and can be combined with any TMF life assessment model.
TMF tests were carried out on EN-GJSA-XNiSiCr35-5-2 at constant minimum temperature (400 °C) and varying maximum temperatures (Tmax = 700 °C, 800 °C, 900 °C) with hold times of 180 s at Tmax and two phase angles (in-phase (IP), 180° out-of-phase (OP)). The results showed a comparable strength under OP- and IP-TMF loading. At Tmax = 700 °C and 900 °C, the lifetime in IP-tests was slightly longer than that of OP-tests, while it is vice versa at Tmax = 800 °C. The IP-tests at Tmax = 900 °C showed a similar lifetime as OP-tests at Tmax = 700 °C and 800 °C, which was unexpected for such a high testing temperature. All IP-tests at Tmax = 900 °C showed a continuous cyclic softening from the beginning on, which was different from all other testing conditions. Complementary metallographic investigations indicated that under this test condition, intergranular creep damage is present in the volume of the test pieces.
Materials testing under mechanical stress, pressure and turbulent flow of impure supercritical CO2
(2013)
The climate change necessitates measures to reduce carbon dioxide (CO2) emissions in the atmosphere, one of which is carbon dioxide capture and storage (CCS). Transmission of pressurized liquid or supercritical CO2 containing residual flue gas constituents in pipelines is an important component of CCS systems. Material testing under conditions as close as possible to real conditions is a prerequisite for reliable and safe implementation of CCS. A novel pipeline corrosion test facility was developed, accounting for major mechanical, physical and chemical influencing factors: elastic deformation, pressure, temperature, gas composition and flow velocity can be independently adjusted. Radial and tangential stress distributions on a round sample were experimentally investigated, indicating a good accordance with expected theoretical values. In this contribution the idea of the novel corrosion test facility and the assembled equipment as well as first results are presented and discussed.----------------------------------------------------------------------------------------------------Die Klimaveränderung macht Maßnahmen zur Reduktion von Kohlendioxidemission (CO2) in die Atmosphäre erforderlich, Rückhalt und Speicherung in tiefen geologischen Formationen (carbon capture and storage (CCS)) stellt eine davon dar. Der Transport von verflüssigtem oder superkritischem CO2 mit Verunreinigungen in Pipelines stellt einen wichtigen Teil von CCS-Systemen dar. Die Materialprüfung unter möglichst realitätsnahen Bedingungen ist eine wichtige Voraussetzung für die Einrichtung zuverlässiger und sicherer CCS-Systeme. Hierzu wurde eine neue Versuchseinrichtung zur Korrosionsprüfung von Pipelinewerkstoffen entwickelt, die folgende wesentliche physikalische und chemische Einflussgrößen berücksichtigt: Elastische Verformung, Druck, Temperatur, Gaszusammensetzung, und Fließgeschwindigkeit können unabhängig voneinander eingestellt werden. Die Verteilung der Radial- und Tangentialspannungen wurden in einer Rundprobe untersucht, und die Ergebnisse zeigten gute Übereinstimmungen mit den theoretisch zu erwartenden Werten. In dem vorliegenden Beitrag werden die Idee zu der neuen Korrosionsprüfeinrichtung und erste Ergebnisse vorgestellt sowie diskutiert.
The low-cycle fatigue behavior of cc-brass CuZn30 was irtvestigated in uniaxial (tension-compression and torional) and biaxial tests under total strain control at room temperature. Planar-biaxial fatigue tests were carried out on a servohydraulic tension-compression testing machine with and without phase shift using a cruciform specimen geometry with fixed principal stress axes. In phase and out of phase tensiontorsion tests were performed using tube shape specimens on a servohydraulic tensiontorsion testing machine. Microstructural investigations were performed by transmission electron microscopy as well as by scanning electron microscope. For all proportional load cases the equivalent strain amplitude based on a maximum shear strain energy criterion results in a similar hardening behavior and in fatigue life times within a scatter band of three. Out of phase loading results in additional cyclic hardening and reduced life time. Planar glide structures were observed in all tested cases as well as areas of pronounced strain localization in the torsional load cases. No evidence of transition to wavy glide behavior was observed.
With the publication of EN ISO 6892-1:2009, the harmonisation of tensile testing at room temperature in Europe was achieved by combining EN 10002-1:2001 with ISO 6892:1998. Essential changes are the introduction of the strain rate and the strain rate control as the significant testing rate or control mode, respectively, referred to as “Method A”. By using this method in the correct way it is guaranteed that in the moment of determination of properties comparable resulting strain rates at the test piece in smaller ranges of specified strain rates are adopted — also if different stiffnesses of the testing equipment are present. This leads especially for strain rate sensitive materials to better comparable results. In this paper it is documented the need for the implementation of method A, methods for the realisation — also by the use of older testing systems — and an example of the comparability of results of the methods for a strain-rate-sensitive-steel.