Thermo‐mechanical fatigue – the route to standardisation (“TMF‐Standard” project)

Abstract

Thermo-mechanical fatigue (TMF) data are needed for dimensioning of safety-critical parts, e.g. in power generation and in transportation by air, rail and road, and for designing of components, which operate at higher levels of efficiency and safety for longer periods of time. However, the current lack of a validated international standard for TMF leads to non-reproducibility of TMF data with negative implications on the design and economy of operation of many components. While a preliminary draft TMF standard has been written under the auspices of the International Standards Organisation (ISO), the "TMF-STANDARD" project addresses this issue at the European level, within the GROWTH programme of Framework Programme 5 funded by the European Commission.The R&D work in the TMF-Standard project has been started with the preparation of an inventory of institutions in the EU and the pre-accession states that perform TMF testing, and the testing methods and procedures which they apply. Afterwards, extensive pre-normative testing work has been carried out, the results of which have been considered in the drafting of a preliminary Codeof-Practice (CoP) that forms the base for the current validation round robin testing. The results of the validation tests are going to be subjected to statistical analysis, in order to substantiate the formulation of a validated CoP on strain-controlled TMF testing. The present paper summarizes the main objectives and achievements obtained so far within the TMF-Standard project, and gives an outlook on the further activities of the project.

TBCs for Gas Turbines under Thermomechanical Loadings: Failure Behaviour and Life Prediction

“…The TMF tests were carried out according to the European Code-of-Practice for strain controlled TMF testing [9] with a mechanical strain range of 0.6% and a temperature range of 350°C-1050°C. The same cooling/heating rate of 2°K/s was used as for thermal cycling.…”

Interaction of thermally induced and mechanical fatigue

“…The test rig must be capable to apply superimposed mechanical fatigue loadings with frequencies up to 5 Hz in total strain control in the complete temperature range covered by the TMF cycle. Further testing conditions and experimental details are the same as described for TMF testing in [6][7][8]. Figure 1a gives as an example of pure TMF loading a trapezoidal shaped out-of-phase cycle with full constraint of the thermal strain ε th .…”

“…The internal constraint within the component is applied as an external constraint of the thermal strain using a servohydraulic or electromechanical test rig. TMF test procedures according to the actual state-of-the-art are Keywords: TMF; HCF; lifetime behaviour; damage; lifetime prediction; described in detail in [6][7][8]. During the recent decades extensive research has been performed on the behavior of cast aluminum and iron alloys for internal combustion engines as well as on cast and wrought Nickel alloys for gas turbine applications, e.g.…”

Material and fatigue life models for thermo‐mechanical loaded components

“…Thereby a lifetime can be calculated by the plastic strain. The hardening coefficient and hardening exponent must be identified, to calculate a lifetime depending on the total plastic strain, see equation (4).…”

“…A European project has been tasked with producing a uniform standard [3]. As a consequence TMF investigations have been performed in several institutions to define standards for this complex issue [4].…”