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Material Model of Case-Hardened Steels for Turbo Gear Applications

Periodic Reporting for period 2 - MatCH4Turbo (Material Model of Case-Hardened Steels for Turbo Gear Applications)

Reporting period: 2020-10-01 to 2022-03-31

CleanSky 2 aims to reduce CO2, NOx and noise emissions of aircrafts in order to maintain the quality of life through environmental protection. At the same time, the competitiveness of the aviation industry must be strengthened in international competition in order to secure and create jobs in Europe. Therefore, the targets of ACARE 2020 will facilitate the first steps to the Flightpath 2050 targets that include 75 % cut of CO2 and 90 % of NOx emissions as well as 65 % noise reduction with respect to the year 2000. MatCH4Turbo will contribute to these ambitious emission reduction goals by providing innovative, knowledge-based calculation methods for gear load capacity to push the Ultra High Bypass Ratio (UHBR) technology. The Ultra High Bypass Ratio technology has been identified as a promising approach to reduce aircraft engine emissions and fuel consumption. UHBR utilises a reduction gearbox that reduces the fan speed and allows the implementation of larger fan diameter. Calculating of gear load capacity will be essential to predict the gearbox behaviour. One of the key challenges of epicyclic gearboxes in UHBR applications is the high number of load cycles reached by the gearbox components. Standards such as ISO 6336 deliver design guidelines for bending strength up to a limiting number of load cycles of NG = 3∙10^6 load cycles which is far less than the expected lifetime of UHBR applications. The lack of testing concepts and models for Very High Cycle Fatigue (VHCF) gear applications leads to uncertainties in the design of highly loaded gears, which is not acceptable in aerospace applications. However, VHCF design guidelines essentially are required for the bending strength of gears as due to reverse bending loads the tooth root breakage is the most likely and critical failure mode for planets in such applications. It is the consortium's conviction that a new VHCF calculation model for bending fatigue of gears can be directly applied in order to optimize UHBR applications in regard of compactness and lightweight design in order to reduce CO2 and noise emission. The VHCF model is validated on a high level in order to guarantee the usage for aerospace components. It is mandatory, that the material data generated within this project is from excellent quality. The material data and developed models can be further used for future gearbox designs in automotive applications (E-mobility), the development of new steels suitable for VHCF as well as future calculation approaches. This contributes to the overall goal of the Clean Sky 2 joint undertaking a reduction of CO2 and noise emissions of aircraft by enabling lightweight aerospace power train designs.
To receive reliable results for the VHCF behavior of case hardened gears in the VHCF regime, gears have to be tested in a high-speed rotational test rig. Therefore, the consortium member WZL developed and designed a high power back-to-back test rig for high rotational speeds, which goes beyond the current state of the art of gear fatigue testing. In a first step, the geometry of the test gear set was designed according to the ISO 6336 and checked regarding the critical failure modes using different standardized and highly detailed calculation methods. The gear design represents the best compromise for the given limits and derives a sufficient safety against unwanted damage types. Based on the final test gear geometry the test rig was designed and critical machine components were optimized. Using the detailed CAD-Model of the test rig, the drawings of the components were derived. For the reliable continuous operation of the test rig, the cooling infrastructure at the WZL has been enhanced. Since the power class of the new high-speed rotational test rig is beyond the state of the art, the available cooling capacity was not sufficient.
Based on the drawings of the test rig the external manufacturing of the housings, shafts and seals was tendered and commissioned. Due to the high estimated costs of these parts, an extensive tendering process was necessary. Furthermore, the other test rig components and components for test preparation, e.g. the oil aggregate, the drive motor and a balancing machine, were tendered and ordered. Performance analyses of the test rig were carried out using the present design status. The thermal and dynamic operational behavior was simulated and the safe operation ensured. The manufacturing of the standard test specimens and the test gears from the first material is finished by 10 %. The procurement of the second material is ongoing and delayed due to the current supply problems on the steel market.
For the understanding of relevant local failure probabilities and mechanisms at up to 10^9 load cycles a VHCF model is developed by Leibniz-IWT. The experimental approach includes fatigue and crack growth investigations on simplified geometries to examine the influence of different carbon contents on the material properties of case-hardening steels under cyclic loading. The preparation of the model taking into account the weakest link concept is in progress. Furthermore, Leibniz-IWT is conducting heat treatment trials in advance for ensuring a high quality of the upcoming heat treatment of gears and standardized specimens.
The main innovation of the MatCH4Turbo project is a validated model for calculating local failure probabilities in the VHCF regime. The VHCF model is built on local approaches in regard of material properties and tooth root stresses. This way, the calculation model can be transferred to other gear geometries with high flexibility and accuracy. The validation is performed using an advanced recirculating gear test rig, which is designed, manufactured, commissioned and operated during the project. In the state of the art, there are no test rigs in this power class available on the market. The advanced recirculating gear test enables an efficient testing of new materials for VHCF applications, which is a major innovation in terms of validation possibilities for gear applications.
The virtual design process for high performance machine elements based on an FE-analysis leads above all to a significantly accelerated and resource-efficient design process. With tools provided in this project, functional tolerances and surface/subsurface characteristics can be pre-defined in the design process to ensure a safe operation of the machine element. Besides productivity, the manufacturing quality can be increased at the same time as tolerances are better controlled. Due to the validated and data-based VHCF calculation model, it is possible to realize gears with reduced geometric dimensions, whereas the transmission of a high power density can be realized. This results in weight savings potential, which in turn leads to higher transport capacity and reduced emissions of the aircraft.
Overall and sub objectives of the project