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Development and Manufacturing of an All-Oxide Inter Turbine Duct for Aeroengines

Periodic Reporting for period 3 - AllOxITD (Development and Manufacturing of an All-Oxide Inter Turbine Duct for Aeroengines)

Reporting period: 2018-12-01 to 2020-05-31

Ceramic Matrix Composite materials (CMCs) have excellent properties at high temperatures. This behaviour is making CMCs interesting for components in the hot gas section of gas turbines. Higher tolerable material temperatures allow a significant reduction of cooling air in contrast to metallic components. This is increasing the turbine efficiency. The objective of the project was the development of an all-oxide Ceramic Matrix Composites (CMC) inter turbine duct for testing in a demonstrator engine.

Within the project CMCs based on filament winding, woven pre-pregs and triaxial braids were investigated in terms of thermomechanical and thermophysical properties at application-relevant temperatures. For ITD demonstrator testing the pre-preg CMC was chosen and further investigated through a verification program. Also a quality assurance program was worked out for this material. In parallel the ITD components were designed by the topic manager, supported by thermomechanical simulations of the consortium to generate a CMC suitable end design. Finally, 25 pre-preg CMC based ITD components for demonstrator testing were delivered to the topic manager (picture).
As a starting point, design and general boundary conditions of a metal ITD were analysed. On this basis design, attachment and sealing concepts of the metal part had to be adapted for the CMC-material. Special care had to be taken to prevent thermal restraints and maintaining a gas-tight seal.
Also, based on a design sketch and boundary condition of a metal ITD detailed thermomechanical calculations were performed, to see if it can withstand thermal and mechanical loads in the gas turbine for the required life-time. The fabrication process of CMC is much more similar to the processing of fibre reinforced polymers than metals. Geometries and fibre architecture, which can be produced from CMC material had to be considered in the design stage. Consequently, at that time, the CMC design looked very different to the metal design, yet fulfilled the same requirements.
During the second period various thermomechanical simulations were generated to comprehend strains and stresses inside the CMC-component and to investigate weak spots. Highest stresses could be localized in the flanks in axial and especially in circumferential direction.
Thermomechanical simulations served to evaluate the CMC-component design. The simulations gave support in the design development with regard to a CMC-appropriate design, thermomechanical loads and manufactural aspects. At the end of the second reporting period most design parameters were defined and manufacturing tools could be developed to produce a first series of hardware parts.
Within the last period of the project remaining simulation activities were finalised. Different approaches regarding strain- and temperature dependent stiffness losses were implemented into the simulation model and applied on the CMC-component.

Manufacturing started with the winding process. Several sample plates with different winding parameters, fibre bundle types and matrix systems were produced for initial testing and subsequent optimisation of mechanical material properties. Also parameter studies for the manufacturing of pre-preg material were carried out. First plates with optimised fabrication parameters were manufactured and delivered to the project partners for testing.
At the beginning of reporting period 2 it was decided to focus on the pre-preg CMC for manufacturing of ITD demonstrator parts. Properties of the wound CMC could not reach the requirements for the dedicated application at that time, while pre-preg CMC showed promising results. The pre-preg CMC was optimised in terms of mechanical properties and manufacturing technologies to realise the designated geometry, which was developed during the project. Standard Operation Procedures (SOP) and acceptance limits have been installed for test materials and process steps for varying geometry levels to ensure material quality.
The last period of the project was dedicated to manufacturing of ITD components for demonstrator testing. Here mainly the upscaling form flat specimen over curves coupons to the final geometry and machining to the required tolerances was in focus. In that part of the project, some of the technical goals could not be fully met. By the end of the project, two sets of ITD components were delivered, in total 25 components. The first for verification tests, the second aimed for potential demonstrator testing.

Parallel to optimising the winding and pre-preg based process a feasibility study for a braid based ox CMC was carried out. In reporting period 1 tri-axial braids of oxide ceramic fibres made by radial overbraiding technology were developed and manufactured for the first time. Due to their high stiffness, ceramic fibres are easily damaged. Here, the most important measure is the enlargement of the deflection radii. Therefore, new bobbin carriers for axial reinforcement were developed that eliminated all contact points at the carrier.
For the fabrication and subsequent testing of oxide CMCs made from braided fibre preforms, a suitable fabrication method had to be developed first. An experimental setup could be realised which combines infiltration and drying of the material in a continuous process.
The up-scaling of the developed process to fabricate larger braided CMC test plates was focused on in reporting period 2. A larger infiltration tool was designed and manufactured. Process parameters were optimised. SOPs for braiding of ceramic fibres and infiltration of braids were installed. Test material of the designated size and quantity was produced and characterised in the last period.

The characterisation of the materials produced within this project is focused on the thermomechanical and thermophysical properties at application-relevant conditions. This data was required for further FEM simulations and verification of the demonstrator component. A suitable test matrix and a time schedule were developed at the beginning of the project. Based on the current test results, the test matrix was constantly reviewed and amended when necessary.
For wound and pre-preg CMC preliminary material data was collected (static thermomechanical and thermophysical characterisation). Dynamic properties (e.g. creep behaviour) were determined for pre-preg CMC additionally.
Also a verification strategy for several geometry levels was worked out. Verification tests were carried out to ensure that the simulations deliver reliable results and reveal that the demonstrator parts could withstand the loads during the application.
The lower specific weight and the higher thermal stability of oxide CMCs compared to standard metal materials promise a potential for use in environmentally sustainable aircraft engines, by reduction of weight and the need of cooling air. The project AllOxITD executes a first approach to bring European oxide CMC in aviation in service. Even though material quality and dimensional accuracy are not yet fully sufficient, first steps in the direction of a future certification of the product were successfully taken.
Also, the developed manufacturing processes and SOPs are already used to produce components for non-aviation applications and the knowledge gained during the project contributed to the designing of a new production line in Heuchelheim, which will go operational in autumn this year.
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