Engine Construction and Materials
Domain 2 - Engine Construction and Materials
The exam tests material selection by temperature zone, blade manufacturing methods, coatings, and structural and containment design.
Core rules
- Aluminium and composites: fan case and cowls. Titanium: fan and LP compressor blades to roughly 550–600 °C; above that titanium loses strength and can burn. Nickel superalloys: rear HP compressor stages, combustor, turbine nozzles and blades. Steel: shafts, casings and gears.
- HP turbine blades are directionally solidified or single-crystal to remove grain boundaries normal to the stress axis and improve creep life. They carry a ceramic thermal barrier coating, usually yttria-stabilised zirconia, over an MCrAlY bond coat.
- Creep, low-cycle fatigue from start–stop thermal cycling, and high-cycle fatigue from vibration are the three life drivers. Disk life is usually fatigue-limited; blade life is creep- and oxidation-limited.
- Containment: fan blades must be contained after release, so casings are designed for a blade-off event. Inspection looks for case rubbing and tip shroud wear.
Worked example
A design change raises T4 by 40 °C on a blade already at its creep limit. Increasing cooling air costs cycle efficiency and may not be available from the existing compressor schedule. A thermal barrier coating gains roughly 100–150 °C of surface protection but does not increase substrate capability. A single-crystal alloy gives real creep improvement but needs a new part number and re-qualification. For a life extension with the cooling schedule already fixed, the material plus coating route is the correct answer.
Traps
- Specifying titanium for a hot section because it is light.
- Treating a thermal barrier coating as a structural load path.
- Ignoring galvanic corrosion where composite cowls meet aluminium structure.