Structural Integrity in Aero Gas Turbine Engines

In Western Europe, the Joint Aviation Authorities, a consortium of avwtlOn authorities from participating member states, publish rcquirements covering all aspects of aviation. The part which has specific application to aero engines is JAR-E. The rules given in JAR-E set standards of strength and margins which must be met by all power plant for civil aviation usage. The main concerns for structural integrity in aero gas turbine engines are identified as behaviour as a result of ingested objects, low cyclic fatigue failures, crack growth and coalescence of cracks. A failure modes and effects analysis identifies those components in the engine whose failure may lead dircctly or indirectly to hazardous effects. It is necessary to demonstratc that the duty of such Critical components and the variation in their strength combine in such a way that failures cannot occur. The approaches used to establish the cyclic capability of such components arc described. With other components, notably blades, failure is designed against by providing a containment shell around the rotating stage which will prevent the release of a single blade outside the engine nacelle. As weil as this, the engine is tested to demonstrate robustness in operation for the hot sections including combustors and turbine blades. With fan blades, howcver. which are the first stage and therefore more likely to be subjected to ingestcd object damage, it is nccessary to show th at they can sustain impacts by small and medium birds but still retain part or all of their thrust capability. For impact with large birds, however. it is simply enough to show that thc cngine can bc shut down safely. In the event of a fan blade becoming detached. the imbalance causes large structuralloads on the engine and enginc mounts. A test is generally required in which a blade is released at the most arduous point in the cngine operation and both the containment system and the ability to shut the engine down safely is demonstrated. Engine mounts, which connect the engine to the airframe. may be designed with damage tolerance where a crack is assumed in the most highly stresscd location and. where applicable. another is assumcd in thc sccondary load path. In such cases the inspection interval is related to the number of fiight cycles to burst from the specified crack sizes. Casings and combustors are two cxamplcs of thin shells used in gas turbine engines and their structural integrity is also described.