• A clean engine is one that, at this time, is not suffering from any performance deterioration.

• Creep is the continuous deformation of materials that occurs under mechanical loads. It is exhibited by iron, nickel, copper and their alloys at relatively high-temperatures, and by zinc, tin, lead and their alloys at room temperatures. All such creep is a permanent deformation of the material.

• Creep-life is the length of operational time before the component is deformed (due to creep) to such an extent that either it is not safe to use it further or the performance has declined to an unacceptable level.

• The engine’s design-point describes the expected values of the influential parameters or characteristics (such as the turbine’s entry-temperature and net thrust) under specified conditions (such as when the aeroplane is stationary and at sea level).

• The engine deterioration-index (EDI) is a hypothetical parameter combining the adverse effects upon the engine’s performance of (i) fouling and (ii) erosion of any gas-path component. It is presumed, for the purpose of this investigation, that a 1% fouling of the compressor(s) accompanied by a 1% erosion of the turbine(s) results in a 1% engine-deterioration index. Linear relationships are assumed: so a 2% EDI represents the combined effect of 2% FI see later for the compressor(s) and 2% EI for the turbine(s).

• The erosion-index (EI) is a hypothetical parameter combining the adverse effects upon the engine’s performance of (i) an increase in flow capacity and (ii) a reduction in efficiency of any gas-path component. It is presumed, for the purpose of this investigation, that a 1% decrease of efficiency accompanied by a 0.5% increase in flow capacity results in a 1% EI. Linear relationships are assumed: so a 2% EI represents the combined effect of 2% fall in efficiency and 1% rise in flow capacity respectively.

• The A/C’s flight envelope indicates the limiting boundaries (mainly in terms of Mach number and altitude) of the flight path followed during the mission.

• Fouling occurs when foreign matter, e.g. carbon, is deposited on a surface.

• The fouling-index (FI) is a hypothetical parameter combining the adverse effects upon the engine’s performance of reductions in (i) flow capacity and (ii) efficiency of any gas-path component. It is presumed, for the purpose of this investigation, that a 1% decrease of efficiency accompanied by a 0.5% reduction in flow capacity results in a 1% FI. Linear relationships are assumed: so a 2% FI represents the combined effect of 2 and 1% falls in efficiency and flow capacity respectively.

• Low-Cycle Fatigue (LCF) is the failure that occurs after a relatively low number (< 5×10⁴) of high-stress applications.

• An outage is the breaking down of a component and thereby stopping the further use of machine (i.e. an engine in this case) until an appropriate replacement or repair is undertaken.

• The spool is the shaft connecting a turbine with its compressor (i.e. HPT with HPC, or LPT with LPC). Spool speed means the speed (normally expressed in revolutions per min) at which the stipulated shaft rotates.

• The compressor’s surge margin is the tolerance between the compressor’s equilibrium running line and the surge line. The surge margin becomes zero if the equilibrium running line intersects the surge line: then the engine will not be capable of being brought up to full speed without some remedial action being taken. Even when clear of the surge line, if the running line approaches it too closely, the compressor may surge when the engine is accelerated rapidly. Among other things to minimize the tendency of a compressor to surge, it can be ‘unloaded’ during certain operating conditions by reducing the pressure ratio across it for any given airflow. One method of doing this is by bleeding air from the middle or towards the rear of the compressor.

There are several types of engine employed in present-day A/C. Military engines are designed for exposure to much more severe extremes of steady state and cyclic usages than are experienced by engines in commercial A/C, as illustrated by the power-setting variations during flight in Fig. 1 The manoeuvres, generally experienced by combat A/C, impose during flight far larger stresses on the engines than encountered normally by either commercial or military transport planes. Consequently, the ability to predict realistically the resulting associated active-life shortening (i.e. life consumption) for these types of engines is desirable from a management viewpoint and it affects the life-cycle costing.

Gas-turbine engines may be subjected to severe operating conditions, which eventually lead to costly and catastrophic failures if a run-to-failure philosophy is adopted. Therefore, military gas-turbines are operated on a safe-life principle, whereby the engine is withdrawn from service for maintenance well before failure is likely to ensue. Early attempts to predict the safe operating life of an engine were primarily undertaken by assessments of engine failure, and upgraded as more engine-usage experience was gained.

However, it soon became obvious, for engines experiencing a wide range of, and frequent, changes in operating conditions, that predictions of the residual life based solely on the EOT were often highly inaccurate. Each resulting anticipated life was consistently underestimated because the prediction was based on a worst-case scenario, regardless of the actual engine usage. The end result was an excessive financial expenditure as a consequence of the associated unnecessarily high maintenance and employed spares costs.

There are many components in a gas-turbine engine, but its performance is highly sensitive to changes in only a few and so only these are considered in this life-usage analysis. The majority of these potentially critical parts are the rotating components: the failures of these are principally due to cyclic and steady-state stresses.

Modern aero-gas-turbines are required to produce extremely high thrusts or shaft powers as well as to withstand the severe thermal conditions and high mechanical loads that arise during military operations. The high-pressure turbine (HPT) blades are the most critical components, because they are subjected to both the highest rotating speeds and gas temperatures, and so have been selected for investigation in this project.

The failure mechanisms, resulting from engine usage, may be considered singly or in combination. Within each mechanism, there is a multiplicity of influential variables. To include all of these that could affect the life prediction is beyond the scope of this investigation, whose aim is to highlight the most important variables and failure mechanisms resulting from the application of mechanical loads at high temperatures. The processes to which high-temperature structural components are subjected are time-dependent (creep) and cyclic-dependent (fatigue) deformations.