The fundamental requirement for an engine part is that it reaches the intended life span safely under the expected operating loads. In order to ensure this, all factors that can influence part life must be considered. This is true for the entire life cycle of an engine type, from its design to its retirement from service (Fig. "Life span influences engine part").
The priority of demands on modern engines changes constantly and becomes ever more rigorous (Ill 12.1-1). With commercial aircraft, the driving forces behind these technical demands are primarily related to environmental concerns (reducing emissions, improving fuel efficiency, ). With military aircraft, the primary motivation is improving tactical value of aircraft, i.e. increasing performance and improving fulfilment of mission requirements. All of the above must be accomplished while minimizing costs. Until now, one could observe general increases in turbine inlet tempratures ( ) and overall pressure ratios (Fig. "Compressor technology and its problems"). It is not certain whether or not this trend will continue, and must be dealt with separately. An increase in the turbine inlet temperature does not necessarily mean that there is a temperature increase in the engine parts. Because the difference between allowable part temperatures and gas temperatures must be balanced by elaborate cooling measures that decrease performance, there is a movement towards using materials with ever greater thermal resistance and creep resistance in serial production (Fig. "Development curve of thermal strength"). Evidently the resistance of materials used in compressors and hot parts, and especially rotor components, has almost reached its limit. This can be seen in the flattening of curves that plot the trends regarding the creep resistance of hot part materials. For this reason, current efforts focus on using thermal insulation coatings (Fig. "Material behavior depending on design and technology") and more effective cooling systems in order to push the temperature limits higher. It must always be considered whether the advantages of a certain technology are worth the new risks it brings with it.
In military applications, especially, the constant demands for weight reduction in combination with increased performance leads to greater stress on engine parts. This decreases the size at which flaws become capable of growing. In other words, weak points that were allowable (according to specifications and design) in previous applications become serious flaws (unallowable, Example 12.1-2). This means that quality requirements must also become more stringent, such as the safe detection of ever-smaller flaws in destructive tests. The increase in usable material strengths also depends on the available destructive testing processes that are suitable for serial implementation. Even damages such as scratches due to handling (manufacture, mounting) become more critical in their effects. This necessitates increased caution during manufacture and installation.
In order to ensure acceptable life spans and overhaul intervals in the future, it will become necessary to incorporate the crack growth phase into the design, giving the field of fracture mechanics a more prominent role (Chapter 12.2).
In order to guarantee the necessary high safety levels despite higher demands on material strengths, it is imperative to have exact knowledge and understanding of all relevant, part-specific operating demands and damage mechanisms. This necessitates extensive testing under conditions that are sufficiently close to those during actual operation (also see ETOPS; Volume 1, Chapter 3), and also statistical analysis of the material data used for design. Because of this, the work (time, costs) necessary for developing materials and technologies (e.g. coatings or surface treatments) increases greatly relative to the demands.
Figure "A pplication specific demands on aeroengines": The demands on the material science and technology of engines depend greatly on whether they are intended for military or civilian use. Even engines of the same type can be subject to very different loads in different applications, causing them to “react” with different weak points.
Civilian use: In this application, environmental concerns are becoming ever more dominant. Emissions in exhaust gases and noise are becoming more important subjects. Due to strict regulations, environmental requirements can no longer simply be ignored, and they also represent a significant portion of a company`s finances. Costs are influenced by fuel consumption, starting windows due to noise control measures, and airport fees.
Naturally, purchasing and operating costs are determining factors for operators. Aside from the fuel costs, these include the purchase of new engines, part replacement (life span), overhaul intervals, and the overhaul and maintenance costs, which depend on hours of flight time. The ecological demand for low fuel consumption (minimizing energy use and emissions) is combined with an economic demand for low costs. These demands dictate the central themes for material technology. The materials used should cost as little as possible. Long term effects are being more closely considered. For example, with corrosion, wear, and erosion, it makes a big difference if the overhaul intervals are 20,000 hours or 2000, as they are in military applications. On the other hand, the shorter standing times, operation at high altitude, and relatively low takeoff frequency in civilian operation have a positive effect on engine life span. Proper assessment of the ways in which these long term effects damage engine parts requires special experience with long operating times.
In order to keep costs down, parts should be repeatedly repairable. For example, labyrinth tips that cannot be welded on are not acceptable on expensive engine parts.
Military applications: These demands are fundamentally different from civilian ones. Mission suitability is the highest priority. In general, this means that the engines should have the greatest possible thrust at the lowest possible weight. Of course, costs are also important, but their priority can usually be discussed. For material technology, this means that:
The materials used have high strength, especially rotor parts have high specific strength (relative to the specific weight). If the weight advantage is great enough, then costs are of minor importance. The high number of cycles (high start-up/shutdown frequency and load changes) requires correspondingly high fatigue strength in the LCF zone. Maximizing the thrust/weight results in high hot part temperatures. Due to the exhausted temperature resistance, new thermal insulation layers and cooling technologies are finding greater use. Fighter aircraft conducting military maneuvers require fast load cycles. This is accompanied by high temperature gradients with correspondingly great thermal strain in the hot parts. For this reason, suitable materials such as monocrystalline materials were needed early, turning military engines into trendsetters. Further examples include cast titanium high-strength powder metallic materials, intermetallic phases, and fiber reinforced metal matrix composites (MMC).
Figure "Life span influences engine part": The life span of a part, and therefore also of an engine, depends on several factors in addition to the operating loads. This problem can be described by using a turbine disk, which is a typical highly stressed engine part, as an example. Turbine disks are primarily subjected to high cyclical loads arising from changes in centrifugal force and thermal strain (mainly during startup/shutdown cycles) and their use in modern engines is usually limited to safe, designed life spans. The loads lie far above the fatigue limit, and are in highly stressed zones in the plastic range. This means that even relatively small flaws can result in cracking (Fig. "Characteristic crack growth"). Several examples (by no means a complete overview!) can make the influences more easily understood:
Operating profile: This affects values that are important for the life span, such as the size, location, and duration of cycles. Important factors include the acceleration time, number of load changes, and the absolute performance, which are in turn dependent on:
External factors: The strength of a part can decrease over the operating life due to influences such as corrosion and fretting wear. External influences are related to:
Overhaul, repair, maintenance, and installation: Here there is the danger that flaws and damages are not detected (in time; such as during crack detection) or that new damage occurs (e.g. due to acid baths or handling; also see Volume 1, Ill. 126.96.36.199-5). These risks also depend on:
Production: The surface of parts, especially, is subject to high dynamic loads (flexural modes). This makes the manufacturing process extremely important, since it has a large effect on surface properties. Factors include:
Bar stock and rough part manufacture: In this process, volume flaws such as pores and spaces and abnormalities in cross-sections (structural) play an important role. These are influenced by:
Construction and design: Naturally, the life span of a part is ultimately determined by its design. However, this is decisively dependent on whether or not the strength data used for the part and the loads are representative and safe. Important factors include:
Figure "Reciprocal influencing temperature dependant damages" (Ref. 12.1-3): Because different stresses and damage mechanisms (also see Chapter 12.4) often occur in combination, the progress of damage is often extremely complex. Depending on the specific material, the same process can be increased under some conditions, while other conditions cause it to weaken. One example of this is creep and damage accumulation under dynamic loads (Chapter 12.5 and Fig. "Dynamic fatigue life span estimations (Miner rule)"). If there is a dwell time, creep deformation can accelerate or decelerate, depending on the temperature and stress levels. This determines whether a “healing” or damaging effect is more dominant. For example, oxidation during slow crack growth can cause the tip of the crack to become rounded out, hindering further crack growth. On the other hand, oxidation can further increase the speed of rapid crack growth.
In order to be on the safe side during the design stage, it must generally be assumed that damaging effects will reinforce one another. Minimizing weight (Fig. "A pplication specific demands on aeroengines") demands sufficient strength, even with little play between the loads and material strength. Safe amounts of play become easier to ensure, the better the operating loads and their influence on the part behavior are understood.
Since the desired behavior of a part can ultimately only be seen by satisfactory performance of a part in an engine with its typical complex operating loads, increasingly elaborate cyclical test runs are necessary (e.g. within the framework of an ETOPS certification; Volume 1, Chapter 3). This shows that the basic axiom “the engine will tell us” holds true even in the computer age.
Figure "Development curve of thermal strength": The top diagram shows the development of the operating temperature of typical turbine rotor blade materials under constant loads over the course of 1000 hours. The development ranged from forged materials to cast materials, the structure of which was optimized to increase their creep strength by directing their structure to a monocrystal in order to achieve higher operating temperatures. The lower dynamic strength of cast materials (example 12.1-2; an example in the compressor) was accepted. One can see that at the end of each development stage, the curves flatten out and indicate the development limit. This is also true for all procedures with all the depicted materials. With today`s technology, the limit is probably at a maximum operating temperature (material temperature) of about 1100 °C.
The lower diagram (Ref. 12.1-2) shows that it is possible to further increase the gas temperature at the turbine inlet even though the maximum material temperature is about 1100 °C (also see Fig. "Historical trends of of fighter engine problems"). The maximum usable material temperature of about 1100 °C cannot be used in supporting cross-sections for reasons of strength. In turbine blades, the edges are typically subject to especially high thermal stress. The thermal strain in these zones is considerably greater than in the cooled supporting areas, which induces internal stress. This internal stress then puts tensile loads on the colder sections. This means that the damage mechanism that limits the life span of high-temperature parts is thermal fatigue and oxidation, rather than creep failure.
There are two primary methods of increasing the gap between the gas temperature and the material temperature:
The reflection of the blade surface has evidently not yet made it into the field of blade design (also see Fig. "Influences on combustion chamber wall heating-up") .
A quantum leap in the increase of the usable material temperature in serial production is not in sight. Future hopes are with operating times of a few hundred hours for monolithic and fiber-reinforced ceramic materials in order to permit their use (see Chapter 14).
Example "Increase of undetectable flaws" (Ref. 12.1-1):
Exerpt: “During the takeoff roll, a…(2 engine civilian aircraft) experienced an uncontained failure of the number 1 engine high pressure turbine wheel. Portions of the turbine disk exited the engine case, producing a 4-inch by 16-inch rupture. Pieces of the disk and the 2nd stage nozzle were fragmented and produced shrapnel like projectiles that pierced the engine core cowling and surrounding engine pylon mount. The first officer, flying the airplane , aborted the takeoff and returned to the departure gate. Examination of the disk disclosed that it fractured due to fatigue from an undetermined origin.
Comments: The probability of fractures originating at undetected flaws will continue to increase along with the loads on engine parts. This is in no small part due to the fact that dangerous flaws, as well as the time required for crack growth to fracture, are both getting smaller (Fig. "Characteristic crack growth").
Example "Poor dynamic strength of cast materials" (Ref. 12.2-4):
Excerpt: „…(The OEM) estimates that corrections to cure cracked compressor stators…will push deliveries…back by almost three months…'It's a cast part, and we have opted to change that to a forged part'…“
Comments: This example shows the relatively poor dynamic strength of cast materials. Evidently this case concerns dynamic fatigue. The higher dynamic strength of forged parts is the reason for the change of materials.
Figure "Material behavior depending on design and technology": The demands on the safety of a technology depend largely on whether its characteristics are included in the design, or whether the technology is used only to ensure that the intended life span is met (like a belt in addition to suspenders). The example here is a thermal insulation coating on a turbine rotor blade. In the case of this “safeguarding” technology, spalling of the coating at the leading edge of the blade causes increased oxidation and thermal fatigue. However, the damage progress is so slow, that an acceptable life span can be expected. In any case, it should be possible to detect the damage during a routine control (boroscopic inspection) before the part fails. The right diagram depicts a case in which the thermal insulation coating is absolutely necessary to ensure safe operation. When the coating spalls, the overheating-induced damage progress is so rapid, that the probability of a blade failure is greater than the probability that the damage will be discovered during a scheduled inspection. Therefore, in this case coating failure must absolutely be prevented. This means that the reliability must be correspondingly greater. This influences the demands on quality control and the performance of the coating properties.
Similar considerations are necessary, for example, in the case of shot peening with regard to the cyclical life span of rotor disks. Shot peening is used as a safeguard to protect against surface
damage (scratches, etc.). As part of the design, it is intended to ensure that the safe life span is reached by giving the disk greater LCF resistance.
12.1-1 NTSB Identification: LAX91IA 376, NTSB microfiche number 466.30A, incident from August 29, 1991.
12.1-2 S.Wittig, R. Schiele, K. Sieger, A. Schulz, „Einfluss der Aerodynamik auf die Wärmeaufnahme konvektionsgekühlter Turbinenschaufeln”, University of Karlsruhe, Faculty and Institut for Thermal Flow Machines, 1995.
12.1-3 A.Rossmann, „Untersuchung von Schäden als Folge thermischer Beanspruchung”, article in J.Grosch „Schadenskunde im Maschinenbau“, ISBN 3-8169-1202-8, Expert Publishing Volume 308, 1995, pages162-187.
12.1-4 G. Norris, „P&W corrects PW4098 cracks and confirms 777-300 delay”, periodical „Flight International“, 27 May-2 June, 1998, page 22.