Aeroengine Safety

Figure "Risk of dinensional inaccuracy" (Example 16.2.2.9-1): It is not always known how close to their strength limits parts are during operation. The greater the utilization of the strength, the more serious the effects of seemingly minor dimensional deviations. In the described case, there was an increase in the number of dynamic fatigue fractures in turbine blades after the profile underwent seemingly unproblematic rework (see Fig. "Minimizing scrap rates throuch reworking").

Figure "Problems by deviation of wall thickness": The vibration behavior of thin-walled parts with large surface areas, as are found especially in older engine types, reacts very sensitively to anomalies in wall thickness. There are several reasons for this, which are primarily related to changes in wall stiffness.

A thinner wall can have a negative influence in two ways (bottom frame):

• Lowering of the resonant frequency can result in resonances that parts with design-conforming wall thickness would not be affected by. This effect is usually not compensated for by the simultaneous reduction of vibrating mass, which increases frequencies.
• Increased dynamic stress on the weaker cross-section at the same excitement intensity.The diagram shows two typical cases of damage. In both cases, damage investigations revealed that the wall thickness was at least locally thinner than in the design requirements.
  

Case 1: In a propeller engine (top diagram), sections broke out of the surface of a thin-walled metal sheet cone that carried exhaust gas. The damage mechanism was HCF dynamic fractures, which are typical for high-frequency plane vibrations. The cause was most likely a sheet metal thickness at the lower tolerance limit, because only those parts were damaged. In critical cases, operating damages can occur even when design drawing requirements are met but the limits are not verified during testing and serial operation.

Case 2: Static labyrinth cones in the bearing chamber of a fighter aircraft engine were made from thin-walled sheet metal. In several cases, sections broke out during operation (middle diagram). These were also HCF dynamic fractures with damage symptoms that were typical for plate vibrations. In this case, as well, the minimum wall thickness required in the design drawings was not met in several areas. This was apparently related to the forming process of the cone.

Figure "Safe operating due to dimensionalaccuracy": Performance concentrations, increased efficiency (lowering specific fuel consumption), and cost reduction all increase the demands on components. Typical effects include higher mechanical loads and/or demands for optimal flow properties and minimal clearance losses. In most cases, this tends to lead to tighter dimensional tolerances. Typical examples include:

Blisks: These are integral bladed disks (left diagram). These parts have been used for a long time in compressors and turbines in smaller helicopter engines. Their use in larger engines up to their use in fighter aircraft engine fans (top right diagram) is a more recent phenomenon. These one-piece parts, as well as welded stators (Fig. "Damaging metal splashes abd sparks") do not have any friction damping like rotors with inserted blades do. This makes high-frequency vibrations a more important factor than before. Effects such as mistuning (Refs. 16.2.2.9-4 and 16.2.2.9-5) become very important. This term refers to a method of changing the resonant frequencies by redistributing the mass of the individual blades. Differences can already be apparent within the usual tolerances. Changes cause the mass distribution and stiffness to shift along with the resonant frequencies that are influenced by these factors. Coincidental unfavorable combinations can cause vibrational loads that are several times higher than usual in individual neighboring blades. In contrast, large dimensional deviations outside of normal tolerances, such as specific dimensional alterations between neighboring blades, can lead to lower dynamic stresses (Volume 3, Ill. 12.6.3.4-6.2).

Variable compressor guide vanes (middle diagram): These systems consist of a large number of components that are moveable and/or connected by positive-fitting. Angle deviations in the blades of less than 1° (also see Fig. "Accuracy demanded for diffusion welding") can already unallowably influence the operating behavior of modern compressors (e.g. surge limit and efficiency). Therefore, bearing tolerances must be strictly maintained over long run times. New part dimensions are a prerequisite for this.

Rotor blade (right diagram): Dimensional deviations in the front edge can cause rotating stalls and dangerous blade vibrations. If a sufficient number of blades are affected, it will have an unacceptable effect on the operating behavior and efficiency of the compressor (Fig. "Accuracy of blades with ultrasonic airflow").
The extreme circumferential speeds of modern compressors stress the contact areas of blade roots at levels near the limit of their dynamic fatigue strength. The wide-chord design with high blade weight contributes to this. In order to prevent local overstressing of the edge zones and transitions of the contact surfaces, these areas are given optimal dimensions. The dimensioning takes into account the elastic deformations and ensures favorably distributed surface contact pressure (Fig. "Limits of demads on blade roots"). This can only be achieved with very tight form tolerances.

Figure "Limits of demads on blade roots": The loads on blade roots increase with the performance concentration/circumference speed and weight of the blades (long chords, see Fig. "Safe operating due to dimensionalaccuracy"). In the root contact surfaces of compressor blades made from titanium alloys, this leads to an increased risk of dynamic fatigue fractures in combination with fretting damage. For this reason, special measures are not required in older engine types to even out the root loads. It is more important to minimize the loads at the edges and the transitions to the contact surfaces (middle diagram). For this reason, the contact surfaces of the blades are given a shape that takes into account the elastic deformations that occur under operating loads. A typical shape is a minimal camber (bowing, right diagram). The task of finishing is to ensure this three-dimensional form with the tightest possible tolerances.

Figure "Accuracy of blades with ultrasonic airflow": Requirements for performance increases have led to ever higher flow speeds in compressors. This means that the efficiency of the compressor, and therefore the fuel consumption of the engine, reacts ever more sensitively to inaccuracies in the flow on the inlet edge (diagram, Ref. 16.2.2.9-2). Even minor deviations (top middle and right details) from the ideal edge shape can have an unallowable effect. In addition to influencing the efficiency, unfavorable inlet edge shapes also increase the risk of rotating stalls and dynamic overstressing of the blade. For these reasons, manual edge rounding is becoming ever more problematic.

Figure "Bore geometry influencing flow resistance": The demands for carrying and distributing the cooling air for hot parts such as combustion chambers and turbine blading are continually increasing. The reason for this is the increase in gas temperature, which is a typical trend in the development of turbine engines (Volume 3, Ill. 11.1-6).

The operating behavior of combustion chambers can react very sensitively to changes in the combustion and cooling air flow (Volume 3, Ills. 11.2.2.1-9 and 11.2.2.1-10). It can be assumed that this will further increase along with the rise in demands for low emissions with shorter combustion chambers and higher energy conversion. The air infeed for the cooling air film on the inner combustion chamber wall usually occurs through a large number of bores (middle diagram). Even seemingly insignificant changes in the boring process can unallowably alter the roughness, shape, and edges of the bores. Experience has shown that this will mean that the necessary operating behavior of the combustion chamber will no longer be guaranteed.
In addition to the cooling air flow, bores are also important for the cooling film on the surface of turbine blades. Next to optimal cooling air flow, an important criterion is preventing blockages from the inside and/or outside. Dust that can cause blockages can be carried by the hot gas and/or the cooling air. Optimizing the shape of the cooling air bores can reduce this danger (bottom diagrams). This places additional demands on finishing to meet form and dimensional tolerances.

Conical bores that are created with the aid of lasers are state of the art, although there is evidently room for improvement.

Sharp edges and burrs on the air inlet side can significantly disrupt the flow relative to rounded edges (top diagrams). This is the case if the possible boring direction on the air inlet side creates undesirable, flow-disrupting sharp edges and/or burrs.

Figure "Accuracy demanded for diffusion welding": Diffusion welding processes are used in special applications such as hollow fan blades made from titanium or dual property turbine disks (diagram, Ref. 16.2.2.9-1). They require especially exact machining of the joining surfaces, especially if the surfaces being welded cannot be sufficiently pressed together by external forces during the welding process. This applies especially to heat-resistant materials such as nickel alloys. This exactness is required because diffusion bonding can only occur if there is metallic contact. Even tiny local gaps will cause bonding flaws (Ills. 16.2.1.3-37 and 16.2.1.3-38). The roughness of the surface and its influence on microscopic contact flaws are important. In this context, it can be assumed that problems will occur when joining materials with different hardening states and different volume proportions of precipitated hardening phases (g'). Even these tiny volumnal changes can cause worrying dimensional changes at the high temperatures of diffusion welding (Fig. "Volume changes by hardening or solution annealing").
These flaws occur as kissing bonds (collections of microscopic flaws). They are not sufficiently reliably detectable with serially-implementable non-destructive testing methods. If the weld is placed under significant shear stress, crack growth can be expected from these flaws under creep and/or cyclical loads. Experience has shown that this means that safe operating life spans can no longer be guaranteed (Ref. 16.2.2.9-1).

Figure "Volume changes during heat treatment": Structural changes such as occur during heat treatments or finishing processes (e.g. welding and diffusion coating) can result in small permanent volume changes. The mechanism is an expansion or relaxation of the metal lattice, during which atoms are moved to other parts of the lattice.

A typical example is the hardening of steels. Martensitic structures (hardened structures) have an expanded lattice relative to the soft annealed state. This is caused by carbon atoms. During case hardening, this creates desirable compressive stresses in the surface (Fig. "Hydrogen embrittlement by coatings"). In steel parts, it affects the final dimensions.

Even in hardenable light metal alloys (aluminum) and nickel alloys, volume changes occur during the reciprocal transition from the hardened state to the solution annealed state (Fig. "Volume changes by hardening or solution annealing"). In these materials, however, the effects are usually limited to blanks or semi-finished parts. For this reason, the dimensional changes are not noticed as they are in hardened steel parts.
If, during the hardening of steels, the cooling process does not result in the complete transformation of austenite into hardened structure, the structural conditions will be unstable. This can result in a later volume increase when the material is heated (below 300 °C, bottom diagram) or cooled (about -25 °C). If the inner part of slip joints with very tight tolerances is affected, they can become jammed. Evidently, this process led to the failure of piston engines in fighter aircraft in World War II. Components of the injection system jammed at very low surrounding temperatures (top diagram). A similar process is described in Example 16.2.2.9-2. In this case, the metering piston of a helicopter engine fuel regulator jammed during testing in a cooled chamber. In this extremely tightly tolerated slip joint, a dimensional increase of only a few m was sufficient to cause damage (middle diagrams).
In roller bearings at high operating temperatures, a 200 mm diameter was observed to grow by 0.01 mm.

The diagram shows an expected dimensional change of up to 1.6 parts per thousand in connection with the annealing temperature for the typical roller bearing material 100 Cr 6 (Ref. 16.2.2.9-6). The lower the austenitizing temperature, the smaller the expected dimensional changes in this case.

Figure "Volume changes by hardening or solution annealing": Hardenable nickel alloys experience permanent volume changes. These are related to the g' hardening phase. If it goes into solution, it will result in expansion of the lattice and an increase in volume. In contrast, hardening leads to a volume reduction. This means that the behavior is the opposite of that in martensitic steels (Fig. "Volume changes during heat treatment"). The volume proportion of the hardening phase should play an important role. It increases along with the heat strength of the materials, i.e. the proportion of g' (left diagram).
If the entire structure has transformed, dimensional changes of between 0.5 and 1 parts per thousand can be expected, depending on the alloy.

Usually, this type of change will not cause problems during the finishing process or operation. This is because heat treatments are usually done on raw parts with sufficient oversize. Therefore, problems should be limited to unique situations.

It is possible that problems could occur during diffusion welding of parts with different heat treatment states or made from different materials. If there are very different volume proportions of the g'-phase, it will threaten the exact dimensional accuracy required by the welding process. The typical very high temperatures of this welding process are sufficient to cause corresponding structural changes (Fig. "Accuracy demanded for diffusion welding").

Figure "Problems due to dimensional changes": The most common causes of dimensional problems (warping) during finishing are residual stresses and/or thermal strain. Residual stresses can occur during raw part or finished part production. In many cases, thermal strain causes plastic deformations that in turn create undesired residual stresses. A typical example is welds.

Residual stresses can change during later operation. This leads to warping, which can have unallowable consequences as shown in the following examples:

Compressor housings of a turboshaft engine with a longitudinal assembly design were made from forged parts with a turned inner side and milled outer side (bottom diagrams). After the installation of new engines, heavy rubbing of the compressor rotor occurred in several cases. During this process, the compressor housings were irreparably damaged by embrittlement resulting from oxygen absorption during overheating. An investigation revealed that the housings evidently permanently ovalized during the first startup, i.e. the first heating to operating temperature. The warping occurred primarily along the axial flange. The deformations were traced back to residual stresses from the finishing process. Apparently, they were related to plastic suface deformations caused by milling. The solution was a suitable shot peening of the threatened area in the parts that had already been produced. The residual stresses from the shot peening overlayed with the residual stresses from the finishing process in a favorable way that did not result in any unallowable warping. For the current series, optimization of the machining process should be a preferred solution.
A sudden shifting of residual stresses with corresponding warping can also occur under the influence of weak external forces. One example is the “jar lid effect” (top middle and right diagrams, Fig. "Effects of residual stresses"). In this case, a thin-walled membrane (lid, disk) enclosed by a stiff ring zone suddenly deforms in an umbrella-like manner.
Disk-shaped structures without reinforcing rings can experience S-shaped deformations (8-shape, top left diagram).

These processes are reversible. They can be observed in thin-walled rotor disks. Destabilizing residual stresses can be created during raw part production (forging stresses, casting stresses) or by intensive machining. This indicates that the machining parameters and/or clamping of the part were not sufficiently optimized.
A solution is to suitably break down the residual stresses. This can be done through stress relief annealing in stabilizing equipment. In the case of rotor disks, it may be possible to pre-spin them until allowable local plastification occurs.

Thermal stresses with disrupting warping of the part can occur during finishing steps that involve heating. Typical examples include heat treatments, welding, and soldering. The cause is thermal strain differences resulting from uneven temperature distribution or differences in strain behavior resulting from the joining of different materials. These dimensional changes can be very problematic and cause bonding flaws in soldered joints or EB welds, which require exact maintenance of the joining gap.

Even small structurally-dependent volume changes can cause problems in case of very tight tolerances. They can occur during finishing (e.g. during diffusion welding; Fig. "Volume changes by hardening or solution annealing") or during operation (e.g. during the jamming of sliding systems; Fig. "Volume changes during heat treatment").
If parts with considerable porosity, such as cast or sintered parts, are subjected to an HIP process (Fig. "HIP of cast parts") that presses pores together and closes them, dimensional changes can be expected. In cast parts, these are zones with collections of porosity, i.e. cavity nests. In these areas, the surface will sink. This must be taken into consideration if very tight tolerances are necessary for optimal functioning of a surface exposed to a flow.
In sintered parts, the shrinking during the sintering process is a process-specific problem. In this case, as well, locally concentrated residual porosity can be the cause of deformations during an HIP process.

In some materials, creep as a relaxation of residual stresses (Fig. "Changes by residual stresses from machining") can lead to warping of thin-walled parts even at room temperatures. One example is titanium alloys that exhibit warping a few hours or days after having sufficiently high residual stresses induced in them (e.g. through shot peening or machining).

Inhomogeneous parts and materials: This refers to parts with design- or construction-specific inhomogeneities that result in differences in physical properties. These properties include stiffness (modulus of elasticity), strength/flow limit, and thermal strain. Typical examples of this are parts with fiber reinforcement in the form of an integrated fiber layer or bandage. This problem has become significant with the use of ceramic-fiber-reinforced metals such as SiC-fiber-reinforced titanium. In flat structures it is important that the fiber layers are positioned symmetrically in order to prevent unallowable warping.

Figure "Surface work limited by tolerances": Rework to save valuable and/or deadline-related parts is not unusual in finishing processes. Although it is trivial, it is often not realized that reworking cannot be repeated as frequently as desired. This situation usually occurs in the case of repairs with dimensional changes on parts that have been run. These processes are most often related with the removal of a seemingly insignificant amount of material. However, this cannot be repeated multiple times on parts with tight tolerances. The processes are primarily etching and abrasive blasting, which are mainly used to remove coatings (stripping, top diagram).

No significant abrasive material removal occurs during shot peening, but the roughness increases considerably relative to a smooth original surface, which “raises” the surface a minimal amount (bottom diagram).