Aeroengine Safety

Figure "FOD damage mechanism": Foreign object strikes can damage rotor blades in different ways. If the blade does not fracture immediately due to overstress, then the biggest danger is from a dynamic fatigue crack from increased dynamic loads (rubbing, stall) and/or from increased tensile stress at the impact notch (see the introduction to this chapter).

Figure "Fatigue fractures" (Ref. 5.2.1-3): This is a diagram depicting the events in Example "Increased sensitivity to FOD" (possible variations depending on the engine and aircraft types). It shows the primary zones for blade failures due to dynamic fatigue fractures originating in notches (FOD, also see Fig. "Notches by FOD"). It is interesting that this foreign object problem especially affects engines mounted on the rear fuselage. Evidently this configuration promotes foreign object ingestion. Possible reasons for the high probability of FOD are objects thrown up by the landing gear (Fig. "Engine configurations") and the possibility that ice from the fuselage or wing could have been carried into the engine by the air flow (also see Fig. "Ice buildup").

Figure "Vibration fatigue": FOD in the compressor is always a potential threat for the aircraft. Extensive damage that makes itself known through the operating behavior of the compressor is easily observable is unpleasant and possibly very expensive. However, damage that occurred at a far earlier time than the ultimate extensive consequential damage can be extremely dangerous. Typical examples of times prone to this type initial damage include a delivery flight or a flight after an overhaul, which experience has shown to be accompanied by an increased risk of FOD. Notches and nicks mean considerably higher local tension (notch effect). The unavoidable dynamic fatigue stress on the blading is less than the fatigue limit of the undamaged parts and can be tolerated indefinitely. Therefore, it does not present any danger of crack initiation to parts without notches, but the notch effect would increase this stress to dangerous levels. Because the vibrations usually only occur for a short time during flight through resonances, these load changes accumulate until a crack is formed and the blade fractures. In titanium compressor blades at the rear area of the compressor, there is the additional danger of a titanium fire occurring, even if this is fairly unlikely. This type of damage should be avoidable with the aid of regular boroscope inspections and a standard for evaluating the findings. It has only been recently that potentially fast and inexpensive techniques have been introduced that smooth out notches without having to open the housing. This type of procedure requires a suitable cutting technique (also see Fig. "Notching effect II"). A suitable cutting tool is fed through the boroscope opening. The reworked area might then be hardened through laser peening in a similar manner.

Figure "Notches by FOD": The promoting effect foreign object impact notches have on dynamic fatigue crack initiation in blades also depends on the location of the notch on the blade. Since the most common type of vibration is the flexural mode, notches in the root platform are especially dangerous, as the highest dynamic stresses can be expected in this area (see Example "Increased sensitivity to FOD"). An 'Eiffel tower-shaped' blade cross-section can reduce this tendency. The profile of the blade also has an effect. Very curved, thin blades of the type used in stators are especially sensitive to damage on the blade edges. Thicker, less curved blade profiles such as those of rotor blades are also sensitive to notches towards the middle of the profile.

Figure "Notching effect I": A notch is not simply a geometric change that affects the dynamic fatigue strength through its shape. Other important factors include its location in relation to the greatest dynamic loads (with cut-like notches, for example), the size and distribution of residual stresses induced by the plastic deformation, any overload fractures or cracks (Fig. "Notching effect I"), as well as material changes such as embrittlement or ruptures of hard surface coatings such as oxide layers. In the cut-like notch in the above diagram, there is an extrusion similar to those that are occasionally observed on a microscopic scale in surfaces subject to extreme dynamic loads. The extrusion consists of material that has been pressed out of the notch through the “fulling” process.

Figure "Notching effect II": This diagram is based on Ref. 5.2.1-5 and shows the effect various kinds of notches and surface damage have on the dynamic strength of steel samples. It can be assumed that similar effects would be observed in titanium alloys and superalloys. These “injuries” are especially dangerous to titanium alloys due to their well-known high sensitivity to sharp notching. On the other hand, the hardness and relaxation at relatively low operating temperatures should create other focal points of the damaging influences. Unfortunately the influence of the impact speed when the creating the notches cannot be deduced from the diagram, since this question was not addressed as with was in “Foreign Object Strikes on Bladings”.
However, several important insights can be gained:
While the permanent strength loss increased along with the depth and sharpness of the notch as expected, these geometric factors were frequently less important than the influence of the way in which the damage occurred. For example, ground areas or notches that have been ground can be especially prone to dynamic crack initiation. However, hammered or chiseled notches affect the fatigue strength of materials that are ductile or not sensitive to notching much less than they affect the fatigue strength of extremely hard, brittle materials.

Figure "Effect on dynamic strength" (Ref. 5.2.1-9): HCF-dynamic fractures in compressor blades are often caused by foreign object strikes. Crack progression data is usually taken from samples with cracks that extend through the entire cross-section. These data do not necessarily apply for damages caused by small foreign object strikes (diagram), which may be only tenths of a millimeter across (Figs. "Notching effect I" and "Erosion damage on suction an pressure side").
Shear bands on the surface of impact craters have only been observed at middle impact speeds. High impact speeds caused raised crater edges. Heavy impact damage such as this results in a considerable loss of dynamic strength.
Cracks of the order of magnitude of the microstructure (grain size) or of the plastic zone at the crack tip can grow at much greater speeds than larger cracks at the same tension intensity. Small cracks can grow even below the threshold value D K_th (diagram). This must be taken into account when determining the dynamic strength of blades during design.

Figure "Influence of impact speed" (Ref. 5.2.1-6): Interesting material behavior was reported in tests in which spherical particles of various sizes and materials were fired at different speeds against blade and sample edges made from a high-strength titanium alloy.
At relatively low impact speeds, largely independent of the foreign object material (!), bulge-like plastic deformations (top right diagram) with little influence on the dynamic strength of the part At high impact speeds the edge was punched out cleanly (bottom right diagram), also with only a moderate decrease in dynamic strength. The greatest losses of dynamic strength occurred at a transitional impact speed (critical impact speed), which was only slightly dependent on the particle diameter, inside of which the edge cracked.

Figure "Influence of impact direction": The impact direction of foreign objects is different in compressors and turbines. This has a considerable effect on the location of blade leaf erosion and impacts on the blade edges.
If one assumes that a larger foreign object (particles larger than about 0.01 mm usually follow the gas flow without being deflected) does not follow an ideal path and travels at a lower speed than the flow speed due to its aerodynamic profile, then this particle will strike the rotating blade against the direction of rotation.
For this reason, foreign object impacts in compressors usually occur from the pressure side. Exceptions may occur if the foreign object is deflected.
In the turbine, the foreign object will strike the suction side of the blade at its leading edge.

Figure "Thermal barrier coating I": This diagram shows typical FOD to turbine rotor blades. The top diagram depicts damage to heavily cooled blades from a small turbine after the impact of carbon from the combustion chamber. Due to the internal cooling ducts, the blade walls are only a few tenths of a millimeter thick and are subject to operating temperatures of up to 1100 °C at the leading edge (see Ref. 5.2.1-7), evidently making them extremely sensitive to seemingly harmless foreign objects such as carbon particles. The impacts were typically on the suction side of the blades.
The bottom diagram depicts the back (suction) side of a turbine rotor blade which has been struck by a ceramic particle from the thermal barrier of either the combustion chamber or a front blade stage. In this case the local plastic deformation led to crack initiation in the relatively brittle protective oxide layer (Al-diffusion layer).

Figure "Thermal barrier coating II" (Ref. 5.2.1-10): Displayed are features around a foreign object impact at the physical vapour deposited (PVD) thermal barrier coating (TBC) of a turbine blade. The material structure of such TBCs is characterized by a columnar structure (Ill. 11.2.3.1-4).

Typical features at the impact region are:

• Permanent indentation. From it can be concluded at the delamination above the undercoating /adherence coating and so enables the evaluation of the deterioration dimension respectively of a possible spalling.

• Compacted zone around the indentation. It is also involved at the introduction of internal stresses into the coating. These increase with the thickness of the coating. In the zone lack the distances between the columnar crystals. It shows a field of concentric cracks, which can penetrate to the surface (Fig. "Thermal barrier coating I"). The compression effect may be especially distinct at PVD coatings. This could be a reason for the comparatively good erosion resistance of this coating type. Erosion does than happen, if the amount of particles is not sufficient to produce a crack formation which leads to the spalling of the coating.

• Crack formation in shear bands runs inclined from the compressed zone to the adhesion coating. Above the adhesion coating these cracks deflect as delamination cracks. A further crack growth during the operation is supported from induced internal stresses of the impact. Delamination cracks determine the size of a possible spalling.
  The influence of the base material at the crack formation seems at sufficient stiffness of the bearing cross section rather minor.
  The delamination can be suppressed with an optimizing of the coating properties at operation temperature. The hardness must be lowered and the “toughness” (yielding) increased. This can be achieved e.g., with an increase of the porosity. However with this the erosion strength an drop unacceptable. For a coating specific threshold of the foreign object energy for delamination conditions can be specified.