Aeroengine Safety

Figure "Residual stresses at edges by machjning": The unfavorable relationship of a heated work surface to the heat-absorbing volume promotes high compressive thermal stresses at the edges. If these stress levels exceed the flow limit, it will result in plastic compression. Subsequent cooling will create high tensile residual stresses which act as a mean stress increase to reduce the dynamic fatigue resistance (Fig. "Increasing fatigue strength by compressive residual stresses").
Another possibility for damage is a strength loss due to structural changes such as solution annealing in hardenable alloys.

Potential machining processes are not only milling, reaming, and grinding. The rounding of edges with hand-guided tools is especially important. If the entire edge is enclosed by the tool, for example, to create an edge profile, an increase in the damaging effect can be expected.

Figure "Hazards from sharp edges": If beveling or rounding is not done, sharp edges promote potential flaws.
Sharp edges are sensitive to passive mechanical damages during handling (Detail 1, Ill. 18-5). These notches are predestined for dynamic fatigue cracks. Sharp edges can also actively damage other parts by creating grooves and notches.
Work processes on edges can create flaws such as overlapping and grain break-outs (Detail 2). Brittle and hard materials are especially sensitive. The more rapid heating and cooling of edges can promote hot cracking (Ills. 15.1-8 and 16.2.2.7-6).
Sharp edges are especially sensitive to plastic deformation and burr formation during subsequent finishing steps such as shot peening, abrasive blasting, or brushing (Detail 3).
Sharp edges complicate assembly and joining. They can promote jamming, especially in fitting diameters, which can subsequently result in damaging of the joining surfaces (Detail 4). The result is assembly problems. If settling occurs in rotors during acceleration, it will result in sudden imbalances and dangerous rubbing. If, for example, joining problems create uneven soldering gaps or welding gaps (EB welding, diffusion welding), it can result in joining flaws.

Figure "Burrs as crack initiations": Burrs promote cracking in very different ways. This creates a risk of dynamic fatigue with safety-relevant part failure (HCF, LCF, Example 16.2.2.2-1) and extensive consequential damages.

Stress increases: Because burrs on edges are often on the outer contour of a part, if they have a large distance to extreme fiber they will be especially highly stressed during bending (top diagram). Burrs can have a stress-increasing effect in case of temperature changes. The relatively large surface area of a burr relative to the volume accepts a lot of heat through radiation and convection during the heating phase. The thin cross-section cannot sufficiently transfer this heat into the massive part. Compressive stresses are created in the burr due to restricted thermal strain. During cooling, the burr cools faster, creating tensile stresses. These are even greater if plastic compression occurred during the heating phase. In this way, high tensile residual stresses form during cooling. This means that burrs are potentially subject to especially high thermal fatigue stress.

If a gas flow becomes turbulent around a burr, the heat transfer will be especially good. This increases the thermal strain and promotes thermal fatigue during operation.
The notch effect of burrs is not obvious at first glance. If one examines a burr more closely, such as with the aid of a stereo microscope, it will usually appear jagged. Small cracks and notches increase the operating stresses and lead to dynamic fatigue cracks even at specified operating loads (Detail).

Embrittlement: Burrs are created under strong plastic deformation. If this deformation occurs at low temperatures, it will lead to strain hardening and a loss of ductility (embrittlement, loss of fracture toughness), which promotes dynamic fatigue cracks and/or forced fractures.
If high tensile residual stresses are created in the burr, it will promote the absorption of hydrogen in sensitive materials during subsequent finishing processes, which promotes embrittlement (Fig. "Hydrogen absorption during etching process").
Embrittlement can also be expected if the burr forms at high temperatures. This situation can occur with separating and grinding processes, as well as high-speed chip removal. Relative to their volume, thin-walled burrs have a large surface area that, when heated, can especially intensively absorb oxygen and create brittle oxides. Titanium alloys are especially prone to this type of embrittlement.

Tensile residual stresses: Plastic deformations with high tensile residual stresses are created in burrs. This leads to increased absorption of hydrogen and increases the risk of stress corrosion cracking (SCC, Fig. "Operation damaging effects of burrs") in case of sensitive materials and specific media. This cracking does not have to occur during the finishing process, but can also appear during storage and operation.

Figure "Formations of dangerous burrs": Burrs are undesired thin ridges that are attached to the material, usually at the edge. They can be created in many different ways. In order to prevent burrs by selecting suitable processes and process parameters (e.g. in work preparation), it is necessary to understand where and when the burrs are created. Burrs are usually only connected with cutting processes. However, more thorough examination of the subject soon reveals that burrs that have an unallowable effect on the operating behavior of parts can be created by a large variety of other production steps.

Chip-removing machining: This usually creates burrs at the exit point from the work surface (Fig. "Burrs as crack initiations"). However, burrs, albeit smaller ones, can also be created at the starting point of the work surface.

Mechanical cutting, punching, beveling: Depending on the direction of the cut, burrs can be created on the separated surfaces.

Shot peening and abrasive blasting: Especially if the blasting media strikes edges at an outward angle, material will be smeared and form a burr (Fig. "Problems by angular shot peening"). Shot peening creates a typical burr shape that is referred to as an elephant tail due to its shape (Fig. "Peened surface extrusion folds"). These burrs can unallowably reduce the fatigue strength of parts such as rotor disks.

Plastic deformation of the surface through handling, transport, joining movements, and impact stress: If equipment is placed against edges, it can create burrs. If these burrs come between sliding surfaces, it is likely that it will result in groove formation and even galling. Experience has shown that these damages reduce the dynamic fatigue strength considerably due to the notch effect. In addition, joining and disassembly forces become very high. Similar burr formation can also occur during careless handling of the parts. Burrs are also created as a result of plastic deformation during forming processes such as drop forging or thread rolling.

Thermal cutting (plasma, autogenous, laser, electron beam): The melt from the cutting gap is transported to the exit of the beam, where it solidifies as a burr and/or melt pearls (Fig. "Preventing contact with welding splashes").

Welding: During friction welding, a pronounced burr is pushed out of the welding gap (Fig. "Flaws of friction welding"). This burr is then removed as much as possible by machining. The burr remains in inaccessible hollow spaces such as hollow shafts. Even during spot and roller seam welding, burrs can form and have a negative effect on dynamic fatigue strength (Ref. 16.2.2.2-2). Unfavorable welding parameters (excessively high welding current) or seams that are too close to the edge of the sheet metal, can cause the melt to escape (Fig. "Welding flaws"). In this case, the burr forms along the surface of the metal sheet and is difficult to remove. If possible, one should avoid welding hollow bodies such as blades made from two metal sheets with burrs running along the inside. The exception to this rule is if the burrs can be accepted as weak points. In this case, they must be included in the design specifications.

Galvanic coatings: Burr-like material buildup occurs at the edges because the electrical field is stronger in these areas and causes more rapid precipitation (Fig. "Typical flaws on galvanic coatinga").

Figure "Burrs at bores" (Ref. 16.2.2.2-1): Up to 30% of the finishing costs of parts made from titanium alloys must be used for deburring. The creation of burrs on bores is especially costly. The poor heat conductivity and tendency to cold welding (galling), as well as chemical reactions, accelerate the wear on the cutting tool. The blade geometry especially influences burr formation. For the selection of appropriate machining parameters, burring and chip formation are both important factors.
Usually, burrs form both at the entrance and exit of bores. The burr at the exit is much more pronounced and therefore requires more attention. Burring is especially dependent on the cooling lubricant (CL, Fig. "Problems with cooling lubticants"). The top diagrams show burrs on bores in a metal sheet made from TiAl6V4. Without cutting fluid, very pronounced burrs are created that have typical forms that can be attributed to the process parameters (bottom diagram).

Example 16.2.2.2-1 (Ref. 16.2.2.2-3):

Excerpt:“ …the operators have been warned that there is a risk of uncontained failure of their …(engines) unless certain high pressure turbine (HPT) components are replaced early. The warning came in a … US Federal Aviation Administration emergency airworthiness directive (AD). In USA 25 engine units with…(known) serial numbers are affected. The AD warns that a manufacturing process change on some …high pressure turbine discs has reduced their `maximum cyclic life'.
Operators must replace the discs with the newly issued cycle limits, the AD states, explaining that some HPT stage 1 discs need replacing between 2,600 and 3,600 cycles, and some stage 2 discs need to be replaced before 3,800 cycles.
…`Rig testing has revealed that the fatigue life of the …turbine discs is more sensitive to a certain deburring process than expected. The life limit of a restricted number of affected discs has been reduced'…“

Comment: Evidently, the problem is not that deburring did not occur, but that an unfavorable deburring process was used. The fact that it had such a strong negative effect on the life span of the turbine disks is an important lesson (Fig. "Operation damaging effects of burrs"). Deburring is an extremely important finishing process. It requires both responsibility and technical expertise.
Unfortunately, the available documentation does not indicate the deburring process, nor does it mention the type and location of the damage.

Figure "Operation damaging effects of burrs": Burrs can have a damaging effect on the operating behavior of a part, as well as other parts. The following examples show this. One case, in which a burr affected the flow and thereby also influenced damages that occurred, is shown in Fig. "Operation damaging effects of burrs".

“1” Reduction of dynamic fatigue strength: Burrs can cause an unallowable decrease in LCF and HCF strength (Fig. "Burrs as crack initiations", Example 16.2.2.2-1). In some cases, this can affect the cyclical part life.
Highly stressed disk zones (left diagram) must be given special attention due to their geometry, which is complicated for deburring and edge rounding (Fig. "Safety of parts by deburring"). The affected areas include disk slots (fir tree, dovetail) and bores (Ills. 16.2.1.1-9.1 to -9.6). The right diagram shows a case in which cracking occurred during friction welding, and was evidently connected to cracking in the friction welding burr. It is suspected that the small cracks in the burr acted together with hand sweat to cause SCC in this titanium part (Fig. "Stress corrosion cracking by process baths and hand sweat").

“2/3” Assembly problems and mechanical damages: Joining movements between fitting surfaces (e.g. running a clamping bolt into a disk bore) can create burrs, as well as grooves and galling marks (Fig. "Hazards from sharp edges"). If cold welding (galling) occurs, considerably higher joining forces can be expected (“2”). If the required axial force is reduced to the point that it prevents contact from occurring, it can lead to dangerous bearing overstress and/or settling movements during operation. If grooves are created by the joining movement, a dangerous reduction of the dynamic fatigue strength can be expected in case of forces acting crossways. Diagram “3” shows a case in which burrs prevent contact between flanges and fitting surfaces. Settling of the flanges during operation possibly during non-steady conditions in which the fastening changes, leads to imbalances and vibrations. Shavings of burrs between flange contact surfaces can move due to relative movements and create notches.
“4” Cause of labyrinth damage: Experience has shown that shavings and burrs in new labyrinths can initiate or promote self-increasing rubbing that can lead to catastrophic failure of the labyrinth ring (Volume 2, Ill. 7.2.2.-4).

“5” Burrs as a cause of damage in sections with flow-through: Foreign objects from broken burrs that are carried by a flowing medium can cause damages to other parts.
If these particles enter into roller bearings and are impressed or rolled over, they can fatigue the roller surfaces and bearing races.
Bearing damages can also be caused by partially blocked oil nozzles and filters. Reduced oil flow leads to damage through overheating and metallic contact of the roller surfaces. Even displaced fuel nozzles can threaten engine safety if the fuel jet shifts and causes local overheating of the combustion chamber and housing (Volume 3, Ill. 11.2.2.2-9).
Burr particles between sliding surfaces such as in gear pumps or regulators (e.g. sliders) can cause blocking.
If wheel vanes and metering bores in regulators (hydraulic, pneumatic) are displaced, it will affect their function.
Cooled hot parts are especially sensitive to even minor reductions in the cooling air flow. This situation can occur if the cooling air bores of turbine blades, which have a diameter of only a few tenths of a millimeter, become partially or completely blocked. It is sufficient, for example, to shift a dust-removal bore to the blade tip (Fig. "Damages in cooling air systems by finishing coused fouling").
Even if burr particles are caught by a magnetic separator, a warning indicator in the cockpit may require emergency measures and create a dangerous situation.
If burr particles enter between the sealing surfaces of a floating ring seal, it can result in leakages and serious consequential damages (e.g. due to an oil fire).

Figure "Disturbance by burrs in pipe lines": Burrs in cross-sections with flow-through can disrupt the flow. In a pipe, a burr can increase resistance and create local turbulence and cavitation (top diagram; Volume 1, Ill. 5.3.1-11.2). This can cause the system to vibrate with the risk of dynamic fatigue fractures. It is also possible that the pressure vibrations in the flowing medium may compromise the functioning of the system (regulator, injection). Cavitation can also cause material removal from the affected surfaces, damaging them.
Sharp edges on an opening cause flow constriction and increase resistance, i.e. they reduce the throughflow amount.
If the throughflow amount is less than the design requires, it can cause damages in many different ways:

• Overheating of cooled hot parts (air)
• Bearing failure (oil)
• Combustion chamber malfunction (fuel, air)

The bottom diagram shows the consequences of a careless change to the finishing process. Lower finishing costs required the use of the parts after a boring method for can-type combustion chamber bores was changed (bottom diagram). The usual inspection methods did not reveal any cause for concern relative to the previous bores. However, the operating behavior of the engine was unacceptable during test runs. The new boring method was determined to be the cause. Analysis of the problem revealed that, unlike the previous established method, the new boring process created seemingly harmless burrs that in fact unallowably disrupted the throughflow and therefore the air supply.

Figure "Safety of parts by deburring": Deburring is usually equated to the rounding of edges. If this task is dismissed as being relatively mundane, it is a dangerous underestimation (Example 16.2.2.2-1, Ref. 16.2.2.2-4). Especially this type of apparently dull task, but which actually requires specialized knowledge, skill, and motivation and involves extremely sensitive part zones, can necessitate specific selection of personnel.

Deburring is important on the entire part, not only in zones under high cyclic stress such as blade slots and bores (top left diagram). Deburring can only be done using specified processes and verified process parameters. In this context, during material-removing deburring by hand, the type and material of the tool are important, as are parameters such as contact pressure, RPM, and any auxiliary materials. These requirements naturally also apply to other deburring methods such as:

• abrasive blasting
• sanding, sometimes with the aid of chemicals
• pressure lapping
• electrochemical machining (ECM)

If the process parameters are too aggressive, especially in the case of material-removing deburring with the aid of high-speed grinders and milling cutters, it can damage edges in the following ways:

• new burrs and/or overlaps can be created (shingles, details)
• overheating can result in a loss of strength due to structural changes and, in extreme cases, cracking
• creation of tensile residual stresses that unfavorably overlay with the operating stresses

Additional problems that are more closely related to tool operation are:

• unfavorable rounding contours
• unfavorable work direction. This is the case when machining grooves run perpendicular to the life-determining operating stresses (bottom left detail). Unfortunately, material-removing deburring processes are often more effectively and easily implemented in directions that are less favorable for the part strength. For this reason, it must be ensured that the personnnel is aware of this danger.

Other deburring processes also have specific potential problems:
Electrochemical processes must be able to control the increased material removal from edges. The cause of this is the concentration of the electric field. Areas that are shielded by the part contour must also be reliably deburred. At the same time, selective attack (grain boundary corrosion) must not create dangerous notches.
Depending on the shot media and process parameters such as intensity, blasting direction, and blasting angle, abrasive blasting can create new burrs or dangerous grooves. Edges of compressor blades are especially sensitive.