Aeroengine Safety

Figure "Adhesive applications in machines": In order to illustrate the varied applications of adhesives in engine construction, this diagram shows the fan and compressor of a modern civilian engine, with areas in which adhesives could be used. It is likely that not all of the applications in this special case would be used. The various applications are marked with letters, which correspond to the adhesive systems in Fig. "Problems of adhesive systems". The most important uses of adhesives are:

Sound damping is mainly used in the bypass duct. This is frequently done with honeycomb structures made from metal (aluminum) or synthetics (e.g. bonded phenolic resin) that are filled and/or affixed to a carrying structure in a sandwich configuration using epoxide adhesive. The filling of the honeycomb structure can also be done with a paste-like mixture of adhesive (epoxy resin) and filler material (e.g. hollow glass beads, hollow phenolic resin).

Abradable coatings are designed to allow the blade tips to gently rub into the opposing surface if the clearance gap is bridged. Usually, they consist of a filled honeycomb structure similar to that used in sound damping, which is affixed with adhesive. The nesting behavior of this honeycomb structure can also be used in containment incidents (Volume 2, Ill. 8.2-14).

Vibration damping: The most common application is the use of elastomers, such as silicon adhesives, to bond the shrouds of compressor stator vanes. In some cases, rotor blade roots are fixed using silicon rubber (Volume 1, Ill. 5.3.2-6 and Volume 3, Ill. 12.6.3.4-5). Especially in older engine types with stator and/or rotor blades made from Al alloys, this was the only method of achieving satisfactory operating behavior without an excessive risk of dynamic fractures.
Another form of vibration damping is the affixing of metal foils (Volume 1, Ill. 5.3.2-6 and Volume 3, Ill. 12.6.3.4-6) to vibration-threatened stator vanes. Under vibrational loads, the inner damping of the adhesive layer absorbs vibration energy during compensation and transferral of the strain differences between the part surface and the metal foil (Volume 3, Ill. 12.6.3.4-14).

Sealing: In compressor rotor blades, the blade root and any slits under the root platform that create a connection between the areas ahead of and behind the stage are sealed with elastomers (silicon rubber).

Fastening and anti-twist protection are primarily intended to secure bolts and screws. Another use is securing press fits between hubs and shafts. This is done using anaerobic adhesives that harden when they contact metal surfaces under the exclusion of air. This type of adhesive bond can be broken when opening the nut by using a slightly increased breakaway torque. However, this may require a new adhesive bond.

Abradable coatings for labyrinths and blade tips: These coatings in the front stages of compressors and in labyrinths usually consist of metal felts and are suitable for operating temperatures up to 450°C (Fig. "Adhesive bond failure of a metal felt"). They are affixed either with solder, or with inorganic adhesive systems filled with Al powder.
For operation at lower temperatures, filled (e.g. with glass beads) silicon rubber coatings are glued into a metal carrying ring to act as abradable coatings for the labyrinth to rub into.

Figure "Problems of adhesive systems" (Refs. 16.2.1.5-1 and 16.2.1.5-2): This is an overview of adhesive systems that can potentially be used in engine construction. Each of these systems usually contains many variations, so that the listed properties may only apply fully to a certain type of production design. For example, it is possible that resistance to chemicals or aging among the variants of an adhesive type will vary greatly. Therefore, the reader may be required to examine and verify the adhesive specifications. The prerequisite for the use of an adhesive system in turbine engine construction is that it has been certified for the specified application by the responsible authorities and OEMs.

The following text discusses several specific characteristics and problems of the different adhesive systems, as described in the available literature:

“A”, Hot melt adhesives: Their working temperature is usually around 150°C. Bonding occurs in seconds. This short time has advantages, because no fastening equipment is required. However, it does not permit readjustments. When bonding materials with good thermal conductivity, the adhesive must not harden too quickly, or it will not wet the surface sufficiently. The greatest drawback of hot melt adhesives is their low strength, especially their low thermal strength.

“B” and “E”, cyanoacrylate adhesives: These systems harden very quickly when they contact minimal amounts of water (instant adhesives, also see “K”), which limits their applications. Their low viscosity demands very small gap widths of only a few hundredths of a millimeter. Porous surfaces such as thermal sprayed coatings may require primers. Their operating temperatures are usually below 100°C, but some special systems can even operate above 200 °C. These bonds can attain very high strength levels, but are sensitive to impact stress and lose strength if they come into contact with water. The sensitivity to impacts must be considered in applications that are subjected to impact stress.

“C”, Epoxy adhesives: These are the most commonly used systems. Their two components require exact dosages and intensive mixing. Inexactness will lead to strength losses. Epoxy adhesive bonds have very high strength (up to 70 MPa). Special systems can tolerate operating temperatures of about 200 °C at low load levels. However, they are cost-intensive due to the time required for cleaning and fixing the joined surfaces during hardening. This must be done in accordance with specifications in order to ensure the optimal strength values.
In general, highly reactive adhesives must be stored at very low minus temperatures in order to attain the necessary storage life. Continual cooling (with dry ice) may even be required during transport. If this cooling fails even relatively briefly (hours), the properties of a later adhesive bond may be unallowably altered. The requirements for cooling naturally also apply to the handling and storage of remaining adhesive. Understandably, there is a possibility of serious problems arising in this situation.

“D”, Dispersion adhesives contain fine distributed synthetic resin in a liquid or solid form, suspended in water. The water is used to transport the particles before it evaporates. For this reason, it is necessary that the materials being bonded have open porosity to absorb the water. This is the prerequisite for bonding with the synthetic resin particles to occur. The penetration of the adhesive into the open pores, if possible under pressure between the joining surfaces, improves the bond strength. Dispersion adhesives are permeable for water vapor (Ref. 16.2.1.5-7). The hardening occurs through the evaporation of the liquid phase. This takes time. The bonds usually have sufficient strength relative to the porous materials being joined. At high temperatures, however, their creep strength is very low. The adhesives can be damaged by solvents or moisture.
“F” Polyurethane adhesives attain similarly high strength levels as epoxy, and are delivered in two components. During mixing, it must be ensured that as little air as possible is mixed in (bubbles act as weak points (Ref. 16.2.1.5-9). The temperature increase during hardening thins the adhesive and is misleading with regard to the work time. Its high toughness can be used in seals and as erosion protection (e.g. coatings on spinners and fan blades made from fiber-reinforced synthetics). Some urethanes are sensitive to moisture, making long-term applications problematic. Their operating temperature is usually below 200 °C.

“G” Silicon adhesives: During hardening, oxides, amines, or acetic acid can be released as decomposition products. These can compromise the functioning of the parts, e.g. by causing corrosion on insufficiently pretreated light metal alloys (aluminum, magnesium). For this reason, the suitability of the adhesive may have to be determined through testing. Silicon elastomers and adhesives can swell in fuel and engine oil. This process may be reversible, but it can lead to excessive compressive stresses and cause thicker coatings to separate (abradable coatings). Silicon adhesives require specification-conforming preparation of the adhesive surface and working atmosphere. Even apparently minor deviations can lead to large separations.
The application process of the silicon adhesives is critical, since they harden in air moisture and form a milk-like skin within about 5 minutes (Ref. 16.2.1.5-6). This skin can affect the wetting and therefore also the bond. For this reason, work (application and joining) must be done within 5 minutes. Depending on the layer thickness, due to water diffusion, complete hardening may require 2-24 hours. Due to the diffusion of the air oxygen, hardening occurs from the outside inward. The hardening effect with air moisture prohibits storage in opened containers in which there is a danger of contact with an air cushion.

“H” phenol- or urea-based adhesives: During hardening at low temperatures, water or formaldehyde split off from phenol adhesives (Ref. 16.2.3.5-8). For this reason, there is a danger of porosity. Phenolic resin adhesives are marked by high thermal stability, but are brittle and therefore especially sensitive to impacts and peeling.
Urea adhesives (aminoplasts) react with formaldehyde. Hardening requires high temperatures and releases water. The temperature/time behavior of the hardening process can be influenced by mixing in latent hardeners. During this process, acids form, which can cause corrosion on light metal alloys.

“J”, Anaerobic adhesives are low-viscosity, rapidly hardening, single-component systems. They harden in the absence of oxygen. This makes them suitable for very tight gaps and they are therefore used to secure nuts and hub seats, for example. Modern variants have high impact resistance with high shear strength. Their maximum operating temperatures are between 150 °C and 230°C. The strength of the adhesive bonds reacts strongly to fouling on the joining surfaces. Some materials require primers in order to prevent reactions with the adhesive.
The adhesives can be mixed with PTFE, and are used in this way as a combination lubricant and sealant for threaded pipe connections.
Another use is the sealing of porous materials such as sintered materials and cast light metals.

“K”, Acrylic adhesives use synthetic resin particles suspended in water, which harden into a waterproof state once the water has evaporated. They have a very high tolerance to fouling of the joining surfaces. This property is due to additives in the adhesive or previously applied primers. These penetrate the surface fouling or react with it (e.g. with oils). The attainable strength, especially the shear strength, is comparable to that of epoxy adhesives. Their working times are short - about 60 seconds for RT (instant adhesive, also see “B” and “E”). Their gap-filling is surprisingly good, and is sufficient for gap widths between 0.7 and 6 mm (Ref. 16.2.1.4-4). Acrylic adhesives may react sensitively to moisture. They give off a distinctive odor.

“L” Inorganic adhesives: These are water-based, paste-like, two-component adhesives with an Al powder filling and acidic phosphate binder. Different varieties have different amounts of environmentally damaging Cr content. Several products are certified for turbine engine construction and have proven themselves for years. Their storage period is roughly half a year. Between mixing and working, a sufficient reaction time of about 20 hours must be observed. Hardening occurs after drying, and is done in several steps at increasing temperatures which, depending on the base material, can be up to about 325 °C. The greater the hardening temperature, the better the water-resistance of the connection. The system can tolerate a maximum constant temperature of about 500 °C (Ref. 16.2.1.5-4).

As with most adhesive joints, the joining surface must be free of grease. Therefore, it must be ensured that no contaminated degreasing bath was used. This is the only way to ensure that the part is not fouled by floating grease when it is removed from the bath (Fig. "Difficult crack detection by fouling on baths").
Abrasive blasting with Al₂O₃ is recommended. In this case, it must be ensured that the pressurized air used in the blasting process is completely free of grease and water.

“M” Ceramic high-temperature adhesives are water-based systems filled with various oxides (Al₂O₃, SiO₂, ZrO₂, MgO). After hardening, they form high-temperature-resistant bonds. There are many different types of adhesives available, and their selection depends on the specific application. Drying and hardening play an important role in determining the quality of the bond (Ref. 16.2.1.5-5). Depending on the filler material, the temperature resistance of the bond can be up to 1800°C. When they are used on hot parts in engines, such as for fastening strain gauges or thermocouples, thermal fatigue, erosion, or impact stress can cause them to fail. Therefore, their use is usually limited to relatively short testing runs.

Figure "Effects at adhesive joint properties": A special problem is non-destructive quality assurance of adhesive bonds. In some cases, it is possible to detect bonding flaws on metals with the aid of thermography. However, this does not reveal poor bond strength. Therefore, quality assurance must be done through adherence to all specifications and the most extensive possible documentation of all procedural steps. This is complicated by the complex interaction of the different influences, some of which cannot be measured. The prerequisite for optimal adhesive joining is proper constructive design (Fig. "Design of adhesive bonds"). The operating demands are the main criteria for the selection of the adhesive system. In addition, the adhesive technology must take into account requirements such as accessibility and gap size. Loads (size of the adhesive surfaces) and elastic deformations (stiffness) are other important factors.
The quality of an adhesive joint is highly dependent on the individual procedural steps. These must take place in specified time periods.

• Preparation of the joining surfaces (blasting)
• Cleaning the adhesive surfaces
• Application of primers (if required)
• Application of the adhesive
• Drying and/or hardening

Even between the procedural steps, there are important time frames that are determined by factors such as the work life or the formation of surface skins (Fig. "Factors influencing adhesive joints").

During working of the adhesive, external factors can have a decisive influence on the quality of the bond. Adhesive joining may require its own rooms with controlled atmospheres (moisture, temperature, vapors, aerosols).

The function of the adhesives themselves is decisively dependent on proper application-specific selection and handling. This includes storage (time, temperature) as well as the treatment of remaining adhesive and opened containers. Proper application, secure contact with the joining surfaces, and any required pressing forces must be guaranteed.

The influence of the (base) materials being joined, as well as their surface structure, are very important and are therefore also important criteria of adhesive selection. For example, the adhesive must not have undesirable reactions with the base materials. Unfavorable structures (e.g. open porosity in some systems) must be taken into consideration.

Pretreatment of the joining surfaces, such as roughing and/or cleaning/degreasing, are decisive for the results.

Figure "Fouling danger at adhesives": A prerequisite for achieving optimal strength in adhesive bonds is clean joining surfaces. Releasing agents such as silicon and PTFE are especially damaging. Unfortunately, this type of media can be distributed unexpectedly.
Silicon is found in oils that are added to cooling lubricants as anti-foaming agents (Chapter 16.2.1.1.1). An especially tricky application is in hand creams (see Example 16.2.1.5-1).

PTFE is used as a sprayed releasing agent and as a liquid solution on forms for parts made from fiber-reinforced synthetics. This application requires high hardening temperatures. These materials can even be found in cosmetics, as a component of hairsprays, for example. These media for personal use must under no circumstances be used in the sensitive environment of the production area.
The special problem with these media is their tendency to float at the top of cleaning baths and to cling to the parts as a film when they are removed (Fig. "Problems by contaminated cleaning baths"). In this way, these materials can be transferred to many different parts, causing very extensive damages.

Figure "Factors influencing adhesive joints": Using adhesives to affix abradable coatings of porous metal felts to the inner surface of rings or bores must overcome similar difficulties as soldering (Fig. "Brazing porous coatings"). The adhesive used for this is usually an inorganic adhesive (L“ in Fig. "Problems of adhesive systems"). In order to attain optimal operating properties, it must be hardened at temperatures up to about 450°C.
The top diagram shows what happens when the adhesive is drawn into the porous metal felt, creating a gap to the base material. In some cases, even after pressing, no bonding will occur if a skin has formed on the exposed surface of the adhesive. The problem with this type of bonding flaw is its poor external detectability. Experience has shown that the adhesive gap can close at the outer edge, making it appear to be a flawless bond.
The diagram in the bottom frame shows situations in which an unbridged gap formed between the porous abradable coating and the surface of the base material. This can occur when a housing (e.g. made from light metal) expands more than the abradable coating. This is promoted by the typical high temperatures during hardening. If the abradable coating shrinks, e.g. during drying and hardening of the absorbed adhesive, a similar effect can occur. Further possibilities are the drawing-in of the adhesive by the porous coating, as shown above, and/or shrinking of the adhesive on the opposing surface during hardening.

Figure "Design of adhesive bonds": The design engineer is responsible for ensuring that the requirements for a sufficiently reliable adhesive joint with the expected strength properties are met. The diagram shows important criteria for this goal.

Operating characteristics are determined by the expected operating loads. For synthetic adhesives, their temperature-resistance is decisive. Their short-time strength decreases rapidly as the temperature increases. Unlike metals, the application of adhesives can be determined by a few °C. Their long-term strength (creep resistance) is accordingly temperature-sensitive. Additional influences, such as aging and damage through fluids such as oil, fuel, or cleaning agents, have a considerable effect.
It must be remembered that adhesive bonds can behave significantly more brittle at high stress rates than under slow load rates. This can be important, for example, in the case of impact loads during containment incidents or bird strikes (e.g. on a spinner made of fiber-reinforced synthetic material).

Properties of the adhesive surfaces: The prerequisite is that a sufficient surface area has a minimum amount of bonding. The bonding surface can be increased with a mounting. However, this increases the costs and may cause problems when the surfaces are pressed together. Another consideration is that the adhesive bonding process does not damage other existing synthetics or adhesive bonds (hardening temperature, solvents). For this reason, the design engineer must properly plan the process sequence and work preparation. The durability of the joint depends decisively on the surface properties. It must be determined whether a certain adhesive-specific surface roughness is required. In some cases, abrasive blasting may be necessary. Porosity may also determine adhesive selection. High or low viscosity are important criteria in this context. Greater roughness does not guarantee better bond strength in every case. Fundamentally, the design engineer should coordinate with the personnel who must later produce the adhesive joint. This can prevent the occurrence of an unfavorable design that does not permit reliable application of the adhesive, for example. If the adhesive joint requires pressing force during hardening, this must be constructively ensured. The possibility of the adhesive being scraped off during joining of the surfaces must be considered. Joining surfaces should not be pushed into hollow spaces that are closed at one end. The trapped air must be able to escape, or sufficient filling of the gap is not possible. If there is uncertainty about the feasibility of an adhesive joint, part-specific adhesive tests should be conducted. These joints should then be tested in a realistic, part-specific manner. The aspect of quality assurance must also be considered.
Fundamentally, stress peaks should be avoided in the adhesive joint, such as in edges. The bottom left diagram shows peeling stress, which must absolutely be avoided, as is the case with soldered joints (Fig. "Peeling of solders and brazings"). More favorable variations are depicted to the right of it. The peeling effect can be minimized through elastic design of the zone with the adhesive joint. In some cases, it is possible to prevent peeling loads through a simple change in position relative to the acting forces.
Special attention must be paid to later repairs during overhauls. The joint must be renewable with acceptable cost and without unallowable damaging of the entire part.

Figure "Operating influence on adhesive metals joints": The strength of an adhesive bond is affected by operating influences, which usually weaken it. Over time, temperature leads to aging processes with embrittlement and strength losses. Moisture, oils, greases, cleaning media, and solvents can cause swelling (possibly irreversible). Permanent damage, such as chemical changes, cracking, and embrittlement, can be expected. The diagram shows the effect of external media on an adhesive joint with an intermediate primer coating (Ref. 16.2.1.5-2).
Moisture resulting from diffusion and/or aging can cause a reaction between the primer and a protective oxide coating on the base material (e.g. in light metals). The destruction of the oxide coating causes separation to occur. Corrosion-sensitive base materials such as light metals and heat-treated steels can be attacked, resulting in a considerable loss of dynamic strength. A typical case is the corrosion of Al alloys in connection with unsuitable epoxy adhesives.

Figure "Adhesive bond failure of a metal felt": The destruction of a turbine disk in a small gas engine (right diagram) was caused by extreme overheating following the failure of a labyrinth seal (left diagram) of the cooling air feed. The metal felt ring, which had been affixed with an inorganic adhesive, separated due to an undectected bonding flaw (Fig. "Factors influencing adhesive joints"). The coating became trapped in the labyrinth and caused catastrophic rubbing (Volume 2, Ill. 7.2.2-4). During this occurrence, the rotating labyrinth part was melted and thrown off.