Table of Contents
3.1.2.1 Mechanical problems
Compressor blades
The components of the compressor ( "Ill. 3.1.2.1-1 and 2" up to "Ill. 3.1.2.1-3 and 4"), depict problem zones according to their correspondingly special conditions of operation. Interesting components are the compressor rotor blades. They underlie, e.g., high mechanical stresses through centrifugal force, gas load, rubbing force, as well as high frequency aerodynamic and mechanical excitation. The influence through air flow impurities and foreign objects are treated in a separate chapter (Chapter 3.1.2.2).
Compressor blades are fatigue endangered through high frequency vibration excitation ( "Ill. 3.1.2.1-4"). Normally these are dynamic loads which cause fatigue cracks or fractures not until some hundred thousand cycles. In this case we speak about a dynamic load in the HCF-region (high cycle fatigue, see "Ill. 3.1.2.1-0"). Those stresses are not far above the endurance limit the material will withstand without a cycle limit. The region of higher stresses with markedly plastic strain is called the LCF-region (low cycle fatigue). Those loads occur rather in rotor discs during start/stop cycles with correspondent low frequency ( "Ill. 3.1.2.1-5 and 6" up to "Ill. 3.1.2.1-7 and 8").
Vibrations of the blades have not exclusively to do with resonance vibrations in the first flexural mode. Fatigue cracks in the proximity of the blade transition to the root platform, but also to do with the vibrations of a higher mode. Typical, are the so called „Lyra mode“ vibrations in the tip area of the rotor blades, or torsional vibrations in the trailing edge area of the stator vanes which are fixed at both sides. There are self excited vibrations („flutters“) that can lead to immediate blade fracture. In the frame of the development of an engine it has to be proved that, at least for normal operation and the test run covered speed and running time, the dynamical stresses are below the fatigue strength. An important aid to the recognition of resonance danger is the Campbell diagram ( "Ill. 3.1.2.1-9"). In this diagram, a dangerous cutting across of the natural frequency of the blades with exciting frequencies, depending on the blade numbers and speed, can be determined.
The case is different, when, e.g., foreign object impacts or scores and notches are formed through careless handling before or during assembly. Such unpermitted stress concentration causes a too high local, vibrational stress. It can also lower the fatigue strength to a dangerous measure locally, e.g. through high production dependent residual stresses, through micro-structural changes, e.g., underneath removed weld spatter or unprofessional repairs, e.g., overheating during removal of notches with power tools. Vibration excitations can emerge frequently and /or stronger than expected. It is thinkable, e.g., that, operation specific, a component, e.g., a rotor blade of a certain stage, runs frequently over a long period of time at a particular resonance speed. A sufficient number of dangerous, high vibration cycles accumulate. Surge ( "Ill. 3.1.1-6"), or an especially intensive rub ( "Ill. 3.1.2.4-3"), can induce fatigue failure or crack, leading to a fracture of the component already under normal load. Experience shows that, if the blade is damaged at the tip during rub in an unduly way, radial fatigue cracks and corner breakages (lyra mode, "Ill. 3.1.2.1-8") can occur. Endangered are especially thin blades and vanes with a wide chord (wide chord blades). If the aerodynamic effective geometry of the blade surface, through erosion, foreign objects or deposits is too strongly altered, a stall can lead to vibration excitation (rotation stall, "Ill. 3.1.1-5").
A further problem area is the blade root. The usual root form in the compressor is the so called dovetail. At the bedding area of the contact surfaces to the disc, ( "Ill. 3.1.2.1-6" and "Ill. 3.1.2.1-8"), low frequency micro movements by the widening of the disc under centrifugal force and temperature follow (low frequency, corresponding to the start cycles) and high frequency micro movements by the vibration of the blade. The so called fretting (corrosion fatigue ) occurs, a local failure up to micro crack formation of mechanically high loaded contact surfaces/thrust faces ( "Ill. 3.1.2.1-10"). Especially sensitive for these failure mechanisms are titanium alloys, as they are typical for blades of modern compressors. Here with a fatigue strength reduction up to 30% must be reckoned. This means an extreme decrease of the bearable number of cycles at a given dynamic stress. The most common remedy is shot peening, with enough intensity, and an additional dry film lubricant. This must not follow only on new parts but also during overhaul, so that the shot peen effect weakened by creep or fretting is refreshed. Through shot peening one gets an advantageous compression residual stress condition in the surface area, as well as a work hardening. Depending on the material, these effects are of a variously large influence. A further positive effect is the especial calotte shaped structure of the peened surface onto which dry film lubricant holds well, delaying wear. There are additional hints that a micro topography by which only peaks come into contact (small loaded volumes, contact zones disengage) is less fatigue endangered. Frequently, however, the cracks originate not directly in the fretting area but close to it in an uninfluenced region. This is accounted for, because in the crack area the created stress at the surface is effective through high shear loads, as a consequence of neighboring friction. This area lies frequently at the transition radius to the blade shank (stress increase through geometrical notch ). The influence of the friction coefficient in the bedding area at the stress level in the critical range, is also the explanation for the additional use and effectivity of sliding paints and lubrication coatings. In the relatively hot rear compressor area many of these coatings age over long periods of time and get damaged (oxidation, decomposition).Thus, they lose their effectivity with age.
Compressor discs
Compressor discs are subject to centrifugal forces and, to a smaller extent, also to thermal stresses leading to fatigue load (LCF, "Ill. 3.1.2.1-6" and "Ill. 3.1.2.1-8"). Since the temperature changes from the ambient temperature at the inlet up to 600°C at the exit, depending on the total pressure ratio, the materials of the discs and blades are correspondingly chosen. In older types of engines, steel is used up to the rear area, frequently 13% Cr steel. Often, heat resistant Fe alloys or Ni base materials are found. Modern engines use Ti alloys or steel in the front area, in the exit area Ni alloys, and in the transition area special steel with a low expansion coefficient to balance the thermal expansion differences. Because steel is mostly insufficiently corrosion resistant (pitting corrosion), components of this material are coated very effectively with aluminium powder filled inorganic paint. There is a problem occasionally in the area of fittings (centerings, "Ill. 3.1.2.1-1") and bolt holes, if, for tolerance reasons, these cannot be painted. Bolt holes, notches with a correspondingly high stress level, are often the LCF life limiting area of the component and consequently, determine the overhaul intervals. The producer must also take care that no corrosion notches (corrosion pits) originate in the unprotected holes. The use of galvanized coatings or other protection demands an exact knowledge of a possible drop of the fatigue strength of the base material. Therefore in case of doubt, the OEM must be consulted. The disc slots ( "Ill. 3.1.2.1-6") are loaded correspondent to the linked blade roots. Similar conclusions are valid for both.
LCF and HCF
Preliminary note: The knowledge of the component specific crack types and positions is as well of high importance for the borescope appraisal on-site for the test personnel in the shop as at the manufacturer (OEM) so far it concerns the blading ( "Ill. 4.1-4", "Ill. 4.1-5" and "Ill. 4.1-6"). Cracks in discs are extreme seldom. The consequences however would be much more serious than after a blade fracture. For this reason the fracture of a disc has to be avoided in any case. During the dimensioning of a disc especially high safety margins must be considered ( "Ill. 3.1.2.1-5 and 6" up to "Ill. 3.1.2.1-7"). An additional crack inspection at every overhaul is essential. The numbers, assigned to the illustrations "Ill. 3.1.2.1-5 and 6" up to "Ill. 3.1.2.1-7 and 8", correlate each a distinct predominant load type.
"Illustration 3.1.2.1-0": The difference between LCF (low cycle fatigue) and HCF (high cycle fatigue) is explained well by the Woehler diagram (top diagram). This applies to materials that have a Woehler curve that becomes horizontal above about 107 load changes, i.e. materials that have a fatigue limit (e.g. steels). Al alloys and titanium alloys do not have a fatigue limit below which no fracture can be expected; in this case the curve continues to decline. Loads in the range of the fatigue strength are above the fatigue limit. After a load-dependent period of time, a dynamic fatigue fracture occurs. Therefore, this breakdown of the Woehler diagram is load-oriented. Another method of classification common in the English-speaking world is based on the expected life span. Dynamic loads that lead to “noticeable plastic deformation“ in the crack initiation zone are defined as lying in the LCF range between 104 and 105 crack initiation load changes. An interesting note is the imprecise definition of the deformation amplitude, which can be seen in the relatively broad life span range. In addition, at first glance it is often not clear how the deformations occurred, since parts that have a limited cyclical life (rotor disks, etc.) do not show any measurable expansion.
The definition of the LCF load also fits to thermal fatigue (TF, TMF, "Ill. 3.3-16"). Here hampered thermal heat strains as a result of temperature changes lead to cyclic plastic deformations.
The bottom diagram uses an everyday situation to make the damage mechanism of (extreme) LCF stress understandable. If one desires to split a wire by hand, then it must be plastically bent several times. This causes the wire to experience an LCF fracture. Of course, the number of load cycles in engine parts is considerably higher, but the mechanism is comparable.
In a steel wire, an increase of bending force can be detected as the material hardens. The wire breaks after several load changes. Heat creation is not important for explaining this process, and only reflects the bending force required to plastify the material. One might also notice that a rusty wire (with corrosion notches) tolerates less load changes to fracture than a smooth wire. Therefore, notches and flaws lower the LCF strength, i.e. shorten the LCF life of a part. For this reason, they must not occur in part zones that are subjected to this type of stress.
HCF fractures (fatigue failure), on the other hand, do not show any noticeable signs of plastic deformation. Even in ductile (tough) materials, these fractures act brittly.
As the bottom left picture shows, LCF-stressed part zones (of a rotor disk, in this case) are limited to small volumes in areas that experience increased stress levels due to a notch effect. These include radii between sectional jumps, bores for connecting bolts, and hub bores. These zones are subject to high stress levels due to centrifugal forces and thermal stress, and always result in plastic deformations at the load levels that are typical in today‘s operating environments. After relief, residual stresses build up in the zones between the plastically deformed areas and the merely elastically deformed surrounding areas. These stresses are in a state of equilibrium when the engine is at a standstill. For this reason, the local plastic deformations are not apparent in external dimensional changes (in outer disk diameter, for example).
In order to avoid a common misconception, it must be pointed out that letter “F“ in the terms HCF and LCF stands for “fatigue“, not “frequency“. Although the large number of fracture load changes and the comparatively moderate stress amplitude in HCF fractures are usually connected with high-frequency vibrations, it is also possible for HCF fractures to accumulate at “LCF-typical“ low frequencies (e.g. startup/shutdown cycles). An example of this would be a peak load gas turbine which is only started a few times a day over several decades of operating time.
On the other hand, LCF fractures can also be caused by high-frequency vibrations with extreme amplitudes. These loads can occur during flutter vibrations or when blades run over a blade fragment that is caught in the housing, for example.
"Illustration 3.1.2.1": Typical integral disc ( bladed disc, abbreviated blisk) of a small performance gas turbine, (less than 300KW). The ‘blisk’ (bladed disk) belongs typically to the derivate of a helicopter engine. Normally are those wheels made in one piece of precision casting. Milled blisks out of forgings are used only by some manufacturers. The blade by the blisk is directly „grown“ at the ring. In the area of the disc membrane, one normally finds seal fins against hot air ingestion and cooling air guidance respectively. The disc frequently has a balancing collar on which, through metal removal, the balancing is undertaken. In the high loaded, clearly thickened hub the centering and torque transmittence occurs often. In such small gas turbines, the compressor often comprises integral discs. Here the centering occurs mostly at the rim. In the development and in the implementation of aero engines, one finds big rotors out of metal cuttings, or electrochemically manufactured compressor rotors in blisk style.
"Illustration 3.1.2.1-2": Compressor discs of bigger engines usually have axial dove tail slots in order to receive the blades in the rim. The diaphragm has normally a spigot for the neighboring discs respectively (spacers, see "Ill. 3.1.2.4-1"). The connection in the rotor formation follows mostly over centering screws/bolts or tie rods through the disc. Here also the highly stressed hub is thickened, so as to decrease the stress level. The bigger the hub, the more this happens.
"Illustration 3.1.2.1-3": Smaller power gas turbines, up to 1000 KW, frequently show one or two centrifugal compressor stages or have a centrifugal compressor as the final stage. The impeller usually has, next to the integral vanes, a labyrinth seal for sealing the compressor outlet air, a balance collar for metal removal, which goes over into a thick hub, often showing a reinforced collar in the rear, and, in the interior of the hub, a strength optimized contour. The two-piece impeller with an inducer is a speciality. A ring at the inlet area over the impeller vane tips (arrow) stiffens primarily against blade vibrations.
"Illustration 3.1.2.1-4": Compressor rotor blades do not have any shrouds. Their blade tips are an element of the blade tip clearance. Not seldom, the optimizing of the tip clearance through rub is foreseen. In order to minimize the damages, ( "Ill. 3.1.2.1-8") the airfoil at the tip is thinned, (squealer tip).Tip -platings that make harder abradables in the casings possible are in development. The, in axial direction very inclined blade, passes towards the root to the platform for gas guidance. The contact faces of the root platform can dampen the blades in order to avoid vibrations, support them and take over sealing functions. The root neck transmits the loads from the wide root platform in the especially loaded dove tail root. The bedding areas of the root usually have a friction lubricant and are work hardened, shot blasted with steel balls, in order to increase the vibration strength.
Preliminary remarks: The knowledge of component specific crack types and their position is, for the inspection personnel in the shop or at the manufacturer’s, with regard to the blades, of special importance for the borescope findings on site ( "Ill. 4.1-5"). Cracks in the discs are extremely seldom. The result of cracks (disc fracture) can be clearly more serious as blade failures. A disc fracture should be avoided in any case. High demands for safety are considered when designing a disc. The crack inspection during overhaul is of particular importance when the disc undergoes many overhauls within it’s foreseen lifetime. The arranged numbers of crack areas in the neighboring pictures correspond to a particular, here predominant, kind of load.
"Illustration 3.1.2.1-5": In small turbine engines in the low power range also the compressor is built on bladed discs (blisk, above left). Compressor rotor discs of bigger engines have up to now typically axial dove tail slots for the blades (above right): (1) cracks through rub and/or ‘lyra mode’ vibrations, (2) cracks through natural frequency blade vibrations in the HCF range (3) disc rim cracks through thermal fatigue, (4) labyrinth cracks through rub and cyclical stress in LCF and / or HCF range. (5 and 6) fatigue fractures through LCF, on grounds of centrifugal force and temperature changes, especially at the start. (7and 8) Crack formation through disc vibrations in the HCF area and/or LCF cracks through thermal change and centrifugal force alterations.(9) Crack formation through fatigue and /or overheating and oxidation.
"Illustration 3.1.2.1-6": Typical disc of an axial compressor (above right): (2) Fatigue cracks in the dove tail slots through blade vibrations in the HCF range or centrifugal load changes in the LCF range. Not seldom, cracks arise in the bedding areas (1) in combination with fretting damage ( "Ill. 3.1.2.1-10"). In the bolt holes, crack formation through centrifugal force changes in the LCF area. (3) Cracks in the labyrinth through rub damage. (4) LCF cracks in hub region through centrifugal force and clearly lesser as by a turbine disc ( "Ill. 3.3-18") through thermal
"Illustration 3.1.2.1-7": Centrifugal compressor impellers (above left), are likewise highly loaded components: (1) Crack formation through blade or disc vibrations in the HCF range. (2) Fatigue cracks through LCF during start and shut down procedures can be supported through disc vibrations. (3) Crack formation through rub, through thermal stresses and overheating failure ( "Ill. 3.1.2.4-7.1"). Crack propagation through centrifugal alteration in the hub region, at the vane edge through vane vibrations.(4) Crack formation through hefty rub of the vanes with overloading of the vane root. (5) Cyclical fatigue through bending stress in the impeller disc and bending overload of the inducer blades.
"Illustration 3.1.2.1-8": Compressor rotor blade: (1) Fatigue crack formation through blade vibrations, (2) crack formation in the bedding area through speed alterations.(3) Crack formation through rub. (4) Crack formation through blade vibrations, mostly in connection with fretting
"Illustration 3.1.2.1-9": The so called Campbell diagram makes it possible, already during design, but also through the detection of causes in the case of failure, to recognize possible resonances and to identify the sources of the vibration excitation. In this diagram, the rotor frequency (Hz) is laid out on the abscissa, corresponding to the revolutions per second. The ordinate contains the natural frequency of the vibrating components and the vibration frequency. For every disturbance, a straight line can be drawn according to the disturbance at the circumference. The curve of the natural frequency ( is not a horizontal line, because, e.g,, the centrifugal force and the temperature dependency of the E-module (modulus of elasticity), respectively, incorporates the stiffness, crosses these straight lines at possible resonance points in the operational speed area (here only one, see arrow). Typical disturbances in the gas flow are struts, guide vanes and extraction of air (bleed openings ). The natural frequency in the development phase of the components is calculated. In cases of failure, the experimental vibration analysis (e.g. modal analysis) is used. Here the mode of vibration is probably the cause of failure, by which the highest stress appears, at the crack initiation area of the failed part. This is where the highest surface expansion emerges (smallest bend radius between the nodal points). According to a measurement technique, this area can be verified with strain gauges. The influence of the operation upon the real height of the produced load is then evaluated from experience values. Naturally, one tries to put the resonance possibilities for important components, beyond the operation speed of the rotor. From start to full load and the amount of flow influencing components an avoidance of all potential resonances is not possible. The test run of the engine must create the last clarity, „the engine will tell us“, as one says so aptly in the English language. It the operation conditions change, compared with the proving tests, the conclusions are limited. The engine is in the position to inform by a failure.
"Illustration 3.1.2.1-10": Typical weak spots of the rotor blades of a compressor are the root bedding areas or the adjacent areas of the dove tail root in the disc slot. High frequency blade vibrations („A“) and /or low frequency elongations under centrifugal load alterations („B“) can produce micro movements with typical wear traces, the so called fretting (fretting corrosion), especially dangerous because it weakens the blade materials. It makes vibration with crack initiation favorable (arrow) and / or the increase of stress in the apparently not directly affected neighboring areas (arrow) of the bedding areas. In comparison with blades of steel, the failures of titanium alloys through fretting is far bigger. In all modern engines the blade roots are, therefore, mechanically treated and / or coated, which keeps these failures within permitted limits for scheduled operation times.
The typical mechanical treatment is a work hardening of the contact area through plastic deformation. Shot peening has become widely accepted. A jet of small steel balls hits through a nozzle at high speed under given pressure and angle at an exactly fixed time onto the surface to be treated. Additionally, these micro rough surfaces are mostly treated with a lubrication system, in order to reduce the friction coefficient noticeably responsible for the load of the neighbouring areas.
The use of wear resistant and plastically molding coatings (e.g. bronze) with low friction coefficient, usually in combination with a peen hardening, is frequently observed.
Because, despite all measures to hinder it, a failure at the contact zone must be reckoned with during longer operation times, it is normal to repeat the treatment of the blade root completely, or partly, or to touch it up during overhaul.
If unusual fretting appearances emerge, hinting at special operation conditions, the expert can evaluate these wear pictures and, if required, give important remedial hints.
A characteristic feature of failure intensity is the length of wear (a) in the microscopic range. Also informative are direction and position of the cracks (detail above left)
Literature of chapter 3.1.2.1
3.1.2.1-1 A.Rossmann, „Die Sicherheit von Turbo-Flugtriebwerken“, Band 3, ISBN 3-00-017733-7, 2003, Axel Rossmann Turboconsult, Bachweg 4, 85757 Karlsfeld.