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5.2.2.2 Bird Strike Damage Mechanisms

This chapter explains the extremely complex sequence of events that occur during a bird strike. This is intended to enable the reader to better understand damage symptoms and more accurately estimate risks.
This chapter begins by considering fundamental processes that occur when an object strikes a beam in bending that has been fastened on one side in order to simulate a rotor blade (Fig. "Laboratory tests on bird strikes").
Fig. "Phases of deformation" shows that the blade is not subject merely to bending loads, but also experiences a dynamic load process in the form of a decaying vibration. The movement of the blade tip can make it come into contact with the surrounding blade rows (Figs. "Unsteady forces during bird strike" and "Damage mechanisms in the compressor") and cause extensive damage.
The flight speed of the affected aircraft, i.e. the impact speed of the bird against the blade screen, can be responsible for damage to various compressor components (Fig. "Contact of rotor with stator vanes"). For example, low flight speeds can put especially high stress on the rotor blading (Figs. "Contact of rotor with stator vanes" and "Influence of flight speed").

It has become possible to simulate the damage process with computer programs ever more realistically. This was and is achieved through extensive tests that make possible an iterative process that brings the calculated results ever closer to the actual physical processes. However, experience has shown that even today surprising damage mechanisms occur that are beyond current mathematical analysis Therefore, it is understandable that certification and acceptance of new engine types requires practical verification of satisfactory bird strike behavior.

These tests are usually conducted by using a pressurized-air cannon to shoot birds into the engine while it runs at a specified performance level. Ensuring sufficiently realistic conditions requires the use of real birds that are normally put to sleep shortly before the test, as this results in the most natural possible mass distribution.

Figure "Laboratory tests on bird strikes" (Ref. 5.2.2.2-4): For laboratory tests with gas pressure guns (air, helium) usually alternative matters instead of real birds are used. Different mixtures of natural rubber, `plasticine' or gelatin with other materials (e.g., wood wool) however show results which are not sufficient realistic for proofs in the aeroengine. A mixture from 85 % water and 15% air reproduce the pressure flow most realistic (Fig. "Phases of deformation"). Anyway in acceptance / certification tests of an aeroengine type, shortly before the test, euthanised birds must be used.
The diagrams below apply for measurement values at sheets during a perpendicular impact with a cylindric shaped water-air bird alternative matter. The shock pressure (hugoniot pressure) is almost 10 times higher than the flow off pressure. The impact velocity increases the pressure exponential. This show measurement values in agreement with the calculated curve. For a better evaluation the practitioner should keep in mind, that the MPa indications correlate the ten times numerical value in bar. So impact pressures up to a multitude of 1000 bar and flow off pressures in the magnitude of several 100 bar. With materials like fiber reinforced plastics (FRPs) of a nose cone (Example "Spinner fragments" and Ill. 14-32), of the linings from the air intake ducts (Fig. "Intake duct liner"), aeroengine nacelles as well as light metals of the walls from aircraft fuselages are even from an angular impact (Fig. "Indirect threats of bird damage") by far overloaded (Ref. 5.2.2.2-6). Its failing can produce dangerous secondary failures of a bird impact (Fig. "Indirect threats of bird damage").
Also typical failure modes at compressor blades, especially fan blades (picture above left), can be amazing good simulated with computer based calculations, which consider the impact effects (sketch above right). This demonstrates impressive the realistic approach. Crack formation at the impact area must be seen in connection with an embrittlement effect of the blade material during high speed deformation (Fig. "Material behavior").

Figure "Phases of deformation" (Ref. 5.2.2.2-4): Already in the seventies extremely high pressures during the impact of a bird at a rigid wall have been measured. Concerned was a perpendicular impact (sketches above). A typical pressure progression above the time is displayed in the diagram. Inside the bird respectively alternative matter during beginning of the shock (phase 1) back running shock waves produce the pressure maximum (shock pressure, hugoniot pressure). Actually extreme high frequency pressure fluctuations occur by the shock wave and its reflection. Then the bird matter at the edge is accelerated outward parallel to the surface (phase 2). The result is a pressure drop inside, which leads to the rip of the bird matter. So it comes to the flow off (phase 3) correspondent a liquid. In the stagnation point the typical pressure progression develops, like it is measured in the impact center of a bird (diagram).
The high frequency pressure oscillation is real and obviously caused by a reflection of the shock waves.

Figure "Material behavior" (Ref. 5.2.2.2-1): The progress of plastic deformation (upper sketch, Fig. "Resulting deformation of impact-bending") after a bird strikes a blade made from ductile, homogeneous metallic material can be schematically explained using the example of the beam in bending fastened at one end (top diagram). Of course, the blade shapes of a compressor are often very different from these idealized conditions. However, the insights gained here can be useful for understanding the damage mechanisms and blade behavior besides the impact behaviour of the bird matter.
For a beam in bending with an even, prismatic cross-section which has been fastened securely on one end, the corresponding moment from a force “F” coming from the side and acting on the beam at distance “l” from the point where the beam is fastened is ME = F.l= sF·h·b2/6 (h = height, b = width of the rectangular face), if the outer fibers are stressed by tension at the yield strength sF , and the cross-section has elastic-linear tension distribution. If the tension increases until the entire face cross-section begins to yield (i.e. is stressed above the yield strength), the corresponding moment is greater. In extreme cases no elastic core remains, and the moment is MP= F·l= sF·h· b2/4.
This case, in which the cross-section is fully plastic, is referred to as a “plastic hinge” (plastic collapse). At the plastic hinge, i.e. the cross-section that the plastic hinge has passed through, the deformation resistance is higher than in the elastically deformed area. Therefore, the plastic hinge runs into this area.
In about half the time of the dynamic reaction to the bird impact, extreme deflection with high strain speeds takes place. The vibrations this incites decay rapidly (middle diagram). The high strain speeds can have a powerful, material-specific influence on the behavior of the part.
The greater the strain speed, the closer the yield strength comes to the breaking strength (bottom diagram). For example at high strength titanium alloys the yield strength increases at yield rates 199-1000 1/sec at 40% (Ref. 5.2.2.2-4). It approaches equivalent the ultimate strength. This behavior results in embrittlement; i.e. the material fractures brittle due to overstress, even though it shows plastic behavior in a normal tensile test. behavior. Because of this, impact of a foreign object at high speed but with relatively little energy can fracture surprisingly thick wall sections. This effect is not very pronounced in titanium alloys, nickel alloys, and high-alloy steels, which are commonly used in compressor blading. Despite this, bird strikes can cause damage symptoms that could be plausibly explained by high-speed embrittlement.

Figure "Resulting deformation of impact-bending" (Ref. 5.2.2.2-1): This picture shows a perception at the processes in the whole blade which is caused by the mass inertia.
As model serves a one-sided mounted beam. If the tip of a beam fastened at one end is struck by a sufficiently large force of deceleration (a mass with high speed and a corresponding amount energy), it forms a plastic hinge that moves towards the fastened end of the beam with decreasing speed (see the curve of plastic hinge progress). At the same time a curve is created as a rotation around the current location of the plastic hinge. When the plastic hinge reaches the point where the beam is fastened, it rotates as though it were stiff above this point. It could be shown that masses which are small in relation to the mass of the beam, but moving very rapidly, cause a distinct curve in the beam tip, but less rotation at the point of fastening (middle diagram). The beam behaves differently when struck by a slow-moving, relatively large mass. This results primarily in curvature at the point of fastening, but only minor permanent bending of the total length of the beam.
If the impact does not occur at the upper part of the blade, then it will usually result in different damage symptoms (Fig. "Influence of blade material"). It causes a local bend, while the mass forces direct the upper and lower parts of the blade against the direction of deformation.
The reaction forces on the blade root (bottom diagram) are important if this is a weak point. For example, fiber-reinforced synthetic blades with a large jump in strength at the connecting point. This might be either an element that forms the hub contour between the blades (Example GE90) or the blade itself.
If the FOD mass strikes below the blade leaf`s center of gravity, it creates a large reaction force at the root in the direction of the impact. If the blade is struck at the blade leaf`s center of gravity, the reaction forces at the root should be small. If the mass strikes the upper blade leaf, then the reaction forces at the blade root will be directed against the direction of impact.

Figure "Influencing parameters": The impact process on a rotating blade row is extremely complex and influenced by a large number of temporally changeable parameters.
An ingested bird can strike the blade row at various speeds, depending on the flight conditions. If it is ingested at a high speed close to the air intake speed, it will follow the airflow until it is chopped into slices by the intake edges of the rotor blades, simultaneously accelerated axially, and guided into the following stator. The thickness of the slices is a function of the distance between blades and the speed at the impact radius of the blade, i.e. the RPM and the speed of the bird “VE in relation to the aircraft. The mass of these foreign object slices can be calculated by the size of the bird and its specific weight. If one assumes that the specific weight of the bird is roughly the same as water and therefore roughly 1000 times that of the air flow (for which the blading strength is designed), then it seems as though the blading would be overloaded.
Even modeling clay has proven to be insufficiently realistic as a substitute for birds. One reason for this may be the plastic behavior of modeling clay, which is highly dependent on the deformation rate and behaves brittle at high deformation rates.
The impact of this mass creates a bending force that acts perpendicularly to the blade chord with a moment arm that corresponds to the distance between the point of impact and the blade root and an accelerating force resulting from the curve of the profile and related deflection of this mass, which acts at an angle in relation to the profile (also see Ref. 5.2.2.3-2). The impact forces due to foreign object strikes depend not only on the mass of the foreign object but also largely on the angle of impact; i.e. on the “incisive projection screen”, the redirection of the foreign object, and the elasticities of the foreign object and the blade (impact digit! Figs. "Unsteady forces during bird strike" and "Damage mechanisms in the compressor").
The slower the birds enters a blade row, the thinner it is sliced, since more blades will be involved (Fig. "Influence of flight speed"). On the other hand, decreasing bird impact speeds are accompanied by increasing impact angles and axial force components.
When a large bird strikes a blade row, the relatively large distance between blades at the tips compared with the spacing at the hub makes the blade tip area do considerably more slicing and transport work than the larger number of engaged blades closer to the hub would have to do.

Figure "Unsteady forces during bird strike": The top diagram schematically depicts a fluid model of the process that occurs when a bird enters a rotating blade row. It clearly shows that the leading blade edge is the first area to be stressed, putting torsional and bending forces on the blade that act diagonally forward corresponding to the angle of the blade. The farther the bird travels through the blade row, the more the force travels along the face of the blade to the trailing edge, changing the torsion moment.
Compressor rotor blades struck by soft foreign objects are usually torqued, bent forward, and finally spring back (up to several centimeters, depending on the blade length). The first deflection of the rotor blade due to the blade leaf bending forward bending around the Imin-axis is due to the angle of the blade face: “the blade row twists into the foreign object.”
After the soft body flows off the trailing edge the blade leaf springs back (bottom diagram).
Brittle-hard foreign objects such as ice burst or spray on impact, resulting in a completely different energy transfer between the foreign object and the blade. The foreign object does not flow off the blade, which results in lower changing torsion stress (see bottom left diagram).
The overlaying of torsion and bending deformations of the blade face reciprocally influences the progress of the impact forces and the resultant vibrations.
Compared with the twisting/untwisting forces on the rotor blade, the internal forces which bend back and square the blade are relatively small. This is especially true with modern compressor designs with typical sharp-edged inlet radii (trailing edge) and small (or located far back) profile thicknesses. This should contribute to the observed characteristic that modern wide chord fan blades are sometimes surprisingly sensitive to bird strikes (Fig. "Low speed impact I").
With hard-elastic foreign objects, the high internal forces cause the backward bending to be especially pronounced.
When blades untwist, especially blades with wide chords, can lengthen radially and the edges may rub heavily against the housing. This can overstress the blades and/or the housing.

Figure "Damage mechanisms in the compressor": The foreign object impact on the rotor blade first bends the latter diagonally forward due to the powerful forces required to transport it. The tip may be deflected up to several centimeters, depending on the size and elasticity of the blade.

The deflection movement of the affected blade (top diagram), and therefore the progress of the impact forces (bottom diagram and Fig. "Unsteady forces during bird strike"), depend on the properties of the foreign object, including:

  • brittle-hard: ice, etc.
  • hard-elastic: steel, etc.
  • soft-viscous: birds, etc.

Extensive research was necessary to find a bird substitute that demonstrates impact behavior sufficiently similar an actual bird strike. It was found that water-filled plastic sacks are not a realistic substitute, since the damages they caused were considerably greater than those caused by actual birds. This is due to a lack of natural air pockets that dampen the development and spreading of the pressure waves during a bird strike, lessening the local damage (in fiber-reinforced synthetics, etc.) and impulse transfer.
The amount of energy transferred during impact depends on the stossziffer. This consists of the relationship of the post-impact speed difference to the pre-impact speed difference between the involved bodies (blade and bird). The stossziffer indicates the elastic or plastic behavior of the bodies during impact and lies somewhere between 0 (completely plastic behavior) and 1(completely elastic behavior). The stossziffer is also dependent on speed.

Figure "Contact of rotor with stator vanes": When a bird strikes a rotor blade, the angle of the blade and the force required to move the bird cause the blade to bend diagonally forward (Figs. "Unsteady forces during bird strike" and "Damage mechanisms in the compressor"). It then springs back elastically (middle diagram). These oscillations can cause the blade to bridge the gap between it and the neighboring stators and touch them (bottom diagram). If the blade touches the front stator the curvature of the blade may cause it to catch and/or be pulled into the stator, increasing the extent of offset. Therefore, this process is self-increasing.
On the other hand, if the blade touches the stator located behind the affected rotor stage, the blade is more likely to be thrown forward again.
In both cases there is a danger of at least several stator vanes being weakened by notches at the circumference near the connection to the root platform. Even if these stator vanes do not spontaneously fracture, they may fracture due to rapid fatigue.

Figure "Influence of flight speed" (Ref. 5.2.2.2-2): If a bird strikes a rotating blade row at high flight speeds, and therefore high relative speed, it is referred to as a high speed impact (top left diagram). This situation occurs, for example, when birds strike fighter aircraft on low-altitude missions traveling at high speeds. In this case the inlet conditions are virtually the same as the aerodynamic configuration (inlet angle and speed correspond to the air flow). Thus the bird is cut by the leading edge of the rotor blade. In extreme cases with proper size ratios, the bird can pass whole between two blades. This puts minimal stress on the rotor blading, but the stator is struck by the entire bird and subject to extreme stress.
At low flight speeds (top right diagram), low speed impact puts especially high bending stresses the rotor blading (see Fig. "Influencing parameters"). However, the following stator blades are only struck by parts of the bird.
Stator blades/vanes, especially those fastened on one end, are bent backward along the circumference due to the intake energy of the foreign object and the reaction speed it received from the preceding rotor stages.
This results in powerful shear stress on the blade fastening, which usually results in cracking or fracturing of the stator blade leaf. This can in turn lead to extensive consequential damages.
If the inner shroud is heavily bent or cracked, it can cause extensive damage to the rotor blading in front of and/or behind it. Because this will damage all of the rotor blades at the circumference, it can result in unexpectedly large damages that greatly exceed the stress levels on components such as housings which are normal for bird strikes. This can represent an uncertainty in acceptance tests.
The bottom diagram is based on calculations and shows how a compressor rotor is deflected by various bird strikes. It is clear that the maximum deflection, i.e. stressing of the rotor blade at relatively low intake speeds, is caused by low speed impact.
In designs with inlet guide blades (compare with Fig. "Influence of inlet guide vanes"), i.e. housing/casing struts in the inlet, the bird may be decelerated in this area. This can create conditions for a pronounced low speed impact with increased stressing of the following rotor blades, even at high flight speeds.

Figure "Low flight speeds" (Ref. 5.2.2.2-2): It was shown in chapter 5.2.2.1 that aircraft are especially prone to bird strikes during takeoff and landing approach flight, i.e. at low altitudes. Also, under these conditions an additional factor makes the engines very sensitive to damage: at low intake speeds and high RPM (climbing after takeoff or direct landing approach flight, flaps fully extended), the bird is cut into thinner slices per blade, since more blades strike it due to the slow entry speed. On the other hand, the angle of impact increases along with decreasing bird entry speed (see Fig. "Influencing parameters"). This increases the ratio of axial force components on the blade, meaning that a greater impulse is transferred to the blade. This effect increases with the difference in bird speed relative to the designed air intake speed, since the aerodynamic configuration was designed in accordance with the latter. At full RPM and low flight speeds (takeoff and climbing), maximum blade stress is reached. This critical flight speed is marked in the diagrams with a gray background.
The impact forces are greater near the hub than at the blade tip due to the greater stiffness of the blade and the smaller angle of attack (middle diagram and Fig. "Damage mechanisms in the compressor"). The effective lever arm is smaller near the root, and the section modulus of the blade profile is greater. Due to the relatively large space between blades at the tips, the number of blades that engage the bird (especially large birds) is less than it is near the hub. This means that the tips are stressed by a larger bird mass per blade. Because of this, the overall blade strains and deformations are greatest when impact occurs at the tips (bottom diagram). While the impact forces at 25% of blade height are greater than at 80% blade height, the leading edge stress, as a parameter for blade failure, is greatest at an impact at 80% blade height.

References

5.2.2.2-1 W. Johnson, A.G. Mamalis, “Gegenüberstellung statischer und dynamischer Schadens- und Deformationserscheinungen”, Fortschr.-Ber.VDI-Z. Row 5 Nr. 32. VDI Publishing, D-4000 Düsseldorf, 1977.

5.2.2.2-2 R.S. Cox, Rolls-Royce Limited, Bristol Engine Division, “Bird Ingestion Problems Relating to Gas Turbine Engines”, September 1969.

5.2.2.2-3 C.A. Huertas-Ortecho, „Robust Bird-Strike Modelling Using LS-DYNA“, University of Puerto Rico, 2006, page 1-236.

5.2.2.2-4 J.Frischbier, „Vogelschlag in Flugtriebwerken- eine impulsartige Fluid-Struktur-Wechselwirkung in der Triebwerksauslegung”, www.mtu.de/de/technologies/engineering_news/entwicklung. Page 1-23.

5.2.2.2-5 J.Frischbier, „Multiple Stage Turbofan Bird Ingestion Analysis with ALE and SPH Methods“, Herausgegeben von „American Institute of Aeronautics and Astronautics”, 2005, Page 1-9.

5.2.2.2-6 P.Starke, G.Lemmen, H.Drechsler, „Validierung von Verfahren für die numerische Simulation von Vogelschlag“, DYNAmore GmbH, LS-DYNA Anwenderforum, Bamberg, 2005, Page J-1-47 up to J-1-52.

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