566 Appendix G (b)(a) (c) (d) Figure G.11. Damage to RB199 fan blades. concentration factors, k t . The distribution of k t is given in Figure G.14 [5]. The aver- age k t is about 4; however, values of up to 10 can occur for the more severe FOD notches. Microscopic features of FOD Metallographic and fractographic examinations were performed on selected samples to determine the extent of local deformation and the possible existence of cracking. It was found that FOD notches encompass a wide range of microscopic features. The impact event leading to the damage site often resulted in non-propagating cracks, as shown in Figure G.15. Non-propagating cracks have been found in laboratory experiments and can have surprisingly little effect on the path of final failure [2, 6]. Appendix G 567 20 0 40 60 120 80 100 0.076 0.003 0.010 0.050 0.100 0.200 0.300 0.400 ln. 1.27 10.2 mm0.254 2.54 7.62 P&W 5.08 FOD depth, mm (top), In. (bottom) Number of occurrences Blend limits SwRI Servicable limits Figure G.12. Distribution of service-induced FOD from two different surveys. Southwest Research Institute (SwRI) conducted study under a USAF contract. 1.20.4 20.8 1.6 6 8 14 16 10 4 12 0 2 FOD root radius, mm Number of occurrences Figure G.13. Distribution of FOD notch root radii. In addition to non-propagating cracks, several notches were found with extensive local deformation but no cracking. Several damage sites were found with little localized damage and some were impacted with enough energy to cause brittle failure and generate debris. In short, post-event inspection revealed that there had been a wide range of impact energies. 568 Appendix G 100% 90% 80% 70% 60% 0% 20% 10% 30% 40% 50% 1 1 0 78 8 10 10 7 9 9 6 6 5 5 2 2 3 3 4 4 Elastic stress concentration factor, k t Number of occurrences Frequency Cumulative % Figure G.14. Histogram and cumulative distribution function for FOD notch k t . Figure G.15. Micrograph showing FOD site with non-propagating crack. The biggest benefit of the microscopic investigation was the identification of impact sur- face details that indicated the angle of incidence. These features indicated that the impactor tended to strike the blade at angles of 30 to 60 relative to the blade leading edge cen- terline, which correlates well with airflow and blade dynamics, as shown in Figure G.16. Appendix G 569 Resulting FOD region Rotational velocity of blade Engine centerline High stress side of blade Leading edge centerline ~3 0 ° Impactor strike angle (30°– 60°) Effective velocity of FOD particle Figure G.16. Illustration of typical FOD impact angles in modern gas turbine engines. Complex airfoil shapes and irregular impactors make it difficult to define accurately the point at which the damage begins. In controlled testing at QinetiQ, it was originally observed that the actual damage size was significantly smaller than was expected. Ini- tially, this was attributed to the deflection of the impactor on striking the blade, thus creating a smaller notch than expected. However, it was observed that the basic method of measuring the notch depth did not always give an accurate representation of the notch depth; large notches need to be viewed from different angles than small notches to measure through the deepest part of the notch. To illustrate this, damage sites on titanium alloy specimens were measured by viewing at a range of angles. Figure G.17 shows the path that the impactor takes in relation to the viewing angles and shows the apparent depth at these angles; the zero degree datum is perpendicular to the engine centerline. It can be seen that the small notches appear largest when viewed at an angle of between −10 and 0 from the datum. This indicates that the notch is in almost the same direction as the incident projectile. However, as the damage gets larger, the angle of the notch changes, reflecting the fact that the projectile has been deflected through a larger angle. The conclusion to be drawn from this graph is that there is no single viewing angle that would give a representative depth for all damage sizes. Indeed, it may be that trying to take a measurement of the silhouette is too simple a method to accurately characterize the notch. It has been suggested that measurements of damage should be made on both sides of the blade. This would enable more consistent measurements for the occasions where material has scabbed off the back. 570 Appendix G Projectile path 0° +ve –ve (a) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 –40 –30 –20 –10 0 10 20 40 50 60 Angle Depth (mm) 30 (b) Figure G.17. Path of projectile and viewing angles. EXPERIMENTAL FOD SIMULATION The FOD problem is extremely complex due to a large number of factors including the random nature of ingested foreign objects, the complex geometries of turbine engine components and the complicated stress states in gas turbine airfoils, particularly with respect to vibratory loading. The typical blade in a large gas turbine engine is a complex airfoil with variable camber and twist. The stresses at the leading edge are the result of complex loads and moments that vary along the length of the blade due to inertial forces, pressure loads and, geometry variations. Centrifugal and gas loads are the dominant LCF loads that control the mean stresses, while vibratory loads produce the HCF alternating stresses. The mean stresses are significantly larger than the alternating stresses in the root and mid-section regions, whereas the tip regions may contain relatively higher vibratory stresses and lower mean stresses. Therefore, the stress ratio, R, may range from R =08 Appendix G 571 (tension–tension) to R =−(fully reversed tension-compression or compression only) at various regions throughout the blade. The accurate simulation of FOD on blades and vanes is also very difficult due to compromises between effectiveness and cost of data. The type and quality of information that is required for analysis, as described in the following sections, determine this cost. Impact simulation Two concerns must be addressed in order to simulate FOD events as accurately as possible. These are specimen design and the process of inducing the FOD damage. During the development of the USAF’s HCF program, six different methods for imparting damage were identified. These are: • notch machining • shear chisel application • quasi-static indentation • solenoid gun usage • light gas gun impact • engine debris ingestion. These methods are nominally listed in perceived order of increasing difficultly. Notch machining One of the most common methods for simulating damage is simply machining a notch into the specimen. The machining process typically results in a notch with a very controlled geometry, such as that shown in Figure G.18. Repeatability, control, and low cost are the primary benefits of this process. However, machining a notch does not produce damage that is representative of FOD. The difference is primarily caused by differences in residual stresses between the machining process and the impact event. Secondary differences include a lack of changes in the microstructure of the material and a lack of impact-induced cracking. Therefore, if a study of notch geometry variance or comparison of the relative merits of different blade designs is desired, machining can be used, provided that the above limitations are taken into account in the final analysis. Studies performed by the USAF indicate that there is little correlation between notch sizes and FOD sizes. Therefore, blade geometry cannot be evaluated by simply machining a notch that is bigger or smaller than an FOD notch size by some factor. Finally, machined notches offer no insight into the impact resistance of a given blade design. 572 Appendix G Figure G.18. Micrograph of a machined notch in a simulated airfoil. Shear chisel, quasi-static impact and solenoid gun Shear chisel, quasi-static impact and solenoid gun impact methods all have similar benefits and drawbacks to each other. Imparting damage via a shear chisel involves attaching an impactor to a pendulum, raising the arm to a predetermined height, and letting gravity accelerate the impactor until it contacts the specimen. A solenoid gun uses a similar method except that an electric coil is used to accelerate the impactor which is attached to the end of a rod. The impactor can be accelerated with a given energy or pushed into the specimen until the notch reaches a predetermined depth. A quasi-static impact typically involves placing the specimen in front of the impactor and driving the impactor into a predetermined depth using hydraulic, mechanical, or both electrical actuation. The speed of the impact in this case is much smaller than both the solenoid gun and the shear chisel. In each of these methods, the shape of the impactor can be selected to deliver a notch shape of a given configuration. The energy or depth of the impact can be accurately controlled and the location of the impact is known beforehand. Finally, once machining has been set up to handle a specimen of a given geometry, several specimens can be damaged in quick succession, resulting in an affordable process. Unlike machining a notch, significant residual stresses can be imparted with any of these three methods. Additionally, material removal can be caused by the dynamics of the impact. These characteristics are much more like actual FOD than the notch produced by machining. An example of a notch caused by solenoid gun impact is shown in Figure G.19. Appendix G 573 Figure G.19. Indentation from solenoid gun. Leading edge radius = 0005in013 mm, indentor radius = 0005 in013mm, impact angle 30 , high damage level. The solenoid gun has good control and repeatability although it is not as good as notch machining. Since the process can be used to generate damage in a large number of specimens very rapidly, it has the additional benefits of quick turnaround time and low cost. The combination of good control, low cost and rapid turnarounds make the solenoid gun ideal for generating large amounts of data. However, like a machined notch, the damage produced by solenoid gun is not an accurate simulation of ballistic FOD damage. The solenoid gun produces some residual stress due to impact, but the amount of stress and cold work are much different than those created by typical FOD. Despite these shortcomings, the solenoid gun can be used to impart a given amount of energy into a blade and can, therefore, be used to evaluate the relative benefits of different blade designs. Light gas gun Ballistic impact from a light gas gun is the most accurate laboratory method for simulating the damage caused to an engine blade from foreign object ingestion. A standard light gas gun uses a compressed gas to accelerate the projectile to a speed that replicates the assumed speed of foreign objects in the gas path. Varying the pressure of the compressed gas regulates the velocity of the projectile. The caliber of the projectile launched by a light gas gun is not governed by the caliber of the gun barrel. Instead, most gas guns use a sabot to hold the projectile. This sabot allows the use of different projectile geometries, 574 Appendix G such as spheres and cubes, and materials, such as glass or steel. In addition, the seating of the impactor in the sabot in a keyed barrel can be used to orient the impact event, such as when using a cube impacting along its side. The velocity of the impact is easily determined experimentally using photodiodes or other optical instrumentation close to the target end of the barrel. The target is held in a vice, which can be rotated to achieve the desired orientation. The repeatability of gas gun shots is illustrated in Figures G.20 and G.21, with quantified results in Tables G.1 and G.2. The targets used were mild steel with a cross section that tapered towards each edge, and the projectiles were hardened steel cubes. The targets were oriented such that the path of the projectile was at 135 to the front face of the target. Two types of shots are illustrated; one type has the cube oriented point first, while Figure G.20. Level 1 repeat shots. Figure G.21. Level 3 repeat shots. Table G.1. Level 1 summary of shots Shot Velocity (m/s) Damage depth (mm) 1 189 0.98 2 186 0.60 3 189 0.69 4 186 0.79 5 190 0.69 Appendix G 575 Table G.2. Level 2 summary of shots Shot Velocity (m/s) Distance from edge to (mm) Start of damage Deepest point Furthest damage 1 187 020 063 393 2 190 040 094 418 3 186 022 072 370 4 187 046 100 396 5 185 060 122 438 the other is oriented edge first. Five impacts were done at each damage level and the size of each measured. The use of cubes has been found to be one of the better ways of reproducing damage that is typical of that observed in the field for certain engines and operating conditions. For this reason, extensive FOD simulations have been carried out with a cube projectile to produce different levels of damage. A 3-mm-size cube has been found to be the one that produces typical damage. Damage Level 1 This type of damage nominally produces a triangular notch 0.75 mm deep by firing the cube corner first. Figure G.20 shows the target with the five shots. The details of the five shots are summarized in Table G.1. The damage depth was measured to the base of the notch silhouette with the specimen in a horizontal position. Damage level 2 As can be seen from Figure G.21, edge-on impact creates a dent that is entirely on the surface of the target. The cube is fired with an edge facing forward, but because the target is positioned at an angle, the cube hits point first and rotates to impact along its edge. This is why one end of the dent appears deeper than the other. Figure G.21 shows the five shots performed at this damage level. Figure G.22 shows the front of an impact and highlights the details of damage summarized in Table G.2. Light gas guns require a significant amount of expertise to obtain repeatability and control of the damage. There may be enough variation in nominally identical damage sites to affect post-damage mechanical properties significantly. This is illustrated by the data in Tables G.1 and G.2. In some of these shots, the location of damage may vary by up to 0.6 mm, despite the careful controls on test parameters. A micrograph of a typical light gas gun impact site on a simulated airfoil is shown in Figure G.23. The accuracy of the light gas gun impact method has been confirmed using extensive material analysis. Five impact sites in a Ti-6Al-4V leading edge test sample were met- allographically investigated. Various diameter steel balls shot against the leading edge at . quasi-static impact and solenoid gun impact methods all have similar benefits and drawbacks to each other. Imparting damage via a shear chisel involves attaching an impactor to a pendulum, raising. an impact and highlights the details of damage summarized in Table G.2. Light gas guns require a significant amount of expertise to obtain repeatability and control of the damage. There may be. Additionally, material removal can be caused by the dynamics of the impact. These characteristics are much more like actual FOD than the notch produced by machining. An example of a notch caused