The Impact Load on Containment Rings During a Multiple Blade Shed in Aircraft Gas Turbine Engines

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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York. N.Y GT-163 The Society shall not be responsible for statements or opinions advanced in papers or in discussior, at meetings of the Society or of its Divisions or Sections, or printed in its publications, m Discussion is printed only if the paper is published in an ASME Journal. Papers are available,lc a from ASME for fifteen months after the meeting. Printed in USA. Copyright 1991 by ASME The Impact Load on Containment Rings During a Multiple Blade Shed in Aircraft Gas Turbine Engines T. B. DEWHURST Department of Mechanical Engineering University of Maine Orono, Maine Abstract In the event of a multiple blade shed in an aircraft gas turbine engine, the impact a load upon the containment ring is critical. A 1/2(a + oultl complete understanding of this load is p density necessary to design optimal rings, which are strong enough to contain blade fragments v plastic wave speed without incurring excess weight penalties. In this study a description of a multiple blade t time shed and subsequent ring failure is given. A finite element analysis of actual engine t ring thickness experiments is then used to determine the impact load on containment rings during a vi blade impact velocity multiple blade shed. The situation modeled here is one where the initial blade fragments ve blade exit velocity are contained but subsequent blade failures create large hoop stresses that result in d plastic wave travel brittle tensile failures in the ring. D ring diameter Nomenclature N number of failed blades KE Kinetic Energy M mass matrix m bblade mass u displacement vector v bblade velocity C damping matrix I bblade moment of inertia K stiffness matrix bblade rotational velocity F applied force vector SE Strain Energy a stress Introduction E strain Objectives The objective of this paper is two fold: a yield strength the first objective is to describe the containment ring response to a multiple blade oult ultimate strength shed: the second objective is to introduce an approach currently being used to characterize o fstrain at fracture the ring response using finite element analysis. V volume of plastic material Presented at the International Gas Turbine and Aeroengine Congress and Exposition Orlando, FL June 3-6, 1991

2 While published statistics show that blade failure is very common, literature describing containment ring response is scarce. Therefore, this paper reviews what information exists in the literature describing blade failure scenarios, but concentrates on the author's observations of both field and experimental data. This leads to a description of containment ring failure modes due to a multiple blade shed. Throughout, comparisons are made to work involving burst disk containment, with similarities and differences noted. Analysis of containment ring response to failed blades is absent from the literature. The author draws on personal experience to describe analysis techniques used in the industry, such as energy balance techniques, and then concentrates on an approach currently utilized to analyze containment ring response to a multiple blade shed. In this approach, displacements observed experimentally are imposed on a finite element mesh of the containment ring, and the time dependent reaction forces are calculated. Again, comparisons are made to work done in the field of disk containment. A discussion of the relevance of this study to the aircraft industry is presented, and supporting evidence is noted from the literature. Relevance of Containment Study The containment of gas turbine missile fragments is critical for safety in both land based and aircraft gas turbine engines. In land based turbines (steam or gas) weight considerations are not important and safety may be achieved through the build-up of containment ring thickness. In aircraft gas turbine engines, however, weight is of primary concern and thus the need exists to optimize the containment ring design to be effective in containing missile fragments without incurring an excessive weight penalty. There are two basic types of missile fragments generated in turbines: fragments due to failed blades and fragments due to burst disks, spacers, or seals. In land based turbines, disk fragments are contained through the use of very thick containment rings; since weight considerations are minimal, safety can be ensured. However, the size of the containment ring required to contain a burst disk in an aircraft engine is excessive. Although on the average the disks in aircraft turbines have significantly less mass than land based turbines, their high rotational velocity makes these high energy missiles very difficult to contain. For this reason, containment of burst disks has not been required of engine manufacturers. To ensure safety then, blades are designed to shed at a speed much lower than that required to burst a disk. Recently however, the FAA [1990] has been pursuing the possibility of requiring some disks to be contained on turboshaft engines. While the problems of containing disk fragments and blade fragments are related, they are different enough to require that the containment of blades be considered as a separate problem. Data A substantial amount of data exists that records the occurrence of noncontained failures in aircraft gas turbine fragments due to failure of blades, disks and/or spacers. Reports describing failures on an annual basis are prepared for the FAA by the Naval Air Propulsion Center (NAPC) [e.g. 1987]. These reports track the number of rotor fragment producing failures and categorizes them according to type of fragments produced, engine section, cause of failure, and whether or not the fragments were contained. The Society of Automotive Engineers (SAE) [1977,1987] has produced statistical summary reports of failures over a period of years. These reports indicate that noncontained failures, occurring at a rate of approximately 0.7 failures per million engine hours, are due to escaped disks and rotors about 55% of the time, and to blades about 45% of the time. However, of all rotor failures, which are responsible for almost 3% of engine shutdowns, 60% produced rotor fragments. Of these, 56% were blade fragments, and the remaining 4% were disk, rim, or seal fragments. About 15% of the blade failures were uncontained, while about 50% of the other fragments were uncontained. These results emphasize that blade fragments are generated much more frequently than other types of rotor fragments; however, they are also much more easily contained. This data, along with FAA certification requirements, demonstrates the need to understand blade failure scenarios so that containment devices can be designed without incurring excessive weight penalties. Furthermore, the above reports indicate that turbine blades fail about four times as often as compressor blades. This is undoubtedly due to the harsh environment in the hot section which is also detrimental to the containment devices. In addition to the above mentioned reports which are industry wide in nature, each engine manufacturer maintains a significant amount of data concerning their own engines. A much more accessible form of data for failed rings has resulted from the Rotor Burst Protection Program performed by the NAPC [1968, Testing has been carried out to understand the problem of containing burst rotors. However, this data has tended to concentrate on large fragments due to disks and seals and is not as informative concerning the containment of blade fragments. Characterization of Containment Ring Response and Failure Modes Based on Field and Experimental Data The results of the Rotor Burst Protection Program conducted by NAPC [1968, 1969] have illustrated the failure of containment rings when impacted by burst disk fragments. Hagg

3 and Sankey [1974] have described the process and proposed a technique for evaluating ring integrity during a rotor burst. During impact of rotor fragments, it was noted the rings can fail in one of two ways. In what is termed Stage I failure, the rotor fragments perforate the containment ring by punching out a segment of the ring in shear. In this case, the shear energy in the ring, over the footprint of the fragment, is less than the kinetic energy of the fragment and thus perforation occurs. In the event that the ring is able to withstand this initial impact, then a Stage II failure must be considered. In what is termed a Stage II failure, the residual energy in the rotor fragments pressurizes the ring, creating large tensile (hoop) stresses which subsequently cause the ring to fail. In the case of a multiple blade shed, a similar, yet different, scenario has been reported by Dewhurst [1989]. When a blade failure occurs it may 1) perforate the containment ring, 2) proceed downstream with the airflow, or 3) be contained and remain in the path of the oncoming blades. The initial containment of the failed blade requires that the ring have enough shear energy over the footprint of the blade to prevent a punching type of failure, and enough strain energy over this area, plus that of the extent of the stress wave, to prevent a petalling type of failure. Both of these modes of failure would be classified as Stage I failures; petalling is the more common of the two. In an ideal situation, the failed blade will not penetrate the containment ring, but will quickly pass downstream before it can severely damage other blades on the same rotor. In the event that the failed blade does not pass downstream, but instead remains in the plane of rotation of the rotor, then severe damage (i.e. failure) can occur to a number of blades; this is termed a multiple blade failure. In many instances, once a single blade fails, all the remaining blades on that rotor also fail due to extraneous debris. This results in a "corn cobbed" rotor. Upon release of multiple blades the containment ring must prevent Stage I failure for each impact. Successful prevention of Stage I failure creates the conditions that could lead to Stage II failure, a more dangerous and energy-intensive situation. The mass of fragments produced during a multiple blade shed pressurizes the ring, creating very high tensile loads (hoop stress) within the ring. If the strain energy in the ring is not greater than the combined residual energy of all the blades, then a Stage II (tensile) failure will result ejecting numerous, high energy blade fragments. The author has observed that a large amount of field data, in addition to experimental testing, has demonstrated that in almost all incidents of a multiple blade shed, containment failures are of the Stage II kind. This indicates that the rings are thick enough to prevent Stage I failure but not sufficient to prevent a Stage II failure. A second key observation is that during a multiple blade shed resulting in Stage II failure, the containment ring shows a minimal amount of stretching in the hoop direction. Whereas NAPC and Hagg and Sankey have demonstrated extensive elongation in the hoop direction in the case of disk fragments, no such elongation occurs due to blade fragments. In addition, the failure surfaces on the rings show a brittle, tensile failure, indicating that the hoop stress is the cause of failure, but also indicating that the strain energy in the ring, necessary to overcome the residual kinetic energy of the blades, is much less than expected. Rather than achieving elongations of 10-15% as expected, the ring exhibits almost no elongation and thus dissipates a minimal amount of strain energy. Even in the vicinity of the failures (axial cracks) there is very little localized elongation, indicating that nowhere does the material attain its expected elongation. This phenomenon is not limited to rings with extended field service (high temperature, property reduction) but is also observed in rings which have seen little or no field service. The failure modes observed during a multiple blade shed indicate that, the material possesses very little ductility, which directly affects the amount of strain energy in the material. Calculation of the strain energy in a ring is difficult, for while the amount of elongation may be readily observed, the stress levels required to cause failure at very high strain rates is totally unknown. It is assumed that this level is significantly higher than low strain rate values. The material behavior of the containment ring material is significantly different at high strain rates, and must be fully characterized in order to accurately understand Stage I and Stage II failure scenarios. Numerical Analysis to Determine the Containment Ring Loading Function Due to a Multiple Blade Shed Current Analysis Techniques Manual techniques. It appears that most engine manufacturers analyze containment rings based on an energy balance approach. However, no analysis stands on its own and experimental data is always used for validation. Since data of this type is extremely expensive to obtain, and yet is so critical, two things result: old test results are used to shed light on new designs; and containment rings are rarely redesigned. The "strain energy - kinetic energy" balance is the most common analysis technique used to analyze containment rings. In this analysis, the kinetic energy of the blade is calculated, accounting for the translational 3

4 kinetic energy and sometimes the rotational kinetic energy, Eq. (1). KE = 1/2 m bvb+ ( 1/2 I bwb ) ( 1 ) This energy must be balanced by the strain energy in the deformed material which can be calculated by Eqs. (2) or (3). SE = fv (a e)dv (2) SE = 1/2 (a y + ault ) E f V (3) Here, V is the volume of material being fully plasticized. The strain energy density is the area under the stress-strai.n curve which is often approximated as shown in Eq. (3). Unfortunately, the stress-strain curves used in this calculation are often taken from simple tensile tests performed at very low strain rates. It has already been noted that these properties do not accurately describe the material behavior at the high strain rates occurring in a multiple blade shed. A second critical component or Eq. (3) is the volume, V, of the plasticized material. The volume of the plasticized material is calculated to be the product of the thickness of the ring and the impact area of the blade enhanced by an increase in area due to the plastic wave traveling for the duration of time required for the blade to pass through the thickness of the ring. The plastic wave speed is given by Eq. (4). v p = [(o/e f )/P ] (4) The time required for the blade to pass t.hrough the ring could be estimated by Eqs. (5) or (6). t = t o / [1/2 (v i + ve )] (5) t = t o / v i (6) The second equation is more convenient In the event of a multiple blade failure, the increase in impact area due to the plastic wave, must not be allowed to exceed the total circumference divided by the total number of failed blades, or d p = nd / N ( 7 ) A more sophisticated approach than the energy balance method has been described by Hagg and Sankey for disk fragments. In that approach, a momentum balance is used first, followed by an energy balance. The momentum of the impacting fragments must be balanced by the momentum of an "effective mass" of the ring after impact. After this inelastic collision, the velocity of the two masses is recalculated. The change in kinetic energy of the fragment is calculated and the reduced kinetic energy must be dissipated in Stage II. This energy change during impact (Stage I) must be absorbed by a combination of shear energy and strain energy within the ring. The difficulty in this process, as in the energy balance approach discussed above, is that the effective mass must be calculated. To do this, test data is used to form a somewhat empirical relationship for this mass. Upon successful prevention of a Stage I failure, the residual energy in the blade fragments must be absorbed to prevent a Stage II failure. The energy is absorbed by uniaxial elongation for short rings, and biaxial elongation for long rings. The volume of plasticized material is the overall ring volume divided equally among the number of disk fragments. This momentum approach appears to be more accurate for disk containment problems than the energy balance approach, but like that approach, the applicability of the momentum balance method to blade containment would raise questions concerning the volume of plasticized material. Numerical Analysis. The most comprehensive report on numerical analysis of containment rings has been compiled by EPRI [1984]. This report presents results of four different codes used to model the containment of burst disks in land based steam turbines, similar to those found in nuclear power plants. The four codes investigated were STEALTH (Science Applications), Whams (Northwestern), CIVM (MIT), and HONDO (Sandia). These codes use a variety of numerical techniques, including finite elements and finite differences to model the problem. All the codes performed fairly well, predicting the slowing of the missile fragment and the deformed shape of the structure. However, there was significant disagreement concerning the local strains within the ring. Again, this study also focused on the containment of burst disk fragments and not on the case of a multiple blade shed. In all cases, the impact of the disk fragments was modeled and significant plastic deformation was observed. In the case of a multiple blade shed, the modeling of all the blades, their fragmentation, interference with other blades, and impact upon the containment ring would be impossible to model. Thus, the approach taken in the EPRI study is not applicable to the problem at hand. A different approach is required. Present Work The major objective of this current study is to accurately predict the time dependent loading function exerted by the blades onto the containment ring during a multiple blade shed. It is believed that an accurate understanding of how the blades load the containment ring is necessary to determine the stresses and strains that result internal to the ring: a necessary forerunner to predicting ring success at containing the blades. In this study, photographed results from a typical multiple blade shed scenario are used as input data to a finite element code to predict the loading function. These 4

5 experimental results are for a medium sized gas turbine engine and the ring modeled is characteristic of the hot section; in particular, a stage of the power turbine in a turbofan engine. The ring is composed of a nickel based alloy, is approximately 50 centimeters in diameter, and ranges in thickness from 0.3 cm to 0.6 cm. In the scenario considered here, all the blades are shed at a speed of about 20,000 rpm. As the experiment progresses from a single blade shed to the eventual failure of all the blades, the ring begins to bulge near the site where the first blade impacts the ring. As other blades impact the first blade, they too fail and impinge upon the containment ring. As the number of failed blades continues to grow, the ring continues to bulge. The magnitude of this bulge grows in time and the location of the bulge travels in the same angular direction as the rotor, albeit not as quickly. A plot of the growth of the ring bulge in time, and the angular location of the bulge peak is shown in Figure by applying the maximum bulge on the one node directly above the blade at the radial location where the bulge is a maximum. The 2.5 msec time for the incident to be completed is divided into small time steps which are varied to ensure that the maximum bulge displacement coincides with the radial location of a node. This displacement is held for one time step and is then released. In the next step, a displacement is imposed on an adjacent node. This process is repeated throughout the 2.5 msec time period. The finite element mesh is shown in Figure 2. For this study, noded shell elements were used with 6 degrees of freedom per node. The mesh used is the largest possible mesh for the version of ANSYS available for this study. The nodes are numbered axially, and six elements are used in the axial direction to meet the bandwidth restrictions of the program. A maximum aspect ratio of 1:4 was used for the elements to provide reasonable accuracy without incurring excessive computational costs. The appropriate ring thicknesses were used for each node. I, Thre (magic) Figure 1. Ring bulge growth and angular displacement as a function of time. To model the incident, the finite element code ANSYS [1989] was used. ANSYS was chosen because of its capabilities to perform a nonlinear, transient dynamic analysis, including material and geometric nonlinearities which will eventually be needed in the analysis. The governing equation for the transient, dynamic solution is Mu + Cu + ku = F (8) A consistent mass matrix is used with minimal damping. In the case studied here, the displacement vector u is generally known as a function of time (see Figure 1) and the loading function in time, F, is to be solved for. To accomplish this, the bulge magnitude and angular displacement as a function of time is imposed on the ring. This is accomplished Figure 2. Finite element mesh, 8-noded isoparametric elements. At this preliminary stage of the analysis, linear, "low strain rate" material properties for the nickel alloy were used. It has already been emphasized that this is inappropriate for an accurate analysis, but at first it is desired to demonstrate the feasibility of the approach before introducing high strain rate, nonlinear material properties. Results The most significant problem in performing the analysis has been the tendency 5

6 of the ring to rebound drastically upon release of a displacement boundary condition. In addition, the response has been very nonuniform in its shape. To overcome these problems has required a combined approach of reducing the time step and refining the mesh. The time steps have been reducgd such that time steps on the order of 5x10 s are being used. Even with this small time step and reasonably refined mesh, the resultant ring shapes tend to be less smooth than desired. With this time step and mesh, applying the ring displacements for the first 0.5 msec requires approximately 40 minutes of CPU time on an IBM Of key interest are the reaction forces exerted by the ring as it resists the time dependent deformation being imposed upon it during the analysis. These reaction forces are an indication of the forces required to deform the ring as seen in experimental testing, and therefore are indicative of the forces exerted by the blades on the ring. A preliminary plot of the reaction forces through the first 0.5 msec is plotted in Figure 3 and may be compared to Figure 1 to relate the reaction forces to the bulge displacement and the bulge angular position. A plot of the ring displacements at 0.34 msec is shown in Figure 4, and a plot of the effective stresses at this time is shown in Figure 5. Figure 4. Ring Displacement at t = 0.34 msec showing ring deflection due to impact, and recovery in region initially impacted. it Impact ite o = 1.14 GPa e = 6.42 GPa 4 N N U 0 LL O a) d Q 10 a = 4.16 GPa e a = 1.89 GPa e Time (msec) Figure 3. The reaction forces on the ring due to imposed displacements. 5 Figure 5. Effective stresses in the ring at t = 0.34 msec. Discussion of Results The actual forces plotted in Figure 3 appear to be unrealistically high. The initial momentum of all the blades is on the order of 3529 N-s. For the blades to be totally stopped in a time of 3.0 msec requires an impulse equal to the initial momentum. This would indicate that an average force of 1.18 x 10 N is needed. This would indicate that the early results for the first 0.3 msec are appropriate, but that later results are not. This drastic increase in the reaction forces is indicative of other problems at this time in the solution. It appears that a few dominant modes are excited resulting in 6

7 unrealistic displacements in the mesh; the ring appears to be trying to respond from the initial enforced displacement. Further analysis is thus prevented. In the past, this behavior has been controlled by a refining of the mesh, and time step reduction; due to the current ANSYS configuration, further refinement is not possible necessitating a reduction in the time step. Thus, significant computational costs will be incurred to further the solution. Other problems are also evident in the approach presented here and must be addressed if a full solution is to be obtained. The applied displacements have only been enforced at one node per time step. It would be desirable to apply a distributed load over a number of adjacent nodes; however, attempts to do this have resulted in the blades "pulling" on the ring which is not physically possible. Further efforts in this area will require iterative approaches to applying the displacements, or the use of a contact/ release algorithm. Conclusions and Future Work Observations of field and experimental data has shown that containment rings most often fail in brittle tensile failure due to large hoop stresses generated by a multiple blade shed. This observation is contrary to what has been reported in burst disk scenarios where significant ductility of the ring has been seen. The lack of literature describing multiple blade shed scenarios has been noted. A second result of the work done to date is that a procedure has been established to determine the time dependent loading function exerted by blade fragments on the containment ring during a multiple blade shed. While the procedure appears to be simple in concept, much difficulty has been encountered in its implementation. The procedure needs to be improved to include more accurate material property data; time step optimization and mesh refinement will continue to be addressed as more accuracy is desired. Furthermore, application of the imposed displacements will be investigated, as will an appropriate plasticity flow law to further improve the accuracy of the results. Acknowledgements The author would like to thank the Faculty Research Funds Committee of the University of Maine for supporting this research. References "ANSYS - Engineering Analysis System," Swanson Analysis, Inc., Houston, PA, May Dewhurst, T.B., "Failure Modes of Gas Turbine Containment Rings," Developments in Mechanics, vol. 15, Proceedings of the 21st Midwestern Mechanics Conference, Houghton, MI, August, EPRI, "Assessment of Turbine-Casing Impact Code Calculations," EPRI N-2744, May FAA, "Background Data and Proposed Advisory Circular, A Development Plan to Support Turboshaft Engine Rotor Burst Protection," April Hagg, A.C., and Sankey, G.O., "The Containment of Disk Burst Fragments by Cylindrical Shells," ASME Journal of Engineering for Power, November NAPC, "Rotor Burst Protection Program, Initial Test Results," NAPTC-AED-1869, April NAPC, "Rotor Burst Protection Program," NAPTC-AED-1901, MAY NAPC, "Statistics on Aircraft Gas Turbine Engine Rotor Failures that Occurred in U.S. Commercial Aviation During 1981," DOT/FAA/CT- 86/42, March SAE, "Report on Aircraft. Engine Containment," AIR 1537, October SAE, "Report on Aircraft Engine Containment," AIR 4003, September In spite of the problems encountered, the approach described here is still more feasible than the alternate approach of modeling each projectile as it impacts the containment. ring. Although this has met with some success in modeling burst disk phenomena, it would be computationally too trying to be applicable here. Using the ANSYS finite element code has been effective in studying this problem. However, application to the containment problem may be pushing the capabilities of the program in terms of accuracy, efficiency, and CPU time usage. Further work in this area may soon be expanded to include the use of DYNA3D [Lawrence Livermore] as it is more suitable for problems of this nature.