A rotor tri-hub burst containment test is an FAA/EASA certification requirement for all aerospace equipment with high speed rotors, such as APU rotors, air cycle machine rotors, engine starter rotors, etc. A photo of an APU compressor rotor obtained from the internet is shown in Figure 5.9. The requirement is that as a result of a containment test, no debris with high kinetic energy shall be allowed outside of the system housing.
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In the disk burst containment test, the rotor is machined with three radial slots 120° apart. The tri-hub fashion is determined based on the maximum translational kinetic energy for the rotor. The radial slots are not completely machined through the rotor, but, instead, with the remaining material they act like “fuses” to break at a designated burst speed. This burst speed should be higher than the maximum operating speed. The exact EASA containment requirements are:
Equipment with high-energy rotors shall be such as to meet one of the following: (1) Failures will not result in significant non-containment of high energy debris, or
(2) An acceptable level of integrity of the design, including the high energy parts, has been established, or
(3) An appropriate combination of (1) and (2).
These requirements are applicable to the following applications: 1. Blade containment only.
2. Tri-hub burst within the normal operating speed (i.e., at the highest permitted speed without failure of the system, but including maximum governor over-swing).
3. Tri-hub burst at the maximum “no load” speed, under all fault or combination of fault conditions (including those affecting fluid supply) other than extremely remote fault conditions.
4. Engine-driven case if more critical than 3. Hub burst containment at the maximum driven speed or the maximum burst speed, whichever is the lesser.
Because containment tests are expensive, containment simulations can be used to predict test results successfully. In addition, simulations are very efficient for design iteration and cost/weight optimization. While certification by analysis only is not currently accepted, simulations can be used for certification of a successful test with minor variations.
Rotor Model
In most cases, the current containment simulations are limited to Lagrangian models. In these analyses, the “fuses” may not be modeled. Instead, initial conditions for rotation may be applied to the three separate rotor pieces. In some cases, “fuse” may be modeled using lower strength material to connect two out of three rotor pieces to simulate the “biased” break-out event.
The rotor hub may be modeled as rigid or deformable bodies using shell or solid elements, depending on the application. If the hub is modeled as rigid bodies, the rim of the hub needs to be deformable in order to simulate the actual damage due to the
high speed impact. If deformable bodies are modeled,
*MAT_PLASTICITY_COMPRESSION_TENSION may be used to avoid excessive element eroding due to the impact induced compressive stress, which will unrealistically reduce the mass of the rotor.
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The rotor blades may be modeled using either shell elements (thin shell or thick shell) or solid elements. If 8-noded brick elements are used, a minimum three elements through the blade thickness should be used.
Stationary Structures Model
The stationary structures may be modeled using solid, shell, beam, or rigid elements as appropriate. The modeled components should be extended beyond the direct impact region to maximize the accuracy. To include the temperature gradient effect, one component may be modeled with several parts with each part assigned specific material properties corresponding to the designated temperature.
Mesh density can significantly affect the results, especially in the case of low mass, high speed debris hitting a target, which results in a ballistic impact. Therefore, the mesh density may have to be associated with specific material properties based on the analysis/test correlation.
Material Models
Material strain rate effect should be included in the material model, if the data are available. The material model input stress and strain should be true stress and strain, which can be converted from engineering stress and strain. The preferred material models are *MAT_PIECEWISE_LINEAR_PLASTICITY, *MAT_JOHNSON_COOK, and *MAT_STRAIN_RATE_DEPENDENT_PLASTICITY.
Contact
Both automatic contact and eroding contact can be used for the containment simulation. Single surface contact where all components are included in a single set for contact checking, should be avoided, because the friction coefficients can be very different for the various contact surfaces.
Initial and Boundary Conditions
Stress initialization to simulate the rotor under centrifugal force may be needed, depending on the application. Section 5.1 of this document covers stress initialization for spinning bodies and should be referenced as needed.
Initial conditions for rotation are applied to the rotor model only using *INITIAL_VELOCITY_GENERATION. For the other rotating components, rotational condition may be applied with *BOUNDARY_PRESCRIBED_MOTION.
Boundary conditions should be applied to be consistent with the test set-up. Running the Analysis
Mass scaling may be used to speed up the analysis, but the locations where the mass is added should be examined. Attention should be paid to the curves for kinetic energy, hourglass energy, and contact sliding energy.
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