|Acquisition Program: ||Joint Strike Fight; PMA-259, Air-to-Air Missile Systems|| Objective: ||Develop a structural dynamics modeling tool which will provide an accurate physics-based solution for predicting non-linear vibration response, and employ the modeling tool for conducting “virtual” vibration testing.
|| Description: ||Because of the complexity and extreme cost associated with "high fidelity simulation" of vibration loads in a laboratory environment, the current practice and goal of the laboratory vibration test is to recreate the effects of the service use vibration environment using electrodynamic shaker systems. Electrodynamic shakers provide input excitation for matching store vibration response measured during captive carriage flight testing.
Vibration excitation resulting from platform captive carriage is transmitted to the weapon through multiple paths and sources; whereas, in a laboratory vibration test, loads are typically induced through a single shaker input. Likewise, the laboratory test boundary conditions and resulting load interface impedances can be significantly different than the “real world” or service use environment. As a result of the differences in load path, boundary conditions, and impedances between flight and the laboratory, input forces generated during test can be much different than those experienced during flight causing unrepresentative failures. Examples where laboratory vibration test loads created unrepresentative failures during all-up-round (AUR) testing include complete failure of forward and aft components and bomb racks for various AUR bomb vibration tests, and lug, hanger, component joint, and launcher failure during HARM, Sparrow, Sidewinder, and JSOW testing. Failures due to insufficient testing have significant impact on design cost and schedule which result in critical delivery impact to the warfighter.
An alternate approach would use a highly accurate dynamic modeling tool to analyze the laboratory test configuration for comparison with the “real world” store / aircraft interface, and allow for “tuning” of the laboratory test configuration to achieve test loads that more accurately represent the service use environment. Tuning of the laboratory test would include test fixture design that more accurately represents the service use store/aircraft interface along with accurate estimates for optimum location of the shaker input forces. Upon completion of the modeling, a "virtual" laboratory vibration test could be conducted which would assess the test configuration and resulting failure modes prior to conducting the actual test. Eventual validation of the virtual test model could then be used to forgo future laboratory vibration testing to qualify airframe or other system modifications which may occur as the weapon system matures.
The current practice of using finite element analysis (FEA) for modeling and predicting vibration response of complex, non-linear structural systems does not provide the necessary accuracy at frequencies much beyond the first few structural modes of the weapon system. Because commercially available FEA tools utilize linear elastic theory only, FEA can not accurately predict vibration response due to inherent nonlinearities associated with either the aircraft/store interface or the laboratory shaker system interface.
In order to exercise the linear-elastic FEA models to output results for use with non-linear vibration problems, the FEA model is typically adjusted by a process which introduces non-realistic structural properties to achieve dynamic response equivalent to output derived experimentally for a unique set of boundary conditions only. Thus, the development of a dynamic modeling tool which combines the ability of linear elastic theory and non-linear problem solving algorithms would provide a robust physics-based solution to process virtual vibration test models, rather than the "trial-and-error" methodology currently in practice which relies entirely on experimental data for each unique structural non-linearity and associated dynamic environment.
|| ||PHASE I: Develop a concept for an accurate non-linear structural dynamics model for a simple non-linear store / aircraft configurations e.g., store hanger and rail.
|| ||PHASE II: Develop and demonstrate an accurate non-linear structural dynamics model for a typical store/platform configuration and apply the information to design an accurate non-linear structural dynamics model for a typical store/shaker interface configuration. Verify results output by the non-linear store/shaker interface structural dynamics model by conducting vibration testing on representative store/platform configuration hardware using various random vibration input levels and spectra.
|| ||PHASE III: Produce a validated virtual vibration test system based on the non-linear structural dynamics modeling tool developed in Phases I and II.
|| ||PRIVATE SECTOR COMMERCIAL POTENTIAL: The structural dynamics design industry e.g., those involved in manufacture of automobiles, heavy equipment, buildings, bridges, space vehicles, weapons, recreational vehicles and accessories, etc. will benefit through extension of their technology base.
|| References: ||1. "Harris'' Shock and Vibration Handbook", 5th Edition, Cyril M.Harris and Allan G. Piersol editors, McGraw-Hill, New York, 2002
2. MIL-STD-810G, "Environmental Engineering Considerations and Laboratory Tests", 31 October 2008
|Keywords: ||vibration; structural dynamics; modeling; non-linear; virtual testing; electrodynamic shaker|
Questions and Answers:
Q: At the request of the Navy, a JASON report (JSR-07-200), Navy Ship Underwater Shock Prediction and Testing Capability Study, thoroughly examined the role of state-of-the-art Modeling and Simulation (M&S) in qualifying military systems to survive harsh operational loads (shock and vibration environments) and for potentially replacing live fire testing requirements. The salient fluid-structure interaction M&S conclusions and T&E recommendations of this report are applicable to this Navy topic.
-- So would it be fair to say that it is a serious oversight to not use this JASON study as a starting point for developing Phase I objectives?
-- Specifically, since the vibration excitations of air to air missile systems during operation are mainly due to relatively high Mach number aerodynamics and launching pyroshock, it is not explained by this topic how a structural FEA-only modeling tool that does not account for time-dependent fluid dynamic parameters, flight attitude/path variables, aeroelasticity responses to external flow fields, etc., is expected to be very accurate predicting dynamic responses of an aircraft weapon store configuration.
-- Is the modeling concept described in this topic required to function accurately without consideration of ‘physics-based’ Navier-Stokes equations of motion?
-- If not, wouldn’t this software development effort duplicate already commercially available coupled Eulerian Lagrangian code?
-- Fine tuning lab testing fixtures has already been done for underwater shock qualifications but the cost effectiveness of this high fidelity test simulation approach seems questionable for aircraft environments unless a capability is available as well to recreate in the lab every conceivable high speed flight condition. Are the tests proposed in Phase II conducted under zero Mach number conditions?
-- Does the concept and software development remain the intellectual property of the developer?
A: With regard to the applicability of the JASON report (JSR-07-200), Navy Ship Underwater Shock Prediction and Testing Capability Study, I believe the information could be beneficial; however, the underwater shock environment likely has unique character with regard to excitation/loading and associated structural response which may not be applicable to the aircraft store captive carry vibration environment.
The desired application for the tool to be derived from the Virtual Vibration Testing Of External Stores SBIR is not for predicting dynamic responses of an aircraft weapon store configuration, but rather for deriving an analytical tool which will aid in the development of a laboratory test configuration for accurately simulating the effects of captive carry vibration. External store vibration response is measured empirically, translated into test criteria, and then a laboratory vibration test is conducted to verify structural integrity and operational integrity of the store for a given captive carry vibration environment. The dynamic modeling tool would analyze the laboratory test configuration for comparison with the "real world" store / aircraft interface, and allow for "tuning" of the laboratory test configuration to achieve test loads that more accurately represent the service use environment. Tuning of the laboratory test would include test fixture design that more accurately represents the service use store/aircraft interface along with accurate estimates for optimum location of the shaker input forces. The dynamic modeling tool would then be applied to a "virtual" laboratory vibration test for assessing the test configuration and resulting failure modes prior to conducting the actual test, and make necessary adjustments to the test configuration depending upon the results of the virtual test.
The modeling concept described in this topic is not expected to include Navier-Stokes equations of motion or Eulerian Lagrangian code.
The tests proposed in phase II will be conducted within a vibration laboratory using electrodynamic shakers to generate vibration excitation. The various random vibration input levels and spectra referred to in the phase II description would be provided by the Government for the representative store/platform configuration.
Yes, the concept and software development remain the intellectual property of the developer.
Q: 1. Are you asking for a methodology for modeling the Non-Linearity of the Store/Platform interface? Commercial Finite Element Software such as Nastran will use a Linear Elastic solution for Eigenvalue/Eigenvector extraction. If you calculate 200 modes, the first 10% (say 20 modes) will be accurate.
2. Are you proposing a Non-Linear Eigenvalue extraction? Direct integration of the Non-Linear equations of motion would require a Transient/Time History Solution. Or the formulation of the equations of motion in State Space Variable Format.
3. Are you proposing a Methodology or the actual writing of an Algorithm in either Fortran or C or C++ language?
A: 1. Yes.
2. A Non-Linear Eigenvalue extraction could be part of the solution. Ultimately, we are looking for a new concept to derive an accurate physics-based non-linear structural dynamics modeling technique which will eventually be used as the basis for a virtual vibration test system.
3. Demonstration of the methodology would like require software development.