Publications

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Modal testing was performed on the uncut BARC structure as a whole and broken into its two sub-assemblies. The structure was placed on soft foam during the test. Excitation was provided with a small modal hammer attached to an actuator. Responses were measured using a 3D Scanning Laser Doppler Vibrometer. Data, shapes, and geometry from this test can be downloaded in Universal File Format from the Sandia Connect SharePoint site.

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The 3D Scanning Laser Doppler Vibrometer (3D SLDV) has the ability to scan a large number of points with high accuracy compared to traditional roving hammer or accelerometer tests. The 3D SLDV has disadvantages, however, in that it requires line-of-sight from three scanning laser heads to the point being measured. This means that multiple scans can become necessary to measure large or complex parts, and internal components cannot typically be measured. In the past, large aerospace structures tested at Sandia National Laboratories typically have used a handful of accelerometer stations and instrumented internal components to characterize these test articles. This work describes two case studies that explore the advantages and difficulties in using a 3D SLDV to measure the same test articles with a much higher resolution scan of the exterior. This work proposes strategies for combining a large number of accelerometer channels with a high resolution laser scan. It explores the use of mirrors and laser head mounts to enable efficient re-alignment of the lasers with the test article when many scans are necessary, and it discusses the difficulties and pitfalls inherent with performing such a test.

The 3D Scanning Laser Doppler Vibrometer (3D SLDV) has the ability to scan a large number of points with high accuracy compared to traditional roving hammer or accelerometer tests. The 3D SLDV has disadvantages, however, in that it requires line-of-sight from three scanning laser heads to the point being measured. This means that multiple scans can become necessary to measure large or complex parts, and internal components cannot typically be measured. In the past, large aerospace structures tested at Sandia National Laboratories typically have used a handful of accelerometer stations and instrumented internal components to characterize these test articles. This work describes two case studies that explore the advantages and difficulties in using a 3D SLDV to measure the same test articles with a much higher resolution scan of the exterior. This work proposes strategies for combining a large number of accelerometer channels with a high resolution laser scan. It explores the use of mirrors and laser head mounts to enable efficient re-alignment of the lasers with the test article when many scans are necessary, and it discusses the difficulties and pitfalls inherent with performing such a test.

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The Structural Dynamics department at Sandia National Laboratories has acquired a 3D Scanning Laser Doppler Vibrometer system for making vibration and modal test measurements. This paper presents the results of testing performed to examine the capabilities and limitations of that system. The test article under consideration was a conical part with two different surface materials which allowed the examination of the effect of angle of incidence and surface reflectivity on the measurement. The system was operated in both 1D and 3D modes, and the results from the 1D scan were compared to a scan performed with a previous generation system to evaluate the improvements between the generations. Data from the laser systems were exported to standard curve fitting software, and modes were fit to the data.

A previous study in the UK demonstrated that vibration response on a scaled-down model of a missile structure in a wind tunnel could be replicated in a laboratory setting with multiple shakers using an approach dubbed as impedance matching. Here we demonstrate on a full scale industrial structure that the random vibration induced from a laboratory acoustic environment can be nearly replicated at 37 internal accelerometers using six shakers. The voltage input to the shaker amplifiers is calculated using a regularized inverse of the square of the amplitude of the frequency response function matrix and the power spectral density responses of the 37 internal accelerometers. No cross power spectral density responses are utilized. The structure has hundreds of modes and the simulation is performed out to 4000 Hz.

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Sandia National Laboratories has recently purchased a Polytec 3D Scanning Laser Doppler Vibrometer for vibration measurement. This device has proven to be a very nice tool for making vibration measurements, and has a number of advantages over traditional sensors such as accelerometers. The non-contact nature of the laser vibrometer means there is no mass loading due to measuring the response. Additionally, the laser scanning heads can position the laser spot much more quickly and accurately than placing an accelerometer or performing a roving hammer impact. The disadvantage of the system is that a significant amount of time must be invested to align the lasers with each other and the part so that the laser spots can be accurately positioned. The Polytec software includes a number of nice tools to aid in this procedure; however, certain portions are still tedious. Luckily, the Polytec software is readily extensible by programming macros for the system, so tedious portions of the procedure can be made easier by automating the process. The Polytec Software includes a WinWrap (similar to Visual Basic) editor and interface to run macros written in that programming language. The author, however, is much more proficient in Python, and the latter also has a much larger set of libraries that can be used to create very complex macros, while taking advantage of Python’s inherent readability and maintainability.

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In the summer of 2020, the National Aeronautics and Space Administration (NASA) plans to launch a spacecraft as part of the Mars 2020 mission. One option for the rover on the proposed spacecraft uses a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) to provide continuous electrical and thermal power for the mission. NASA has prepared an Environmental Impact Statement (EIS) in accordance with the National Environmental Policy Act. The EIS includes information on the risks of mission accidents to the general public and on-site workers at the launch complex. The Nuclear Risk Assessment (NRA) addresses the responses of the MMRTG option to potential accident and abort conditions during the launch opportunity for the Mars 2020 mission and the associated consequences. This information provides the technical basis for the radiological risks of the MMRTG option for the EIS. This paper provides a summary of the methods and results used in the NRA.

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The transmission simulator method of experimental dynamic substructuring has the capability to couple substructures with continuous connections. A hardware example with continuous connections is presented in which the method is used to couple an experimental substructure with a finite element substructure to predict full system response. The predicted response is compared with frequency response functions measured on the full system hardware. The experimental substructure captures the motion of a component packed in foam. This is coupled to a finite element model of a cylindrical metal case which contains the foam and is attached through a flange to a plate and beam structure. © The Society for Experimental Mechanics, Inc. 2014.

This paper presents an example of the transmission simulator method of experimental dynamic substructuring combining two substructures of the Substructures Focus Group's test bed, the Ampair 600 Wind Turbine. The two substructures of interest are the hub-and-blade assembly and the tower assembly that remains after the hub is removed. The hub-and-blade substructure was developed from elastic modes of a free-free test of the hub and blades, and rigid body modes were constructed from measured mass properties. Elastic and rigid body modes were extracted from experimental data for the tower substructure. A bladeless hub was attached to the tower to serve as the transmission simulator for this substructure. Modes up to the second bending mode of the blades and tower were extracted. Substructuring calculations were then performed using the transmission simulator method, and a model of the full test bed was derived. The combined model was compared to truth data from a test on the full turbine. © The Society for Experimental Mechanics, Inc. 2014.

In previous work, a modal test of a large beam like structure on a vibration slip table was analytically constrained to fixed base providing estimates of the first three bending modes active in the direction of slip table motion. This work extends the frequency band of the method to extract the first ten fixed base modes of the test article. All ten fixed base modal frequencies are within two percent of the truth test fixed base modes. When compared to the truth test, the estimated damping of the lower modes has large error, but at higher frequencies the estimated damping converges on the truth value. © The Society for Experimental Mechanics, Inc. 2014.

In previous work, a modal test of a large beam like structure on a vibration slip table was analytically constrained to fixed base providing estimates of the first three bending modes active in the direction of slip table motion. This work extends the frequency band of the method to extract the first ten fixed base modes of the test article. All ten fixed base modal frequencies are within two percent of the truth test fixed base modes. When compared to the truth test, the estimated damping of the lower modes has large error, but at higher frequencies the estimated damping converges on the truth value. © The Society for Experimental Mechanics, Inc. 2014.

In the summer of 2020, the National Aeronautics and Space Administration (NASA) plans to launch a spacecraft as part of the Mars 2020 mission. One option for the rover on the proposed spacecraft uses a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) to provide continuous electrical and thermal power for the mission. An alternative option being considered is a set of solar panels for electrical power with up to 80 Light-Weight Radioisotope Heater Units (LWRHUs) for local component heating. Both the MMRTG and the LWRHUs use radioactive plutonium dioxide. NASA is preparing an Environmental Impact Statement (EIS) in accordance with the National Environmental Policy Act. The EIS will include information on the risks of mission accidents to the general public and on-site workers at the launch complex. This Nuclear Risk Assessment (NRA) addresses the responses of the MMRTG or LWRHU options to potential accident and abort conditions during the launch opportunity for the Mars 2020 mission and the associated consequences. This information provides the technical basis for the radiological risks of both options for the EIS.

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