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MODAL TESTING


Understand Modal Frequencies

Modal testing allows engineers to understand the dynamic behavior of large structures by physically testing and measuring fundamental mode shapes and frequencies. Quartus uses industry leading software and hardware for all phases of modal testing during your Ground Vibration Test (GVT). Our engineers perform pretest analysis, create modal survey test plans, supply all test equipment for data acquisition, data reduction for mode shape extraction, and correlate the finite element model (FEM) to the measured modal frequencies.

Maximize Modal Testing Results

Quartus Engineering uses industry leading LMS hardware and software to execute modal testing data acquisition activities. Quartus utilizes multiple input random (MPR), step sine, sine sweep, and hammer excitation to excite the structure’s modal frequencies of interest and generate frequency response functions (FRF). During the modal test Quartus engineers perform preliminary extraction of modal results using LMS PolyMAX to calculate orthogonality and cross-orthogonality in NASTRAN to verify data quality. Prior to test completion, calculated modal parameters will be used to synthesize measured FRFs and generate reciprocal mode shapes and modal participation factors to validate finite element model assumptions.

Mode shape extraction methods include:

  • LMS PolyMAX
  • Least square complex exponential (LSCE)
  • Lease square frequency domain (LSFD)

Mode shape validation methods include:

  • Modal assurance criterion (MAC)
  • Modal phase collinearity (MPC)
  • Mean phase deviation (MPD)
  • Mode over complexity (MOV)

Accelerometer placement methods include:

  • Iterated Guyan reduction (IGR)
  • Iterated kinetic energy (IKE)
  • Effective independence (EI)
  • Mass-weighted effective independence (MWEI)
  • Triaxial effective independence (TEI)
  • Genetic algorithms (GA)

Pretest Analysis Delivers Efficient Modal Testing

In order to maximize the results from your modal test, pretest analysis in NASTRAN is performed to optimize the accuracy of the test analysis model (TAM). Results from the pretest analysis include predicted test modes, frequency response functions, optimum accelerometer locations for modal testing measurements, recommended exciter locations, TAM mass matrices, and expansion matrices for measured test results display.

Correlate Theory with Modal Testing Reality

Quartus performs model correlation and updating to achieve good agreement between analysis results and modal test data. Both qualitative (mode shape appearance) and quantitative (cross-orthogonality, modal assurance criteria, etc…) techniques are used to assess correlation accuracy. NASTRAN design sensitivity methods are used to accelerate the correlation process by identifying optimum changes that minimize test-analysis errors.


Experience on Many Tests

Quartus has extensive experience in performing pretest analysis, modal testing, and model correlation. Some of the structures our engineers have worked on include:

  • Gulfstream G-V aircraft
  • Dawn Spacecraft
  • Indostar Payload
  • Space Shuttle SRB Segment Test
  • OSC Pegasus XL - Numerous Configurations
  • SpaceShipOne