Boundary Condition Testing

By J. Lukens

Outline

  • Strain gage theory overview
  • Select strain gage locations
  • Perform controlled static testing
  • Execute operational testing
  • Apply test data to correlate model
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Electronic Resistance Strain Gage Theory

Electrical resistance of a wire is a function of geometry

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Longitudinal strain is a function of change in resistance

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Strain can be found directly by voltage measurements

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Temperature influences strain gage resistance

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Select Strain Gage Locations to Generate High Fidelity Data

  • Install at least one strain gage for each boundary condition
    • Additional strain gages provide redundancy
    • Redundancy protects against gage failure
  • Select locations with large amplitude response
    • Wish to maximize signal to noise ratio
  • Find locations with low strain gradients
    • High strain gradient locations are difficult to correlate
  • Implement appropriate strain gage setup
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Develop Linear Relationships with Static Testing

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  • Install strain gages onto structure
  • Account for environmental effects
    • Apparent strain due to temperature
    • EMF causing current and voltage shifts
  • Apply boundary conditions through static testing
  • Generate Strain Versus Load Plots


Verify Operational Boundary Conditions Through Testing

  • Replicate static test setup on operational structure
  • Install data acquisition system (DAQ) on structure
  • Route wires and instrumentation such that structural behavior is unaffected
  • Account for environmental effects such as EMF and temperature
  • Perform operational testing
  • Examine data and look for ‘events’ of interest
Operational Test Data image


Apply Test Data to Correlate Model

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  • Apply measured loads from operational testing
    • Correlate measured strain gage response to FEM response
    • Adjust material properties, boundary conditions, and theoretical connections as required
  • Set goals for correlation
    • Reality != Theory
    • Correlation goals depend on structure
  • Retest if necessary
    • More gages
    • At least one gage for each load

Summary

  • Select strain gage locations
  • Perform controlled static testing
  • Execute operational testing
  • Apply test data to correlate model

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