Boundary Condition Testing

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

Longitudinal strain is a function of change in resistance

Strain can be found directly by voltage measurements

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

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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