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

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

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

DEVELOP LINEAR RELATIONSHIPS WITH STATIC TESTING

• 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

APPLY TEST DATA TO CORRELATE MODEL

• 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