Strain Gauges Example: Bone has a Young’s modulus E of about 100,000 psi. 10 psi of stress will stretch a 20 inch femur Compare to F = K*Δx … Hooke’s Law.

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Presentation transcript:

Strain Gauges Example: Bone has a Young’s modulus E of about 100,000 psi. 10 psi of stress will stretch a 20 inch femur Compare to F = K*Δx … Hooke’s Law

stress-strain curve: stiffness, yield strength, fracture

Consider incompressible material (constant volume…) Transverse Strain and Poisson’s ratio

What to expect on the strain gauge quiz: Given stress, Young’s modulus, gain, Vex, Poisson’s ratio, calculate Vout…

Another deformation: torsional twist

What about the piezoresistivity term? It is normally not zero. In fact for some semiconductors it dominates the expression on the right of the equation. For metals, with a Poisson ratio around 0.3, nonzero piezoresistivity of 0.4*ε basically results in approx: In the Strain Gage Quiz the gauge factor will be 2.

from from

Unknown material properties vs strain gauge material properties: Gauge factor: of the strain gauge Poisson’s ratio: of the unknown strain: of the unknown Young’s modulus = Ym… Young’s modulus: of the unknown > Ym of SG Next consider a Wheatstone bridge made with 4 strain gauges… Two voltage dividers to consider:

That is the term to finish the expression for Vout = G*Vex*ΔR/4R for one longitudinal strain gauge in a "positive" location in the bridge.

Sign of influence on Vout given SG placement: Table reads: if R1 is the LG strain gauge then how does Vout change? *dR/4R …

SG web links: Week06Lecture1-StrainMeasurement-Introduction.pdfhttp://me.queensu.ca/courses/MECH215/documents/MECH215- Week06Lecture1-StrainMeasurement-Introduction.pdf Methods/NI_Strain_Gauge_tutorial.pdfhttp:// Methods/NI_Strain_Gauge_tutorial.pdf

FINAL REPORT TO THE NATIONAL OPERATING COMMITTEE ON STANDARDS FOR ATHLETIC EQUIPMENT (NOCSAE) 1999 “THE USE OF SHOE ORTHOTICS TO REDUCE BONE STRAIN RATE”