1. 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

2. stress-strain curve: stiffness, yield strength, fracture www.tpub.com/doematerialsci/materialscience22.htm

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

5. What to expect on the strain gauge quiz: www.solware.co.uk/blog Given stress, Young’s modulus, gain, Vex, Poisson’s ratio, calculate Vout…

6. Another deformation: torsional twist http://home.gna.org/getfem/shots.html

9. 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.

10. from www.sensorland.com from www.natmus.dk

12. 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:

13. 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.

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

15. SG web links: http://me.queensu.ca/courses/MECH215/documents/MECH215- Week06Lecture1-StrainMeasurement-Introduction.pdfhttp://me.queensu.ca/courses/MECH215/documents/MECH215- Week06Lecture1-StrainMeasurement-Introduction.pdf http://www.eidactics.com/Downloads/Refs- Methods/NI_Strain_Gauge_tutorial.pdfhttp://www.eidactics.com/Downloads/Refs- Methods/NI_Strain_Gauge_tutorial.pdf http://www.thorlabs.com/images/TabImages/Strain%20Tutorial.pdf

16. FINAL REPORT TO THE NATIONAL OPERATING COMMITTEE ON STANDARDS FOR ATHLETIC EQUIPMENT (NOCSAE) 1999 “THE USE OF SHOE ORTHOTICS TO REDUCE BONE STRAIN RATE” http://www.nocsae.org/research/rep/Burr_final.htm