Strain Gauges Hooke’s Law 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 …
stress-strain curve:stiffness, yield strength, fracture www.tpub.com/doematerialsci/materialscience22.htm
Transverse Strain and Poisson’s ratio Consider incompressible material (constant volume…)
What to expect on the strain gauge quiz: Given stress, Young’s modulus, gain, Vex, Poisson’s ratio, calculate Vout… www.solware.co.uk/blog
Another deformation: torsional twist http://home.gna.org/getfem/shots.html
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 www.sensorland.com from www.natmus.dk
Unknown material properties vs strain gauge material properties: Gauge factor: of the strain gauge Poisson’s ratio: of the unknownstrain: of the unknownYoung’s modulus = Ym… Young’s modulus: of the unknown > Ym of SG Next consider a Wheatstone bridgemade 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: • http://me.queensu.ca/courses/MECH215/documents/MECH215-Week06Lecture1-StrainMeasurement-Introduction.pdf • http://www.eidactics.com/Downloads/Refs-Methods/NI_Strain_Gauge_tutorial.pdf • http://www.thorlabs.com/images/TabImages/Strain%20Tutorial.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” http://www.nocsae.org/research/rep/Burr_final.htm