# Poissons Ratio help

1. Oct 18, 2013

### lizr1

Hi. I can't seem to see where I am going wrong or if i have gone wrong as the answer I got for Poissons Ratio looks incorrect. Can anyone help or point me in the right direction? Thanks.

An axial load of 28.5kN is applied to a bar of length280mm, width 38mm and thickness 21mm. After the load is applied, the bar elongatesby 4.85mm and the width contracts by 2.98mm. At he prescribed load, note thatthe stress in the bar is less than its proportional limit.

1) Calculate the modulus of elasticity
2) Determine Poisson’s Ratio

1) Calculate the modulus of elasticity

Cross Sectional Area = 38 *21 = 798mm2

Stress(σ) = (F)/(A) = 28.5 / 798 = 35.71 MPa

Strain(ε) = (e)/(L) = 4.85 / 280 = 0.01732 mm/mm

Modulus of elasticity: σ = Eε

35.71 = (E)(0.01732) = 2.06 GPa

2) DeterminePoisson’s Ratio

εlat = Δwidth/width = -2.98 / 38 = -0.0784

v = -εlat / εlong = -(-0.0784 / 0.01732) = 4.53

2. Oct 18, 2013

### Staff: Mentor

Maybe the 2.98 is a typo. Maybe it should be 0.298. That would make the poisson ratio 0.453 which seems much more reasonable. 4.53, as you noted, is way out of the ballpark.

Chet

3. Oct 21, 2013

### lizr1

Thanks you were right there was a typo in the question!