How to find a value for poisson's ratio

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Discussion Overview

The discussion revolves around calculating Poisson's ratio for a rod subjected to axial loading, based on given dimensions, strains, and loads. Participants explore the relationship between lateral and longitudinal strains, addressing potential errors in calculations and unit conversions.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant presents a homework problem involving a rod's dimensions and the effects of an axial load on its strains.
  • Several participants point out potential errors in unit conversions for both lateral and longitudinal strains.
  • Another participant proposes a calculation for lateral strain and longitudinal strain, leading to a preliminary value for Poisson's ratio.
  • Concerns are raised about the validity of the calculated Poisson's ratio, as it appears inconsistent with known material properties.
  • Participants request confirmation of their calculations and whether they are on the right track.
  • One participant emphasizes the importance of correctly defining Poisson's ratio as the negative of the ratio of lateral to longitudinal strain.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the accuracy of their calculations and the correct values for strains. There is no consensus on the final value of Poisson's ratio, and multiple viewpoints on the calculations remain unresolved.

Contextual Notes

Participants highlight issues with unit conversions and the need for careful attention to definitions. Some calculations may depend on assumptions about the material properties and the accuracy of the provided data.

Who May Find This Useful

This discussion may be useful for students or individuals studying material mechanics, particularly those interested in understanding Poisson's ratio and strain calculations in practical applications.

shortshanks
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Homework Statement


The question is: A rod of 50mm in diameter and 0.8m in length is subjected to an axial load of 150kn. If the rod entends by 1.4mm and there is a decrease in diameter of 0.01mm, determine: The value of poissons ratio?


Homework Equations



v=lateral strain/longitudinal strain

e= change/original

The Attempt at a Solution



My attempt is: e(lat)=0.01/0.050
=0.2
e(long)= change in length/original length
= 0.014/0.8
= 1.75x 10-3
 
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It appears that you didn't convert delta_d from 0.01mm to 0.00001m.
 
And you converted 1.4 mm to meters incorrectly as well.
 
how about: 1.4mm/800mm = 1.75x10-3 (0.00175) (longitudinal)
-0.1mm/0.50mm = -2x10-3 (0.002) (lateral)

lateral/longitudinal= -0.002/0.00175 = -1.1428
 
Is the decrease in diameter 0.1 mm or 0.01 mm?
 
oops! 0.01mm.
-0.01mm/0.050mm = -0.02mm (lateral)
1.4mm/800 = 1.75x10-3 (0.00175) (longitudinal)

lateral/longitudinal = -0.02/0.00175 = -11.428

Im not sure if this is right though as -11.428 does not tallie up with any ratio for any material for poissons ratio. What do you think?
 
Can anyone let me know if I am heading in the right direction with my answer please?
 
Check your lateral strain calculation again, particularly the units, and note that Poisson's ratio is defined as the negative of the ratio.
 
0.001/0.050 = 0.02 (lateral)
1.4/800 = 1.75x10-3 (0.0075) (longitudinal)

0.02/0.0075 = 2.666
I think I finally have it, can someone confirm please?
If I do then thank you to everyone who helped!
 
  • #10
Check your lateral strain calculation again, particularly the units.
 

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