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-   -   bulk modulus (http://www.physicsforums.com/showthread.php?t=303970)

ABoul Mar31-09 01:51 PM

bulk modulus
 
1. The problem statement, all variables and given/known data
derive the equation for the bulk modulus, K = E/3(1 - 2v), where v is poisson's ratio.


2. Relevant equations
E = stress/e, where e is strain


3. The attempt at a solution
e_v = e_x + e_y + e_z
e_y = e_z = -v*e_x
e_v = (1 - 2v)*e_x

K = stress/e_v
therefore K = stress/[(1 - 2v)*e_x]

i am out by a factor of 1/3. where have i gone wrong?

CFDFEAGURU Mar31-09 02:12 PM

Re: bulk modulus
 
Here is a hint. e = epsilon sub x + epsilon sub y + epsilon sub z. Look at the equations for epsilon sub x, epsilon sub y, and epsilon sub z.

For instance, epsilon sub x = (sigma sub x) / E - (v*sigma sub y) / E - (v*sigma sub z) / E.

CFDFEAGURU Mar31-09 02:17 PM

Re: bulk modulus
 
Perhaps this will bring the answer out.

A body subjected to a uniform hydrostatic pressure all three stress components are equal to a -p.

ABoul Mar31-09 03:08 PM

Re: bulk modulus
 
Quote:

Quote by CFDFEAGURU (Post 2141274)
Perhaps this will bring the answer out.

A body subjected to a uniform hydrostatic pressure all three stress components are equal to a -p.

i see. so the total hydrostatic pressure is the sum of all components, and that's where the factor of 3 comes in, right?

CFDFEAGURU Mar31-09 07:51 PM

Re: bulk modulus
 
Yes that is right. But it is a negative 3. You are in compression.


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