# Homework Help: Mechanics of materials-permanent set

1. Aug 8, 2008

### quyn123

An Al bar has length=4ft and diameter=1inch. Modulus of elascity=10x10^6psi. The bar is loaded by a tensile force 24k and then unloaded. what is the permanenet set of the bar.

welll stress=F/A=210.77MPa with this value I made a dot on the corresponding stress strain curve. Then I shifted the linear part of the curve to the right until it hit the point. A vertical line at the 210.77Mpa is drawn on the graph and the small gap between the shifted line to this vertical line represents the elastic recovery strain. (sorry if its hard to understand I tried my best). So this elastic recovery strain region is calculated e.g to be 0.001. I know permanent set= elastic recovery strain x intial length........wouldn't this then state that permanent set is a representation of contraction length?

2. Aug 8, 2008

### PhanthomJay

I don't know if you're from the USA, but I like your units of psi and feet and kips, so please don't fool with Paschals if you're given stuff in the old fashioned imperial units that America has grown to know and love.
Are you given anything else in this problem like a graph or the yield stress of the aluminum? Deformations are fully recoverable within the elastic region, which i assume may be at strains less than .001. What you are looking for is the inelastic recovery that results in the permanent set. Draw a line parallel to the straight part of the stress strain curve, that passes through the point of max stress. Where that line hits the x axis, read off the value of strain. This is the number to use for your permanent set calculation.

3. Aug 8, 2008

### quyn123

Cheers for the help. It all makes sense.