Elongation of Tie Rods: LO or L?

[Trenthan]

Hey guys, girls stupid question

Whats the actual formula for elongation of a tie rod
$$\delta = \frac{FL_{O}}{EA}$$

or

$$\delta = \frac{FL}{EA}$$

When searching the net, I've found some use the tie rod length "L"
Others use the original length of the tie rod L_O (In my lecture notes the lecturer has used both interchangeably...)

I don't have a textbook on hand to check it against so I'm not 100% sure which one i can use.

Cheers Trent

 

[rock.freak667]

Your formula is derived using

ε = σ/E

and σ = F/A so

ε = F/AE

ε = change in length/original length

so δ= FL₀/AE

However, if there is an extension δ, and then it extends again, you'd need to change the value of L₀ to get the new value for δ.