# Queston on workng out Stress in a Beam

1. Aug 2, 2010

### a5hl3yi

Question:

A bar has a unknown tensil load. A guage shows that the strain is 400x10^-5.
Work out the Stress & Tensile Load (there should be an image attached of the beam. Also the Youngs modulus is given as 180GNm^-2, but I belive this is for a later part of the question)

Relevant equations:

The question does not come with any equations, but I belive I have to use the following.
M/I= σ/Y = E/R. However, I never got the hang of using this formula, and so I'm having some trouble.

The attempt at a solution:

So far all of my attempts have come up trumps, mainly because I'm not sure how to use the formula, or if its the correct one to use. Any help is apriciated.

Thanx :D

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2. Aug 2, 2010

### PhanthomJay

This is not a bending problem...the bar is under an axially applied tensile load, T, causing tensile stresses T/A. What is the relationship between stress and strain using Young's modulus and a linear curve?

3. Aug 2, 2010

### rock.freak667

Well that formula is for bending, it doesn't look like you have bending taking place at the moment.

If you know the strain, you can get the stress from using the Young's Modulus. Then the force using the definition of stress.

4. Aug 3, 2010

### a5hl3yi

First of all, thnx, and in my question the strain measured by the guage is an axial strain.

Secondly. Soo I need to work out the strain first. Ok. This is a formula that that I found on a different thread asking a similar queston: σ = Eε (Strain = Young's modulus x Axial strain)? Is this the way that I get the Strain? And then after the stress?

Thanx

5. Aug 3, 2010

### PhanthomJay

See corrections in red above

6. Aug 3, 2010

### a5hl3yi

Thanx, dont know what I was going on about then. The more I go into it, the more it all seems the same to me. Ill use this n put what i get on

7. Aug 4, 2010

### a5hl3yi

Ok, so I'v worked out the stress in the beam :

Stress = Young's modulus x Axial strain
Stress = 400x10^-5 x 180
Stress = 0.004 x 180 = 0.72 (dont know what units to put it in though :S)

Now I have to work out the tensile load that is applyed. Is there a formula for doing this, because rock.freak667 says to use the deffinition of stress but Im not sure what that is.

Thanx for all your help :)

8. Aug 4, 2010

### rock.freak667

Stress is measured in N/m2, which is a force/area

9. Aug 4, 2010

### PhanthomJay

Be careful with your math. Strain is a dimensionless quantity given as 400(10^-5). Young's modulus is given as 180 GN/m^2, which is 180(10^9) N/m^2.

10. Aug 5, 2010

### a5hl3yi

Right. thanx. So this is what I have now:

(400x10^-5) x (180x10^9) = 720000000N/m^2

Now this is correct I hope, how do I go about workng out the magnitude of the tenslile load? (dont realy wanna keep asking loads of questions, sounds like I'm just leaching but I'm soo stuck :S)

11. Aug 5, 2010

### rock.freak667

That stress looks a bit hight, but you'll need to multiply by the area where the force is acting to get the force.

12. Aug 5, 2010

### a5hl3yi

Great thanx the both of you, I think I understand it now :D

Just wondering, is there a text book or something along those lines that ether of you would recomend that covers this kind of physics? I spent £35 on one that a tutor recomended and its not very good at all :)

Thanx

(also im assuming that when you say "force is acting" its the 720000000N/m^2 answer I gave and the "get the force" force is the tensile load)

13. Aug 5, 2010

### rock.freak667

I am not sure about books, but if you know what topic your question covers,you can try searching that topic on Google. If not, post on PF and we'll help if we can.

My words did sound quite confusing.

Your stress= Force/Area so that the tensile load is the force. So multiply by the area.