# Breaking point of material?

Discussion in 'Physics' started by Ben_C, Feb 17, 2011.

1. ### Ben_C Thread Starter Active Member

Oct 19, 2010
65
4
Hello

I need to know the equation for breaking point of material... The point where all stress is had and the material starts to deform or break.
I've looked all over and cant find it anywhere, I heard that its something like Strength x Area = Force needed to break, but I cant see it being this simple.

The question is:

An aluminium alloy has a tensile strength of 200MPa. What force is needed to break a bar with a cross sectional area 0f 250mm^2

I know this is very simple and I can do a lot harder math than this but resources on this are few.

Jul 7, 2009
1,585
141
The general topic is strength of materials. For the basics, consult any college-level physics text. If you read through that wikipedia article, you'll probably realize that there's a lot to the topic, much more than you can pick up without taking a bunch of college courses and getting some practical experience working under an experienced professional.

In the real world, predicting exactly where something breaks is, in general, a difficult problem. The equations are not too bad for simple geometries, but they only hold if their assumptions are met. Often, another problem is not knowing enough about the material's properties or having too much dispersion in the properties.

You don't give any details about what you're trying to do or calculate. Is the stress involved in tension, compression, or shear or a combination?

And there are lots of resources on the web; you probably just didn't search using the right terms. Once you know what you want, a site like Engineer's Edge can supply some handy calculators.

3. ### Papabravo Expert

Feb 24, 2006
10,145
1,791
We had machines that would run the experiment. The sample would deform plastically, then neck down, and finally fracture. The actual amount of force required was neither uniform nor consistent.

We did get a good measure of Young's Modulus from the slope of the curve. That result was actually pretty consistent across a range of samples.

http://en.wikipedia.org/wiki/Young's_modulus

4. ### Rbeckett Member

Sep 3, 2010
205
32
The math you are looking for is readilly available. The question needs a little more clarification to give you the correct Formulae. I have a copy of Machineries Handbook (27th Edition) for engineers and machinist as well as technicians and riggers too, and it contains ever one of the equations you will need. But first you must decide exactly what you are trying not to break and how close you are coming already and whether this will occur once off or under continous stress cycles. There is a calculation for even that if you should need to know that too. So, if you will give us a little more info we will be glad to help you find the correct calculation and you will be able to prove your results in an experimental or laboratory environment.
Bob

Last edited: Feb 18, 2011
5. ### Ben_C Thread Starter Active Member

Oct 19, 2010
65
4
Sorry I haven't replied sooner I've been away..
The topic were looking at is to do with compression, an axial load presented to the bar.

This is the only information I have on this I hope it helps. I'm just too confused looking at articles that are available on the web :S

6. ### shortbus AAC Fanatic!

Sep 30, 2009
4,015
1,531
Tensile strength and compressive strength are two completely different things. Which are you after?

From your first post "An aluminium alloy has a tensile strength "

From your last post "The topic were looking at is to do with compression"

7. ### Ben_C Thread Starter Active Member

Oct 19, 2010
65
4
Oh my bad :/ the other questions on the sheet are to do with compression and elasticity, its definitely tensile.

What other information is needed as the only information I have is in the question..?

I thought it would be simple to find the force that is required to break the bar seeing as the strength of it is 200MPa and a cross sectional area of 250mm^2.

I've just googled "force needed to break a bar" and this very topic is the 7th one down on the first page xD