# Determining yield stress via a bar subjected to tensile force

1. Jan 6, 2017

### Al_Pa_Cone

1. The problem statement, all variables and given/known data

2. Relevant equations
All Equations Below

3. The attempt at a solution

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Views:
21
2. Jan 6, 2017

### PhanthomJay

You are using the wrong area for the cylinder. What should it be?

3. Jan 7, 2017

### Al_Pa_Cone

Do I use the area of a circle as the force is acting on the circular end? pi r^2

4. Jan 7, 2017

### Staff: Mentor

Of course. The force is equal to the stress times the cross sectional area.

5. Jan 9, 2017

### Al_Pa_Cone

Does This look better?

6. Jan 9, 2017

### mjc123

Once again, 1 m2 is NOT 1000 mm2! And GN m is not a unit of force.

7. Jan 9, 2017

### Al_Pa_Cone

I should have noticed that given the fact I only did it last week! As for the units, I think GPa is the correct unit to use in my answer? I am getting mixed up by force measured in Newtons and Newton meters as a measurment. I have reworked my answer again?

8. Jan 9, 2017

### Staff: Mentor

Looks OK, except for the units. GPa is a unit of stress, not force. It would be better to just give the answer in N.

Last edited: Jan 9, 2017
9. Jan 9, 2017

### Al_Pa_Cone

That would be 117.6 Pascals then.

Thanks!

10. Jan 9, 2017

### mjc123

No it wouldn't. You have a stress in GPa and you multiply it by an area in m2. What are the units of the answer?

11. Jan 9, 2017

### Al_Pa_Cone

Sorry I did not read the last comment properly,
I only caught half of it in my emails (Looks OK, except for the units. GPa) and assumed it was ok.

As my undertanding goes with these units:

A Pascal is a unit which can be used for internal pressure acting in all directions equally within a material
A Newton is a unit which can represent force which acts upon a structure or an object
meters and millimeters is obviously a measurement of distance
meters^2 is a measurement of 2 dimentional area
meters^3 is a measurement of 3 dimentional area

The part where I get mixed up is when I have a GN m^-2

a Giga Newton is type of unit and a m^-2 is another but when do you choose to drop the distance/ area measurement out of the final answer giving a Force answer in simply Newtons or Giga Newtons rather than Newton meters ect

So... I am unsure whether my answer should be in Newtons or Newton meters
As the calculations go I presume they are ok so my attempted answer is:

12. Jan 9, 2017

### Staff: Mentor

Much better.

13. Jan 9, 2017

### mjc123

You don't "choose to drop the distance/area measurement", as if this was optional. It disappears as a result of being multiplied by a distance/area measurement. N m-2 * m2 = N.
Remember, quantity equals number plus unit. Until you get the hang of this I would strongly advise you to write your units in your calculations, e.g.
Force = 6 x 10-3 GN m-2 * 1.96 x 10-5 m2 = 1.176 x 10-7 GN
And you should just learn the basic units for common quantities, e.g:
Force: N = kg m s-2
Energy: J = N m = kg m2 s-2
Pressure and Stress: Pa = N m-2
Strain: m/m = dimensionless (sometimes we use "units" like με = 10-6 m/m)
etc.

14. Jan 9, 2017

### Al_Pa_Cone

I think I need to read up some more on common units and measurements, It was introduced into the earlier assignments without covering in the maths bridging course I completed before starting this HNC,. Once again I thank you!