Help Needed With Stress Calculations for Steel & Brass Rods

[donkey131]

I'm having problem knowing how to work out this question, can anyone point me in the correct direction.

Question
14mm diameter steel rod carries a load of 3.08kN in tension. calculate the stress.?

I ended up with this:
Stress = Force/Area.
Stress = 3.08/154 = Answer = 0.02N/mm²

if the rod is 2.5m long and the modulus of elasticity is 200GN/m2
how much will it extend?

so far i have know idea of the formula or how to work this out?
can anyone help please

I also have one other question related to stress that is

A brass rod measures 10mm in dia, and 150mm length. The rod extends 0.3mm when subjected to a load. given that brass E=85GN/m2
Calculate the stress in the material and the magnitude of the load:
if you could help with the formula that would be a big help.
I'm not able to find any thing on the net and the books i got from the library are over the top in calculations
please help
many thanks

 

[Mapes]

Hi donkey131, welcome to PF. For a rod undergoing axial loading, stress [tex]\sigma[/itex] and strain [tex]\epsilon[/itex] are related by the equation [tex]\sigma=E\epsilon[/itex].

Make sure to check your units in your calculations.

 

[PaintballerCA]

Another formula that you will need is $$\epsilon$$= [l(final)-l(initial)]/l(initial)

 

[donkey131]

so how do i now find the Magnitude.

 

[donkey131]

not sure about this as I am trying to teach my self, it an NVQ course. NOt good teaching your self something you don't know.

Calculate the stress in the material and the magnitude.
10mm = Diameter steel rod
?? = Area of the rod
3.08kN = Tension
?? = Stress
?? = Strain
150mm = Length
0.3mm = Extends
85 GN/m² = Elasticity

Area:
Area = pi x radus² There for: Area = Pi (3.142) x 2.5² (6.25) = 19.637mm² (19.64)

Strain:
Strain = extension (change in Length)/Original Length
Strain = 0.3/150 = 0.002


There for we now can find the Stress:
Stress = Strain x Elasticity
Stress = 0.002 x 85 =0.17N/m²

Answer = 0.17N/m²

so how do i find Magnitude:?

 

[Mapes]

The magnitude is just the absolute value of the number. If the stress is -5 N/m², for example, we describe it as a compressive stress with a magnitude of 5 N/m².

Again, recheck the units in your calculations - I see several errors.