Finding the maximum load of a bar from the yield stress

Homework Statement

I have a bar, 500mm long (0.05m) 10mm wide (0.01m) and 3mm deep (0.003 m)

The bat has a load applied in the centre of 30n

I know the maximum yield stress is 150 mpa 150x10^6

How do I calculate the maximum load the bar can take?

Homework Equations

I used stress = m y / I

The Attempt at a Solution

I calculated the bending moment to be 3.75 at 250mm (.25m) using a shear bending diagram,

I then reversed the equation above to get:

150x10^6 = 0.0015m/2.25x10^-11

Which gives m as : (150x10^6 x 2.25x10^-11) / 0.0015

M = 2.25

I know the bending moment is at .25 m

So divide 2.25 / .25

I get 9

The answer can't be 9n as the question uses 30n

Can someone please show me how to solve this?.

rock.freak667
Homework Helper
How is the bar supported? Are the two ends simply supported or fixed, etc?

The type of supports affect the maximum bending moment, which will affect your maximum load.

Sorry, the bar is simply supported at either end

rock.freak667
Homework Helper
For a simply supported beam, the maximum bending moment occurs at the center and is given by

Mmax =PL/4 where P= applied load and L= length of beam

So I can then use this to find the max load from the yield stress??
If so, how?

rock.freak667
Homework Helper
So I can then use this to find the max load from the yield stress??
If so, how?

Use your first equation and rearrange for M.

σ = My/I

thank you for this...