Finding the maximum load of a bar from the yield stress

[Lap9387]

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?.

Thanks in advance x

 

[rock.freak667]

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.

 

[Lap9387]

Sorry, the bar is simply supported at either end

 

[rock.freak667]

For a simply supported beam, the maximum bending moment occurs at the center and is given by

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

 

[Lap9387]

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

 

[rock.freak667]

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

 

[Lap9387]

thank you for this...