# Units of Force Confusion lb/in

1. Mar 11, 2012

### Corsan

Hi all,
I have a spreadsheet which says failure will occur in my composite piece when a load of 3500 lb/in is applied.
If I were to tensile test this piece and the thickness has already been input (0.138 in) does this mean for every inch wide the piece can take 3500lb of force?
That seems high?

I'm confused about the units of measurement and if someone can clarify this I'd appreciate it,
Regards

2. Mar 11, 2012

### AlephZero

This is impossible to answer the general question without a complete description of what you are calculating.

For the particular number you got, it's impossible to say whether 3500 lbf is a sensible load to apply over a 1 x 0.138 inch area unless you tell us what material the test piece is made from.

3. Mar 11, 2012

### Corsan

Ah right okay, its a 10" x 1" x 0.138" thick piece of CFRP (12 plies).

I'm planning on tensile testing unti failure and a spreadhseet I have from the internet says the failure load is 3500 lb/in but nowhere on this does it state the width/length of the piece so its obviously working it out from the ply properties (E1,E2...etc).

What I'm asking is, if that is the 'failure load' then does that mean that for every 1" width that the plies can take 3500 lb/in force?

4. Mar 11, 2012

### AlephZero

I would have thought a 1 x 0.138 section of CFRP would take a lot more than that 3500lb. Even mild steel would do that.

Possibly your "3500 lb/in" is for one ply of CFRP, not for 12 .... ????

5. Mar 11, 2012

### Corsan

Okay, I'll keep that in mind during the test.

Am I right in what I say though?
i.e. the 3500 lb/in refers to the width of the cross section, so if it were 2 inches wide the predicted failure would be 7000 lb/in?

Regards

6. Mar 11, 2012

### AlephZero

That seems a reasonable guess, but without seeing the complete spreadsheet (or at least the relevent equations) and understanding how it works, it is only a guess and I don't accept any responsibility if it turns out to be wrong!