# B Units involving metres

1. Apr 16, 2016

### richard9678

Hi. Stress is measured on Pascals. It's force divided by area. Area of course is m x m. So when stress is given it's in units of MN / m2 or MN m-2.

Okay. I'm looking at something called the Hall Petch equation. There a constant in it labelled k.

In an example, k is given as 0.45 MN m-3/2

Does this make any sense or is there an error with the description of k?

I mean, what is m-3/2? Thanks.

P.S. Possible that there is an error in the text. And that k is MN m1/2. However, if it is, I still don't know what m is as a unit. In other words I know m2 is - square meters.

Last edited: Apr 16, 2016
2. Apr 16, 2016

### pasmith

The Hall-Petch equation asserts a relation of the form $$(\mathrm{stress}) = \frac{(\mathrm{coefficient})}{(\mathrm{length})^x}$$ or $$(\mathrm{coefficient}) = (\mathrm{stress})(\mathrm{length})^x.$$ It follows that in SI units the coefficient is measured in units of $\mathrm{Pa}\,\mathrm{m}^x$ or $\mathrm{N}\,\mathrm{m}^{x-2}$. $x$ need not be an integer.

3. Apr 16, 2016

### SteamKing

Staff Emeritus
In fracture mechanics, the stress intensity factor for different types of flaws is expressed in units of MPa-m1/2.

https://en.wikipedia.org/wiki/Fracture_mechanics