# Force resistance in shells

1. Jul 30, 2009

### BrentMW

To measure the force required to break a shell I have a simple device in which a bolt rests on the shell attached through a piece of plywood. This piece of plywood slides through 4 bolts on the plywood on the bottom. There is a bucket on top which I fill with water at a constant rate until failure at which point I stop the water flow. Once calculating the mass being placed on the shell (plywood, bucket, water etc.) I am left with the mass required to break the shell. I was just wondering how to convert this into a measure of force needed to overcome the shell. I assume you use the F=MA equation but I'm unsure what you use for A. Thanks.

2. Jul 30, 2009

### Q_Goest

Hi Brent. Welcome to the board. I don't know what you mean by "shell" but I'm assuming it's either a flat, cylindrical or spherical shell. Also, I don't understand how you're applying this load. However, the point at which it breaks has nothing to do with F=MA. This is a stress analysis question. Most stress analysis today is either done by hand using something like Roark's, an applicable code, or by FEA. For this, I suspect the best method will be to do the FEA, but it largely depends on how the point loads are distributed into the shell. More simple methods such as doing the analysis by hand may very well be applicable depending on how the loads are applied. However, if you want to know how to determine stress in such a material, the way to do that is a much more complicated than simply writing out one equation like F=MA.

3. Jul 30, 2009

### Staff: Mentor

Use the gravitational acceleration (g = 9.8 m/s²). But Q_Goest is right, it is the stress that is important, not simply the force.