# Grip mechanics problem

1. Mar 25, 2012

### ljaeggi

I have a small dilemma with a statics problem that isn't really static...

I have been asked to design a grip for a tensile testing machine specifically for rubber that will supply up 1kN of force through the rubber
I have chosen aluminium to make the main body of the grip and will be driven pneumatically.

I have gone for a grip design of two flat plates that will hold the rubber in place.
One grip would be stationary at the bottom of the test piece and the other would be moving at about 1cm/s gripping the top of the test piece.

The test piece is in a dumbbell format according to standards and the test piece will weigh no more than 10 grams.

Therefore, assuming a 0.5 coefficient between rubber and aluminium, is it a simple static problem and the force required to keep the rubber in place would be 2kN?
Or do I have to treat the two systems independent of each other?

2. Mar 26, 2012

### OldEngr63

Have you thought about how you will shape the edges of the flat plates in order to avoid (or at least reduce) stress concentration at the edges where the plates grip the rubber sample? This can have a profound effect on your results, I would think. It should relate also to the profile of the rubber sample.

3. Mar 26, 2012

### ljaeggi

I have shaped the plates to account for the phenomena known as necking where by too much pressure to the rubber makes the rubber spill over the plates. A simple round and adjustable pressure accounts for this. this has been previously documented

However, multiple test pieces will be used ranging in material and overall size. This also is why I want to calculate the force required. (μ=0.5 for the lowest coefficient of rubber on test btw)

But while the rubber is approaching its ultimate tensile strength (the point of rupture) including a factor of safety, at the 1kN max, I am assuming the dimensions of the rubber would not change however during the test its elongation is about 400% (very roughly). I want to be very close to the equilibrium point of the system to ensure the least stress to the test piece.