## Butt Test

### Quick Start

The butt joint is a great example of how NOT to think about adhesion. Countless papers report a force F to break an area A so give a value of F/A MPa. Butt joints do not fail evenly across the area. They fail with forces concentrated at the edges, and small defects, or small deviations from a 90° pull can cause large concentrations of forces and earlier failure. So the quoted MPa values are meaningless.

Playing with surface energy W, A and modulus E may not cause much surprise, but the thickness of the adhesive layer, d, has an unexpected effect: the thinner it is the better.

### Butt Test

The Butt joint is not the greatest of joints and the Butt test is not the greatest of tests. The idealised version of it from Kendall's *The Sticky Universe* is included here because it is another reminder that testing doesn't test what you think its testing. A related, but different pull (Butt) test is described in the Weak and Strong page.

Take an adhesive of modulus E (and therefore a bulk modulus, `K=(3E)/(1-2ν)`, where ν is the Poisson ratio, taken here to be 0.33), a Work of Adhesion W and thickness of d. Assume the joint has a cross-sectional area of A (i.e. πa^{2}). Then the force needed to pull it apart is given by:

`F=A.sqrt((KW)/d)`

Famously this shows that an adhesive of zero thickness requires an infinite force, so not only do you save on cost of adhesive but you get a very strong joint. Clearly the assumptions behind the model break down at very low values of d. But for small thicknesses, such joints can be amazingly strong - provided you stay strictly in pull mode. If you pull with a slight tilt then the bond will break (in this idealised system) with a peel force of ~W.A^{½}.

For guidance, a typical strong adhesive (such as an epoxy) has a modulus ~1GPa.