30 Mar 2015 Why simple adhesion tests aren't simple.
It's an exciting time for me because my book, Adhesion Science: Principle and Practice is appearing in 18 days from now. I'm trying something new for myself and the publisher - the book is linked to my Practical Adhesion website so that just about any significant formula or idea is brought alive with one click (from the eBook) or a relatively easy hop (from the printed book) to the relevant app. This means that the reader isn't shielded from hard formulae - it's important to know what the science is saying in the language of science - but the formulae aren't dead on the page - they are alive in the app so their meaning can become clear, even to those, like me, who aren't great at understanding the implications of a formula just by staring at it.
Because authors have to make their own publicity, I've prepared a series of 9 videos covering a selection of the ideas in the book. Two of them are the topic of this blog (I'll update the blog with the links once they are launched the same day as the book.)
In terms of complicated formulae, two rather simple-looking adhesion tests stand out. The first is the classic 90° peel that we all do as though it's the simplest thing in the world - you just peel, and the value you measure is the adhesion! The second is the classic lap shear joint - pulling things apart through pure shear, again with a value which tells you what the adhesion is.
In fact the peel formula (from Kaelble, one of the greats of adhesion science) is rather complex and, as shown in the Peel app there is the phenomenon that ahead of the peel front (tensile pull) is a compression zone - something that is surprising to all of us, but readily demonstrated experimentally. It is probable (but you'll have to read the book for the long explanation) that a lot (50%+) of PSA action takes place in this zone and not in the more spectacular peel zone where there is a lot of stringing, cavitation and general drama
The lap shear test is even further removed from intuition. The formula (Goland-Reisner) is so horrendous that I don't include it in the book or app - it means nothing to most of us who stare at it. The exploration of the formula in the Lap shear app, however, makes a lot of things immediately clear.
- In most circumstances the peel stresses are larger than the shear stresses, and are highly concentrated at the ends of the joint. So failure is seldom due to shear.
- A lot of the adhesive used in a lap joint does no good at all. This is because the forces in the middle of the joint are effectively zero. Experiments described in the book show that you can remove 60% of the adhesive in a lap joint (as long as you choose the right places!) with no change in the failure stress.
- The mechanical (rather than adhesive) properties of the joint dominate what's going on.
- A "weaker" adhesive (with a lower modulus) reduces the stress concentration at the ends and can give a stronger resistance to failure (though with the obvious danger of creep)
The aircraft industry provides an indication of some of those complexities. It is rather important that bits of aluminium stuck together for the airframe don't fall apart via shear. So for decades the industry carried out the lap shear test as it is simple to set up and convenient to run. The experts knew that the test was really a peel test (along with all the other issues above), but no plane had ever fallen out of the sky due to shear failure of joints that met their standard tests, so they carried on using this "wrong" measurement technique. But then along came composites. These fail the standard test instantly - because composites, while great in shear, are useless in peel - the strands come apart with ease. So the composites people had to do what the Al people never bothered to do - created a genuine shear test, using the more complex double lap test.
The moral of all this is that Adhesion is a property of the system, which is the mantra that runs throughout the book. If you find yourself focussing on one aspect of an adhesion issue (e.g. the surface energy of the interface) you are almost certainly focussing on the wrong thing. To find out what the right things should be, explore the apps or, even better, read my book!