Interesting find. The earlier testing I referred to in the other thread (post #11) were with Araldite, 2011, 2014 and 2020 to be exact. Did lots of testing with varying clamping pressures, cure cycles etc (surface prep is simple, mechanicallly break it and and keep it contamination-free. Simple at least in testing...)It did take some time, but as the question about bond line thickness surfaced again, I did recall where I had read something about it. The datasheet for Huntsman on Araldite 2011 epoxy glue. A search and I quote:
I do interpret this as a recommendation to get the adhesive layer between 0.05 to 0.10mm thick for maximum shear strength of the bond. To consider excessive clamping pressure a possibility causing a to thin adhesive layer would not be far fetched when reading this datasheet.
Bottom line is that even with utterly ridiculous clamping pressure and very smooth surfaces, you won't be able to get even near 50 microns of bond thickness. Tried plenty of materials and all the possible combinations between them (steel, alu, TSA, VE, VE with a perpendicular PE fiber, PE, epoxy, phenolic, all blasted, sanded and untreated) and even at 10 MPa (that's 100 times atmospheric pressure or about 1000 tonnes per square metre), you won't get to too thin bond layers. Test strength obviously increases with decreasing bond strength, though that effect is much more prominent with materials with different thickness*stiffness figures.
Though much of that testing is proprietary, I can probably answer most specific questions.
Below a picture of a test sample I did today. Mostly put in to scare the **** out of people who want to glue unclamped.
On the right the original surface of the core, on the left the broken out core. Surrounding it is the area (VE+fibers) that were clamped under pressure against a metal surface.
All surrounding surface is clamped under a pressure of .16 MPa and that glue joint is extremely strong. The core is (obviously) unpressured. All those air inclusions will make your glue joint to go hell and it shows in testing. Orders of magnitude less strength...
Adhesive is Araldite 2014 cured under a heat cycle.