
What I have found is that you really have to look very carefully at a broken test joint to see. Sometimes it looks like it broke at the glue bond, but if you look very closely with good light you might actually find a very thin "fuzz" of foam adhered to the glue. Sometimes it is not as obvious as others, but if this fuzz exists, IMO, that is still a foam failure and not a bonding failure.
Okay, now where do I collect that beer?