georgepowell(at)hotmail.c
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 Posted: Tue Aug 07, 2007 3:28 pm Post subject: Araldite "expiry" - shear test |
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There was a thread some time back concerning Araldite 420 and whether it was any good after the "expiry date" on the can. My Araldite has a listed expiry date of April 21, 2006. The gist of the thread was that as long as the inert materials were not mixed and were stored properly, the Araldite should work fine. Several people suggested testing the bond on scrap to determine if "old" Araldite would hold.
In order to test the adhesion integrity of Araldite 420 past “expiry date” on the can, I set up a test as follows:
I cut two pieces of scrap F04 PVC foam approximately 3 inches wide and 10 inches long. I coated both sides of each piece with micro slurry, then laid up two pieces of bid at 45 degree bias on each side (similar to the construction of the aft bulkhead). The two pieces (I will refer to each piece as a “mini-bulkhead”) were laid up on June 10 and 12, 2007. After more than a month of post-cure, I mixed a batch of Araldite 420 from containers opened on July 25, 2007, and mixed in flox per the manual direction. I used this mixture to bond the two previously laid-up foam/glass “mini-bulkhead” pieces together with a 6 inch lengthwise overlap on each piece.
After nearly two weeks of post-cure, I clamped the glued-together sandwich to a piece of half-inch plywood on my shop bench. The piece was clamped in such a fashion that when weight was applied to the end most distant from the bench, force would be applied so as to try to pull apart the bond between the two pieces (in other words, the test of the bond was in tension rather than compression).
The first test involved hanging 23 lbs of weight 2 inches from the outer end of the piece (8 inches from the edge of the plywood on the shop bench). The bonded sandwich deflected approximately an inch, with no sign of delamination or failure of the bond. The second test upped the weight to 36 lbs. A slight additional deflection occurred, with no delamination or failure of the bond. Finally, 57 lbs. of weight was added. The piece deflected several inches and held. A slight cracking noise was heard, then the piece failed, approximately a quarter inch beyond the edge of the plywood away from the clamps. The piece did not fail all the way through, but remained deflected in approximately a 40 degree bend. The fiberglass layers held the parts together, and when removed from the clamps, some flexing was required to break the part completely in two. The Araldite bond remained completely intact.
I then took the remaining piece and clamped it in a steel vise bolted to another bench. I then pulled on the upper “mini-bulkhead” piece as hard as I could, and was able to make the sandwich fail. Once again, the failure occurred just beyond the point of clamping, and the failure was limited to a break in the bid/foam laid-up piece. The Araldite bond again remained completely intact.
I don't have the math or engineering skills to calculate the forces in the my (admittedly) informal test, but it was comforting to see that a properly laid up fiberglass/foam sandwich will fail before the Araldite bond. As one poster to the earlier thread questioned, is there a "life" to the other materials we are using in our planes, such as fiberglass cloth? It seems that if anything on my plane is going to break, the Araldite bond will be the last thing to go.
I feel pretty comfortable using my Araldite as is.
Any comments to this post would be appreciated, and if anyone would like digital pictures, I could try to upload them.
Thanks,
George Powell
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