The Glue Strength Test: Results

We conducted the glue strength test today using a new set of test articles designed to fail well under the maximum load capable of being applied by a tensile strength test machine here at the University of Minnesota. My cohorts in this project were undergraduate engineering student Aaron Bardon, and Professor Jeffrey Schott of the Chemical Engineering and Materials Science Department at the University. I said in my previous entry that my assumptions and calculations were simplified, and that part failure would probably be based on a number of modes of failure rather than on one simple cause. The results were surprising to us to say the least.

The results are summarized here. I should state up front that no particular glue or brand is advocated or disparaged here. Different varieties of glue bring their own unique characteristics to a project, and this test is simply one of strength comparison when applied to a set of identical parts constructed of quartersawn red oak with mortise and tenon joinery.

The three varieties of cyanoacrylate glues will be discussed first since this project was designed to test a new brand of these glues specifically formulated for commercial woodworking applications against the traditional water-based polyvinyl acetate wood glues. These are marked four through six in the photo here, and their test results are summarized below.

4. Cyanoacrylate glue with short open or working time. The glue failed at around 2000N or about 450 lbs. of force in tension.

5.  Cyanoacrylate glue with medium open or working time. The glue failed at around 2400N or about 540 lbs. of force in tension.

6. Cyanoacrylate glue with long open or working time. The glue failed at around 1800N or about 405 lbs. of force in tension.

The important thing observed here is that all three varieties of cyanoacrylate glue failed rather cleanly when a force similar in magnitude was applied, and failed before the wood did. I predicted that the wood would fail in shear first, not the glue. This was not the case though.

Two common water-based polyvinyl acetate glue formulations are marked one and two respectively in the accompanying photo here. Test results are summarized below.

1. PVA Type-I woodworking glue. This part did not fail even at the maximum test force of 5000N or 1124 lbs. applied in tension.

2. PVA Type-III woodworking glue. The part containing the mortise itself failed parallel to the grain of the wood approximately at the mortise depth. The section containing the mating tenon remained intact. The glue joint did not fail. The wood failed either in tension or shear perpendicular to an applied force of 3900N or 877 lbs. A stress concentration at the mortise probably contributed to failure here.

I included two special case parts for comparison, and their test results are described below.

3. Polyurethane glue. The glue failed at around 2000N or about 450 lbs. not too unlike the three cyanoacrylate glues. I included this glue in the test because it is specifically promoted for its high strength. In this test though, not so much.

We concluded the test with the part marked #20, and any woodworker knows what's coming. The part was joined using a #20 biscuit glued with PVA Type-I woodworking glue. What most woodworkers would not expect is that the part shown in the photo did not fail when tested to 5000N or 1124 lbs. I now have a much healthier respect for biscuit joinery, and would consider using a biscuit or two for strength rather than simply for alignment as is usually the case in my shop.

The cyanoacrylate glues however, do not get that same respect. They might have been useful to me in the construction of an organ case, but case frame members have to carry heavy loads, and these glues appear to be significantly inferior to the polyvinyl acetate glues with respect to strength. Although all joint tolerances were kept close, the water-based polyvinyl acetate glues may in the end have superior bonding strength because they are capable of swelling the joint. Because they do not cure within minutes, the polyvinyl acetates may also penetrate the wood pores deeper resulting in a better and stronger bond.

We debated these ideas as we concluded the test, and to be honest, this is where the fun in all this is. You make a guess about something and test it. Sometimes you get it right, but sometimes the test can lead you in a completely new direction. I learned a lot today.

Much thanks goes to Professor Jeffrey Schott and Aaron Bardon at the University of Minnesota for graciously agreeing to be part of this test project.

Bibliography.

Higdon, A., Ohlsen, E., Stiles, W., Weese, J., & Riley, W. (1976). Mechanics of Materials. New York: John Wiley & Sons.

Hoadley, R. Bruce. (2000). Understanding Wood. Newtown: The Taunton Press.