The Glue Strength Test: Engineering Test Articles

I introduced the glue strength test project in my previous entry. Working with an engineering professor and undergraduate student at the University of Minnesota, the project is designed to test a new woodworking glue against traditional glues by tensile strength testing a series of identical mortise and tenon joints to failure. The problem with the initial test was that the part was built too strong for the test machine which could not exert enough force to cause it to fail.

I decided then to apply engineering to the design of a new part that would hopefully experience failure, and therefore provide a meaningful test of one glue against the other. I had to make assumptions about where failure would occur to model the modes of failure most likely to occur, and decided that the glue could fail where the mortise and tenon were joined, the wood itself could fail at the tenon due to the force in tension being applied by the test machine, or the wood could fail at the tenon due to the shear force applied by the test machine.

I began by looking up the equations and published strength data for both the glue and wood used in the test samples pertaining to the three modes of failure I identified as possible. What I found was that there was little published data on the strength property of wood in tension parallel to the grain as this number was always much greater than other strength property figures for other modes of failure of wood in general. The more likely modes of failure then were going to either be failure of the glue, or failure of the tenon in shear.

I set up the document shown here in the iPad app PocketCAS pro to calculate the force at which a joint would fail due to both glue failure, and failure of the wood due to shear force. My goal was to design a joint that would comfortably fail within the test machine force limit of 5000N or 1124 lbs. There is an axiom among woodworkers that glue is stronger than wood, and the equations prove this true. The wood will fail in shear before the glue will fail according to the math, and I designed the tenon dimensions to comfortably fail well under the maximum force the test machine is capable of exerting on the part assembly.

An interesting find obtained by the mathematical model is that such a small tenon can withstand a force up to 667.5 lbs. due to the shear strength of red oak parallel to the grain. I expect this number could be different by some because there may not be a single and simple mode of failure, but rather failure may be multi-modal. In other words, the glue that joins mortise to tenon may play a role in how the joint fails.

I sort of hope this is the case because that would mean that the strength properties of different glue types factor in, and are somewhat represented in the test result data obtained. If so, their effect on joint strength will factor into my work. If not, there is no strength restriction placed on the type of glue I can use in any one construction.

A last word about my assumptions and corresponding calculations. They are simple, and do not represent anything near the complex mathematical models that may more accurately describe the behavior of the glued mortise and tenon joint under tension. I am at this point looking for a ball-park figure so that we can get one of the assemblies to fail during test. An analysis of test results will later provide a more accurate direction to take with regard to the interactions between force, glue, and wood in this type of joint.