The Glue Strength Test: Introduction

I recently came across an article in a trade journal about a company that had begun producing a new glue for commercial woodworking applications that promised quick dry time thereby negating the lengthy times normally associated with clamping glued parts. The new glue is a cyanoacrylate glue, and is far different from the traditional polyvinyl acetate woodworking glue I normally use. Consumer formulations of cyanoacrylate glue are commonly referred to as super glue, or simply as CA glue.

The quick dry time got me interested though because there are applications where this could be useful if the glue proved to be strong enough. I contacted the company making the new glue for more information, and they provided me with a sample pack of the three varieties they offer differing only in the open or working time of the glue, short, medium, and long.

I also happened to recently meet engineering student Aaron Barden who specializes in materials science at the University of Minnesota. Materials Science at the University is part of the Department of Chemical Engineering. Aaron put me in touch with Professor Jeffrey Schott who agreed to be part of a tensile strength test using the new glue against the traditional glue in parts joined from each.

I decided to make six t-shaped parts using mortise and tenon joinery for the test such that two were glued with commonly used polyvinyl acetate glues, one with polyurethane glue, and the remainder with the three varieties of the new cyanoacrylate glue. I chose a tenon size that seemed typical for a normal frame design. With six test articles built, we set about to test the strength of the glue joints.

Testing was done on the tensile test machine shown above. A part is clamped in the machine which applies a force that increases steadily over time, and because most materials stretch some when a force in tension is applied, the machine plots that force against the change in length of the part until ultimate failure occurs. We didn't expect the wood to stretch much over the course of the test, but we did expect the joint to fail before the maximum force limit of the machine was reached. The joint astonishingly did not fail under the maximum force applied to it of 5000 Newtons, or about 1124 pounds. We agreed to run the test again with new parts designed with smaller joints that would actually fail.

So walking back after that initial test conducted during the lunch hour, I concluded that I needed to understand the shear strengths of both wood and glue involved, and make an attempt to apply some real engineering to the problem. There are some surprising numerical results for the size of a joint that should fail under the load parameters capable of being produced by the test machine, and I will publish those in my next entry.

The next step is to produce new parts based on the numerical results obtained from published data on the shear strength of both glue and wood, and then test those parts to catastrophic failure. In doing so I can get an idea of the relative strength of the new glue, and be comfortable using it as a substitute method where appropriate.