Testing Joints to the Breaking Point
Surprising results surface in an analysis of commonly used furniture joints
Synopsis: Using the laboratory at the Wood Science and Technology Center of the University of New Brunswick, Bruce Gray tested joints and talked to experts about why joints fail. They tested traditional mortise-and-tenon joints, floating-tenon joints, biscuit joints, and dovetail joints for load, displacement, rate of failure, strength after failure, and rigidity. He explains how the samples were prepared and tested and how to interpret the charts. The article also offers a guide to the anatomy of the joints tested.
Most furniture makers have an entrenched point of view regarding the strength of different types of wood joints, based as much on speculation as on observation. As a puzzled furniture designer who wondered about the relative strength of various joints, I wanted some reliable answers. To find them, I sought expert technical advice and access to the laboratory testing facilities of the Wood Science and Technology Center of the University of New Brunswick. What we discovered may surprise you and should help you design and build stronger furniture.
The types of joints tested
We chose to examine what I believe are the joints most commonly found in tables and chairs, carcases and face frames, doors and drawers—all products of amateur and professional woodshops. We tested traditional rectangular mortise-and-tenon joints; floating (or what some people call loose) tenons, both rounded and straight-edged; double #20 biscuit joints; narrow and wide dovetails and low- and high-angle dovetails. Let’s look at which joint proved to be the strongest and where and why the joints failed.
Traditional mortise-and-tenon joints—The traditional rectangular mortise and tenon served as a base of comparison for the other similar joints. I figured that this joint (which is more time consuming to make) wouldn’t provide a significantly stronger result when compared with floating tenons of the same size. I was wrong. In the tests, the traditional mortise and tenon beat all other joints hands down. The joint proved to be significantly stronger than either variation of floating tenons and twice as strong as the double #20 biscuit joint.
For the 3/4-in.-thick by 4-in.-wide samples we tested, the average maximum working load (how much stress the joint can take without failing) was nearly 5,000 lbs., and the average failure load was right around 6,000 lbs. One of the traditional mortise-and-tenon samples accepted a load of 6,605 lbs. before it failed. That’s like piling up three cars on top of that test sample.
If you look at the graph on p. 77 (for more on how to read the graphs, see the story on p. 76), the line in the elastic region (in which the joint is stressed but not damaged) is sharply vertical, meaning the joint is very stiff and resists deflection. This may or may not be a desirable characteristic, depending on the application. The top of the curve is broadly rounded and the tail long and high. This means that failure was gradual, and the joint was able to support a heavy load long after it was damaged.
From Fine Woodworking #148
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