Testing the ultimate bending strength of joints:
How five different wood components fared
By Jilei Zhang, Ph.D.
Information related to strength and stiffness properties of various types of joints in upholstered furniture design is becoming increasingly essential. This new priority has emerged because more plywood and engineered composites are being used as furniture frame structural materials. Furniture manufacturers continue to seek new materials in order to reengineer their products. Also, the strength of the joints constructed with composites should be investigated and compared with joints traditionally constructed with solid wood. In joint strength study series, we begin with the dowel type joint since the two-pin dowel joint is still one of the standard methods of joining critical structural members in upholstered furniture frames.
In order to understand strength and stiffness behaviors of two-pin dowel joints constructed of composite materials under static and fatigue bending stresses, tests were performed in a sequence of joint static strength investigations first, followed by joint fatigue tests. This article presents the study results from the first part of a study of joint static bending tests. This part of the study serves three purposes: understanding joint moment-rotation behaviors under static bending stress; obtaining joint ultimate bending strength data; and providing joint static strength data for continuation of joint fatigue studies. Therefore, the objectives of this study were: 1) to record joint moment-rotation curves, 2) to obtain joint ultimate bending strengths, and 3) to compare static bending strengths and stiffness characteristics of two-pin dowel joints constructed of solid wood and wood composites.
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| Figure 1 - Diagram showing construction of the T-type, two-pin dowel joint specimen for the study. |
Method of loading and testing
To determine the ultimate bending strength and moment-rotation characteristics of two-pin dowel joints, a special set-up fixture was designed. The post of the specimen was bolted to the fixture attached to the table of the testing machine. Two linear variable differential transformers (LVDT) were attached to the rail by a bracket, one on the top of the rail and one on the bottom of the rail. The distance between the two LVDTs was 10 inches. Loads were applied to the rail 12 inches in front of the post, i.e., the moment arm was 12 inches. The load-rotation curve of the tested joint was recorded through a data acquisition system. All specimens were tested on a hydraulic SATEC universal-testing machine and bending tests loading rate was 0.10 inch per minute.
Results and discussion
Joints constructed of red oak and plywood had similar moment-rotation behaviors, i.e., the bending resistances of the joints sharply declined right after reaching their failure loads. But, joints constructed of yellow poplar, ESL and particleboard behaved differently compared to red oak and plywood joints. The moment-rotation curves are smoother, i.e., the bending resistances declined gradually instead of dropping sharply.
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| Figure 2 - Comparison of average ultimate bending strengths of tested joints constructed of red oak, yellow poplar, plywood, ESL and particleboard. |
Joints constructed of red oak failed due to shear failure of the dowel in tension — half of the joints with dowel shear failure from posts and half from rails. All joints constructed with yellow poplar failed due to dowel shear failure from the posts. Seven plywood joints failed due to dowel shear failure from posts, and three due to withdrawal of dowels from the rails. Half of the ESL joints failed by withdrawal of the dowels from posts with ESL core material attached, and half from rails with core material attached. Joints constructed with particleboard had the same types of failure as ESL joints.
The results of joint stiffness Z-value study indicate that the joint stiffness was in the magnitude of 10-6 range. Joints tested in this study are relatively stiff and could be treated as rigid joints.
To compare bending moment resistances of the joints constructed of different materials, bending strength means and their 95% confidences interval were plotted as shown in Figure 2. This joint bending strength data gave no evidence of a red oak joint/plywood joint difference in mean, and also yellow poplar joint/ESL joint. The bending strength of the joint constructed of plywood was significantly higher than the joint of ESL. The joint constructed of particleboard was the weakest joint in bending strength among tested joints constructed of red oak, yellow poplar, plywood, ESL and particleboard.
Approved for publication as Journal Article No. 199 of the Forest and Wildlife Research Center, Mississippi State University.
| Materials and design of joint specimens |
The T-type, two-pin, end-to-side dowel joint specimen for this study is shown in Figure 1.
In general, each specimen consisted of two principal structural members, a post and a rail, joined together by two dowels symmetrically spaced in the rail. The distance between the centerlines of the two dowels is two inches.
Joint specimens were constructed of red oak, yellow poplar, southern pine plywood, aspen Engineered Strand Lumber (ESL) and particleboard. Both the rail and the post were constructed of the same material and measured 16 inches long by 4 inches wide by 3/4 inches thick. All joints were constructed with spiral groove yellow birch dowels with a nominal diameter of 3/8 inches and a length of two inches. Depths of embedment of the dowels in both the rail and the post were one inch.
Ten replications were randomly selected from each material blank source.
An attempt was made to minimize dowel-hole clearances. Differences between dowel and hole diameters averaged 0.003 inch for the joints constructed of red oak, 0.003 inch for yellow poplar, 0.001 inch for plywood, 0.001 inch for ESL, and 0.003 inch for particleboard.
All specimens were assembled with a polyvinyl acetate emulsion adhesive with 52 percent solids content. Joint assembly began right after the drilling operation. Before assembly, the holes in the members were cleaned with compressed air. The walls of the holes and the sides of the dowels were liberally coated with glue prior to insertion of the dowels. In all samples, a piece of wax paper was included between the two members to prevent any possibility of the members adhering. |
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