| Summary: | 碩士 === 國立嘉義大學 === 林業研究所 === 91 === Abstract
The purpose of the study was to investigate the joint behavior of dovetail joints between the back post and the side rail of chairs and furthermore to explore the possibility of preventing the failures. T-type joints and L-type joints were designed to conduct the withdrawal test and the bending test. The joint strength, the joint rigidity efficiency and different failure conditions were examined in terms of different wood species, tenon sizes, dovetail angles, adhesives and fitnesses.
The materials used in the study were mainly the hard maple and partly the Japanese cedar. Besides, the tenon sizes included: width (30, 25, 20) mm, length (15, 12, 9) mm, and thickness (30, 15) mm. Also, there were five kinds of dovetail angles: 8°, 11°, 14°, 17°, 20°. Moreover, the fitnesses included: +0.3mm clearance fit, +0.0mm transition fit, -0.3mm interference fit. As to the adhesives, one group had joints with modified PVAc; another group had joints with UF resin; the other group had joints without glue. Each test, containing 12 samples, was conducted and compared by single factor experiment instead of factorial design. Among the test samples, the standard conditions of the joint are: W30L15T30, 14°, modified PVAc ,+0.0mm transitional fit for tension sample, and +0.2mm clearance fit for bending sample .
The withdrawal test was used to measure the withdrawal strength, and the bending test to measure the bending moment and the joint rigidity efficiency. According to the collected data, the differences of the variables were compared by SPSS (Statistical Package for Social Science). In addition, based on the curve of load and displacement drawn by Excel, the process and the failure modes were explored and realized.
In terms of the wood species, the result showed that the hard maple was significantly greater than the Japanese cedar in both the withdrawal strength and the bending moment. However, no significant difference in the joint rigidity efficiency was found between the two materials. It was also found that there was no significant difference in the withdrawal strength among the factors, except for the group of W30L15T15 in the Japanese cedar dovetail joints.
In the bending test, the results showed that the bending moment became greater with the increase of the tenon length. Especially, the tenon of L15 has the greatest bending moment, that of L12 the second greatest, and that of L9 the weakest. Moreover, the bending moment changes with the fitnesses. The order arranged from the greatest to the weakest was interference fit, transitional fit and clearance fit. As to the other factors, they put no significant influence on the bending moment. With regard to the joint rigidity efficiency, the results indicated that only three factors had significant correlation with it: the tenon width, the tenon thickness and the adhesives. The joint rigidity efficiency increased with the increase of the tenon width; that is, the tenon of W30 had the highest efficiency, that of W25 the second, and that of W20 the lowest. Similarly, the joint rigidity efficiency showed significant changes with the increase of the tenon thickness: the tenon of T30 showed higher efficiency than that of T15. Moreover, the joint rigidity efficiency was significantly correlated with the adhesives. Both the joint with UF resin and that without glue were superior to that with modified PVAc in the rigidity efficiency.
In the withdrawal test and the bending test, the failure modes of the hard maple joint contained (A) the longitudinal split in the mortise, (B) the longitudinal split in the mortise combining the shear off in the tenon tip, (C) the shear off in the tenon tip, and (D) the compression of the tenon tip.
Among which, the most frequent mode was mode A and the second was mode B. For the Japanese cedar, there were only two kinds of failure modes: mode B and mode C. Among which, the most frequent mode was mode B. Thus, the findings showed that the failure modes of dovetail joints for the two materials all resulted from the failure of the materials rather than the joint. For hard maple, the withdrawal strength was affected by the failure found in the mortise; for the Japanese cedar, it was affected by the failure found in the tenon. Consequently, it could be suggested that the dovetail joints strength was governed by the material strength. Besides, it usually took several stages for a set of joints to be completely failed. Therefore, the joints could maintain their final strength to a certain degree in spite of the fact that the joints were not well-made or the materials got some failure in the process of the manufacture.
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