木材接着系の面内せん断破壊に関する研究

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The mode of fracture is classified into three types, opening mode (Mode Ⅰ), sliding mode (Mode Ⅱ) and tearing mode (Mode Ⅲ). In general, the fracture of single mode seldom occurs, the fracture of complex mode is considered to occur mostly. Each fracture of single mode seems to affect intricately with one another under the fracture of complex mode. Accordingly, each fracture of single mode must be investigated to solve the mechanism of fracture. Many studies have already been made on the fracture of Mode Ⅰ. The mechanism of Mode Ⅰ is brought the almost whole aspect to light. But,because of the difficulty of making the state of pure shearing stress, study on two other modes is considerably behind. As many fractures seem to occur usually complex mode in combination of Mode Ⅰ with Mode Ⅱ, this study was intended for the development of the test method for the measurement of fracture toughness of only Mode Ⅱ.The measurement of fracture toughness of single mode must be made under the condition of stress as pure as possible. As the bending and the torsion are considered to be able to make the state of the purest shearing stress of Mode Ⅱ at present, the measurement of fracture toughness was made by these two test methods.This study was intended for the fracture of glue line of wood abhesive joints on account of being able to make easily crack of any shape and any size as we wish.Polyvinyl acetate emulsion easy to handle was used as the adhesive. The bending specimen reinforced with mild steel plates to prevent bending fracture was used for the measurement of fracture toughness. The crack lengths of specimens were varied from 2cm to 14cm at an interval of 2cm.The fracture toughness, <special>G∥c </special>were estimated experimentally by employing the compliance method. As the fracture seemed not to occur starting from the crack tip when the crack length was less than 6cm, in both the kaba and the hinoki specimens, <special>G∥c </special>of the case of crack length not exceeding 6cm were excepted. As the experimental formula of compliance is difficult to fit experimental results at the both edges of crack length, <special>G∥c </special>of both edges were excepted,too. <special>G∥c </special>obtained were about 1.4kgf・cm/c㎡ in kaba specimen and were about 1.8kgf・cm/c㎡ in hinoki specimen. Besides, the stress distribution of specimens was investigated by the finite element method, <special>K∥c and G∥c </special>were estimated by employing the stress method, the displacement method and J-integral method. As a result of stress analysis, the possibility of fracture occuring from the other points than the crack tip proved to be large, because the rate of stess concentration at near the crack tip was very low when the crack length was short. Little difference was found among <special>G∥c </special>measured by various method. <special>G∥c </special>obtained were about 1.1kgf・cm/c㎡ in kaba specimen and were about 1.6kgf・cm/c㎡ in hinoki specimen when the crack length were 10-12cm. As the energy spent in plastic deformation increases with the increasing of crack length, the fracture toughness measured by the bending test increased with increasing the crack length. Accordingly, the intact fracture toughness could not be used as a characteristic value of strength. Although to define a practical basic fracture toughness was possible by revising in consideration of the plasticity at the crack tip, the bending test seemed not to be satisfactory for the measurement of fracture toughness.Then the measurement of <special>G∥c </special>were made by the torsion test. <special>G∥c</special> of kaba specimens measured experimentally by the compliance method were about 1.2kgf・cm/c㎡, and a constant regardless of the crack length. The compliance of specimen with an optional crack length was estimated accurately by eq.(3-10), the measurement of <special>G∥c </special>was possible without making experiments with specimens with various crack length. <special>G∥c </special>calculated using these estimated values were about 1.3kgf・cm/c㎡. This was almost equal to <special>G∥c </special>obtained experimentally. Besides, the measurement of <special>G∥c </special>was made with hinoki specimens of different sizes but similar figures by the torsion test. There were no significant variations of <special>G∥c</special> among these specimens. Accordingly, the torsion test proved to be applicable enough for the measurement of fracture toughness of glue lines. The average <special>G∥c </special>of hinoki specimens of similar figures were about 1.7kgf・cm/c㎡. All <special>G∥c </special>measured by the torsion test were close to those measured by the bending test when the crack length was 12cm. In comparison with two test methods-the bending and the torsion -as the method for the measurement of <special>G∥c</special>, the torsion test proved to be superior in easiness of the preparation of specimens and the test method and in stability of <special>G∥c </special>obtained to the bending test.Bending tests with concentrated center loads were made on taiwan hinoki and moutain ash double-notched (beveled) beam specimens. The failure criterion at the notch apex was discussed with stress distribution calculated by the finite element method. When the angle of the notch apex is <special>90°</special>, the fracture is caued mainly by opening deformations (Mode Ⅰ). The role of the opening deformation of the fracture of beams decreases with increasing angles of the notch apex. As the fracture load predicted by eq.(4-6) was close to that measured by experiments, eq.(4-6) seems to be useful enough for the prediction of the fracture load. Furthermore, an improvement in strength of the beam was made by reinforcing the notches with a flexible epoxy-resin adhesive. Filling kerfs made at the notch apexes with flexible adhesive proved to be greatly effective in improving the strength of notched wooden beams.
三重大学農学部附属演習林の論文
1988-03-01

著者

鈴木直之
三重大学生物資源学部

木材接着系の面内せん断破壊に関する研究

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