PSI - Issue 68

Masayuki Arai et al. / Procedia Structural Integrity 68 (2025) 3–8

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M. Arai et al. / Structural Integrity Procedia 00 (2025) 000–000

beak of the red-cockaded woodpecker, osteoderm of the shell of the leatherback turtle, sutures of the skull, and connective tissue of diatoms exhibit excellent fracture toughness despite being composed of proteins (Wickramasinghe et al., 2022). A common feature of these structures is having a regularly interconnected microscopic structure like puzzle pieces; when tensile stress is applied, the crack propagates along the interconnection boundary, resulting in higher toughness (Malik and Barthelat, 2016). Our research group focused on the sickle joint, which is used to attach columns and beams to Japanese wooden architecture without the use of a nail (Arai, 2023). Fig. 1 shows a schematic of a sickle joint. This joint can be considered a type of interlocking structure in which wood is fastened by hooking the convex (kerakubi) on the male part (M in the figure) to the recess (mortise) on the female part (F). In this study, we attempt to improve the toughness of brittle materials using an interlocking structure with a sickle joint. To this end, we study the deformation behavior and fracture process of a Compact Tension (CT) specimen with a regularly repeated sickle joints introduced at the tip of a pre-crack. First, a finite element (FE) analysis is performed on CT specimens with different heights ( ℎ ) in the sickle shape to verify the effectiveness of the interlocking structure in improving the fracture toughness. The effect of the sickle-joint height on the load displacement curve and fracture energy will be discussed. Then, we print CT specimens using a stereolithography 3D printer, and perform tensile tests that confirm the increase in the toughness of the CT specimens upon introducing the sickle joint.

Fig.1. Sickle joint used as a fastening technique without steel nails for beams and columns in Japanese wooden constructions, and geometry of the sickle-joint structure. 2. Finite element analysis of CT specimens with interlocking structure using sickle joints 2.1. Numerical procedure Fig. 2 shows the geometry of a CT specimen with an interlocking structure using a sickle joint. The opening angle of the kerakubi in the sickle joint was fixed at = 10°, and the height ( ℎ ) from the root to the tip of the kerakubi was varied to 3.2, 4.8, 6.4, 9.6, 10.4, and 11.2 mm. In addition, a CT specimen for the base material ( ℎ = 0.0 mm) was prepared. Finite element analysis was performed under plane-strain conditions using commercial FE analysis software MARC (MSC Software Corporation). Fig. 3 shows the FE model for ℎ = 3.2 mm. The element size near the sickle joint was 0.1 mm, and that in the other areas was 1 mm. The element type was a three-node isoparameter (element 6). The Young's modulus of the base material was 2650 MPa, the Young's modulus of the pin was 300 GPa, and the Poisson's ratio was 0.3 for both. As boundary conditions, the lower pin was fixed in displacement, and a constant displacement speed of 10 mm/min was applied to the upper pin in the vertical upward direction. The contact surface between the pin and hole of the CT specimen was set as smooth under the coefficient of free friction. To simulate the crack propagation behavior, the fracture mechanics function of the MARC software was used to delaminate the elements when the normal and tangential cohesive stresses between the elements reached certain critical values that were set to 100 and 150 MPa, respectively, which were determined by trial and error based on the tensile test results of the CT specimens of the base material.