Mixed-mode fracture properties of parallel laminated bamboo lumber

  Objective  This paper aims to explore the fracture performance of parallel bamboo glulam along the fiber direction under complex stress, and analyze its failure mechanism according to the fracture surface, finally get the basic law of composite fracture of parallel bamboo glulam, and establish the corresponding model.

  Method  Fracture behavior of parallel butterfly laminated bamboo lumber specimens along the fiber direction was tested with Arcan system. Several fracture modes of parallel laminated bamboo lumber were realized by adjusting loading angle, including type-I, type-II and mixed-mode fracture. The effects of fracture ligament length on fracture toughness of specimens were studied by setting different crack-to-height ratios. The fracture toughness was calculated by the measured extreme load and the failure mechanism of different forms of fracture of parallel laminated bamboo lumber was analyzed by combining the fracture section. Finally, the composite fracture criteria along the fiber direction of parallel laminated bamboo lumber was obtained by the analysis of mixed-mode fracture envelope diagram.

  Result  The fracture of parallel laminated bamboo lumber along the fiber direction of type-I, type-II and mixed-mode fracture was brittle fracture, and the specimens were more likely to be damaged under complex stress. The cracks in type-I fracture test mainly developed along the fiber direction between the parenchyma cell layers and the fiber/parenchyma cell tissue interface. In the type-II fracture test, cracks developed at the fiber/parenchyma cell interface, and at the same time, they also developed among the parenchyma cell layers, forming the bridging mechanism of parenchyma cell tissue and enhancing the bearing capacity of the specimen. In type-I and type-II fracture tests, the fibers were basically not involved in fracture. With the change of crack-to-height ratio, the fluctuation of type-I fracture toughness was small, type-II fracture toughness had the maximum value at 0.5 crack-to-height ratio. With the increase of loading angle in the parallel laminated bamboo lumber mixed-mode fracture test, the type-I fracture toughness component decreased gradually, and the type-II fracture toughness component increased gradually. With the increase of loading angle, the equivalent fracture toughness increased when the crack-to-height ratio of the specimen was 0.3, and decreased when the crack-to-height ratio of the specimen was 0.6.

  Conclusion  When the crack-to-height ratio is 0.3, the fracture criterion curve of the mixed-mode fracture coincides well with fracture characteristics of the parallel laminated bamboo lumber specimens.