The trumpet sample modeling crack11/7/2022 ![]() ![]() The finite element analysis (FEA) of the hierarchical model of softwood was developed as an effective model to predict deformation and stress concentration in the cell wall that might become the sites of later damage initiation. The trumpet sample modeling crack crack#Therefore, it is evident that the initial crack and crack propagation in the cell wall is not fully understood. However, the previous research results also showed that, when wood bore longitudinal tensile stress or bend, the initiation cracks of cell walls occurred in the S 2 layer. The cell wall fracture, as well as propagation under longitudinal tensile load, was reviewed by Mark, and the results indicated that the initiation fracture occurred in the secondary wall S 1 layer or the S 1/S 2 interface, rather than in the ML layer. Four types of cell fracture are recognized according to the rupture position: intercell, intrawall, transverse, and longitudinal transwall. Although the initial crack and crack propagation in the wood macrostructure have been well understood, very few attempts have been made to investigate the crack initiation and growth modes along with fracture mechanisms at cellular level.įractures must proceed through the wood cells, and, therefore, the degree of stress and location of the cell wall play an important role in characterizing the fracturing of wood. The stick–slip type of crack growth has also been observed, and new crack planes are often formed at the growth ring border. Furthermore, as has been determined by the in situ scanning electron microscope or light microscope, the initial fracture in wood rays occurred under longitudinal tensile and bending loads. However, this method does not take into account the influence of the initial crack on the wood fracture. For wood, the critical stress-intensity factors ( K IC and K IIC) are a function of the wood species and are affected by many of the same factors that affect other wood material properties, e.g., grain, specific gravity, and moisture content. Many researchers have invested a significant amount of effort in the analysis of the structure and fracture relationships of wood and have sought the recipes for the optimal design of composite materials and construction in the framework of biomimicking concepts.įracture mechanics provides a rational method to quantificationally predict the failure of wood macrostructures when propagating crack is a major contributor to the failure. Further, the prediction of fractures in wood or wooden components has become increasingly more important for practical application. Moving toward the macroscopic scale, solid wood can be viewed as a honeycomb network of cells cemented together by the middle lamella (ML). The cell wall, as a mesoscale structure, is a concentric laminated structure that is comprised of four discrete layers in which the orientation of cellulose microfibrils varies, as does the chemical compositions. At the molecular level, the cell wall mainly consists of cellulose, lignin, and other organic molecules, such as hemicelluloses. 1), i.e., from the honeycomb structure to the chemical polymeric structure. The excellent material properties of wood are closely related to its hierarchical structure (Fig. It has long been used as building materials. Wood is an extremely stiff and tough natural composites material in relation to its density. Furthermore, the tracheids of the tensile parts outermost of bending specimen were subjected to the longitudinal tension and shear coupling stresses that led to the two kinds of cracks occurring, including trumpet-shaped cracks in the S 2 layer, and S 1/S 2 interface debonding. ![]() ![]() In the cases of pure longitudinal–radial (LR) or longitudinal–tangential (LT) in-plane shear loading, the highest stresses are observed in the S 1/S 2 interface and the S 3 layer, but the initial fractures of the tracheids of the neutral layer under the LR or LT shear stress only occurred in the S 1/S 2 interface. The stress concentration and initial fracture of the tracheid wall under longitudinal tensile stress occurred in the S 2 layer. The results indicated that the simulated stress concentration regions of the tracheid wall approximately matched the experimental initial fracture locations. A hierarchical model of softwood was developed to effectively analyze stress concentration and predict initial fracture of the wood cell wall under different loading scenarios. ![]()
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