Fiber-reinforced bamboo composite materials exhibit remarkable properties that make them attractive for various load-carrying components. Despite having several styles, bamboo composites generally exhibit nonlinear stress-strain behaviors in compression, shearing, and tension. Therefore, it is imperative to study the stress-strain relationship of bamboo materials to get more insights that can help advance their properties and expand their applications. The currently available stress-strain models for bamboo composite materials can be classified into two categories, namely, two-segment and there-segment. Nevertheless, these models have several limitations that make them unsuitable for studying stress-strain behaviors of bamboo composites materials. Recent research has shown that nonlinear modeling of bamboo composites can be enhanced by developing continuous and slide nonlinear stress-strain models that reflect the nonlinear compression and shearing performances.
Ramberg-Osgood equation is a common method for representing stress-strain curves with no distinct yield points. However, this approach cannot be used to model strain-softening curves. As such, its applicability in modeling bamboo composite material needs more research and clarifications. Also, it is important to note that apart from stress-strain performances, flexural performances of bamboo beams also exhibit nonlinearity characteristics. Equipped with this knowledge, Dr. Zhenyu Qiu from the Army Engineering University together with Professor Hualin Fan from Nanjing University of Aeronautics and Astronautics developed a new a nonlinear constitutive model to predict the nonlinear deformation and load-carrying capacity of bamboo flexural beams. Their research work is currently published in the research journal, Composite Structures.
In their approach, the nonlinear Ramberg-Osgood model was explicitly built for studying and modeling the characteristics of the bamboo fiber-reinforced composite materials. The authors modeled various types of stress-strain curves of the bamboo composites by simply adjusting the key control parameters under different loading conditions. Additionally, they also discussed the influence of the variation of the nonlinear stress-strain curves.
Results showed that the Ramberg-Osgood could accurately model different types of stress-strain curves of bamboo composites under different loading conditions. This could be successfully achieved by adjusting the four key parameters: stress (σ), reference stress ), constant n, and the ratio between the reference stress and Young’s modulus (ε). Similarly, the authors also observed that by appropriately adjusting these parameters, Ramberg-Osgood equation could be efficiently used to model curves with different slopes. Since bamboo composites are not uniform, their mechanical behaviors exhibit great dispersion, which results in engineering design difficulties. Thus, the presented stress-strain curve envelope can be used as a guiding reference in the design of bamboo structures. On the other hand, it was worth noting that the nonlinear flexural beam theory based on the Ramberg-Osgood equations can be successfully used to analyze the nonlinear flexural behavior of both the bamboo beams and fiber-reinforced polymer strengthened bamboo beams, as well as predict the ultimate loads.
In summary, Qiu & Fan used Ramberg-Osgood equation to model the nonlinear stress-strain relationship of bamboo composites. Results proved that the Ramberg-Osgood equation can be used to accurately model nonlinear stress-strain curves and flexural performance of different bamboo composites. In a statement to Advances in Engineering, the authors stated that the stress-strain curve envelope based on the Ramberg-Osgood equation could be used as a reference by design engineers. This would surely lead to the design of high-performance bamboo structures for various applications.
Qiu, Z., & Fan, H. (2020). Nonlinear modeling of bamboo fiber reinforced composite materials. Composite Structures, 238, 111976.