Comparative study of toughness and energy evaluation of laminated bamboo composites and toughening mechanism

As toughness becomes an important mechanical property of construction materials, scientific toughness assessment is vital to the development of new materials and helpful for material innovation. For laminated bamboo composites in which the failure modes are relatively uncertain, different methods were applied to evaluate the toughness of bamboo laminate. A comparative study of various toughness evaluation methods was conducted for laminated bamboo. The meaning and error of the toughness expressed by the energy release rate or J-integral determined by different methods were interpreted and analyzed. Based on the calculated toughness, the toughening mechanism of bidirectional laminated bamboo was explored. The results show that the ratio of transverse bamboo strip volume V_f vertical to the initial notch direction has a positive impact on the initial stiffness, maximum load, and energy release rate. Besides that, the layup or arrangement of bamboo strips would also affect the mechanical properties and fracture toughness of laminated bamboo, of which the effect is investigated by dividing the fracture parameters by V_f. It is concluded that the major mechanism for increasing the ductility and the toughness before the maximum load (expressed by D₂/D₁ and G_Ic^un/V_f or G_Ic^0.8d/V_f) is the crack deflection caused by the hierarchical structure composed of vascular bundles (fiber) and parenchyma tissue (matrix) in a single layer of bamboo laminae. However, the ductility and the toughness after the maximum load (expressed by D₃/D₂ and G_Ic^0.8d/G_Ic^un) are mainly enhanced by the alternatively arranged bamboo laminae of different fiber directions.