Fracture mechanisms of wood/polyester laminates under quasi-static compression and shear loading

There is increasing use of natural fiber/polymer composites as alternatives to traditional structural materials like concrete and metals and to the inorganic fibers like carbon. While the fracture mechanisms during crushing of synthetic fiber/polymer composites have been thoroughly studied, limited information is available on post-fracture investigation and identification of the dominant fracture mechanisms of wood/polyester composites. In this study laminates of Douglas-fir veneer were fabricated using a catalyzed polyester resin and their potentials as energy absorbers have been investigated and discussed. Factors for this study were (i) laminates symmetry (face layers of 0° or 90°), (ii) lay-up balance (balanced and unbalanced) and (iii) number of lamina (8, 11, and 12). Samples were tested under quasi-static Combined Loading Compression (CLC) and their compressive performances were compared to control specimens using glass fiber as reinforcement. Results indicated that the effect of symmetry on compressive properties of wood veneer/polyester laminates was significant with laminates with face layers of 90° and core layers of 0° had the highest deflection to failure. Increasing the wood/polyester laminate thickness enhanced their energy absorbing ability by bringing more fracture mechanisms into play but it noticeably reduced the laminates compressive modulus. Despite the brittle failure of glass fiber composites wood laminates exhibited a progressive fracture mechanisms with shear buckling as the dominant mode of failure in symmetric samples. This progressive failure with high energy absorbing ability make wood/polyester laminates a good candidate to be used as an energy absorber structure where high deflection to failure and longer failure time are required.