It is well-known that the presence of the delamination in a plant fiber-reinforced composite is difficult to detect. However, the delamination introduces a local flexibility, which changes the dynamic characteristics of the composite structure. This paper presents a new methodology for composite laminate delamination detection, which is based on dynamic mechanical analysis. A noticeable delamination-induced storage modulus reduction and loss factor enhancement have been observed when the delaminated laminate was subjected to a forced oscillation compared to the intact composite laminate. For delamination area of 12.8% of the whole area of the composite laminate, loss factor of approximately 12% increase was observed. For near-to-surface delamination position, loss factor of approximately an 18% increment was observed. The results indicate that the delamination can be reliably detected with this method, and delamination position shows greater influence on the loss factor than that of the delamination size. Further investigations on different frequencies and amplitudes configurations show that the variation of loss factor is more apparently with low frequency as well as the low amplitude.

The application of plant fiber-reinforced epoxy plastic is severely limited by its relatively low mechanical properties, especially for its brittleness and weak interlaminar properties. The physical and mechanical properties in both nanoand macro-scales of polyurethane/epoxy interpenetration networks were deeply investigated through dynamic mechanical analysis, atomic force microscope and the tensile tests. The atomic force microscope analysis shows that proper amount of polyurethane and evenly distribution could improve the mechanical properties of the resin cast due to the polyurethane/epoxy interpenetrating networks fix the phase zone and increases the strength and stiffness of the system and this was proven with macroscale tensile tests. Moreover, it is impressive to notice that with addition of 15 wt% of polyurethane into epoxy, the initial fracture toughness of its corresponding composite exhibits 48% enhancement. The results in this study indicate that polyurethane/epoxy interpenetration networks show positive effect on improving the interfacial and interlaminar properties of plant fiber reinforced composites. K E Y W O R D S dynamic mechanical properties, interlaminar fracture toughness, interpenetrating polymer network, nanomechanical properties, plant fiber-reinforced plastic