This work reports on the static mechanical and modal analysis by free vibration response of hybrid natural fiber reinforced polymer composites. Modal analysis may predict the failure of structures prone to dynamic loading conditions. The composites were compression molded using recently identified short Sansevieria cylindrica fibers and coconut sheath mats in a polyester matrix. The study focused on the effect of the stacking sequence and fiber chemical treatment (alkali and silane) on the composite properties. Hybridization of coconut sheath and Sansevieria cylindrica fiber significantly enhanced all properties compared to single-fiber composites. For all composites (pure or hybrid), the increase in mechanical strength and natural frequency was noticed after chemical treatment. The trend in mechanical strength followed: untreated < alkali treated < silane treated. Interestingly, regarding vibrational behavior, coconut sheath/Sansevieria cylindrica/coconut sheath offered superior natural frequency in all three conditions. This highlights that higher modulus composites can be designed by keeping fiber mat as skin and short fiber as core. Morphological investigation revealed that the attraction between fiber–fiber and fiber–matrix is greatly enhanced after silane treatment.

This work addresses the tensile, impact, and free vibration properties of sisal/coconut sheath fiber hybrid-reinforced unsaturated polyester composites. The hybrid composites are fabricated using a compression molding technique with varying stacking sequences under as-received or chemically treated conditions. The chemical treatments (alkali and tri-chloro vinyl silane) have shown better performance than untreated composites. Furthermore, the silane-treated composite shows enhanced static mechanical and free vibration properties for all stacking sequences relative to the other two cases. From the experimental results, the silane-treated coconut sheath/sisal/coconut sheath hybrid stack is found to be an optimum stacking sequence for better properties. Further, an encouraging damping factor value is also observed for the optimum stacking sequence. The failure mechanism of interfacial de-bonding between the fibers and the matrix is analyzed with the aid of scanning electron microscopy.