In the paper, a new Euler-Bernoulli theory is presented, in which the bars eigenpulsations with rectangular section increase with the ratio between the bar width and thickness. This theory is experimentally verified for composite bars made of Dammar reinforced with cotton and flax plus one or two layers of fiber glass fabric. For the considered bars, we have experimentally determined the eigenfrequencies and the damping factor values. We have experimentally determined the Young modulus and breaking strength for the used resin and the obtained composites.

The production of composite materials through the reuse of waste or by-products from the agri-food industry will be a challenge for environmental protection. This study focuses in that direction. In the first stage, composites were made with the hybrid resin matrix (with three major volume proportions of natural Dammar resin and epoxy resin) and the reinforcing from shredded shells of sunflower seeds. Based on the tensile and compressive stresses, the stress–strain and strain–strain diagrams were obtained. The surface area of the rupture was investigated with stereo-microscopic analysis, and the absorption/loss of water was studied with a high precision balance. The vibration behavior was investigated experimentally, determining the damping coefficient and its own frequency. In the second stage, the study of these materials was extended. Sandwich composites were made with the same type of hybrid matrix as in the first stage. The core was made of shredded shells of sunflower seeds and the outer faces of linen fabric. These composites were applied to the bend (in three points), being obtained the force-deformation diagrams. The determined mechanical properties allow the complete or partial realization of these composites of some furniture components or of some equipment used in the field of constructions.

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In the paper it is presented a new Euler - Bernoulli theory based on an assymetrical distribution of strains towards the median plane, the area subjected to compression has a transversal stretching, while the area subjected to tensile has a transversal contraction. This theory takes into account the transversal contraction factors and consequently the eigenpulsations of bars vibrations with rectangular section depend on the ratio between the bars width and thickness. This theory is experimentally verified for Dammar based composite bars reinforced with cotton, flax, silk and hemp fibers. For the considered bars, we have experimentally determined the eigen frequencies and the damping factor values.

The increasing concern for the protection of the environment, through the use of renewable natural resources, has led in the last decades to the realization of a number of natural polymers or hybrids to be able to replace petroleum-based polymeric materials. Rosin is a low-cost wood resin extracted from conifers. In this paper, based on FTIR and Raman analyses, the spectra of natural Rosin resin and some hybrid resins with volume proportions of 45, 55 and 65% Rosin were studied. Some mechanical properties such as modulus of elasticity, elongation at break, tensile strength were determined from stress-strain diagrams obtained from tensile stress. Increasing the volume proportion of Rosin leads to a decrease in stiffness and strength properties and obtaining a visco-elastic behavior.