  Citation: Bolcu, D.; St ˘ anescu, M.M.; Miri¸ toiu, C.M. Some Mechanical Properties of Composite Materials with Chopped Wheat Straw Reinforcer and Hybrid Matrix. Polymers 2022, 14, 3175. https://doi.org/10.3390/ polym14153175 Academic Editor: Emin Bayraktar Received: 1 July 2022 Accepted: 29 July 2022 Published: 3 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Some Mechanical Properties of Composite Materials with Chopped Wheat Straw Reinforcer and Hybrid Matrix Dumitru Bolcu † , Marius Marinel St ˘ anescu * ,† and Cosmin Mihai Miri¸ toiu † Department of Mechanics, University of Craiova, 107 Calea Bucure¸ sti, 200512 Craiova, Romania; dbolcu@yahoo.com (D.B.); miritoiucosmin@yahoo.com (C.M.M.) * Correspondence: mamas1967@gmail.com; Tel.: +40-74-035-5079 † These authors contributed equally to this work. Abstract: Modern agriculture produces a very large amount of agricultural waste that remains unused. The use as a reinforcer of these renewable resources for the realization of composite materials, and the finding of useful industrial applications, constitutes or provokes the groups of researchers in this field. The study conducted in this article falls in this direction. Composites were fabricated with the chopped wheat straw reinforcement and epoxy resin matrix or hybrid resins with 50% and 70% Dammar volume proportions. Some mechanical properties of this type of composite materials were studied based on tensile strength, SEM analysis, water absorption/loss, vibration behavior and compression strength. The strength–strain and strain–strain diagrams, the modulus of elasticity, the breaking strength and the elongation at break were obtained. Compared to the epoxy resin composition, those with 50 and 70% Dammar, respectively, have a 47 and 55% lower breaking strength and a 30 and 84% higher damping factor, respectively. Because the values of these mechanical properties were limited, and in practice superior properties are needed, sandwich composites were manufactured, with the core of previously studied compositions, to which the outer faces of linen fabric were applied. These composites were applied to the bend (in three points), obtaining the force–deformation diagrams. The obtained properties show that they can be used in construction (paneling, shells, etc.), or in the furniture industry. Keywords: composite materials; chopped wheat straw; hybrid resin; mechanical properties 1. Introduction Reinforced polymer composite materials are widely used in the industrial field for their excellent properties that allow for superior fatigue behavior over the lifetime [1]. Existing research has focused on the long-term evolution of single-type fiber-reinforced polymer composites, such as the glass fiber-reinforced polymer, GFRP [2], carbon fiber-reinforced polymer, CFRP [3], and hybrid fiber-reinforced polymer, HFRP [4]. In recent years, considerable attention has been paid to the production of new com- posite materials whose reinforcement materials are made from the recycling of waste or by-products in the agricultural-food industry. This increases the economic value and envi- ronmental benefits for their use in industrial applications [5–7]. The use of straw biomass in composites is increasing due to cost efficiency and the fact that they are lightweight, have low density and have a lower impact on the environment during production [8]. To date, the most commonly used material for the manufacture of organic composites is wood [9], but wheat straw as a renewable material has the potential to successfully replace wood in various applications. In general, wheat straw has chemical components similar to those of wood; more precisely, wheat straw contains 33.4–41.93% cellulose, 25.2–30.2% hemicellulose and 14.55–18% lignin, respectively [10–12]. Wheat straw is a by-product of the dry wheat stalk that remains after the grain and husk have been removed. Such natural fibers are extremely cost effective, with high Polymers 2022, 14, 3175. https://doi.org/10.3390/polym14153175 https://www.mdpi.com/journal/polymers