Composites are mixtures of at least two materials, such as reinforcing elements, fillers, and composite matrix binder. Compared to concrete and timber, composites are relatively lightweight. Different composites differ in form and/or composition on a macroscopic scale. Each component retainsits identity and does not dissolve or merge completely into one another.
Composites are not generally used as fibres by themselves. They are often impregnated with a matrix material that transfers loads to the fibresand protects the fibres from abrasion and environmental attack. The useful properties of composites are diminished by the matrix to some extent but not fully.
Thermoset resins, formed by chemical reactions, are inherently brittleand therefore cannot be re-melted or reformed once synthesized. The fibre volume fraction can be varied from 0 to ~ 55%. In addition,thefibre combination and the fibre volume fraction can be adjusted to achieve the exact physical and mechanical properties for a product as required by a certain application.
The composite samples are generated through a compression molding process in an open mold cavity. Pressure is applied to the mold from the top to force the starting material to make contact with all mold areas. The mold is usually heated to around 300 °F to 400 °F, in order to start the chemical reaction. Heat and pressure are maintained until the starting material cures. The process employs thermosetting resins in a partially cured stage. However, in this scenario the mold is exposed to heat for a shorter period of time than the usual recommended time. This is because the mold starts curing before it is placed in the pressurizedhot mold cavity. The pressurized hot process helps cure the material faster,and the shortened time periodsuggests that the sample is partially rigidwhen it is not fully cured.
The dimensions of the formed composites arerecorded after the molding process was completed and the resin hasfully cured. The average values are used for determining the ultimate tensile strength and the strain
Wood is a common building material in the construction industry. Wood from different species of trees possesses different physical and mechanical properties, based on which it can be selected for various applications. The strength of wood is dependent ona lot offactors, includingwood species, grain direction, direction and duration of loading, content of moisture and temperature.Testing methods defined by standards, such as ASTM D143, are used to determine the mechanical properties, including stiffness, hardness, flexure strength, tensile strength and shear strength of wood. This allows engineers to compare the properties of various wood species before choosing one that best fits the engineering requirements.
Experimental procedures of determining wood bending strength
The static bending test is carried out by the central loading method. The timber samples are classified into two types, with or without defects. The dimensions of the test pieces are also measured at three different points, and the averagesare then calculated. Specimens with defects have their knot on the lower side so that it is subject to tension. The cross-sections of the samples with defects are marked with an arrow indicating which face should be up when the test is carried out.
Wood is frequently cut into beams whose bending strength is an important parameter. The strength and deformation properties of wood and timber products can be determined by bending tests in a quick and simple way. The beam is attached to the test instrument using simple mechanical interfaces which allows it to be removed when not in use. The specimen beamisanchored onto a base by two anvils. Force is then applied atthe center of the rod (three-pointbending) at a cross-head speed of 6mm/min for a 2cm standard. A deflection measurement yoke and a suitable extensometerare used to determine the deflection of the specimen rods.