Composite materials, as the name indicates, is a combination of at least two different materials combined together in a unit that has various properties. Namely, if a composite material is made out of different materials, then each material will keep its specific and unique properties- and this is what makes composite materials versatile and applicable for various engineering projects. Composite materials can have their strength increased if they are additionally reinforced with particulates or fibers.
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Particulates reinforcement is not the best option for composites reinforcement as it leads to weaker and more brittle or ductile materials due to its inexpensive costs of fabrication. On the other hand, fiber reinforcement is a much effective but also a more expensive option for composites reinforcement. The quality and improved strength of the reinforced composite depends on what kind of fibers was used. Fibers can typically be categorized in continuous and discontinuous types. Continuous fibers have a much larger ratio between their length and their diameter compared to the discontinuous type of fibers. If one acknowledges that each fiber has a specific set of characteristics in different directions, one may realize that a composite built out of continuous fibers will have somewhat uniform properties in certain directions. On the other hand, composites built out of shorter, discontinuous, fibers will have many properties in various directions. Depending on the purpose of the built composite, reinforcement with continuous or discontinuous fibers may be preferred, but overall preference leans more towards reinforcement with continuous fibers. One may believe that the more fibers are added, the stronger and better reinforced material will be. However, research has proven differently as it has been indicated that extremely large fiber volume ratio (volume of the composite taken by fibers compared to the overall composite volume) can, in reality, reduce the strength of the material due to the lack of the binder/resin that is connecting the fibers; they may become loose and the composite itself may become brittle. Composite materials have been introduced in the early 1960s and they have immediately received high recognition and were acknowledged as potentially the most useful approach in building products for high-performance use. Due to their relatively unpredictable nature of binding between fibers and polymers, there has been no specific guidance that one should follow in order to produce a high-quality composite material, but the decision-making process is based on performing different tests to acquire as many characteristics about the composite as possible.
Exceptional composite material is achieved when its weight is minimized enough to keep its original properties and yet when this reduction does not affect the quality of the high-performance product. Given the numerous possible combinations between fibers, polymers/resin and the laminae themselves, there was an urgent need to specify some universal criteria for characterizing composite materials. This paper specifically revolves around shear, tension and compression analysis for in-plane loads. In order to perform any kind of the aforementioned test, the characteristics of each individual lamina need to be known (type of fibers and the resin binding them together). After obtaining this information, one may proceed with testing the composite by applying normal and shear stresses to the material. Strains and stresses are not uniform throughout the composite, which may cause irregular deformation of the material, so the stress applications must be taken on a specific, as uniformly combined as possible, part of the composite volume (also known as average volume). It is worth noting that during the testing, material can become damaged, which in return will result in its lower overall strength. Increase in trials, although time consuming and less cost effective, will decrease any measurement error and will lead to more accurate results. Different fiber directions during testing will have different stress properties, with some possibly having the same directional stresses, which will lead to the concept of symmetry. A symmetry between different directions makes testing the composite specimen more reliable since it cuts down the data processing time. Composites cured at high temperatures, but tested at normal/room temperatures, most likely also have thermal stresses residing in them, which can influence the test results by indicating a higher strength of the composite. In attempt to reduce test outliers, researchers are reaching out to more digitalized and modernized methods of obtaining characteristics of composite materials. An effective method in testing the composite materials and their strength properties is the digital image correlation (DIC), used to scan an area of the both undeformed and deformed composite specimen with a laser, compare the images from the same surfaces and use the distances between the misaligned ones to calculate the displacements caused by the applied stresses and, consequently, composite bending and strength properties at a given area.  More on the DIC method is in the paper summary below.
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