Testing and Applications of Composite Materials

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Young’s modulus is an important giveaway of the material’s capabilities to withstand changes in the length of its specimen when a stress is applied along its length, whether it is compression or elongation. This valuable information is crucial in making decisions about which composite material is suitable for what application. Therefore, the combination of DIC and FEA methods analysis is a great approach if one wants to obtain a stress-strain curves that include fracture strain in a composite material, which makes the determination of Young’s modulus very easy.[4]

Tensile testing proves to be one of the most successful traditional methods of testing for composite’s strength and its corresponding characteristics. However, not every composite material can be tested equally and under the same experiment setup. Namely, the type of reinforcement of a composite material plays a great role in this determination. If one recalls that there are continuous and discontinuous fibers used for reinforcement, it is important to remember that when the continuous fibers are used, the material obtains unidirectional properties, meaning that the properties of the material are specifically oriented to one location. Different properties are obtained after applying loads in different directions; along, across and normal to the direction of the fibers. Because of these characteristics, unidirectional composites are challenging to test under tensile testing. Namely, the traditional clamps for tensile testing used to hold the specimen in place may induce unwanted stresses at the clamped specimen areas, which will disrupt the experiment results and accuracy of the further results analysis. This paper proposes several ideas on how to improve the existing setup for tensile stress testing. One recommendation is to use a thin specimen for testing in order to reduce the stress concentration across the depth of the specimen, reducing numerically any unwanted induced stresses. However, it is important for the specimen to still retain the original material properties, meaning that one needs to carefully choose the minimum depth that one wishes to test. This approach requires more details and information about the tested composite, specifically the minimum thickness of the specimen that can successfully represent the materials properties. Another recommendation is to use longer clamps in order to have uniform stress distribution over a bigger part of the specimen area, meaning that the stress concentration will not be contained only in a small outside area of the specimen. This approach is somewhat limiting as it will leave a very small specimen area that can be actually tested. Lastly, this paper indicates that American Society for Testing and Materials (ASTM) has suggested a different approach as a solution to this issue. ASTM suggestion is that if the clamps used for tensile testing are made of the same material as the composite, the induced stresses will be minimized. This approach has proven to be successful in several experiments, but it can be limiting to experiments where the specimen composite is relatively weak and not well bonded or reinforced. For most common applications, such as high-performance applications, this would not be the case,

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