Testing and Applications of Composite Materials
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.
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, so the approach will be valid. 
After reviewing several existing testing methods, this paper suggested a new method for testing the composite materials for their strength characteristics. Namely, this experiment will test the composite under fatigue and static loadings. The main concept behind this experiment is to keep the end-to-shear load ratio constant along with the load translation. This is possible by using a specific mechanism that is able of controlling a gripping pressure of the loaded specimen. The main idea behind this experiment setup was a great success of a similar experiment used for static loading; now, a similar but significantly updated setup will be used for fatigue load testing. The clamps and several other fixtures that are used to secure the specimen in place were updated and advanced so that the experiment results are not influenced by a specimen slipping from the frame or by having clamps slide over the specimen surface. This modification from previous tests allows for accurate results of the fatigue strength. By keeping the end-to-shear ratio constant, the test can be repeated for several trials (or cycles) in order to yield different fatigue strengths that will help in creating a strength model specific for the composite material in question. Fatigue strength, also known as endurance limit, is a value of the stress that a material specimen can endure for several trials (or cycles) without breaking apart. By performing a specified number cycles of testing with different strength values, one can expect to have the specimen fall apart, as the strength and hardness of the composite material will decrease with the increase of cycles. Obtaining the highest possible stress values that a material can withstand for a specific amount of time is a very valuable and important piece of information to know when making decisions about which composite material to attribute to a given application. If the application for which the material is intended requires a strong material that can withstand many stresses applied over time (for example, bolts of the lawn mower must have high endurance because of the constant vibrations coming from the mower), choosing a strong material is crucial. However, applications that do not require extremely strong materials used can be performed with a lower-cost composites that will satisfy the required performance and be cost-effective at the same time. The above described method is, therefore, very useful in the analysis and categorization of the composite materials for various applications. Such experimental setup yields accurate fatigue strength results, which greatly help in making cost-effective decisions. 
Several testing methods for composites have been explored and presented, but if one thinks in terms of applying them to every kind of composite available, a question arises: all of these methods mainly rely on applying force to the specimen, so how can a brittle composite specimen be properly tested? Many of the current methods do not allow or may not be suitable for low force applications and testing, so some new methods must have been introduced. One of them is a Modified Short Beam Shear specimen, also known as MSBS. This specimen is used to suitably test brittle specimens for their interfacial or attachment strength. This specimen is made as a sandwich of two reasonably thick aluminum plates and the brittle composite that is supposed to be tested is laying between them. This setup allows for safe and accurate measurement of the strength of the composite material in question. One of the most brittle composites are made of thermosetting vinylester resin that is reinforced with carbon tubes (multi-walled), also known as MWCNT. This sandwiched configuration allows for this brittle composite to be properly tested and explored for its strength characteristics. However, this setup does not produce a pure shear stress state in the composite, but the result is still applicable since the characteristics of this type of composite can be altered with chemical additions and changes. The whole experimental setup post-analysis is performed by using a finite element analysis method. This allows for the experimental results to create a suitable model that can be applied throughout different scenarios where this composite is being used. The failure mode is also a combination of normal and shear stresses, which is a typical testing result for very brittle composites like this one. Overall, this setup provides for a safe and experimentally successful environment in order to test composite materials for their strength properties even if they are categorized as highly brittle composite materials. Although this test specifically refers to thermosetting vinylester resin that is being reinforced with multi-walled carbon nanotubes (MWCNT), considering the success of this experimental method, it is worth exploring if it will be applicable to other composite materials of the same or even higher level of brittleness.