Non-contact Measurement Of Tensile Strain And Modulus Of Single Filament Of Carbon Fibers By Laser Micrometry

Carbon fiber is one of the most promising reinforce materials for composites in diversified fields. Accurate measurement of its mechanical properties is essential in understanding not only mechanical performance of carbon fiber reinforced composites, but also the microstructure-properties relationship for carbon fiber materials. In the thesis, a non-contact Young’s modulus measurement method for single filament and various influencing factors were investigated, aiming at providing a test method with higher accuracy and precision for high modulus fibers.A non-contact laser micrometer was successfully applied to a conventional single filament tensile testing instrument to build a new test system. A laser micrometry methodology is proposed based on the new system. Its accuracy and reproducibility is found to be comparable to the conventional method at the moment, while complicated system corrections can be eliminated.Experimental results obtained from medium modulus fibers(T300, T700) and high modulus fibers M40 J are agreed well with the data provided by their manufacturer, and the difference is within ± 5%. However, a non-linear behavior of the carbon fiber samples was clearly observed during extension, using the new system. Young`s modulus of carbon fiber increased approximately linearly with increasing strain for T300 、 T700 and M40 J. Stress-strain curves fitted with a non-linear model allow for the determination of a coefficient describing the modulus dependency on strain. It is found that the coefficient varies with the fiber type.Several influential factors for the laser micrometry method were investigated and it is found that the materials of specimen card may affect the tensile strength considerably, but little for Young’s modulus. The markers attached to the fiber specimen appeared to be a key to accurate measurements. The effects of the shape, size and material of markers were experimentally studied. It is showed that markers should be opaque with straight parallel sides and perpendicular to the fiber axis. The distance between two markers also affected the reproducibility of measurements. It is recommended to keep the same mark distance and the same sample length, in order to make testing results comparable between different samples.