Method of Dynamic Tensile Testing of High Strength Fibers

Abstract

The work proposes and tests an experimental method for determining the high-speed tensile strength of high-strength fibers, based on a combination of the modified Kolsky compression method and the method of a thin ring made of the test material expanding under internal pressure. To create a pulsed internal pressure that causes expansion of the ring, accompanied by stretching and rupture of its constituent material, hydrodynamic loading is used in a split Hopkinson bar system under compression. To do this, between the ends of the measuring bars there is a sealed cavity with a liquid that transmits compressive pressure to the inner surface of the ring. Machine oil was used as a medium transmitting the compression impulse from the measuring rods to the inner surface of the thin-walled annular sample. The transmitting pressure bar performs the function of a dynamometer, which allows, based on the deformation pulse registered in it, to measure the amplitude of the pressure developed in the oil. Since the liquid inside the ring is practically incompressible, the measured stress transmitted into the liquid based on the transmitted pulse is equal to the pressure developed in it, which causes an axisymmetric expansion of the annular sample with the development of tangential stress in the body of the ring, leading to the rupture of high-strength reinforcing fibers. The maximum tangential stress, which corresponds to the maximum of the transmitted pulse recorded in the transmitting pressure bar, can be considered the required tensile strength of the composite ring material. To evaluate the performance of the proposed technique, samples in the form of thin rings made of four layers of carbon fibers impregnated with polymer glue were used. Trial tests of a composite material with unidirectional reinforcement were carried out. The stretching was carried out in the direction of the fiber. As a result of the tests of the ring sample, based on the transmitted pulse in the transmitting pressure bar, the time dependence of the development of circumferential stresses in the ring was obtained. The maximum value of tangential stresses in the ring, corresponding to the tensile strength of the carbon fibers, was approximately 2120 MPa. The rate of stress growth in this case is 35000 GPa/s. The combined use of the Kolsky method and the expanding ring method made it possible to obtain a characteristic of the tensile strength of the composite material.