Stress-strain relationships in yarns subjected to rapid impact loading: 4. Transverse impact tests

Abstract

If a textile yarn segment clamped at each end, is impacted transversely at the midpoint, the stress-strain curve for this yarn can be obtained from measurements on a high speed photographic record of the motion of the yarn. This paper describes the apparatus and pro­ cedure used. Stress-strain curves for high rates of straining, of the order of 5,000 percent per second, obtained by this method are given for high-tenacity nylon, Fortisan, and Fiberglas. Comparison with stress-strain data obtained at conventional rates shows that these materials have higher initial moduli, and that their stress-strain curves remain linear up to higher stress values when the testing rate is high. The breaking tenacities are slightly greater and breaking elongation slightly smaller at these high test rates. J the behavior of textile yarns subjected to longitudinal impacts of order of magnitude 50 m/sec was dis­ cussed. Equipment using high speed photography was described. A procedure for obtaining stressstrain curves at initial rates of straining of the order of 5,000 percent per second (300,000% per min) was given. The concept of a limiting breaking velocity, approximating the lowest velocity at which a textile yarn will rupture immediately upon tensile impact, was introduced, and values of this characteristic quantity were given for several different yarns. Equipment for studying the behavior of yarns impacted transversely at velocities of order of magnitude 50 m/sec has now been constructed. This equipment is described here. A method of obtaining stress-strain curves from photographs of the succes­ sive configurations of a clamped yarn subjected to transverse impact is outlined. Stress-strain data obtained in this way are given for high-tenacity yarns of nylon, Fortisan,2 and Fiberglas.3 2. Apparatus An assembly of the transverse impact equipment is shown in figure 1. The yarn specimen is clamped to a rigid massive table, A, on which a coordinate grid system is inscribed. Central transverse impact is made by a freely flying projectile that has been struck by a rapidly rotating hammer, H. Apparatus for rotating and stopping the hammer is separate from the specimen table in order to avoid jarring. The 6-in. hammer rotates under a powerful torque through an arc of 270° before striking. The force to rotate the hammer is applied at the surface of a 2-in. shaft by straps from four springs, which can be extended up to 20 in. by a motor. At full extension the total tension in the four springs is 800 lb. The hammer is held in place by a latch, which can be