THE EFFECT OF STRAIN RATE ON THE DEFORMATION AND RELAXATION BEHAVIOR OF 6/6 NYLON AT ROOM-TEMPERATURE

The influence of strain rate changes in the range from 10(-3) to 10(-6) 1/s on the zero-to-tension loading and unloading behavior as well as short term relaxation properties is investigated using cylindrical specimens of circular cross section. A clip-on extensometer measures and controls the axial strain in an MTS servohydraulic, computer-controlled mechanical testing machine. Strains do not exceed twenty percent and all deformation is macroscopically homogeneous. An increase in strain rate causes an increase in stress level. Surprisingly, the total stress drop in a 20 min relaxation period increases with prior strain rate. When the relaxation test is star-ted in the inelastic region with low tangent modulus the total stress drop is nearly independent of the stress and strain at which relaxation commences. Unloading to zero load is not linear but curved and the strain recovery at zero stress is significant. It occurs at an ever decreasing rate and does not exceed three percent in a 12 h period. Like the relaxation behavior the recovery rate increases with prior strain rate. Repeated relaxation periods during zero-to-tension cycling can show a stress magnitude decrease during loading but a stress magnitude increase during unloading. The results suggest that a unified model with an overstress dependence of the inelastic rate of deformation could be useful in modeling.