RELATION BETWEEN THERMAL STRESS AND HEAT DISTORTION TESTS OF TEXTILE FIBERS

Abstract

Heat distortion test is similar to that of tensile creep test except that the temperature increases at uniform rate.Under some range of loads and a constant heating rate, the textile fiber(e. g. hot drawn P. P. monofilament) shortens its length because the thermal shrinkage overcomes the creep. At the softening or heat distortion temperature, the textile fiber begins to stretch at a rapid rate over a narrow range of temperature.The maximum shrinkage and the load of heat distortion tests are similar to those of the thermal stress measurements, when the load under which the extension begines and this heat distortion temperature is equal to the maximum stress (fmax) and temperature (Tmax) of the thermal stress measurement.Comparing these two sorts of test, we obtained the following results:1. The relation between the load (P) and temperature under which the extension begins (T) satisfies the experimental formula: P=A•T+B, where both A and B are constants.This formula is similar in shape to that of thermal stress; i.e. fmax=K•Tmax+C, where both K and C are constants.2. Under the load equivalent to the value of fmax and the same heating rate as on thermal stress measurement, the fibers never shrink by heating until the maximum temperature in the thermal stress measurement.It is therefore, assumed that free thermal shrinkage and the creep under this load are just in equilibrium until the maximum temperature of thermal stress, and the free thermal shrinkage may be equivalent to the creep recovery introduced by A. Ribnick3).3. Four-parameter model (Fig. 10 can be applied to explain these phenomena, but the viscosity of dashpot η_??_ follows Bingham flow: η_??_={fe(Tmax-fmax)}/D, where fe(Tmax) is the equilibrium thermal stress at Tmax and D is the rate of shear.