Accurate measurement of creep of nylon-6,6 at constant temperature and humidity

Abstract This paper describes a purpose-built environmental chamber housing three creep machines for accurate measurements of creep in a hygroscopic polymer. The environmental chamber controls the temperature to within ±0.1°C and the relative humidity to within ±1% r.h. or better. The creep machines are sufficiently accurate to limit the error in the measured modulus to ±0.4% at the 0.1% strain level. Also described are new designs of lateral extensometers and of a simplified zero-load control machine based on similar principles to those of the creep machines, but which can also be adapted for accurate creep tests on very small test pieces using loads of as little as 1 N. Creep results are presented of nylon-6,6 at several constant stresses and relative humidities at 23.5°C, each test lasting for six weeks. Evidence is given, in the form of 24 h creep curves, of an aging phenomenon in nylon-6,6: a progressive increase in stiffness with storage time at the test condition following humidification at 90% r.h.