Swept Sine Humidity Schedule for Testing Cycle Period Effects on Creep1

The stacking life of corrugated containers under load decreases as relative humidity (RH) increases and when RH cycles, compared with when RH remains stable. Conventional RH test schedules that rely on fixed cycle periods do not adequately reflect the significance of moisture diffusion and hygroexpansion rate phenomena on the creep of corrugated containers and general wood fiber products. With cycle periods shorter than a critical period, materials are relatively unaffected by cyclic changes in RH. Longer periods amplify material hygroexpansion and accelerate creep. In this study, we propose a swept sine RH schedule in which the cycle frequency varies logarithmically with time and set forth equations for generating a numerical control signal. Data from creep tests of corrugated fiberboard reveal the frequency dependence of the amplitude and phase relationships between cyclic hygroexpansion and cyclic RH. A swept sine RH schedule yielded the continuous form of the response characteristics varying with cycle period that could not be acquired with multiple constant period schedules. The critical cycle period and the hygroexpansion response to moisture as a function of cycle period are proposed as criteria for discriminating among hygroexpansion-creep models.