SWEPT SINE HUMIDITY SCHEDULE FOR TESTING CYCLIC PERIOD EFFECTS ON CREEP

The stacking life of corrugated containers under load decreases as rel ative humidity (RH) increases and when RH cycles, compared with when R H remains stable. Conventional RH test schedules that rely on fixed cy cle periods do not adequately reflect the significance of moisture dif fusion and hygroexpansion rate phenomena on the creep of corrugated co ntainers and general wood fiber products. With cycle periods shorter t han a critical period, materials are relatively unaffected by cyclic c hanges in RH. Longer periods amplify material hygroexpansion and accel erate creep. In this study, we propose a swept sine RH schedule in whi ch the cycle frequency varies logarithmically with time and set forth equations for generating a numerical control signal. Data from creep t ests of corrugated fiberboard reveal the frequency dependence of the a mplitude and phase relationships between cyclic hygroexpansion and cyc lic RH. A swept sine RH schedule yielded the continuous form of the re sponse characteristics varying with cycle period that could not be acq uired with multiple constant period schedules. The critical cycle peri od and the hygroexpansion response to moisture as a function of cycle period are proposed as criteria for discriminating among hygroexpansio n-creep models.