FATIGUE AND CREEP IN CHIPBOARD .2. THE INFLUENCE OF SLOW CYCLIC FATIGUE ON THE CREEP-BEHAVIOR OF CHIPBOARD AT A RANGE OF STRESS LEVELS AND MOISTURE CONTENTS

Matched samples of chipboard were loaded in four-point bending under e ither a 7 hours loaded/17 hours unloaded cycle, a 17 hours loaded/7 ho urs unloaded cycle, or constant load in order to asses the effect of c yclic loading on creep deformation. Tests were carried out at three hu midity levels (30, 45 and 90% RH) and at three stress levels (30, 45, and 60% of the short term strength). The creep rate of samples under c onstant load was greater than under cyclic load, with the 7 hours load ed/17 hours unloaded samples giving the lowest creep rate. Analysis of the data on an accumulated time-under-load basis showed that the law of superposition did not apply to the 7 hours loaded/17 hours unloaded samples at the 30% and 45% stress levels, with these samples giving l ower deflection than for the other loading conditions. The 17 hours lo aded/7 hours unloaded samples also did not appear to obey the law of s uperposition, although the difference between them and the constantly loaded samples was slight. Considerable scatter in data accumulated at the 60% stress level did not allow any differences in the superpositi on analysis of cyclic and constant loaded samples to become apparent. Increasing the stress level applied to samples produced an almost line ar increase in relative creep for all loading regimes. The 7 hours loa ded/17 hours unloaded samples gave consistently lower relative creep v alues at all levels of stressing, but particularly at the 30 and 45% s tress levels. Increasing the relative humidity from 30% to 65% RH had an almost negligible effect on relative creep of all samples, but incr easing the humidity to 90% RH caused a marked increase in relative cre ep.