ON THE GENERATION OF STRUCTURAL DAMAGE IN WOOD BY CYCLIC COMPRESSIVE LOADING

We investigated the effect of cyclic loading in mechanical pulping ope rations by observing morphological changes in western hemlock samples compressed along the radial direction. To compare the effect of pure c ompression with that of compression and shear; we conducted experiment s in which samples were restrained so as to prevent deformation along the longitudinal and tangential directions. Under identical loading co nditions, restrained samples display much less damage than unrestraine d ones. This indicates that damage accumulation results primarily from the shear stresses induced by the compressive action. Since the unres trained condition represents the conditions prevailing in mechanical p ulping operations, the effect of frequency, type of waveform and strai n rate have been investigated using unrestrained samples compressed to 50% strain. We found that square waveforms produce greater structural damage and modulus reduction than sine waveforms. As frequency was in creased, the amount of damage induced by the sine and square waveforms became fairly similar At a strain rate of 20.8 s(-1), numerous cracks were generated in the samples close to the specimen-platen interface but only a small portion of the earlywood fibres showed visible effect s of loading. At a strain rate of 2.94 s(-1), however little or no cra cking was observed but we observed widespread damage to the earlywood fibres. Although measured changes in the overall modulus reflect the g eneral degree of breakdown, they can Sail to characterize the fine dif ferences in the degree of destructuring. Furthermore, it appears that cracking contributes more to modulus reduction than other forms of dam age. Hence, a characterization method based only on changes in overall modulus cannot be used to assess all forms of damage generated under repeated loading conditions.