Effect of Stress Level on Bending Creep Behavior of Wood During Cyclic Moisture Changes

Yaguang Zhou, Masami Fushitani, Takafumi Kubo

Abstract


This study investigated the effect of stress level on bending creep behavior of Japanese cedar (Cryptomerica japonica D. Don) during cyclic moisture changes. Tests were made at 20° with four cyclic relative humidity changes between 65% and 95% under four stress levels: 12.6, 18.5, 24.3, and 30.2 MPa, corresponding to 17, 25, 33, and 42% of short-term breaking stress, respectively. The effect of moisture content (MC) change on elastic compliance and mechano-sorptive (MS) compliance was examined. The results indicated that the total compliance revealed different behavior under various stress levels over the history of cyclic moisture changes and appeared to be greater under higher stress. Elastic compliance increased linearly with MC and affected the behavior of total compliance during MC change. As MC change increased, the MS compliance during the first adsorption and all desorption increased linearly, while during subsequent adsorption revealed a decrease in lower stress and an increase in higher stress. At a given stress level, the first adsorption led to greater MS deformation than did desorption and subsequent adsorption. The elastic parameter KE for MC effect on elastic compliance and the MS parameter KM for relationship between MS compliance and MC increased as quadratic functions of applied stress. The rate of increase in the parameters accelerated above about 25 to 33% stress level. As a result, when MC is cycled, MS compliance increase occurs at lower stress levels relative to creep at constant MC.

Keywords


Creep;cyclic moisture changes;bending;stress level

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