Influence of Frequency on Wood Viscoelasticity under Two Types of Heating Conditions

During lumber drying, heating speed affects the dried lumber quality and properties such as strength, elasticity, and viscoelasticity. The dynamic viscoelastic properties of Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) were investigated under two types of heating conditions: (1) specimens were conditioned at different constant temperatures ranging from 25 to 180°C for a long time, which is referred to as iso-heated wood, and (2) specimens were heated rapidly from 25 to 40–180°C, which is referred to as rapid-heated wood. Specimen dynamic viscoelastic behavior was measured in the frequency range between 0.1 and 100 Hz. Changes in storage modulus and loss factor with frequency were examined at each temperature level. The results indicated that the storage modulus decreased as temperature increased and decreased as frequencies dropped from 100 to 0.1 Hz. The minimum value of loss factor slightly shifted toward higher frequencies at higher temperatures. In general, rapid-heated wood exhibited lower stiffness and higher damping than iso-heated wood. This indicated that the unstable state was caused by rapid heating. The difference in stiffness between rapid-heated wood and iso-heated wood increased progressively with higher temperatures. The damping differences had a maximum value at 1 Hz. The minimum loss frequency that corresponds to the minimum loss factor for rapid-heated wood was higher than that for iso-heated wood. It could be deduced that the unstable structures were formed in the wood cell walls because of rapid heating and consequently increased mobility of molecular chains.

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