Quantifying fatigue generated in high strain rate cyclic loading of Norway spruce

Papermaking, especially mechanical pulping, consumes much energy. To reduce this energy consumption one has to understand and exploit the phenomena present during the pulping. An important phenomenon to understand is wood fatigue. We quantitatively measure the fatigue generated during high strain rate cyclic loading of spruce wood performed under conditions resembling those present during mechanical pulping. We impacted the samples with 5% strain pulses at 500 Hz. The radial direction stiffness drop in the samples was quantified by 500 kHz ultrasonic through-transmission postimpacting. The depth profile of the generated fatigue was also determined. A dependency of the amount of fatigue generated during cyclic straining on the moisture content was detected. A hypothesis about the temporal and spatial evolution of the fatigue during the process is presented. The results, supporting the hypothesis, provide insight into wood behavior under mechanical pulping conditions.