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We report for the first time on the existence of a localized reduction in elasticity caused by repeated compression impaction applied along the tangential wood direction. Previous research indicates that localized strain profiles are generated by such cyclic impacting on wood along its radial direction. This finding is significant for the paper/board-making industry where wood is exposed to cyclic unipolar compression during grinding. However, the effect of the impacting direction, with respect to the orientation of the annual rings, on the localization phenomenon is unknown. In addition, the shape of the developing fatigue layer is unclear. We revisit the localization phenomenon with a focus on tangential impacting. We employed ultrasonics and x-ray tomography to quantify the induced fatigue. An interlacing technique increased the precision of the ultrasonic stiffness depth profiling technique. We studied both radial and tangential wood annual ring geometries. We used ultrasound to quantify the drop in shear modulus resulting from impacting. Both radial and tangential geometries featured strain localization, but the shape of the fatigued layer was different in the radial and tangential wood geometries (steeper profile in the radial geometry). The fatiguing reduced the shear modulus. These results tell us about the ratio of the number of hits that need to be delivered along the radial and tangential direction of the ground wood, respectively. This insight may translate into a design pattern for an energy saving grind stone surface.

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