Ultrasonic stiffness measurements of single plant fibers during humidity cycling

The mechanical properties of single fibers are important when estimating the behavior of fibrous materials under stress. Appropriate tests are tedious and alter the fibers structurally and mechanically. To avoid this problem we developed a non-destructive ultrasonic method to determine the stiffness of 4-5 mm long, and 30 ± 10 ¿m thick, wood fibers during R_H cycling (90% - 30% - 90% R_H, at 25°C). The stiffness estimate was obtained by measuring the ultrasonic propagation velocity and fiber density. The stiffness of larch fibres was 75-170 GPa during the humidity cycling. The maximum stiffness increased by 3.7% per cycle during 99 cycles. Our results correspond to earlier results from pull tests of plant fibers. We believe these to be the first reported ultrasonic stiffness measurements on individual plant fibers.