Acoustic emission from softwoods in tension

SummaryAcoustic emission (AE) monitoring is a non-destructive testing technique widely used to detect flaw development and crack propagation in metals, ceramics, polymers and composite materials.This paper relates the AE-strain characteristics from three softwoods tested in tension to mechanisms of deformation observed by scanning electron microscopy. All wood specimens are identical in size and radial-longitudinal in orientation, enabling the path of failure through planes of earlywood and latewood to be examined.It is found that the proportion of earlywood to latewood in each species has a marked effect on the shape of the AE-strain curves. Parana pine, containing very few latewood tracheids, exhibits a close to linear relationship between log cumulative emissions and strain until close to failure when the count rate increases rapidly. Douglas-fir, which has well-defined earlywood-latewood boundaries generates many AEs at low strain and there is greater variation in the shape of the AE characteristic between samples.Parana pine and Douglas-fir are tested at 20 °C (12.5 % EMC). Scots pine is also stressed at 20°C (12.5%EMC), 20°C (0.7%EMC) and 80°C (0.7%EMC), to assess the effect of moisture content on AE.Values of Young's modulus, stress at failure and work of fracture for the three softwoods are compared with the AE-strain data. Although the work of fracture is related to the total AEs to failure, no direct proportionality exists between the two parameters.Finally, the AE-strain data for plywood and glass-reinforced plastic (GRP), both man-made composite materials, are compared with those of wood, the natural composite material.