Damage evolution and acoustic emission of wood at tension perpendicular to fiber

This paper presents the tracing of damage evolution in spruce loaded in tension perpendicular to fiber direction by means of acoustic emission (AE) analysis. The 2 dimensional burst source location in cross-sectional slabs of boards utilized full wave form recording of AE signals monitored by six simultaneously triggered multiple resonant longitudinal (p-)wave sensors and fast transient recorder PC cards. The location algorithm was based on the minimization of the run time differences between experimental and theoretical signal travel times. The latter accounted for the global polar material anisotropy leading to considerably variable off-axis wave velocities along the wave propagation paths, assumed to be straight; the employed orthotropic on- and off-axis p-wave velocities were: obtained experimentally from ultrasound pulse measurements. Below 50% of the ultimate load generally very few burst events were recorded, ht further increased load, burst agglomerations were predominantly associated to local areas of elevated stresses here resulting from the specific configuration of anisotropy and loading. For all specimens a distinct burst localisation was obtained in the range of 80 to 90% of the ultimate load, coinciding well with the theoretically highest stressed areas of the fracture plane at brittle failure. The correlation of AE event rates with global strain allowed a tracing of the damage evolution especially when events in confined areas are regarded. Distinct damage localisation was then denoted by a clear AE rate increase, however, not accompanied by a global stiffness change. The investigations revealed that failure of spruce - and most probably of similar softwoods - in tension perpendicular to grain, although being very brittle hom a macroscopic perspective, is preceded by progressive and localizing damage evolution on the micro-level which can be traced accurately in space and time by acoustic emission burst source analysis.