Non-contact Ultrasonic Characterization of In-plane Density Variation in Oriented Strandboard

In-plane density variations exist in oriented strandboards (OSB) due to random flake deposition in mat forming. The integrity of structural performance of the board in service depends on the degree of homogeneity of its horizontal density. In this paper, non-contact ultrasound (NCU – air-coupled) in through transmission was used specifically to correlate the NCU parameters to the spatial density elements for the laboratory-made southern pine OSB. Test boards consisted of two replicates at each of three resin content and three nominal density levels. The state-of-the-art scanner with high transduction NCU transducers were used to locate and map spatially the internal structure distribution of each board type. NCU variables velocity and attenuation are effective to predict density for all resin content and density levels. The predicted average density on each grid in the panel was in situ validated within ±10% tolerance limit from the panel average density. Control charts and spatial graphic plots produced reasonable resemblance to the measured density distributions in OSB. NCU is therefore a sensitive tool to characterize the local representation of internal properties of the wood composites and other biofiber-based particleboards as well. Introduction: Nondestructive evaluation technologies have been utilized to study internal material properties such as defects and bonding quality for wood composite products (Bucur et al. 1998). Among various scanning methods (Szymani 1985), only ultrasonic and electromagnetic methods have the potential to reveal defects that are not visible on the surface. Because the propagation of stress wave is a mechanical phenomenon, techniques based upon them are commonly used in wood-related research such as forced vibration, impact stress wave, acousto-ultrasonics, and acoustic emission (Vun and Beall 2004). In addition to these techniques, the ultrasonic through-transmission (UT) is simply done by transmitting ultrasonic pulses on one side through a specimen and capturing the response signals on the opposite side (Bhardwaj 2002), which allows continuous scanning of relatively large surface areas for spatial material properties. The mechanical strength of oriented strandboard (OSB) is strongly related to mean panel density and in-plane density distribution. In-plane density variation exists due to random particle deposition in mat forming process (Suchsland and Xu 1989). Contributing directly to density variation, intrinsic raw material attributes, such as strand type and geometry, amount of fines, strand dimension and configuration, form the basic modelling factors for spatial structure of wood composite (Steiner and Dai 1993). Expensive γ-ray computed tomography was successfully used to diagnose density variation …