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UBC Theses and Dissertations

Nondestructive evaluation of veneer quality based on acoustic wave measurements Wang, Jianhe


Veneer quality is critical to the performance of veneer based wood composites. In some engineered applications, lathe checks and knots have been identified as two most critical veneer grade factors affecting the shear strength which generally controls the load carrying capacity of these products. Currently, a nondestructive evaluation (NDE) method to detect veneer lathe checks and assess veneer overall quality is unavailable. In this thesis, a NDE method for veneer overall quality assessment using stress wave and acousto-ultrasonic (AU) techniques has been developed. This method is based on the detection of lathe checks and knots with wave propagation in both parallel and perpendicular to grain directions. The sensitivity of stress wave and AU techniques for detecting lathe checks and knots through observed differences in the shape of waveforms, frequency components, stress wave timing (velocities) and attenuations was evaluated. The severity of lathe checks and size of knots were also quantified with wave parameters using multiple regression models. Further, an observed veneer overall quality criterion (Q) defined by averaged lathe check depth (LCD) and percentage of knot area (PKA) was established. The significant findings of this research included: 1) wave propagation perpendicular to grain is sensitive to the presence of lathe checks, but cannot accurately detect the existence of knots; whereas wave transmission parallel to grain is sensitive to the existence of knots, but cannot reliably detect the presence of lathe checks; therefore to evaluate veneer overall quality based on the detection of both lathe checks and knots, measurements should be taken in both directions; 2) there is no significant difference in wave timing (or velocity) measurements between stress wave method and AU method with both methods showing strong promise to detect lathe checks and knots; 3) wave timing and attenuation perpendicular to grain are strongly affected by averaged lathe check depth (LCD), but quantification of LCD cannot be significantly improved by incorporating both parameters into regression models; 4) a satisfactory NDE approach of knots in veneer has been achieved by using percentage of knot area (PKA) and incorporating wave parameters such as the standard deviation of parallel wave timings; 5) a regression model based on wave velocities in two orthogonal directions can predict the observed overall quality criterion (Q) with r2 from 0.392 to 0.500 for the stress wave method, which shows promise to nondestructively evaluate the veneer quality for engineered applications.

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