Grading of lumber using stress waves

Abstract

The goal of this research was to develop stress wave grading technology suitable for small lumber mills. Specific goals include: 1) develop an ultrasonic probe configuration to facilitate real-time grain angle and edge knot measurement, 2) determine the statistical correlation between localized stress wave indices and lumber tensile strength and 3) compare the ultrasonic technique with other nondestructive evaluation (NDE) measurements including static MOE, impact stress wave and transverse vibration. Two hundred pieces of 2 x 6 Southern Pine lumber were randomly sampled. Material properties and NDE measurements such as static MOE, impact stress wave and transverse vibration MOEs were collected for the lumber. Before proceeding with final ultrasonic testing, pilot studies were done to study the effect of the strength reducing factors, such as grain angle and edge knots, on ultrasonic wave velocity. Wave velocity decreased as grain angle increased, with more apparent loss taking place at lower angles. The presence of edge knots decreased the wave velocity as measured along the narrow edge of the lumber. Using the knowledge gained from the pilot studies an ultrasonic probe configuration was devised to detect gross grain angle and edge knots. The tests were carried on the lumber using the configuration. Statistical models from localized stress wave indices were developed to predict the tensile strength. The linear correlation between predicted and actual ultimate tensile strength was 0.724. Ultrasonic testing was a slightly better predictor of ultimate tensile strength than shortspan bending, impact stress wave and transverse vibration techniques which had linear correlations of 0.716, 0.696 and 0.716 respectively. Separately including impact stress wave and transverse vibration MOEs into the ultrasonic model resulted in improved linear correlations of 0.769 and 0.787, respectively. In summary, knowledge from this study will be useful in the continuing development of stress wave lumber grading technology. Even though the results were only slightly better than those with short span bending and transverse vibration techniques, the ultrasonic technique appears to be promising for grading of wood.