Investigation of stress wave propagation mechanism in American red pine trees

Stress wave technology is quickly becoming one of the main wood quality evaluation tools for plantation trees. However, the mechanism of stress wave propagation on standing trees is not well understood and the research in this area is lacking. In this study, we investigated how stress wave propagate along a standing tree through a series of laboratory experiments. Three freshly-cut red pine (Pinus resinosa) logs were used to simulate the standing trees and the time-of-flight data were obtained using a stress wave microsecond timer (Fakopp Inc.). It was found that the wave front of the stress waves on the longitudinal-radial (L-R) log plane, represented by the time-of-flight contour curves, was oblique (relative to the longitudinal axis of a log) in the beginning, with the wave leading along the surface of log. As the travel distance increased, the wave front became less oblique, and eventually became vertical to the longitudinal axis of the log. The difference of time-of-flight on a cross section was affected by both the travel distance and log diameter, and the difference of time-of-flight was smaller for long travel distance and small log diameter. The results also showed that on the log surface layer along the longitudinal mid-section, there was a very good linear relationship between travel distance and the time-of-flight.