Evaluation of finger-jointed lumber strength using critically refracted longitudinal waves and constituent wood properties

Finger-jointed lumber is one type of reconstituted wood product which has been studied using acoustic methods of nondestructive evaluation (NDE). While past research with acoustic NDE has established correlations with finger joint tensile strength, predictive capabilities remain weak. The goal of this research is to apply a critically-refracted longitudinal wave (L{dollar}sb{lcub}rm CR{rcub}{dollar} ultrasonic technique to finger joints in structural lumber and evaluate L{dollar}sb{lcub}rm CR{rcub}){dollar} with respect to other methods for determining joint strength. This study will also help in understanding the effect of wood microstructure on joint strength, and should advance the understanding of the relation between wood microstructure and stress wave propagation.; This work has shown that L{dollar}sb{lcub}rm CR{rcub}{dollar} inspection of wood is feasible and offers several advantages over the current methods of wood evaluation. Highly localized inspections provide reduced signal attenuation leading to higher inspection frequency. This may be particularly beneficial to the inspection of finger joints where the wood fiber discontinuity at the joint typically results in reduced sound energy transmission.; Results indicate that an inspection frequency of 1.30 MHz may be too high for inspecting finger joints and marginally high for inspecting clear wood. Further, studies of the beam radiation patterns suggest that ultrasonic energy penetrated only a small portion of wood closest to the inspection surface, leading to limited knowledge of wood properties in the thickness direction.; In a study using 325 finger-jointed samples of southern pine 2 x 6 structural lumber, the L{dollar}sb{lcub}rm CR{rcub}{dollar} ultrasonic method has shown a slight, but statististically significant, sensitivity to finger joint tensile strength. Physical wood properties were also related to joint strength, and when combined with ultrasonic data, provided a linear correlation of R = 0.78. A measure of the static bending modulus provided a correlation with tensile strength of R = 0.55. While several potential measures of joint strength were examined, scatter in the data prevents an accurate prediction model from being developed. The relation between wood properties and joint strength are not yet fully understood. However, a relation between wave behavior and wood microstructure is proposed where the predominant microfibrillar orientation in the cell wall affects the observed wave velocity.