Nondestructive evaluation of green wood using stress wave and transverse vibration techniques

Longitudinal stress wave and transverse vibration nondestructive testing (NDT) techniques have proven to be accurate means of evaluating the quality of wood based products. Researchers have found strong relationships between stress wave and transverse vibration parameters (e.g., wave velocity and modulus of elasticity predicted using NDT measurements) with the actual static bending properties (e.g., modulus of elasticity from static bending tests) of dry as well as green wood based materials. Therefore, these techniques may be used to presort or grade structural wood prior to drying, provided that cross correlation equations are developed to relate wood properties in the dry state with NDT measurements in the green state. Discarding wood with inferior properties prior to drying can result in significant cost savings in the drying process. This paper presents the results of an experimental program designed to examine the relationships between stress wave and transverse vibration characteristics (i.e., stress wave velocity, and modulus of elasticity predicted from stress wave velocity and transverse vibration measurements) with the static bending modulus of elasticity (MOE) and modulus of rupture (MOR) of green as well as dry southern pine dimension lumber. Also, cross correlations have been developed between green stress wave and transverse vibration properties with the dry static bending MOE and MOR. Such cross correlations are important for grading wood in the green state itself, so that drying costs can be minimized. This research resulted in good cross correlation relationships between the static bending MOE values of dry southern pine and the nondestructive parameters in the green state (e.g., stress wave velocity and derived MOE, and transverse vibration based MOE values for green wood). Also, ultrasonic tests were conducted over limited distances (300 mm [12 in.]) at three locations on the 2.4 m (8 ft) long specimens to see if good correlations can be achieved. The low coefficients of determnination lead to the conclusion that ultrasonic testing is not suitable for grading of 13 to 2.4 m (6 to 8 ft) long dimension lumber.