Application of modal analysis by transfer function to nondestructive testing of wood I: determination of localized defects in wood by the shape of the flexural vibration wave

This study was intended to detect nondestructively some defects such as knots and grain deviations in wood using modal analysis. The shapes of flexural vibration waves at the first mode generated by the tapping of wooden beams were determined using the transfer function. The wave shapes obtained were compared with the theoretical wave shape for a uniform material; and the possibility of detecting defects in wood was examined. The results are summarized as follows: (1) The shapes of flexural vibration waves at the first mode of wooden beams free of defects coincided almost completely with the theoretical wave shape. (2) The shapes of flexural vibration waves of wooden beams containing defects such as knots clearly differed from the theoretical wave shape, especially near the defect. (3) Based on these results, it should be possible to detect the presence of defects and to determine their location in wood.