Modal frequencies to estimate the defect position in a flexural wooden beam

Abstract

An inexpensive methodology is proposed to identify and locate a single defect within a wooden beam using free a flexural vibration technique. A similar approach has been introduced in the literature based on free longitudinal vibration, which was selected to be a leading frontier for the present research. The flexural vibration technique was tested for five groups of the absolutely clear specimens while holding a manually drilled hole at 0.1, 0.2, 0.3, 0.4, and 0.5 of their total span. The beams were tested in free flexural vibration with both ends in a free condition before and after drilling, and relative shifts of modal frequencies due to the presence of the defects were measured and compared to their mathematically calculated values in a sinusoidal equation. Using the method of least squares, a coincidence factor was developed based on the differences of the measured and calculated shifts of the four initial resonance frequencies where the minimum district of the coincidence factor curves successfully indicated the defected area. Though the longitudinal vibration approach was promising enough to estimate the position of the defect, its combination with the flexural vibration might increase the degree of confidence in the identifications.