Experimental Modal Analysis of a palm tree trunk under axial vibrational excitation

Palm trees are living entities that may be subject to the attack of several natural agents which affect the strength of their trunk. The most important of these damaging agents are rot fungi and parasites which proliferate into the substance of the stem and weaken it. A tree with a weak trunk and standing in an urban space or in a public park may constitute a hazard to by-passers. Furthermore, date palm trees in oases require regular control of their health status, and the use of a reliable investigation technique may in this case be decisive prior to felling a sick tree. All studies conducted previously on wood and trees, and using various vibrational and acoustical techniques for assessing the internal state of tree trunks, confirm the fact that strength weakening defects in wood lower the speed of propagation of acoustic waves in the material. However, only very few studies have been devoted to investigating the vibrational modes in the stems of standing trees, and of which almost no publication has considered the case of palm trees (monocotyledons, and which develop no growth tree rings as do dicotyledon trees). The aim of this research project is to make a contribution into this research field through presenting the results of a thorough investigation into the vibrational modes of a palm tree trunk. The study, which is purely experimental, aims thus at identifying the various vibrational modes and characterize them in terms of shape, frequency and relative response amplitude. In more detail, the stem of the palm tree is set into vibration by means of a shaker fastened to it at a fixed position, its shaft being directed normally to the axis of the stem. The response signal to the excitation is then recorded by means of an accelerometer which is moved consecutively from position to position at several points evenly distributed on a mesh on the surface of the stem. After studying the frequency spectra for the particle displacement at all the measurement locations, the most pronounced modes of vibration will be extracted and analyzed in terms of both frequency and attenuation. The study will encompass both the cross-sectional modes (which show on the circumference of the stem), and the flexural modes (which propagate along the fiber and bend the trunk as a whole). Depending on the geometrical size of the test sample, the study will be limited to the few lowest and most prominent modes of vibration, which usually occur below 2-3 kHz. Afterwards, similar experimental studies will be conducted on palm tree logs of similar geometrical and physical characteristics but subject to various degrees of decay attack, the goal being to establish a possible relationship between the extent of rot infection and the subsequent alteration of the acoustical properties of the tree trunk. Finally the purpose of the present study is to examine the feasibility of developing a novel non-destructive vibrational technique for detecting rot in standing date palm trees.