Effect of structural defects on the strength and damping properties of a solid material

In material sciences solid materials are known to be more or less dispersive, i.e., the Modulus of Elasticity, MOE, and the loss factor η are frequency dependent. Furthermore, these two parameters are not totally independent of each other as their frequency variations exhibit some interrelationship. Early studies have revealed that the elasticity and damping properties of wood, unlike many of other solid materials, start to show some frequency dependence already at a few KHz and that these variations depend among others on the species, the drying process, and the size of the specimen. In the present study the variations of the MOE and the loss factor are studied in terms of the number of defects in a wooden element. To this end an increasing number of holes is drilled in a wooden beam, and the major resonance frequencies for the longitudinal mode of vibration are localised on the frequency response curve permitting the determination of the MOE and η. The loss factor is evaluated by means of a room acoustical technique using the concept of reverberation time. A refined procedure permits to evaluate in an efficient manner the reverberation time from a single measurement of the impulse response. This latter is also shown to be easily assessed through the use of a cross-correlation operation between the response signal of the system and the input signal to it, this latter being taken as a random broadband noise. As an application, these concepts are used for the study of a wooden beam, and the results obtained for the longitudinal mode of vibration are presented and discussed. It is found that the MOE and η values are dependent on the number of defects present in the test sample, and that for an increasing number of these defects the MOE's value decreases steadily whereas the loss factor increases, although to a lesser degree. Some possible explanations of the phenomena underlying such behaviour are addressed and discussed.