Rolling shear modulus and damping factor of spruce and decayed spruce estimated by modal analysis

Modal analysis was used to determine the rolling shear modulus of Norway spruce samples that were either untreated or inoculated with fungi. The resonance frequencies of centimeter-range cuboids were measured using contact-less laser interferometry. A three-dimensional theoretical model describing the orthotropic behavior of the material was used to calculate the resonance frequencies. Using an iterative scheme based on the least-squares method, the value of the rolling shear modulus was then extracted. In this first investigation, the decrease in the rolling shear modulus and the weight loss of Norway spruce inoculated with white-rot fungi Heterobasidion annosum and Ganoderma lipsiense were studied for three different exposure times ranging from 4 to 12 weeks. Comparison of measured and theoretical resonance frequencies confirmed that operation was in the applicable range of the theoretical model for the inoculated specimens. A decrease in rolling shear modulus of up to 10% (H. annosum) and 50% (G. lipsiense) was found.