Pre-grading of spruce logs containing frozen and unfrozen water by means of frequency-based nondestructive testing (NDT)

The possibility for pre-grading of logs throughout the year has been evaluated based on dynamic modulus of elasticity (MOEdyn or simplified E-dyn) by means of frequency-based nondestructive testing (NDTfreq). The challenge is that E-dyn is variable in the frozen, partly frozen, and unfrozen states of logs (state of matter). In the unfrozen state, NDTfreq is reliable if the moisture content does not fall below fiber saturation. The state of the matter was tested via temperature measurements of air and log by means of an infrared technique and sensors, and information was evaluated from NDTfreq. The temperature measurements proved to be inefficient for industrial application because of unreliable data and long measurement times, whereas the NDTfreq measurements allowed the determination of meaningful damping factors. A sufficient level of predictability for more than 60% of the measurements was found. The log measurements in the partly frozen state are inconclusive because E-dyn data fluctuate in the presence of both liquid and frozen water. For the completely frozen logs, a correction is introduced as a precaution that avoids an overestimation of E-dyn in the unfrozen state.