Development of an in-kiln monitoring system based on acoustic emission rates of wood during the low-temperature drying process

The purpose of this study was to determine if an in-kiln monitoring system based on the acoustic emission rates of wood as it dries could be developed to monitor the traditional kiln drying process. Recording the acoustic emission rates for each of the kiln steps and then analyzing the recorded data with statistical process control (SPC) provided the means to monitor the wood drying process;The study examined the acoustic emission rates of Quecus macrocarpa Michx. (bur oak) as it was dried in a 500 board foot commercial kiln using the white oak drying schedule presented in the Dry Kiln Operator\u27s Manual. Polyvinylidene fluoride film was used as the acoustic emission (AE) sensor to pick up the AE given off as the wood dried. The first seven steps of the kiln schedule were monitored for AE. The last two steps, equalizing and conditioning, were not monitored;A typical AE wave form was captured using a digital storage oscilloscope and printer. The wave form was analyzed and interpreted;It was found that significant differences existed between the mean acoustic emission rates of the first seven kiln steps. Nineteen differences were found;Statistical process control charts were created for the first seven kiln steps. The control charts indicate that steps one, four, six, and seven had a controlled drying process while steps two, three, and five had uncontrolled variations in the drying process;A slice test was done to identify the amount of stress/strain buildup in the kiln samples. Normal results were found. In the early stages of drying, the shell was in tensile stress and the core was in compressive stress. Approximately midway through the drying process the shell reversed to compressive stress and the core reversed to tensile stress;The slice test identified compressive and tensile stress. However, when the prong test was performed at the end of the drying process, no casehardening was found. When the kiln samples were examined for tissue rupture and collapse at the end of each step, no measurable amounts were found