Microwave scanning as an additional grading principle for sawn timber

Abstract

Prediction of strength is an important aspect in machine strength grading of timber. This property is strongly influenced by grain direction: Deviations from the main direction can cause a dramatic loss of strength. Therefore, an essential demand of the timber industry is finding the direction of wood fibres in a fast and non-invasive way. For three decades now the use of polarized microwave radiation has been investigated, revealing the main direction of fibres in a non-contact and non-destructive way. This paper describes the use of an innovative prototype able to detect grain deviations on a local scale using a stereoscopic method. The results of this prototype are evaluated to identify specimens with severe local slope of grain. Additionally, the prediction of strength using the microwave unit is compared to existing grading principles concerning prediction accuracy and other aspects.