Measurement of internal stress in Radiata pine sapwood during drying using an improved online sensor

Internal stress in wood as a result of kiln drying is an important factor that affects the quality of the final product. Internal stress is inherently difficult to measure and it is usually either assessed through destructive sample evaluation after drying, or is predictively modelled using extensive material property relations which require much effort to obtain. This study describes improvements of the design, accuracy, and precision of a previously constructed stress sensor for the direct measurement of internal stress in wood during drying. The sensor is designed to be placed directly into a wood block and it records online internal uniaxial stress. Lurethane® was found to be the best material for transmitting internal stress to the sensor because reduced creep was observed, compared with Teflon® which was originally used. Stress modelling showed that a linear relation could be theoretically obtained between the stress from the sensor and the actual primary axis stress that is likely to occur in an intact wood block. Accordingly, the response of a sensor placed in a certain area could be successfully calibrated. However, rotation of the sensor away from the primary stress axis caused a decrease in recorded stress but did not affect stress trends during drying, provided the angle remained constant. Finally, stress results from drying Pinus radiata D. Don at 70°C are also described and discussed.