The ability to measure the stiffness of wood is important as it can be used to determine the optimal usage of the timber sample to maximise profitability and increase sustainability. The stiffness of trees and logs is measured in order to segregate them into different grades. Stiffness measurements are also made on juvenile trees and seedlings for breeding trials to improve the stiffness quality of future plantations. The traditional static bending test is considered the gold standard for measuring the stiffness of wood. However, this method is destructive, costly and difficult to use. Non-destructive testing (NDT) techniques have therefore been developed to mitigate these issues. Acoustics is the most common NDT technique used to measure wood stiffness. The time-of-flight method is the only acoustic method which can be used on standing trees. However, literature has shown that stiffness measurements obtained using the time-of-flight method can have a significant overestimation. Studies have reported the potential causes of this overestimation but the exact cause is still not known. In recent years, NDT techniques such as guided wave techniques have been developed for other industries. Guided wave testing is extensively used on metallic structures such as pipes and bars. However, there have been very few studies that utilize guided waves for wood. This thesis investigates the use of guided wave knowledge to identify the cause of the overestimation and to obtain improved NDT measurements. This thesis contains some of the first reported works to perform guided wave measurements on cylindrical wood samples. The results from guided wave experiments show that enhancement and suppression of desired wave modes can be achieved using a ring array of shear transducers. The effects of dispersion on ToF measurements are investigated and it was found that dispersion can be a potential cause of overestimation. Guided wave techniques were developed to obtain acoustic velocity and stiffness measurements for wood. The measurements were compared with the traditional resonance, ToF and static bending methods and improved measurements were obtained. More work can be done to further develop guided wave tools and techniques to be used in the wood industry.
