Wood stiffness is both a key sawn wood quality criterion and an important parameter when studying the biomechanics of trees. This project is investigating how we can use ultrasonics and image analysis to rapidly study the variation in wood stiffness on a large number of tree cross sections.
Due to time and handling costs, our traditional approaches to measuring the variation on wood stiffness within and between trees limit the number of trees that can be included in a study. We are investigating the use of ultrasonic equipment to rapidly measure stiffness on wood discs cross cut at various points along a tree’s length.
The use of ultrasound to determine the stiffness of materials is a well-established principle in engineering. There are challenges however in doing so with wood due to its inherently variable structure, e.g. grain direction or localised changes in grain angle and wood density. This project aims to develop more efficient methods of determining wood stiffness in order to make effective comparisons within and between trees. We are continually cross checking our results with traditional methods.
Figure 1. Our basic prototype testing rig applying a signalling and receiving transducer to a radial slice of a tree. An ultrasonic pulse is sent along the longitudinal axis of the wood and detected at the opposite end to measure the speed of sound.
Moving forward from the rudimentary rig (shown in the picture above) which was used to establish a proof of concept, we are finally in the process of automating our ultrasonic instruments. We are developing an electronically controlled frame in-house which holds, moves and tests full-diameter samples with operators only required to load the sample and press go. We have developed control software for the system and are currently integrating lots of moving components. A working machine is expected in January 2020… so watch this space!
As expected by theory, results indicate that stiffness data obtained from ultrasonic testing show a strong correlation with that those derived from traditional mechanical testing across a range of conifer species.
Figure 2. Sound waves detected on two wood samples of identical length. The shorter arrival time of sample 2 equates to a faster speed of sound and a stiffer section of wood.
Crucially for us, the results to date suggest that data produced using ultrasonic testing is suitable for use in comparative studies of wood stiffness between groups of trees (i.e. stands, sites or species) and between individual trees. Comparative analyses produce very similar results to analyses based on traditional mechanical testing. We will continue to validate this approach for the range of material that we are testing and improve on the practical application of the testing on mass.
Funders and partners
Forestry Commission policy
This work will support:
FC Scotland policy: The Scottish timber industry and Scotland's timber resources, particularly via The timber development programme
FC England policy: Protecting, improving and expanding England's woodland resource
Welsh Government policy: Woodlands for Wales