Acoustic (stress wave) velocity
Acoustic tools are a non-destructive method of predicting the stiffness of materials. They work by measuring the propagation of stress waves through wood, either by time-of-flight (over a fixed distance) or via resonance (vibration at the wood’s natural frequency). Specific tools using these technologies have been developed for forestry and wood processing applications. We use tools designed for standing trees, felled logs and sawn timber of a range of sizes.
Measurements on standing trees are desirable as they offer early screening of wood stiffness; unfortunately they are also the least reliable as only the time of flight method can be used. There are questions about the path of the stress wave meaning the velocity is ambiguous. This is particularly true at the tree level. We are working on ways to improve the reliability of these measurements. Measurements on logs and sawn wood are more reliable because the path is more certain, or the more reliable resonance methods can be used. Resonant instruments are sufficiently accurate to grade structural timber.
- Forestry Commission Technical Note: Using Acoustic Tools in Forestry & the Wood Supply Chain
- Forestry Commission Research Note: Developing Methods for Assessing Scots Pine Timber Quality
Airborne laser scanning (LiDAR) and photogrammetry
Airborne LiDAR can provide forest resource information on tree heights, crown width, number of trees, stand volume, and forest biomass. We are evaluating the potential for using data obtained from sensors mounted on Unmanned Aerial Vehicles (UAVs) to assess timber properties.
Kiln for timber drying
The Tree & Wood Properties group have a small kiln at the Forest Research Northern Research Station. The kiln is a scaled-down version of those found at modern sawmills and is suitable for replicating industrial kilning on small batches of timber, so it is ideal for research projects! The kiln is used for the drying of 3m batons used in large scale mechanical testing in many of our studies, in collaboration with Edinburgh Napier University. The kiln is capable or drying timber up to 3.1m length and stacks of approximately 1m wide by 2m high. It is fully kitted with temperature (dry and wet bulb), relative humidity and wood moisture content sensors. Drying schedules can be programmed according to prefixed schedules by species or custom to user requirements using the dedicated control computer.
Magnetic resonance imaging (MRI)
Magnetic resonance imaging (MRI) is a non-destructive technique that can locate hydrogen atoms within a material. This makes it particularly useful for studying internal water without damaging the sample. MRI is commonly used in hospitals for medical imaging of soft tissue, but is also a valuable tool for studying a wide range of other biological and non-biological materials. In a forestry context the sensitivity of MRI to water makes it the best method of measuring moisture inside trees and felled timber. These measurements provide that can help us understand processes such as timber drying, sap flow, the effects of drought, and injury response in trees.
Forest Research has been collaborating with the University of Surrey Soft Matter Group on magnetic resonance imaging of wood and trees since 2005. This collaboration has resulted in one of the few MRI systems in the world designed for in-situ imaging of living trees – the Treehugger - which is currently based at our Northern Research Station in Roslin
Wood strength, stiffness and density measurement
The suitability of timber for structural purposes is jointly determined by its bending stiffness, bending strength and density. These three properties are considered and tested under Forest Research’s Tree and Wood Properties programme.
Density is determined from measured mass and volume. We use destructive testing to determine both stiffness and strength using a benchtop 3-point bending machine and we have an environmental conditioning chamber to ensure that the moisture content of the wood is adjusted relative to the purpose of the test Small samples (20x20x300 mm) of clear wood are typically tested.
Terrestrial laser scanning (TLS)
Terrestrial Laser Scanning can potentially be used to make stand-level assessments of tree numbers, diameters, crown shape, stem profiles and stem and branch quality related attributes. We are actively investigating the potential applications and limitations of hand-held scanners and the combination of data obtained from these TLS devices and airborne devices in production forestry.
We use X-ray densitometry to measure the density and width of individual tree rings. This provides information about the tree and how it has been affected by its immediate environment. Wood density also has a very important impact on how we use trees. Studying growth rings therefore allows us to assess the past influences of forest management and climate, for example, on tree growth. X-ray densitometry allows us to measure density on a much higher spatial resolution than is feasible by other means.
Forest Research has a batch scanning X-ray densitometer specifically made for wood samples. We can produce radiographs with a maximum 25 micron spatial resolution. From these radiographs we can extract information about tree growth and timber properties. We can measure growth rings on cores taken from trees or radial sections prepared from discs.
The Itrax Multiscanner
Up to 15 wood samples can be measured in a single batch according to a user-defined path. Samples are scanned with a fine x-ray beam to produce a density image, in which each pixel has a defined density value.
Calibrated radiographs viewed with image analysis software and used to measure both growth ring width (blue and red bands) and within-ring wood density the charts to the left of each image are density profiles).