Down to the woods - quality control
COST, the intergovernmental framework for European Cooperation in
Science and Technology, has been running a series of R&D programmes
in wood. Martin Parley reports on the final Cost Action E53 conference,
held at Dynamic Earth in Edinburgh, UK.
Entering Dynamic Earth feels like taking a walk outdoors. Interactive exhibitions of the planet’s history take you on a journey showing why this world has sustained itself for so long. It was fitting that the Cost Action E53 conference The Future of Quality Control for Wood & Wood products, was staged here from 5-7 May, aiding the development of the industry.
‘The aim is to improve existing methods and techniques for fast, accurate quality assessment at every link in the forestry wood chain,’ said Professor Robert Kilger from Chalmers University of Technology, Building and Environmental Engineering, Steel and Timber Structures, in Gothenburg, Sweden. He presented the keynote speech entitled ‘Timber Quality for the Construction Industry’.
Kilger started by asking questions, such as are we focusing on the correct requirements and right properties? What kind of knowledge does a producer of timber or a trader have of enduser needs? Does timber come up to end-user expectations?
To this end, Chalmers University has conducted an internet survey with the help of the Norwegian Institute of Wood Technology (NTI), encompassing 25 countries to discover end users expectations and demands for timber products. The research has revealed that the companies in the processing industry feel that the timber market is producer driven, as customers have to accept what is offered. Over half the companies see wood as a product that is prone to defects and the end-users should just accept this.
Nonetheless, Kilger says techniques such as scanning, drying, advanced measurements and monitoring, as well as new grading equipment, are making it possible for more consistent properties to be achieved. However, he highlights the fact that moisture content is one of the most important areas when catagorising timber quality.
He says, ‘More than 75% of the companies in the secondary processing industry do not use limits News regarding distortion in quality control, do not know and do not answer questions relating to twist distortion, bow, spring and cup distortion.’ Yet, twist distortion was found to be the most important defect for the companies.
According to the survey, the strength grading of the timber is rarely used and visual grading is the most common, but it is rarely accompanied by scanning. In general, the importance of a low price for timber products varies for trading and building companies. Kilger concluded by saying, ‘All of us working with building companies must have a mission – to speed up and improve communication between timber producers and users of timber as a building material. We need to improve the education of builders, buyers of building products and engineers.’
However, it is not possible to manufacture products in sawmills according to the customers’ specific needs, meaning that profits in the area are sensitive to economic conditions says Professor Arto Usenius from the VTT Technical Research Centre of Finland.
He states that delivery times of timber to the sawmill can be lengthy and that there is an emphasis on high volume and cost minimisation in the production process. Therefore, new business concepts and processes for the industry are needed to increase value through a more customeroriented approach.
‘Very few small mills can manufacture special wooden components according to the actual needs of customers,’ said Usenius, ‘Production and business are not adaptive, feedback is not generated and cannot be utilised. There is a limited volume of reliable, and less reliable data, measured, however, it is only used locally.’ If this is to be improved, the following things need to be put in place, he suggested – sawmills manufacturing more value-added components, control and optimisation of information and material flows in planning and production systems, and also, the introduction of integrated information systems covering entire conversion and supply chains.
Wood drying was also a primary focus at the event, with talks surrounding moisture content and the sorting of logs and planks high on the agenda.
Johannes Welling from the Johann Heinrich von Thuenen-Institut, Institute of Wood Technology and Wood Biology, in Hamburg, Germany, outlined that, ‘the process of drying does not only involve the removal of water, it strongly influences the properties and usability of the timber. Drying needs a lot of energy and time, it is costly and involves high risk, requiring a lot of expertise and practical experience. False drying conditions may lead to serious devaluation or even complete loss of the timber.’
Welling stated that in the 1950s many scientific publications were produced describing the drying process of wood, predicting the drying time and moisture distribution within the material, and exploring stress development during the process – known as ‘drying quality’, coined by the European Drying Group.
In 1996, the Group published a recommendation for assessing drying. This was the first framework that allowed practitioners to specify and control the drying quality of timber, and led to the European Committee for Standardisation forming a set of technical standards for nations in the EU. The European Committee for Standardisation (CEN) Technical Committee produced standards for ‘round and sawn timber’ in 1996, specifically for moisture content, casehardening, drying quality and acceptance control.
However, acceptance and use of these standards has not been widespread. Welling says that this is due to the new standards not being sufficiently visible to, and not fully understood by, practitioners, who fail to change their attitude that specification, and quality control, and additional requirements might lead to extra cost.
Pieter Rozema from Brookhuis Micro-Electronics, in Enschede, The Netherlands went on to explore the various moisture detectors available, such as resistance and capacitance type hand-held moisture meters, both of which are relatively accurate. While inline moisture measurement is a contact-free method conducted on production lines. It has higher reliability because every board is measured more thoroughly by testing more than one area. The final moisture content of the drying can be found and also the spread.
Other industry issues, such as drying processes and strength grading, were also covered and further testing procedures explained in research group technical analysis presentations (see below).
Untangling the knots
Senior scientist Franka Bruechert at the Forest Research Institute in Baden-Wurttemberg, Germany, has explored the use of computer tomography (CT) scans on Scots pine wood from southern Sweden, to enable automated detection of branches using image analysis, uncovering the shape of knots in the wood. As sapwood areas have high water content, they absorb radiation in a similar way to branch wood and the areas both appear grey, meaning CT use has been restricted. An algorithm has been developed to remove the sapwood areas from the images, using a polar transform with the pith as pole for each slice, followed by pinpointing the heartwood-sapwood boundary on the radial coordinate. To cut out the whorls, the values are corrected using interpolation. Threshold values and morphological procedures are used to detect the knots, giving a 3D image of the log and shape and position of all the knots in the heartwood.
This work has been conducted under the Woodvalue project that aims to develop a standardised methodology in Europe to define, measure and value the efficiency and profitability of key wood supply chains.
Shear tests are used on glue lines for quality control of the wood adhesive glulam. However, within the standards to be met, the testing method is not specified, and is only given in principle. Existing techniques mean that the stress in the glueline is not equally distributed, but is a mixture of shear and normal stresses. Rene Steiger, Senior Scientist at Materials Science and Technology Institution, Empa Wood Laboratory, Dubendorf, Switzerland, outlines an improved method.
In Europe, the block shear test is commonly employed with stresses perpendicular applied to both ends of the sample. In Steiger’s new version a small angle of 14 degrees is introduced so compression stresses result in lower shear stresses. The test principle is to perform axial compression tests with an angle between the grain and the loading direction. The new method has shown that it could provide reproducible shear strength values, and that the shear strength of bond lines displays lower variations.
Regarding wood failure, no difference has been reported. Steiger says that target limits of shear strength and percentages of wood failure in glulam quality control standards needs to be assessed.
Thomas Tannert from Bern University of Applied Sciences, Switzerland and his associates presented research that arose from the need to inspect glue-laminated timber regularly as degradation and subsequent structural failures can occur.
In 2006, in Europe, there was high occurance of timber buildings collapsing due to the heavy snow lying on the structures, although tests had not found any cracks in the framework. In many of these, the load bearing elements were found to be glued-laminated beams where visual inspection missed the fact that the moisture content was too high, allowing fungi to grow, destroying the wood. In buildings that survived, replacement timber was commissioned to support the existing structure and used glulam adhesive.
The team has performed bending tests in the form of three-point tests with a free span between the supports. For large-scale shear tests, the specimens are loaded with three glue-lines exposed to the shear stress between the support and the loaded plate. These have been undertaken in displacement control at a constant loading rate of three millimetres per minute. The results have demonstrated that tests on core samples can be used to determine the shear strength of glue-lines, but this will only give information on the area sampled. Therefore, existing assessment methods need questioning and strength estimates must be taken from the damaged areas or areas of expected stress.
What a strain
The aim of this research was to discover to what degree strain fields around a knot can be revealed using a digital image correlation (DIC) and finite analysis. Conducted by Jan Oscarsson from SP Technical Research Institute of Sweden and Professor Anders Olsson and Berti Enquist, both from Linnaeus University, Sweden, wanted to increase the accuracy of machine strength grading. This is currently judged on measured characteristics and predicted strength properties. Knots have the greatest effect on grading and are the major cause of strength defects.
The study has focused on the Norway spruce which is subjected to tensile forces. White light DIC uncovers the strain fields, the link between strain fields measured on different sides of the sample, and their distribution. Using the ARAMIS materials measuring system, strains have been detected on the surface in both longitudinal and lateral directions and recorded to an accuracy of 0.05-200%. Two cameras record displacement and deformation at fixed intervals (with load increments of 400N). Section diagrams and contour plots are used to show the strain results, and by overlaying the contour plots onto the image, strain distribution can be shown. The results demonstrate that DIC is good for qualitative and quantative information about knot behaviour and that knots act as barriers to strain.