SECA WoodWeld

Structural Engineering and Construction Applications of Wood Welding

Wood Welding Dissertation (O'Lionsigh)

 

excerpt from funding proposal 2013

Products such as glue-laminated beams and cross-laminated timber (CLT) panels have created new markets for undervalued softwood timbers, as the lamination of smaller section sizes can offset deficiencies in scale or material properties. This has enabled the construction of large-scale timber buildings through the use of massive wood construction methods, surmounting previous concerns regarding fire safety. The circle, however, has not been closed because the adhesives used in these products militate against their reuse as biomass at the end of their lifecycle. This has consequences for the efficient use of wood resources in Europe, since the most successful means of addressing the competing need for wood in construction products and as biomass is to enable a “cascade” approach to the use of these resources, first for wood products, which are later recovered for reuse, and, at the end of their lifecycle, used as biomass for energy [1]. Post-consumer wood for biomass has been identified as having significant potential for mobilization and considered simple to achieve [1]. SECA WOODWELD will extend the markets and recyclability of these products through the introduction of advanced technologies and processes to their manufacture, while expanding on the wood species currently used in these applications.

            Rotational wood-dowel welding by mechanical friction is an advanced technology of assembling wood laminates into larger sections without the addition of synthetic adhesives or mechanical fasteners, thus ensuring the end product can be fully recycled and finally recovered for use as biomass for energy production. Significantly, the weld produced between the wood dowels and wood substrate, formed through lignin fusion with the cellulose fibres and wood cells, has been shown to yield joints of considerable strength, comparable to those obtained from adhesives [2]. SECA WOODWELD brings the potential of this welding process to applications in structural engineering and construction products owing to the development of a new water-resistant weld, which has been made possible through the addition of rosin, a non-toxic wood resin, at the weld interface [3,4]. This innovative water-resistant wood-weld technology makes the manufacture of all-wood dowel-laminated beams and CLT panels for use in the construction of large-scale buildings possible and ensures a more sustainable product for the entire length of its service life and final use as biomass, owing to the elimination of contaminants such as adhesives.

            To enlarge the market for wood resources, beyond what had been achieved with the introduction of glue-laminated assemblies, SECA WOODWELD will promote the use of unexploited regional softwoods, hardwoods and broadleaf thinnings. The principal parameters that influence the strength and effectiveness of rotational wood-dowel welding are wood species and the moisture content of joined wood members, as well as drilling speed and the speed of dowel insertion [5]. SECA WOODWELD will identify the most viable species for use as substrate and dowels in CLT panels and dowel-laminated beams through the analysis of weld performance in a variety of regional hardwood and softwood species. The moisture content for various species and species mixes will be optimized during these studies. Species to be investigated include Norway and Sitka Spruce (France, Sweden, Ireland), Ash, Beech and Oak (Ireland), Ash, Larch and Scots Pine (UK), Black Spruce, Jack Pine, Eastern White Pine, Balsam Fir and Trembling Aspen (Canada) and European Beech (Austria, France). Welding with hardwoods has proven particularly successful, achieving greater strength values at the weld interface than when softwoods are used as a base material [5,6]. The use of regional hardwoods in the production of dowel-laminated beams or as finish surface on softwood CLT panels will also reduce the introduction of finishes, owing to the aesthetic value of the timber, facilitating the reuse of these products as biomass at a later date. As hardwood dowels are employed in the welding process, regardless of the species used for the laminations, a new market for poor quality growth and thinnings from broadleaf forests will be developed for the production of dowels.

            SECA WOODWELD will optimize the service life of these products using full-scale prototypes in situ, and components in laboratory environments, to assess the resistance of the weld interface to water and weather. These will include in-depth studies of water movement across the weld interface, fracture mechanics studies of the waterproofed welded dowel interfaces, accelerated durability tests of components and joints through CT scanning in a controlled climate, and in situ climate exposure studies on full-scale prototypes to assess long-term behaviour and durability. The development of appropriate techniques for drying of welded timber is also an important objective to establish the viability of welding using green timber, which could eliminate some aspects of timber processing, thus reducing energy use and production time. Dowel-welded joints in construction products also need to perform to prevailing Eurocodes and national regulations under variable loading and service conditions. SECA WOODWELD will assess the behaviour of dowel-welded wood construction products using lab-scale and full-scale prototypes together with the development of detailed mathematical models, using nonlinear finite element analyses, which will be validated by correlation with results from the prototype testing. Together these tests and mathematical models will explain the load transfer mechanisms and modes of failure associated with the dowel-welded systems, leading to the optimisation of dowel size, placement and junction designs in CLT panels and dowel-laminated beams in conformance to relevant regulations.

            Additionally, the process of rotational wood-dowel welding enables more efficient and energy saving manufacturing processes, as the welding process is rapid and does not require the stringent pressing and drying times associated with glue-lamination methods. SECA WOODWELD will improve current prototype equipment developed for automated rotational wood-dowel welding equipment, suitable to industrial use, and hand-drills, appropriate for smaller fabricators or on site use. The degree of automation and flexibility, tolerances, speed, volumes and safety will be evaluated and optimized. Evaluation of process times for open fresh cut surfaces, conditioning and surface treatment will form part of the study.

Progress beyond State-of-the-Art: Wood welding is a term that has become associated with mechanically induced linear vibration or rotation welding processes, which allow for the assembly of timber without the need for adhesives. First discovered in the labs of the School of Engineering for the Wood Industry (Switzerland), the chemical and mechanical properties of the weld were analysed and further developed in the past decade in concert with Dr. Pizzi of ENSTIB (France) and affiliated researchers across Europe and Canada. To date this technology has had success in the furniture industry, however, due to the vulnerability of the weld to moisture, it has not been applied to construction products. This innovative technology has reached maturity through the development of a weld that is both weather and water resistant, remaining free of adhesives and that is entirely wood-based. Through application of this innovative technology to CLT and dowel-laminated beams, SECA WOODWELD will enhance the environmental and economic profile of these products, increase the use of unexploited wood resources, and expand their use in building due to the increased aesthetic value associated with hardwoods.

[1] Directorate General for Agriculture and Rural Development (2010): Good Practice Guidance on the Sustainable Mobilisation of Wood in Europe. European Commission. Brussels, Belgium.

[2] Pizzi A, Leban J-M, Kanazawa F, Properzi M, Pichelin F (2004): Wood dowel bonding by high-speed rotation welding. Journal of Adhesion Science and Technology. Vol 18.

[3] H.R.Mansouri, A.Pizzi, J.M.Leban, L.Delmotte, O.Lindgren, M.Vaziri (2011): Causes of the characteristic improved water resistance in pine wood linear welding. Journal of Adhesion Science and Technology. Vol. 25.

[4] A.Pizzi, H.R.Mansouri, J.M.Leban, L.Delmotte, P.Omrani, F.Pichelin. (2011): Enhancing the Exterior Performance of Wood Joined by Linear and Rotational Welding. Journal of Adhesion Science and Technology. Vol. 25.

[5] Belleville B, Stevanovic T, Pizzi A, Cloutier A, Blanchet P (2013): Determination of optimal wood welding parameters for two North American hardwood species. Journal of Adhesion Science and Technology. Vol. 27

[6] Stevanovic, T. (2012): Sustainable Chemistry in Wood Transformation. Proceedings of the 55th International Convention of Society of Wood Science and Technology. Beijing, China.