Christelle Ganne-Chédeville

Natural and artificial ageing of spruce wood as observed by FTIR-ATR and UVRR spectroscopy

Abstract Spruce samples, naturally aged for 200, 400 and 500 years, artificially aged by a hydrothermal treatment (at 180, 160 or 130°C, relative air humidities of 14%, 40%, or 60% and for treatment times between 1 to 50 h), as well as reference samples, were analysed by Fourier transform infrared spectroscopy (FTIR) attenuated total reflection (FTIR-ATR) and ultraviolet resonance Raman (UVRR) spectroscopy. Natural ageing mostly affected the hemicelluloses and lignin, as observed from the FTIR-ATR and UVRR spectra, respectively. The UVRR spectra of the same samples after acetone extraction indicated that lignin was partially degraded and quinone structures were possibly formed. Artificial ageing at 160°C showed a significant change in the lignin structure, a well-known effect in the thermal treatment of wood, whereas treatment at 130°C did not alter the wood structure to any significant extent. Principal component analysis of the UVRR spectra confirmed that the spectra of artificially aged wood up to 160°C are dissimilar to naturally aged wood and which are also dissimilar to unaged wood.

Vibration welding of heat-treated wood

Vibration welding of wood that has been preheated according to an industrial two-step process indicates that such wood can be welded and can yield welded joints of good strength. The joint strength is, however, markedly lower than obtained when welding non-heat-treated timber. In general, weld strength of the timber is poor if welding is done on hydrothermolyzed wood. The strength results are instead much better if welding is done at the end of the complete heat treatment process, i.e., after the dry heat step. The weld lines of heat-treated wood show entangled cells where there is none or very little of the molten matrix intercellular material usually observed in welded timber. Furthermore, in weldlines obtained after hydrothermolysis an increase in rigidity and brittleness of the wood cells is observed. Hence, the wood cells are not entangled at all or very little. Both observations indicate that heat treatment has affected the main melting region of the wood, namely the intercellular material. As most of this material is already either lost or heavily cross-linked during heat treatment, only little of it is now available to melt and bind the wood surfaces during vibrational wood welding.

Wood Welded Connections: Energy Release Rate Measurement

The energy release rates of beech specimens bonded by linear friction welding were determined using double cantilever beam (DCB) tests. The analysis of the results was carried out with the experimental compliance method, which is based on the linear-elastic fracture mechanics. The compliance relation was approximated to a third-order polynomial equation for smoothing and followed by calculation of least squares. This analysis method had not been previously used for glued connections in mode I. It proved to be an ideal method for the results obtained from the study of the energy release rates of the welded joints. The value of the energy release rate G Ic obtained is, on average, 106 J/m2. The variation in the results is less than for most energy release rates of the wood–adhesive couples using beech studied previously. This proved that this method of measurement of the energy release rate adapts well to DCB wood welded specimens. It is the first time that fracture mechanics is applied to the study of linear friction welded wood joints.

Parameters of wood welding: A study with infrared thermography

Abstract Welding of wood is a well-known joining procedure that offers several advantages over traditional mechanical fasteners or gluing. During welding, extensive solid-state transformation phases occur in the so-called melting zone and the heat-affected zone. The nature and the extension of such transformations are correlated to the energy input and thus to the heat generated during the process at the wood joint interface. In the present work the influence of the welding parameters and wood grain orientation on the temperature profile and distribution and final strength of welded connections was investigated. For this purpose, the characteristics of the joints were evaluated with both destructive and non-destructive techniques. Non-destructive evaluation was performed with infrared thermography, which allowed measurement of the maximal and average peak temperature, temperature profile and distribution, and rate of temperature increase. Thus, this technique can also be used to detect welding defects and to provide information on material modification during welding.

Wood Welding: Chemical and Physical Changes According to the Welding Time

Wood welding using linear friction is a technique that has been developed in the past five years. The goal of this study was to analyze the microstructure development in the interphase enabling the wood-to-wood adhesion without any adhesive. Chemical and physical analyses have been carried out using infrared thermography, mechanical shear tests, transmitted light microscopy and X-ray densitometry. They have been considered as efficient to qualify the characteristics of the welded joints. The aim of this paper is to present a study using these analysis methods to observe the physical modifications of the wood in the interphase according to the welding time. The welding process of beech wood (Fagus sylvatia) with a welding time between 0 and 11 s could be divided into three different phases. The first phase describes changes in the physical and chemical characteristics of the wood. Densification and anatomical modifications occur in this phase. The second phase represents stabilization of the welded joint. The last phase of the cycle is a conditioning phase. All phases are controlled by the heat spread in the interphase and the time of heat exposure. Various parameters such as welding time, shear strength, temperature and width of the welded joint have been correlated and a hypothesis on the chemical reactions occurring in the interphase has been put forth. This study allowed discovering a window of parameters in which the quality of the welded joint is quite stable. Improving the quality of manufactured welded wood products without adhesive can now be done more easily due to this method.

Temperature and density distribution in mechanical vibration wood welding

Nondestructive evaluation of wood joints welded by linear vibration welding was performed with an infrared (IR) thermography technique, which allowed the maximal and average peak temperature profile/distribution to be measured. The density profile/distribution at the joint interface was measured by X-ray microdensitometry. The results show that the width of the welded zone also varies as a function of the maximum temperature reached during welding and that the maximum temperature reached at the ends of the specimens is lower than that obtained in the central part of the specimens.

Predicting the Thermal Behaviour of Wood During Linear Welding Using the Finite Element Method

A numerical model to simulate the temperature behaviour of wood welding samples during the welding process was developed to understand the influence of material parameters on the welding temperature. A finite element method and the CAST3M software were used to simulate and model the temperature changes during welding of beech wood. This model takes into account the different properties of the wood welded bondline, the geometry of the sample and the external conditions. The energy produced by the friction welding of the wood samples was determined from infrared thermography measurements for the welding process and inputted into the model. The comparison between the predicted and experimental results shows that the model is reliable. The applied pressure, the vibration, the extrusion of material and the chemical reactions, particularly exothermic reactions, are not taken into account in this model and thus probably explain the differences existing between actual and simulated values. However, this numerical simulation gives information on the distribution of the temperature in the sample. The model predicted that the temperature difference between the centre and the side of the sample is not higher than 4°C. This means that the border effects are negligible. The model was tested for different welding times. According to the model a heat flow about 70 kW/m2 is necessary at the welding line to ensure a satisfactory bonding for the chosen sample geometry. Welding of large wood pieces has also been simulated in this study.

Viscoelastic behaviour of aged and non-aged spruce wood in the radial direction

Abstract The mechanical behaviour of non-aged (modern) and aged spruce [Picea abies (L.) Karst.] wood was investigated in the radial direction using a microtensile testing device. The size of the samples was 50 × 3 × 3 mm (radial × tangential × longitudinal). Elastic, creep, relaxation, rupture and mechanosorption tests were carried out under controlled temperature and relative humidity. Optical and electronic micrographs were produced in order to correlate the mechanical behaviour with the wood structure. Results indicate that the overall mechanical properties such as Youngs modulus and the time-dependent behaviour of wood in the radial direction do not change significantly with age. However, the strength of aged wood shows a decrease of about 25% in the radial direction. This loss of strength in aged wood might be explained by localized damage at the microstructural level in the wood.

Potential Environmental Benefits of Ultralight Particleboards with Biobased Foam Cores

A new generation of ultralight particleboards (ULPB) with an expanded foam core layer produced in an in-line foaming step is under development. The environmental impacts of three types of ULPB containing foam based on 100% polylactic acid (PLA), 100% expanded polystyrene, and 50% PLA/50% polymethyl methacrylate, as well as a conventional particleboard (PB), have been compared in an LCA. Two approaches were chosen for the assessment: first, the “EPD-approach” in accordance with EN 15804 for EPD of building materials and second, a holistic-approach which allows an expansion of the system boundaries in order to forecast the consequences of a broader replacement of PB with ULPB. The results show that most of the environmental impacts are related to raw materials and end-of-life stages. Both approaches show that the exchange of PB with ULPB with a foam core based on PLA leads to a reduction of greenhouse gas emissions. On the other hand, the PLA is responsible for higher ecotoxicity results in comparison to non-bio-based polymers mainly due to agricultural processes. Both approaches allowed the drafting of complementary advisories for environmental impact reduction addressed to the developers.