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Creep and Mechanosorption of Wood at High Temperature
2011 (English)In: Thermo-Hydro-Mechanical Wood Behaviour and Processing, 2011Conference paper, Oral presentation with published abstract (Refereed)
Abstract [en]

Creep and mechanosorptive behaviour of wood are two important phenomena that occur in wood during thermo-hygro-mechanical processing such as drying or heat treatment. However, these variables are inherently difficult to study and quantify experimentally due to equipment design difficulties and material behaviour complexity. For example, for elevated temperatures, pressure rated equipment is required for humidities up to 100% and experimental design must account for any overlap between creep and mechanosorption at elevated temperatures and moisture contents. A study is described in which the material properties of creep and mechanosorption of radiata pine (Pinus radiata D.Don) were experimentally obtained and statistically modelled as functions of temperature, moisture content and stress (Pearson et al 2004).

Tensile creep and mechanosorption displacement data was collected over time for tangential grain radiata pine sapwood at temperatures between 70 and 150°C for a variety of moisture contents and applied stress. A full description of three-dimensional mechanical behaviour for a single wood species is complex in that it requires all combinations of tension, compression, and shear behaviour for the three grain directions, for heartwood and sapwood, for all elements of the total strain equation and as functions of the main control variables temperature, moisture content and stress. This study reduced as many of these variables as possible to focus on equipment design and development and to keep experiments within achievable limits. Radiata pine was used because it is an important plantation grown species in New Zealand.Sapwood was used because of the high recovery rate, the tangential grain direction was chosen as it exhibits the greatest shrinkage range compared to longitudinal and radial directions and tension testing was used as it avoids buckling failure and therefore yields more uniform results. Matched samples were taken from one representative tree for which as much wood quality information as possible was recorded. The wood quality information was then used as a covariate during statistical analyses of the results. The use of samples from one tree instead of many also helped to evaluate the efficiancy and accuracy of the equipment that was specially designed for this work.

For the creep studies the moisture content was kept constant at varying levels but for the mechanosorption studies the moisture content was varied cyclically. A modified central composite experimental design was used for both the creep and mechanosorption studies to reduce the number of experiments. Wood moisture content was controlled using a high temperature dew point sensor and a preliminary set of experiments was performed to quantify the relationship between dew point, humidity and equilibrium moisture content whilst under pressure. The results were then applied to the central composite design. The same levels of temperature were used in both the creep and mechanosorption tests but applied stress was significantly reduced for the mechanosorption tests compared to the creep tests in order to reduce the presence of creep.

Both creep and mechanosorption tests were performed in a modified Moldrup variable pressure kiln. Tensile samples were connected to a specifically designed and constructed tensile rig that was able to accommodate ten samples at a time. The rig included an external jack to enable either weight removal or application during operation in order to control applied stress. The whole unit, including the Moldrup kiln was controlled through a specifically developed control and data acquisition system called MechanoSpec.

As expected, creep results revealed a dramatic increase in creep strain for higher temperature, stress or moisture content. Creep results were best described using a power law which interestingly indicated the lack of a creep limit at higher temperatures. The amplitude response of the creep power law equation was found to be primarily dependent on stress, temperature, moisture content and cycle number. Cycle number related to the number of times a load was added or removed. Stress and temperature were best described by an exponential term, which showed that the creep strain rapidly increased with an increase in either of these variables, whilst MC was best described by a power term. The power term could be expressed as a linear function of stress. Interestingly a small degree of morphing between the instantaneous elastic and creep curves was apparent at higher temperatures. This was due to curvature in the elastic region for high, compared to low temperature results. A greater displacement was statistically recorded for the first cycle of loading and unloading compared to all succeeding cycles. This may have been due to irreversible plastic deformation but it is interesting to note there may be a limit as later cycles usually reached the same level of displacement. The overall coefficient of determination for the statistical creep equation was 0.740.

Mechanosorption displacement magnitude results were best described using a common low temperature relationship where mechanosorptive displacement is a function of the absolute value of moisture change and stress. The curvature to reach the mechanosorption displacement was modelled using an exponential function containing temperature and mean moisture content terms. However, mechanosorption displacement was not found to be dependent on temperature but the results revealed that an opposing phenomenon to mechanosorption occurred due to reduced hygroscopicity. This was labelled as thermo-sorptive-aging and only occurred when a change in moisture took place which was different to the change in moisture when drying from green without adsorption. Thermo-sorptive-aging was a function of temperature and moisture content oscillation cycle number and affected the final amplitude level of EMC, but not the rapidity to which the final amplitude was reached. The cycle number was the number of times a moisture change had been effected through a setpoint change and required some form of moisture content steady state to have been reached before successive changes were made. The thermo-sorptive-aging effect was thought to be due to the loss of hydrogen bonding sites in cellulose that occurred every time bound water and cellulose bonds were alternatively broken and reformed through wetting and rewetting. It is likely such a phenomenon could be used to reduce drying times and internal checking through some sort of bond shuffling, where wood structure is massaged during oscillatory drying. This would theoretically allow as much of the wood structure as possible to be gently morphed into a modified form without placing excessive stress on the remaining original crystallite bound water bonds that occur when drying without adsorption. The overall coefficient of determination for the statistical creep equation was 0.880.

Place, publisher, year, edition, pages
2011.
Keyword [en]
Creep, mechanosorption, elevated temperature, radiata pine, thermo-sorptive-aging
National Category
Engineering and Technology Wood Science
Research subject
Technology (byts ev till Engineering), Forestry and Wood Technology
Identifiers
URN: urn:nbn:se:lnu:diva-41651OAI: oai:DiVA.org:lnu-41651DiVA: diva2:800179
Conference
COST Action FP0904, Thermo-Hydro-Mechanical Wood Behaviour and Processing, February 16-18 2011, Biel(Bienne), Switzerland
Available from: 2015-04-02 Created: 2015-04-02 Last updated: 2016-05-03Bibliographically approved

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