Hygro-Mechanical Behavior of Red Spruce in Tension Parallel to the Grain

The principal objective of the project was to provide a reliable testing protocol for determination of the material-level (e.g. local and decoupled from the artifacts of the test protocol) mechano-sorptive properties of wood in the longitudinal direction that could be used for modeling of the long-term structural response of wood and wood composite elements. The method also involves determination of the hygro-mechanical characteristics of free shrinkage and swelling and short-term viscoelastic characteristics from reference tests performed on matched specimens. Tensile creep tests in the longitudinal direction at varying climate conditions were performed on small (1-mm x 25-mm x 300-mm) clear specimens of red spruce (Picea rubra). All tests were conducted in a temperature-controlled environment. Optical deformation measurement techniques were used. Strains were calculated by comparing successive digital images using Digital Image Correlation (DIC) principles. The mechano-sorptive component of total strains measured on the loaded specimens was separated by: 1) subtracting free shrinkage/swelling measured on matched reference specimens; and 2) subtraction of the magnitude of viscoelastic creep measured separately on matched specimens at constant MC (in 'dry' and 'wet' conditions). The results confirmed earlier findings reported in the literature by other researchers that the effect of cumulative moisture content change on mechano-sorptive compliance is not linear. However, no fundamentally different governing mechanisms during the first and consecutive moisture cycles were observed. The effects of applied stress level and initial moisture content on the mechano-sorptive response of wood in tension were found insignificant at the 95% confidence level. The experimentally determined mechano-sorptive compliances were expressed in terms of generalized rheological model equations with cumulative moisture content change (rather than time) as the independent variable. Based on these findings, a minimal testing protocol was proposed for routine determination of hygro-mechanical characteristics for other structurally important species

Creep parameters of spruce wood in high temperature environment

The paper presents partial results of an ongoing project dealing with thermal diffusion and mechanical behavior of spruce wood in high temperature environment (up to 180°C). The objective of this part of study was to obtain rheological properties of spruce wood which will be used in future modeling of wood performance in high temperatures. Oven dry samples (30 x 30 x 700 mm3) underwent constant mechanical loading in bending parallel to grain (σmax = 7.3 MPa, load span 600 mm) at three temperature levels, namely 120°C, 150°C and 180°C, respectively. Deflection of the samples was measured using optical non-contact method. Four rheological parameters of Burger’s model, describing immediate elastic, delayed visco-elastic as well as plastic (permanent material change) behavior, were determined which show the strong influence of the temperature on creep propagation. Experimental setup, results and further application in the modeling of high temperature treatment are discussed

Development of Small Scale Experimental Protocol and a Multi-Physics Model to Predict the Complex Hygro-Mechanical Behavior of Wood Under Varying Climates

The reliability of wood structures is strongly affected by duration of loading and environmental conditions. The goal of this study was to develop a simple method for measuring the mechano-sorptive character of the hygro-mechanical behavior of wood and to develop a model capable of predicting long term beam behavior under changing climates. The model predictions were compared with experimental results. Red spruce was the species selected for investigation. The mechano-sorptive properties were measured in tension and compression on thin specimens where moisture content variation within the material was minimal when exposed to a varying environment. The measured mechano-sorptive deformation in compression was significantly greater than that measured in tension (4 times higher at cumulative MC 60% than in tension). However, the developed compression protocol was less accurate, with a tendency to overestimate the magnitude of subsequent experimental creep behavior. A multi-physics model of hygro-mechanical uniaxial beam behavior was developed that rigorously couples spatially varying time-dependent moisture content with the uniaxial stress-strain relations. To verify the model and accuracy of the measured uniaxial mechano-sorptive characteristic of red spruce, the behavior of beams loaded by four point bending was measured in cyclically varying climate over 2.5 months. Although the model did not properly account for the immediate effect of moisture content change on mid-span deflection, as was observed in real beams, the overall trend of the predicted deflection was in good agreement with experimental results. An additional part of the thesis dealt with development of an analytical model to predict the hygro-mechanical behavior of multi-directional polymer matrix composite laminates which incorporate the mechano-sorptive effects of a phenol resorcinol formaldehyde resin. E-glass / phenol resorcinol formaldehyde resin composite material parameters were used in the model. Simulation of wetting showed that the contribution of resin compliance change due to moisture content change is approximately half of initial resin compliance which decreases the stiffness of multi-directional laminates accordingly, based on lay-up sequence

Creep parameters of spruce wood in high temperature environment

The paper presents partial results of an ongoing project dealing with thermal diffusion and mechanical behavior of spruce wood in high temperature environment (up to 180°C). The objective of this part of study was to obtain rheological properties of spruce wood which will be used in future modeling of wood performance in high temperatures. Oven dry samples (30 x 30 x 700 mm3) underwent constant mechanical loading in bending parallel to grain (σmax = 7.3 MPa, load span 600 mm) at three temperature levels, namely 120°C, 150°C and 180°C, respectively. Deflection of the samples was measured using optical non-contact method. Four rheological parameters of Burger’s model, describing immediate elastic, delayed visco-elastic as well as plastic (permanent material change) behavior, were determined which show the strong influence of the temperature on creep propagation. Experimental setup, results and further application in the modeling of high temperature treatment are discussed