Short-Term Creep Tests on Phenol-Resorcinol-Formaldehyde (PRF) Resin Undergoing Moisture Content Changes

The objective of the study was to develop an experimental technique that would allow determination of the hygro-mechanical properties of thin uniform resin films undergoing moisture content changes; and to use the technique for assessment of the hygro-mechanical performance of phenol-resorcinol-formaldehyde (PRF) resin.Creep tests on 6 small specimens of PRF film under constant stress (50% of the short-term ultimate stress level), at room temperature (23°C ± 2°C) and controlled relative humidity (RH) conditions (drying or wetting) were carried out. Digital images of the specimens were acquired using a CCD camera at discrete time intervals during the experiments. Displacements were then measured by comparing successive images using digital image correlation principles. Separation of strain components from total strain recorded during the creep tests was carried out by using data from two reference tests performed on the same material: 1) free deformations of unloaded specimens during drying or wetting conditions, and 2) creep under equilibrium conditions.The experimental method developed for the study provided a tool to determine hygro-mechanical properties of thin resin films. Quantitative data on material properties of hygroscopic resins determined by means of the technique may be used for modeling the behavior of adhesive bonds as well as adhesive bonded materials in varying climate conditions. The PRF resin revealed a distinct mechano-sorptive behavior, though it seems to be less significant than that reported for wood in transverse directions

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

Acquisition of Laser Scanning Confocal Microscope for Biological and Materials Research

Biological and Materials research at the University of Maine will be strongly impacted by the acquisition of a Laser Scanning Electron Microscope as a result of this NSF-MRI award. The Leica confocal unit, along with an upright and inverted microscope and digital camera will form a multi-user facility for campus researchers working with a range of biological and materials problems. Initially, 13 faculty members from 8 academic departments have projects planned for the instrument. The microscope will be the first of its kind on the University of Maine campus. A wide range of research problems will be attacked through use of this instrument in conjunction with existing instrumentation. Examples include: Biological research projects involve understanding the maintenance of bone structure through mapping of the distribution of proteins, the reproduction of algae in troubled marine ecosystems, and bacterial or viral diseases of fish. Additionally, improved understanding of fundamental microbe-plant symbiosis and wood decay processes will allow for future applied research to attack economically and socially important problems. The development of biofilms for sensors of biological and chemical warfare agents will be aided through film characterization using this instrument. This $10 million effort is a University/private industry/Department of Defense partnership. Materials-related research includes determination of morphology and fracture of wood-based composite material, microfracture characterization of cement-based materials, and characterization of paper roughness. This work, along with environmental scanning electron microscopy and X-ray microtomography, is focused on the measurement of microstructural mechanisms of material behavior and its improvement through subsequent processing changes. The ultimate benefit will include more efficient use of natural resources, better performance and lowered product costs

Nondestructive evaluation of wood properties by stress wave spectral analysis

The influence of selected properties on the propagation of stress waves in wood was investigated. Waveform analysis of the stress waves was performed using spectral analysis techhniques developed for stationary random processes. Information analyzed from the stress waves included wave velocity, energy spectra, and the frequency response function. Three wood properties investigated as to their influence on stress waves propagation were grain angle, moisture content, and weight loss caused by decay. Significant relationships between grain angle and the wave properties of velocity, amplitude gain, and total gain were obtained. Significant damping of the stress waves was observed at large grain angles and moisture content values above the fiber saturation point. No significant equations were found for consistent prediction of moisture content. The results of the decay study showed that as weight loss increased, the ratio of energy of the stress wave to that input to the specimen decreased for the perpendicular to grain case. Two approaches toward prediction of wood strength were investigated. The first method employed prediction of wood properties from the stress wave spectral characteristics. Known relationships between these wood properties and strength were then utilized. The second approach involved direct correlation of the stress wave spectral properties with strength. Significant correlatlons with strength were obtained using both approaches. Application of basic results are discussed as to their applicability toward development of an [sic] nondestructive evaluatlon (NOE) procedure for wood poles used in transmission line structures

A Technique to Measure Strain Distributions in Single Wood Pulp Fibers

Environmental scanning electron microscopy (ESEM) and digital image correlation (DIC) were used to measure microstrain distributions on the surface of wood pulp fibers. A loading stage incorporating a fiber gripping system was designed and built by the authors. Fitted to the tensile substage of an ESEM or a Polymer Laboratories MINIMAT tester, it provided a reliable fiber straining mechanism. Black spruce latewood fibers (Picea mariana (Mill) B.S.P.) of a near-zero microfibril angle displayed a characteristically linear load elongation form. ESEM was able to provide real-time, high magnification images of straining fibers, crack growth, and complex single fiber failure mechanisms. Digital images of single fibers were also captured and used for subsequent DIC-based strain analysis. Surface displacement and strain maps revealed nonuniform strain distributions in seemingly defect-free fiber regions. Applied tensile displacements resulted in a strain band phenomenon. Peak strain (concentration) values within the bands ranged from 0.9% to 8.8%. It is hypothesized that this common pattern is due to a combination of factors including the action of microcompressive defects and straining of amorphous cell-wall polymeric components. Strain concentrations also corresponded well to locations of obvious strain risers such as visible cell-wall defects. Results suggest that the ESEM-based DIC system is a useful and accurate method to assess and, for the first time, measure fiber micro-mechanical properties

Moment-Curvature Analysis of Coupled Bending and Mechanosorptive Response of Red Spruce Beams

In this study, an expanded comprehensive numerical approach to predict hygromechanical behavior of beams is proposed that rigorously couples spatially varying time-dependent moisture content fluctuation with uniaxial stress-strain relations. The constitutive model, consisting of elastic, viscoelastic, and two mechanosorptive strain elements connected in series, was used in a layered moment-curvature flexural analysis. The procedure is numerical and is able to take into account effect of moisture content changes, different mechanosorptive behavior in tension and compression, and cross-sectional hygroexpansion. The overall trend and magnitude of predicted deflections are in good agreement with experimental results. Results demonstrated that complex beam behavior in a varying environment can be predicted by a simple model with well-defined material characteristics generated through relatively simple 18-h uniaxial experiments

Selected Volatile Organic Compound Emissions and Performance of Oriented Strandboard from Extracted Southern Pine

The impact of a hot water extraction procedure on select volatile organic compound emissions during pressing, as well as on properties of oriented strandboard (OSB) was evaluated. Southern pine strands were extracted with hot water using a rotating digester at 160°C for 22.9 or 53.6 min. Weight loss for the two extraction conditions was 6.3 ± 0.1% (short time) and 9.3 ± 0.9% (long time). The extract contained a mixture of hemicelluloses, acetic acid, and lignin. OSB panels were manufactured both with and without adhesive. The emissions (phenol, methanol, acetaldehyde, and formaldehyde) without adhesive present decreased from 38.2 to 24.2 mg/kg (oven-dry wood) as a result of the high severity factor (HSF) extraction. When adhesive was used, emissions totaled 22.1, 17.0, and 15.6 mg/kg (oven-dry wood) for control, low severity factor, and HSF, respectively. Water sorption and thickness swell were significantly reduced in panels made from extracted strands. Flexural modulus of elasticity of extracted panels exhibited significant increases in both dry and wet conditions. The flexural modulus of rupture and internal bond were slightly reduced in the dry condition as weight loss increased. The extraction procedure shows promise for improving a variety of properties of OSB, including performance, reduced environmental impact, and generation of a valuable chemical feedstock byproduct

Influence of log temperature in irregularities on strand geometry detected by digital image analysis

Oriented Strand Board (OSB) wood strands, while often idealized as being rectangular and slender objects, are in fact typically very complex in shape. This complexity is important to the manufacture and performance of OSB as it influences forming, screening, blending, formation and ultimately performance of the panel. In a mill setting, strand geometry is defined by average length, width and thickness values as determined through simple caliper measurements and/or screen analyses (Gilson). The application of CCD cameras and digital image analysis techniques to rapidly acquire and analyze complex strand geometry will allow the processing of large amounts of data, thereby creating the potential for statistical process control applications. Aspen (Populus grandidentata) strands were produced from logs subjected to -6ºC (20F), 21ºC (70F) and 60ºC (140F). Grey scale digital images of individual strands were acquired using a CCD camera (1296 x 1016 pixels) under direct lighting. The complexity of strand geometry was characterized by a variety of automated measuring procedures. As a result of this research the feasibility of applying this methodology to study geometrical distributions in strands was established. Area, length and width presented changes in their distributions due to the effect of log temperature. It was found that the irregularity ratio of strands was variable and strongly influenced by log temperature at time of stranding as well as other geometries. Statistical correlation between strands irregularity and log temperature was found (p-value<0.001)

Properties of Transfer-Molded Wood-Fiber/Polystyrene Composites

Transfer-molded composites combining polystyrene, wood particles, and three bi-functional coupling agents were prepared and evaluated for physical and mechanical properties. Pure 685D polystyrene (PS) (75-100% by weight) was combined with 100-mesh (0.15-mm sieve opening) particles prepared from thermomechanically pulped quaking aspen (Populus tremuloides) (0-25% by weight). Three coupling agents, polystyrene/poly(methacrylic) (both low and high molecular weight) and polystyrene/poly(vinyl acetate) developed at Michigan Technological University, were added in an effort to promote compatibility between the hygroscopic wood fiber and the non-polar hydrophobic polystyrene. Mechanical tensile testing was used to assess the respective composite's tensile elastic modulus and tensile strength. A polystyrene/poly(methacrylic) acid (PS-PMAA) coupling agent was found to be the most effective with regard to enhanced tensile elastic modulus at higher fiber-loading levels (enhancement levels of 11.3-23.8% over pure PS). A fiber/PS composite using low molecular weight PMAA (PS-PMAAL) as a coupling agent demonstrated the best tensile strength retention characteristics at higher fiber-loading levels. Initial results show high variability in material properties over the range of fiber-loading levels, and between coupling agent type. It is clear, however, that certain coupling agents do have a positive effect on composite properties