Audrey Zink-Sharp

Properties of compression-densified wood, Part II: surface energy

Surface energy of yellow-poplar wood from three wood treatment groups was studied using sessile-drop contact-angle measurements and the Chang acid–base model for surface energy calculation. Water, glycerol, formamide, ethylene glycol and α-bromonaphthalene were used as probe liquids. Wood treatments were control, hygro-thermal treatment and densification. Contact angle trends for densified and hygro-thermally treated wood were found to be the same. Total surface energy decreased with both hygro-thermal treatment and densification.

Sample preparation and image acquisition using optical-reflective microscopy in the measurement of fiber orientation in thermoplastic composites

A complete sample preparation procedure used to determine three‐dimensional fiber orientation from optical micrographs of glass fiber‐reinforced thermoplastic composites is presented. Considerations for elimination of irregularities in the elliptical footprints, contrast enhancement between fibers and surrounding polymer matrix, controlled‐etching that allows the identification of small shadows where fiber recedes into the matrix, and topographical reconstruction of the elliptical footprint are described in the procedure. This procedure has produced high‐quality optical micrographs employed to obtain accurate fiber orientation data for thermoplastic composites using the method of ellipses. The optimal definition of the nonelliptical footprints’ borders allows an accurate measurement of orientation in small sampling areas.

Properties of compression densified wood. Part I: bond performance

Bond performance of hygro-thermally compression densified wood was studied using hygro-thermally treated and control yellow-poplar wood (Liriodendron tulipifera). Opening mode double cantilever beam fracture testing and cyclic boiling were used to evaluate bond performance. Phenol–formaldehyde (PF) film and polymeric diphenylmethane diisocyanate (pMDI) adhesives were used to bond specimens for fracture testing. Fracture toughness of hygro-thermal samples bonded with PF film was significantly higher than control samples, while no difference was found for densified samples. Fracture toughness of densified samples bonded with pMDI was significantly higher than control samples, however no change was seen for hygro-thermal samples. Cyclic boiling reduced the fracture toughness of hygro-thermal fracture samples only, irrespective of adhesive type.

Applying digital image correlation to wood strands: influence of loading rate and specimen thickness.

Abstract Previous studies were devoted to various applications of digital image correlation (DIC) to wood and wood-based composites. However, the focus of these studies was qualitative strain distribution. Overall, there is a lack of testing protocols of DIC for quantifying the elastic properties of woody materials. The objective of this study was to investigate the effects of different specimen thicknesses and loading rates on measurement of Youngs modulus and Poissons ratio by DIC. Youngs modulus from DIC decreased as thickness increased at a loading rate of 0.254 mm min-1. Comparing the different loading rates at a thickness of 0.794 mm, Youngs modulus from DIC was not in agreement with the value obtained by means of the extensometer regardless of loading rate. However, Youngs modulus from DIC at a thickness of 0.381 mm and a loading rate of 0.254 mm min-1 was in good agreement with the corresponding Youngs modulus obtained by an extensometer. Poissons ratio measured from different loading rates and specimen thicknesses was not significantly different between the two measurement systems. From the testing arrangement applied for this study, it is recommended that DIC should be applied at a loading rate of 0.254 mm min-1 or slower for strands with a thickness of 0.381 mm or less.

Characterization of mixed-mode I/II fracture properties of adhesively bonded yellow-poplar by a dual actuator test frame instrument

Abstract Experimental results for the fracture behavior under mixed-mode in-plane loading conditions of adhesively bonded wood specimens are reported. The material systems considered involved yellow-poplar (Liriodendron tulipifera), a hardwood of the Magnoliaceae family, as adherends bonded with two different adhesives, a moisture-cure polyurethane (PU) and a phenol/resorcinol/formaldehyde (PRF) resin. A dual actuator test frame permitted fine scanning of fracture behavior over a full range of mixed-mode I/II levels for double cantilever beam (DCB) geometry specimens. These tests showed that, in the considered material systems, the critical strain energy release rate, c, tends to increase as the mode-mixity of the loading increases. In particular, the increase is steeper in proximity to pure mode II loading for the PRF bonded specimens. The experimental values of c obtained were fairly scattered, as is common when testing wood systems. This variability is due in part to the natural variability of wood but also to other factors such as the orientation of the grain in the bonded beams and variations of bondline thickness. In particular, measurements of adhesive layer thickness were performed. This analysis was implemented with microscopic examination of samples cut from untested DCB specimens, where the bondline had not been disrupted by the test. Although the wood parts were power planed prior to bonding, rather large variations of the adhesive layer thickness were observed: on the order of 1–100 μm for specimens bonded with the PU resin and 10–50 μm for specimens bonded with the PRF resin, which showed somewhat more consistent fracture behavior.

Frequency and Factors of Earlywood Frost Ring Formation in Jack Pine (Pinus banksiana) across Northern Lower Michigan

Abstract Late spring frost disturbances have significant ecological and physiological impacts on forests. Frost-induced cambial damage that occurs when cells are actively dividing can result in the formation of frost rings, abnormal modifications to wood anatomy within the annual growth rings of an injured tree. Frost rings are indicators of growing season frost damage to the cambium and therefore have potential to be used both as a proxy in the reconstruction of extreme climatic events and to identify frost-prone environmental conditions. In this study, we measured the occurrence of earlywood frost rings across cambial age and diameter class in 11 jack pine (Pinus banksiana) populations of northern lower Michigan. Earlywood frost ring formation was greater in younger trees and in trees with smaller diameters. Biotic (cambial age, diameter, and ring width) and abiotic (elevation, initial site-related growth rate, and minimum temperature) factors demonstrated significant influence on the probability of earlywood frost ring formation. When using frost rings as a proxy of historical climate, susceptibility to abrupt freezing temperatures during the growing season and thus the ability of an individual tree to record a frost disturbance should be considered.

Fiber modification by steam-explosion: 13C NMR and dynamic mechanical analysis studies of co-refined wood and polypropylene

Wood-plastic material from a novel reactive processing method (co-refining by steam-explosion) was investigated by 13C solid state nuclear magnetic resonance (NMR) and dynamic mechanical analysis (DMA). NMR spectra indicated chemistry of the material changed as a result from co-steam-explosion. It was also observed from NMR analysis that the crystallinity of the cellulose increased in the presence of iPP during steam-explosion co-processing. By using variable contact time cross pulse experiments, the relaxation parameters (T CH and T 1ρ ) for the constitutive components were evaluated to reveal the level of interactions. T 1ρ values for steam-exploded wood had values similar to those published in literature, which are independent relaxation values for lignin and cellulose. However, for co-steam-exploded wood and iPP, the independent value of lignin relative to the amorphous cellulose was absent. It is proposed that lignin adopts an alternate arrangement during co-steam-explosion processing. This arrangement is transient because the independent relaxation of lignin is recovered after the application of heat during compression molding. DMA demonstrated a mechanical reinforcing effect of the steam-exploded wood without influencing the glass transition of polypropylene for the compression molded co-steam-exploded sample. The paper concludes by describing a hypothetical scheme for a meta-stable interaction of wood bio-polymers and iPP.

Compression wood formation in Pinus strobus L. following ice storm damage in southwestern Virginia, USA1

Abstract In 1994, heavy ice storms hit southwestern Virginia, causing extensive damage to trees. Larger trees tended to experience branch breakage while smaller trees were more commonly bent under the weight of the ice. Bent trees formed compression wood in the years following the ice storm. To evaluate the influence of tree size and location on the formation of compression wood, a commercial wood defect, 47 Pinus strobus L. trees were felled and cross-sections were collected at 0.5 m above the root collar. Disks were sanded and scanned, and the cross-sectional area of compression wood within each tree ring was quantified using image analysis software. In addition, wood anatomical features were quantified in the 3 years before and after the storm. Compression wood formation was significantly related to tree diameter and the 6–9 cm diameter class formed more compression wood area than any other size class (this class was 4–8 cm at the time of the storm). Trees > 18 cm at the time of the storm did not form any post-storm compression wood, suggesting a lack of stem bending at 0.5 m. Trees < 6 cm were able to right themselves with less compression wood formation than the 6–9 cm class. Post-storm compression wood contained significantly more cells per unit area than pre-storm normal wood, but no significant differences existed in cell wall area. Therefore, compression wood had a higher cell wall to lumen ratio, creating wood that is more structurally sound on the lower stem side. This research identifies which size classes of P. strobus are most vulnerable to post-ice storm compression wood formation, and sheds light on cellular characteristics that contribute to its ability to return gymnosperm trees to an upright position after displacement.

Radial growth changes following hemlock woolly adelgid infestation of eastern hemlock

Abstract• ContextHemlock woolly adelgid (Adelges tsugae) is an invasive insect that is defoliating and killing eastern hemlock (Tsuga canadensis) in the USA.• AimsWe quantified changes in tree-ring growth rates and wood anatomy for living trees infested with hemlock woolly adelgid across six sites from Massachusetts (42°41′N) to Georgia (34°53′N) to identify growth responses of eastern hemlock that had survived infestation.• MethodsAnnual ring widths from infested eastern hemlocks were cross-dated and measured. Growth rates before and after infestation were compared. Two infested trees from Virginia were cut, and thin sections were prepared to identify changes in cell properties.• ResultsAt three sites, trees experienced a significant decrease in radial growth after hemlock woolly adelgid arrival; however, the other three sites showed no change or increase in growth. Latewood produced after hemlock woolly adelgid infestation had significantly smaller cells with reduced cell wall thickness compared to latewood prior to infestation.• ConclusionAt half the sites where hemlock woolly adelgid infested eastern hemlock trees were sampled, radial growth increased or remained unchanged. This unexpected response may be due to reduced competition due to mortality of other eastern hemlocks or physiological compensatory responses of increased photosynthetic rate and increased water use efficiency experienced by eastern hemlock infested with hemlock woolly adelgid.

INTRA-RING COMPRESSION STRENGTH OF LOW DENSITY HARDWOODS

Engineered wood composites are being crafted with increasingly smaller and smaller components, yet a search of the literature indicates a lack of intra-ring mechanical property data for almost all commercial wood types, particularly the underutilized low density hardwoods. In addition, there is no universally accepted testing regime for determining micromechanical properties of wood samples. As a result, we developed a testing system for determining compression, tension, and bending properties of growth ring regions of wood samples. Our microtesting system consists of a 45.4 kg load stage, motor drive, data acquisition system, motor control, load cell, strain transducer, and software. In this study, intra-ring compression strength parallel to the grain was determined for small samples (a few millimeters3 in volume) of sweetgum (Liquidambar styraciflua), yellow-poplar (Liriodendron tulipifera), and red maple (Acer rubrum). It was determined that compression strength is weakly correlated with specific gravity but unrelated to growth rate. Specific gravity was also unrelated to growth rate. Sweetgum values were intermediate between yellow-poplar and red maple