Meng Gong

Measurement of wood shrinkage in jack pine using three dimensional digital image correlation (DIC)

Abstract Dimensional stability is one of the most important properties of lumber. Shrinkage is especially critical in this regard. In this study, longitudinal, radial and tangential shrinkages in jack pine (Pinus banksiana) have been found to be 0.4, 3.3 and 5.7%, respectively, by means of three dimensional digital image correlation (3D DIC). Full field shrinkage data were extracted from two types of specimen. It was demonstrated that the 3D DIC method is a suitable method of providing localized shrinkage values. The shrinkages in the three anatomical dimensions exhibit different variation patterns from pith to bark. Detailed shrinkage measurements provided data for a better understanding and modeling of shrinkage within a tree stem, which could be useful for lumber distortion simulation.

Comparison of NDE techniques for assessing mechanical properties of unjointed and finger-jointed lumber

Abstract Non-destructive evaluation (NDE) methods are common for grading structural lumber with static bending as the traditional NDE method for strength. More recently, longitudinal and transverse vibration techniques have also been proposed for grading lumber. In this study, unjointed and finger-jointed sawn lumber has been evaluated by these traditional and relatively new NDE methods. In total, 188 pieces of 38 mm×89 mm black spruce lumber were tested. Of these, 40 were unjointed, 47 had 2–3 finger joints, and 101 had 5–7 finger joints. The main objective was to evaluate the reliability of the various NDE techniques in predicting the bending stiffness and tensile strength of finger-jointed lumber with different number of finger joints. Results show that all NDE methods provide stiffness values of unjointed and finger-jointed lumber that correlate well with laboratory measured static bending stiffness with R2 values ranging from 0.76 to 0.97. Moreover, lumber with finger joints has lower bending stiffness than unjointed lumber. Based on the correlation coefficients, there is no evidence that finger joints affect the precision of the strength prediction by NDE methods.

Simultaneous measurement of elastic constants of full-size engineered wood-based panels by modal testing

Abstract Engineered wood-based panels are widely used in structural applications. Accurate measurement of their elastic properties is of great importance for predicting their mechanical behavior during structural design. In this study, an efficient non-destructive test method for measurement of effective elastic constants of orthotropic wood-based panels is proposed based on a modal testing technique. An algorithm was developed based on an improved approximate frequency equation of transverse vibration of orthotropic plates under the boundary condition, in which two opposite sides are simply supported and the other two are free (SFSF). The method is able to predict the frequency ranges and mode indices as well as corresponding normalized sensitivity to elastic constants based on initial estimates of orthotropic ratios with uncertainties and measured fundamental natural frequency. Full-size engineered wood-based panels including cross laminated timber (CLT), oriented strand board (OSB), and medium density fiberboard (MDF) were tested with the proposed method. In general, the measured elastic constants of the three types of panel based on modal test agreed well with those corresponding values measured by static tests. More tests are needed with a range of panel sizes and types for further validation of the proposed test method.

Exploratory study on fatigue behaviour of laterally loaded, nailed timber joints, based on a dissipated energy criterion

Abstract Exploration of damage accumulation and reliable prediction of the fatigue lives of laterally loaded, nailed timber joints, are important to proper engineering design of wood structural systems subjected to earthquakes, cyclones/hurricanes or other loads causing fluctuating force flows in such joints. Failure of nailed timber joints typically involves the combination of yielding at plastic hinges in nail and/or the crushing of wood under nails. Force-based criteria can predict the static strength of such joints but cannot reliably predict fatigue behaviour because that depends on loading history and the dissipation of energy within plastic nail hinges and/or crushed wood. In this study, the failure modes, damage accumulation and fatigue life of nailed timber joints subjected to reversed cyclic loads under load-control condition were studied. The results showed that there are two failure modes of nailed timber joints, i.e., ductile failure of nails when the applied load level was higher than 85% of the static maximum load to failure (Pmax) and brittle failure when the applied load level was <85% of Pmax. The damage accumulation involved three phases, i.e., damage initiation, damage propagation and failure. Fatigue life of nailed joints under reversed cyclic loads was modelled by an energy criterion that separated dissipated energy into non-damaging and damaging components demarcated by the fatigue limit. This approach replicated the behaviour of nailed joints tested at 20 load levels of fully reversed repetitive cyclic loading.

Comparative study on measurement of elastic constants of wood-based panels using modal testing: choice of boundary conditions and calculation methods

Modal testing based on the theory of transverse vibration of orthotropic plate has shown great potentials in measuring elastic constants of panel products. Boundary condition (BC) and corresponding calculation method are key in affecting its practical application in terms of setup implementation, frequency identification, accuracy and calculation efforts. To evaluate different BCs for non-destructive testing of wood-based panels, three BCs with corresponding calculation methods were investigated for measuring their elastic constants, namely in-plane elastic moduli (Ex, Ey) and shear modulus (Gxy). As a demonstration of the concept, the products used in this study were oriented strand board (OSB) and medium density fiberboard (MDF). The BCs and corresponding calculated methods investigated were, (a) all sides free (FFFF) with one-term Rayleigh frequency equation and finite element modeling, (b) one side simply supported and the other three free (SFFF) with one-term Rayleigh frequency equation, (c) a pair of opposite sides along minor strength direction simply supported and the other pair along major strength direction free (SFSF) with improved three-term Rayleigh frequency equation. Differences between modal and static results for different BCs were analyzed for each case. Results showed that all three modal testing approaches could be applied for evaluation of the elastic constants of wood-based panels with different accuracy levels compared with standard static test methods. Modal testing on full-size panels is recommended for developing design properties of structural panels as it can provide global properties.