Mechanical Performance of Wood Construction Materials

The paper aims to illustrate the relevant use of infrared thermography as a nondestructive, non-contact and real time technique (a) to observe the progressive damage process and failure mechanism of wood, and (b) to detect the occurrence of intrinsic dissipation localization. The investigated parameter is the heat generation due to intrinsic dissipation caused by inelasticity. Thanks to the thermomechanical coupling, this useful technique provides a very convenient tool to detect mechanical phenomena depicting wood damage and failure. It allows a measure of the limit of a progressive damaging process under load beyond which the wood material is destroyed. Introduction: Damage and failure behavior of wood in tensile, compressive or shear loading is an important consideration in connection with designs or regulations of wooden structures subjected to high allowable working stresses or in cases where the dead loads form a smaller part of the total load capacity. In such a situation, failure of the construction material may occur at stresses below its static strength. Accurate knowledge should therefore be obtained of the mechanical behavior of wood subjected to various loadings. As with other materials in response to these problems, diverse damage analysis methodologies have been developed in recent years, which isolate the factors affecting crack initiation and growth, and enable the prediction of their cumulative effect on the mechanical performance of structural components. They are based on (1) the formulation of analytical models for damage crack and growth, and (2) the acquisition of supporting baseline data and validation of such models by means of a comprehensive testing procedure. The present paper proposes the use of infrared thermography as a non-destructive, non-contact and real time technique to examine the mechanisms of damage and the processes of failure of wood. The aim of this study is to illustrate the onset of damage process, stress concentration and heat dissipation localization in loaded zones. In addition, this technique can be used as a nondestructive method for inspection and evaluation of stress concentration in wood construction and engineering. Wood is a natural product of biological origin [2-3]. It is a very variable and heterogeneous material. Its mechanical properties are affected by the presence of knots, checks, shakes, splits, slope of grain, reaction wood and decay, etc., and anisotropy. Stress concentrations occur because the knot interrupts wood fibers. Checks, shakes and splits all constitute separations of wood fibers. Slope of grain has a marked effect on the structural capacity of a wood member. Reaction wood or abnormal wood is hard and brittle, and its presence denotes an unbalanced structure in the wood. Decay is a disintegration of the wood, caused by the action of fungi. These damages are generally very difficult to quantitatively evaluate. When the span becomes long or when the loads become large, the use of sawn lumber may become impractical. In these circumstances and possibly for architectural reasons, structural glued-laminated timber or glulam can be used. Glulam members are fabricated from relatively thin laminations of wood. These laminations can be glued together and spliced in such a way to produce wood members. Glues are capable of producing joints that have horizontal shear capabilities in excess of the capacity of the wood itself. The efficient use of materials and the long length of many glulam members require that effective end splices be developed in a given lamination. Butt joints are poor splices. They are considered ineffective in transmitting both tension and compression. Finger joints are poor splices and are considered ineffective in transmitting both tension and compression. Finger joints can produce high strength joints when the fingers have relatively flat slopes. With scarfed joints, the flatter the slope of the joint, the greater the strength of the connection. Veneer is the thin sheet of wood obtained from the peeler log. When veneer is used in the construction of plywood, it becomes a ply. The cross-laminated pieces of wood in a plywood panel are known as layers. Layers are often simply an individual ply, but they can consist of more than one ply. It is the cross laminating that provides plywood with its unique strength characteristics. It provides increased dimensional stability over wood that is not cross-laminated. Cracking and splitting are reduced, and fasteners, such as nails and staples, can be placed close to the edge without a reduction in load capacity. A connector or fastener is a mechanical device (nails, bolts, screws, etc.) or mechanical assembly (bolted shear plates, nailed metal truss plates, etc.), a glue or an adhesive used to hold together two or more pieces of wood or wood based products. Wood frame buildings consist of several components such as walls, floors and roofs joined by intercomponent connections. The performance of wood frame buildings is influenced by the behavior of the individual components and their connections. Therefore an understanding of the behavior of the different structural components and connections is essential to accurately predict the performance of a housing unit under different types of loading. Wood frame buildings perform well under gravity loads. Considerable damage, however, has been observed in such structures under severe and moderate earthquakes and major hurricanes. Experiments are therefore required to verify and refine the numerical tools. The end result promises improved design standards that enable buildings to resist the expected earthquake and wind loads without significant damage or collapse. Damage and failure may thus be viewed as a micro structural process through the activation and growth of one preexisting flaws or of a site of weakness, or through the coalescence of a system of interacting small defects and growing micro cracks. Macroscopically it occurs a localization of intrinsic dissipation before a visible failure. The stress level, corresponding to the activation of the defects, is related to the defect size and connected with the encompassing microstructure. Non-destructive and non-contact tests are thus needed to define wood and wood product properties (1) to establish strength, (2) to optimize design values and (3) to insure quality control. Infrared thermography is a convenient technique for producing heat images from the invisible radiant energy emitted from stationary or moving objects at any distance and without surface contact or in any way influencing the actual surface temperature of the objects viewed. It is successfully used as an experimental method for detection of plastic deformation during crack propagation of steel plate under monotonous loading or as a laboratory technique for investigating damage or failure mechanisms occurring in engineering materials. The work, reported in this paper, considers the intrinsic dissipation as a highly sensitive and accurate indicator of damage manifestation and assumes that intrinsic dissipation and damage present the same evolution under fatigue loading up to failure. In the framework of thermodynamics of irreversible processes, the development of thermoelastic-visco-plasticity equations leads to the coupled thermomechanical equation [6]: ( ) I e 2 o v E : S T E : D : T K r T C + β − ∇ + = ρ (1) where ρ denotes the mass unit in the reference configuration, Cv the specific heat at constant deformation, T the absolute temperature, ro the heat supply, K the thermal conductivity, ∇ the Laplacian operator, β the coefficient of the thermal expansion matrix, D the fourth-order elasticity tensor, E the elastic strain tensor, S the second Piola-Kirchhoff stress tensor and E the inelastic strain tensor. The superposed dot stands for the material time derivative. The volumetric heat capacity of the material C = ρ Cv is the energy required to raise the temperature of a unit volume by 1°Celsius (or Kelvin degree). This equation shows the potential applications and various uses of the infrared scanning technique in engineering problems [4-5-7-8-9-10-11-12]. Temperature changes result from four distinct physical phenomena: heat source, conduction effect, reversible thermo-elastic coupling and intrinsic dissipation. A scanning camera is used, which is analogous to a television camera [1]. It uses an infrared detector in a sophisticated electronics system in order to detect radiated energy and to convert it into a detailed real time thermal image in a color and monochrome video system. Response times are shorter than 1 μs. Temperature differences in the heat patterns are discernible instantly and represented by several distinct hues. The quantity of energy W (W.m.μm), emitted as infrared radiation, is a function of the temperature and emissivity of the specimen. The higher the temperature, the more important the emitted energy. Differences of radiated energy correspond to differences of temperature. Since the received radiation has a non-linear relationship to the object's temperature, and can be affected by atmosphere damping and includes reflected radiation from object's surroundings, calibration and correction procedures have to be applied. Knowing the temperature of the reference, the object's temperature can then be calculated with a sensitivity of 0.1°C at 20°C. The infrared scanner unit converts electromagnetic thermal energy radiated from the tested specimen into electronic video signals. These signals are amplified and transmitted via an interconnecting cable to a display monitor where the signals are further amplified and the resultant image is displayed on the screen. Results: Three series of monotonic unconfined compression tests have been conducted on square specimens of pinewood, prepared along its three anisotropy directions (longitudinal L, radial R and transverse T). The corresponding compressive force F (kN) versus axial displacement d (mm) curves are respectively presented in Figures 1, 2 and 3. The wood specimens were especially designed (cross section S0 = 256 mm, h0 = 20 mm) with enlarged ends to prevent from sliding, bending or premature buckling, caused by heterogeneity, bad alignment of compression loading, or others significant end effects. Longitudinal 0 20 40 60 80 100 120 140 160