Abstract
The main issue of wood is its sensitivity to Relative Humidity (RH) variations, affecting its dimensional stability, and thus leading to crack formations and propagations. In situ structural health monitoring campaigns imply the use of portable noninvasive techniques such as acoustic emission, used for real-time detection of energy released when cracks form and grow. This paper proposes a calibration method, i.e., acoustic emission, as an early warning tool for estimating the length of new formed cracks. The predictability of ductile and brittle fracture mechanisms based on acoustic emission features was investigated, as well as climate-induced damage effect, leading to a strain-hardening mechanism. Tensile tests were performed on specimens submitted to a 50% RH variation and coated with chemicals to limit moisture penetration through the radial surfaces. Samples were monitored for acoustic emission using a digital camera to individuate calibration curves that correlated the total emitted energy with the crack propagation, specifically during brittle fracture mechanism, since equations provide the energy to create a new surface as the crack propagates. The dynamic surface energy value was also evaluated and used to define a Locus of Equilibrium of the energy surface rate for crack initiation and arrest, as well as to experimentally demonstrate the proven fluctuation concept.
Highlights
Mechanical damage in materials is procured under the action of stress and deformation.During laboratory tests, a continuously supplied tensile load to a material sample provides energy to the material, and the specimen continuously stores this energy until the ultimate energy storage capacity is reached and energy release occurs [1]
Samples were monitored for acoustic emission using a digital camera to individuate calibration curves that correlated the total emitted energy with the crack propagation, during brittle fracture mechanism, since equations provide the energy to create a new surface as the crack propagates
Power law best fits from experimental data; shaded areas, 95% confidence band for experimental data
Summary
Mechanical damage in materials is procured under the action of stress and deformation.During laboratory tests, a continuously supplied tensile load to a material sample provides energy to the material, and the specimen continuously stores this energy until the ultimate energy storage capacity is reached and energy release occurs [1]. The driving force which tends to enlarge the crack can be defined as the rate of change of strain energy over the crack length The measure of this driving force is the stress intensity factor taking into account both the applied stress and the geometry of the sample under consideration. The conditions under which a crack can propagate occur when the elastic energy released by the material is higher than the energy required to form a plastic zone or, when the energy release rate exceeds the Materials 2020, 13, 3775; doi:10.3390/ma13173775 www.mdpi.com/journal/materials. Materials 2020, 13, 3775 critical strain, a characteristic value for each material It is at the tip of the crack, in this plastic zone, that the stress exceeds the elastic limit of the material. It is this plastic zone that effectively controls crack propagation
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