Characterisation and categorisation of the diversity in viscoelastic vibrational properties between 98 wood types

Context : Increased knowledge on diversity in wood properties would have implications both for fundamental research and for promoting a diversification of uses as material. Aims : The objective is to contribute to overcoming the critical lack of data on the diversity of wood dynamic mechanical/viscoelastic vibrational properties by testing lesser known species and categorising sources of variability. Methods : Air-dry axial specific dynamic modulus of elasticity (E′/γ) and damping coefficient (tanδ) were measured on a wide sampling (1,792 specimens) of 98 wood types from 79 species. An experimental device and protocol was designed for conducting systematic (i.e. rapid and reproducible) characterisations. Results : Diversity at the specimens' level corroborates the "standard” relationship between tanδ and E′/γ, which is discussed in terms of orientation of wood elements and of chemical composition. Diversity at the species level is expressed on the basis of results for normal heartwood, with specific gravity (γ) ranging from 0.2 to 1.3. Axial E′/γ ranges from 9 to 32GPa and tanδ from 4 × 10−3 to 19 × 10−3. Properties distribution follows a continuum, but with group characteristics. The lowest values of tanδ are only found in certain tropical hardwoods. Results can also suggest alternative species for musical instruments making. Conclusion : The variations in specific gravity, in stiffness or in "viscosity” appear to be predominantly linked to different levels of diversity: between species or between wood types (reaction wood or taxonomy-related differences in heartwood extractives

Characterisation and categorisation of the diversity in viscoelastic vibrational properties between 98 wood types

International audienceContext Increased knowledge on diversity in wood properties would have implications both for fundamental research and for promoting a diversification of uses as material. *Aims The objective is to contribute to overcoming the critical lack of data on the diversity of wood dynamic mechanical/viscoelastic vibrational properties, by testing lesser-known species and categorizing sources of variability. *Methods Air-dry axial specific dynamic modulus of elasticity (E'/γ) and damping coefficient (tanδ) were measured on a wide sampling (1792 specimens) of 98 wood types from 79 species. An experimental device and protocol was designed for conducting systematic (i.e. rapid and reproducible) characterizations. *Results Diversity at the specimens' level corroborates the "standard" relationship between tanδ and E'/γ, which is discussed in terms of orientation of wood elements and of chemical composition. Diversity at the species level is expressed on the basis of results for normal heartwood, with specific gravity (γ) ranging from 0.2 to 1.3. Axial E'/γ ranges from 9 to 32 GPa and tanδ from 4×10-3 to 19×10-3. Properties distribution follows a continuum, but with group characteristics. The lowest values of tanδ are only found in certain tropical hardwoods. Results can also suggest alternative species for musical instruments making

Fields of stored energy associated with localized necking of steel

This paper describes an experimental procedure for the simultaneous determination of heat sources and mechanical energy involved locally during a heterogeneous tensile test. This procedure involves two complementary imaging techniques: digital image correlation (DIC) and infrared thermography (IRT). The first technique gives displacement fields from which strains are derived while the second provides temperature fields with which the heat sources are estimated using a local form of the heat equation. Moreover, a method based on integration of equilibrium equations under the plane stress assumption is used to determine the stress distribution during the test. The distribution of the local deformation energy developed by the material is then assessed using stress and strain-rate fields. Tensile tests were performed on thin flat steel samples. The results revealed early and gradual development of strain localization within the gauge part of the specimen. Energy balances were performed inside and outside the necking zone based on the assumption that the thermoelastic part of the behaviour remains linear and isotropic. Finally, indirect estimate of the stored energy led us to compute the time course of the local Taylor-Quinney coefficient. mechanical energy involved locally during a heterogeneous tensile test. This procedure involves two complementary imaging techniques: digital image correlation (DIC) and infrared thermography (IRT). The first technique gives displacement fields from which strains are derived while the second provides temperature fields with which the heat sources are estimated using a local form of the heat equation. Moreover, a method based on integration of equilibrium equations under the plane stress assumption is used to determine the stress distribution during the test. The distribution of the local deformation energy developed by the material is then assessed using stress and strain-rate fields. Tensile tests were performed on thin flat steel samples. The results revealed early and gradual development of strain localization within the gauge part of the specimen. Energy balances were performed inside and outside the necking zone based on the assumption that the thermoelastic part of the behaviour remains linear and isotropic. Finally, indirect estimate of the stored energy led us to compute the time course of the local Taylor-Quinney coefficient

QIRT & DIC association to analyze the thermomechanical behaviour of a semicristalline polymer

International audienceThis paper first presents the characteristics of a new experimental set-up using digital image correlation and infrared thermography. The kinematical data are used to track the temperature variations of material surface elements. They are then combined to construct local energy balance. To illustrate the interest of such an approach, the paper then describes the calorimetric effects accompanying the propagation of necking in a plasticized PolyAmide 11. A thermodynamic analysis of cyclic loading finally aims to show the existence of an entropic elastic effect generally associated with rubber-like materials

Bidirectional antimonide laser diodes: application to the development of an infrared probe based on absorption spectroscopy

International audienceWe present a study of a sensor probe based on tunable diode laser absorption spectroscopy, using antimonide-based diode lasers emitting at 2.3 and 2.6 μm. The lasers were fabricated by molecular beam epitaxy in the IES laboratory. The active regions are based on InGaAsSb/AlGaAsSb quantum wells grown on a GaSb(N) substrate. The diode lasers operate at room temperature in a continuous wave (CW) regime and exhibit 5 mW of emitted power. A linear optical setup using the two emitting facets of the diode lasers was developed. By using a second derivative detection by wavelength modulation spectroscopy, we obtained a CH4 detection limit of 9 ppm m. The sensor is designed to be used in soil and to measure CH4, CO2 and H2O, which are important constituents of the soil atmosphere generated by anaerobic digestion, microbial respiration or water transfer

Microplasticity and energy dissipation in very high cycle fatigue

International audienceThis paper present the ongoing DISFAT project financially supported by the French National Agency ANR and its white program. It aims at a deeper understanding of mechanisms leading to crack initiation in metals and alloys in Very High Cycle Fatigue (VHCF). In contrast to conventional fatigue tests, detecting irreversible microstructural changes using cyclic stress-strain curves analysis is no more possible, at least with conventional measurement systems, since the macroscopic behavior is quasi-elastic. To cope with this problem, we propose to use the energy signature of the deformation mechanisms. The main challenge of this project results from the fact that the energy and mechanical manifestations as well as the microstructural "evidences"of the mechanisms of interest involve very low signal. In this paper, we explain our strategy and present some results on fcc and bcc single-phase polycrystals