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Mechanical and Thermomechanical Characterization of Different Leathers: Proceedings of the 2018 Annual Conference on Experimental and Applied Mechanics

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a b s t r a c t This paper deals with the calorimetric analysis of deformation processes in filled styrene-butadiene rubbers. More especially, the study focuses on the effects of the addition of carbon black fillers on the calorimetric response of "demullinized" SBR. Temperature variations are measured by infrared thermography during cyclic uniaxial tensile tests at ambient temperature. Heat sources 1 produced or absorbed by the material due to defor-mation processes are deduced from temperature fields by using the heat diffusion equa-tion. First, the results show that no mechanical (intrinsic) dissipation is detected for weakly filled SBR, meaning that the heat produced and absorbed over one mechanical cycle is the same whatever the stretch ratio reached. Second, the mechanical dissipation in highly filled SBR is significant. The quantitative analysis carried out highlights the fact that it increases quasi-linearly with the stretch ratio. Finally, a simplified framework is pro-posed to discuss the identification of the heat sources, in particular the mechanical dissipation.
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The fatigue of dual phase steel was examined in terms of calorimetric effects in order to match the energy manifestations of fatigue and constitutive equations drawn up in a thermomechanical framework. A simplified method, assuming a homogeneous fatigue test, is proposed to determine heat source development from a temperature field provided by an infrared camera. Thermoelastic and dissipative sources were then separately identified. Experimental results concerning thermoelastic effects are in close agreement with theoretical estimates. Dissipation depends on the loading frequency and stress amplitude applied to the fatigue specimen. However, as the marked decrease in dissipation observed when testing a block at high stress was not easily interpretable in terms of material effects, we questioned the homogeneous fatigue test assumption.
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The paper aims to illustrate three advantages of infrared thermography as a non-destructive, non-contact and real-time technique (a) to observe the progressive damage processes and failure mechanisms of leather, (b) to detect the occurrence of intrinsic dissipation localization, and (b) to evaluate a threshold of acceptable damage. The parameter, investigated in this paper, is the heat generation due to intrinsic dissipation caused by anelasticity and/or inelasticity of leather. It readily describes the damage location and the failure evolution of leather for sport foot-wear.
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The paper aims to illustrate three advantages of IR thermography as a nondestructive, noncontact and real time technique, (a) to observe the progressive damage processes and mechanisms of leather failure, and (b) to detect the occurrence of intrinsic dissipation localization. The parameter, investigated in this paper, is the heat generation due to intrinsic dissipation caused by inelasticity and/or inelasticity of leather. It readily describes the damage location and the failure evolution of leather for sport foot-wear.
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A theoretical study is made in this paper of the formulation of constitutive equations describing the thermo-mechanical response of solid polymers in the temperature range in which they exhibit rubberlike behaviour. An expression for the Helmholtz free energy of such a material is first constructed on the basis of two assumptions which are motivated by physical arguments concerning the relation of the molecular structure of a cross-linked polymer to its bulk response. The constitutive equations for the stress and the entropy generated by the proposed form of the free energy function are then employed in a detailed investigation of the extension of a cylindrical specimen under prescribed conditions of temperature and pressure, a situation which serves as a model of the experimental arrangement most frequently used in laboratory studies of the mechanical and thermo-mechanical properties of rubberlike solids. Qualitative consistency of the theoretical predictions with observed behaviour is shown to be assured, over the full range of circumstances for which measurements have been reported, by two simple inequalities affecting one of the three response functions appearing in the stress-deformation-temperature relations. The function concerned is closely related to the strain energy governing isothermal deformations of the material at a selected reference temperature and it is associated, by the physical considerations referred to above, with the contribution to the stress of the polymer network. This conclusion shows that a rational macroscopic theory of rubberlike thermoelasticity can be developed in rather general terms. In particular, the requirement that the constitutive equations shall reproduce the anomalous thermo-mechanical effects which are characteristic of solid elastomers imposes restrictions on the response functions no more severe than those which ensure that the purely mechanical behaviour of the material is physically realistic. In the remainder of the paper the capability of the basic theory for furnishing results quantitatively agreeing with experiment is examined. Empirical forms of the three response functions are presented which accurately represent measurements made in tests involving compression at different fixed temperatures and stretching at the reference temperature. Numerical calculations relating to the analysis of the extension of a cylinder, given earlier, are then described and compared with the results of experiments in which thermoelastic inversion phenomena occur. Satisfactory agreement is secured, but it is noted that insufficiency of material data for the rubbers used in the tests precludes an exact correlation of theory and experiment. The final section of the paper is concerned with isothermal deformations of rubberlike materials which are mechanically incompressible (in the sense that volume changes can be brought about by thermal expansion but not by loading at fixed temperature). This property closely approximates the typical behaviour of natural and synthetic rubbers, but its incorporation into a general treatment of rubberlike thermoelasticity presents difficulties and places an undesirable limitation on the scope of the theory. An analogue is shown to exist between the constitutive equations for deformations at the reference temperature and their counterparts in respect of isothermal deformations at other temperatures, and with its aid the problem of the combined extension, torsion and uniform heating of a circular cylinder is solved. Again, a numerical evaluation of the solution is compared with available experimental data.
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A soft material testing, as leather is more difficult than a rigid material one, especially for the variable shape. A complex processing is actually performed to accomplish the industry requirements and much more information about the internal structure is requested than simple visual inspection. Pulsed Phase Thermography is a suitable tool for a fast and reliable testing of such materials. A first step is the Non Destructive Testing devoted to the identification of hidden scratches. A more complex analysis is required to visualize the texture in depth, evaluating the quality of the final product. Thermal diffusivity and effusivity proven their usefulness to distinguish among different leather types. Experimental results on different specimens containing intentionally fabricated defects and normal samples are presented. The used experimental procedure is described. A mathematical model of the leather covered by a finishing layer has been applied to interpret experimental data.
Article
Previously developed constitutive equations are presented and discussed for shoe upper leather which undergoes large deformation. An energy potential function has been established using total Lagrangian strains for computer modelling of finite deformation of leather in shoe lasting. A practical last simulator, composed of a cylindrical last and uni-axial leather strip, has been developed to model a one-dimensional simulation of the lasting process. Experiments have been carried out to study the effects of different friction characteristics at the leather-last interface. The experimental results for the variation of the lasting force with the last displacement are presented and shown to be closely predicted by the computer models based on the multi-axial constitutive equations.
Article
The relation of rubbers to other classes of polymers and the molecular basis of rubber elasticity are briefly examined. The methods used in the quantitative development of the statistical-thermodynamic theory of a molecular network are outlined, and the main conclusions, in the form of stress-strain relations, etc, are presented and compared with experimental data. Also examines the photoelastic properties of rubbers from both theoretical and experimental standpoints and discusses in detail the evidence derived from photoelastic studies on the statistical segment length in the molecular chain and its relation to intramolecular energy barriers. The thermodynamic analysis of stress-temperature data for rubber and other polymers, with particular reference to the internal energy and entropy changes during extension under constant pressure or constant volume conditions is studied. The phenomena of swelling in liquids is considered.
Article
Four bovine leathers subjected to five stretching procedures in water at different temperatures between 60 and 80°C and drawing ratios between 1.20 and 1.40 were analyzed to demonstrate the suitability of the generalized Maxwell model to fit the different stress-relaxation processes related to the structural hierarchy of leather. This consists of a set of three Maxwell units connected in parallel to represent the high-rate, the medium-rate, and the low-rate stress-relaxation processes, and a Hookean spring in parallel to represent the residual stress at the equilibrium. The high-rate, the medium-rate and the low-rate relaxation times were approximately of 0.6, 10, and 200 s, respectively. Stretching of leather under different conditions to gain area yield in addition to a reduction in thickness also produces a decrease in leather density but not always induce hardening of leather. Leather stretching increases the initial stress when leather is strained 20% for stress-relaxation tests. The effect of stretching on the residual stress depends on the level of stretching. The hardening effect of stretching measured by relative softness showed a good relationship with the medium-rate relaxed stress. The harder the stretched leather the higher the decrease in the medium-rate relaxed stress. Softness also showed a good relationship with the low-rate relaxation time. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 6000–6008, 2006
Article
1° Nous avons avons examiné la tension du caoutchouc étiré en fonction de la température.
Article
The stress-strain behaviour of partially processed leather which has been dried under a range of uni-axial strains has been investigated. It has been found that the dependence of the tensile modulus on the strain applied during drying is non-linear, increasing slowly at first then more rapidly later on. A two-dimensional microstructural model based on an idealised fibre network can describe this non-linear relationship and account for differences between samples. High values of set are produced by drying under strain and some of this set is retained even after soaking in water. It is suggested that this is due to the formation of stable crosslinks between the chemically modified collagen fibres that comprise leather.
Article
Chrome-free leather such as glutaraldehyde-tanned leather behaves very differently from chrome-tanned leather. Information regarding its viscoelasticity has not been previously reported. Hysteresis and stress relaxation are two essential properties associated with viscoelasticity. We have designed a cyclic tensile test to measure these properties to gain insight into the structural difference between chrome-free and chrome-tanned leather. Observations revealed that chrome-free leather tanned with glutaraldehyde has a higher hysteresis than chrome-tanned leather. Stress relaxation experiments, on the other hand showed chrome-free leather has very similar relaxation curves as chrome-tanned leather. Both leathers demonstrate a rapid decrease in stress for the first few seconds followed by a much slower decay thereafter. The chrome-free leather, however, has a greater initial stress than chrome-tanned leather, indicating a higher stiffness than chrome-tanned leather. Moreover, observations showed the viscoelasticity of leather was affected significantly by its fatliquor content. A decrease of loading energy in a cyclic stress–strain experiment resulted from higher fatliquor content in leather.
Article
This paper presents an infrared data processing developed to analyse the calorific manifestations accompanying elastoplastic transformation during tensile tests. The surface temperature images are provided by an experimental set-up essentially made of a testing machine coupled with an infrared camera equipped with a home-made numerizer. The `inverse' passage from temperatures to heat sources is detailed in the case of flat and thin parallelepipedic samples. The infrared image processing, based on Fourier’s techniques, was checked using spectral solutions of the heat equation in the case of realistic examples close to experiments. Numerical simulations are shown which attest coherence and efficiency of the method for several heat source distributions and different sets of noisy data. The method is then applied to experimental data files coming from tensile tests on mild steels at the room temperature. Sudden dissipative effects due to the propagation of the Lüders bands during the plastic plateau can be observed. Then, during the strain hardening, gradual and precocious concentrations of dissipation are shown; they herald the local necking of the sample. Finally, the interest of such experimental results is briefly discussed by referring to the specialised literature dealing with localisation phenomena and behaviour identification.
Article
During the high cycle fatigue of aluminium alloys, an energy dissipation occurs. This dissipation is hard to be estimated because of the high diffusivity of such alloys and the importance of the thermoelasticity effects in comparison with others standard metallic materials (e.g., steels). Nevertheless the study of the energy balance gives valuable information about the nature of deformation mechanisms facilitating the construction of constitutive models associated with the microplasticity and damage of the aluminium alloy. In this work, the different energies involved in the energy balance were deduced from two complementary imaging techniques. The dissipation and thermoelastic sources were derived from an infrared thermography system, while the deformation energy was estimated from a digital image correlation system. Three tests with various loading blocks were carried out and a comparison between deformation and dissipation energies was systematically performed.
Article
Energy balances of pseudoelastic behaviour of a shape memory alloy are experimentally determined using IR techniques. In fact, these balances, established for CuZnAl alloy at constant room temperature and under quasi-static tensile conditions, allow us to study the relative importance of the intrinsic (mechanical) dissipation involved in austenite-martensite phase changes compared with thermomechancial couplings. An analysis of the experimental data shows that the thermoelastic austenite-martensite phase transition can be considered as a non-isothermal process involving no or little (hardly measureable) mechanical dissipation.Assuming a non-dissipative phase transition, numerical simulations are done in the case of non-isothermal processes using two existing models. This numerical exercise gives mechanical, thermal and energetic responses that can be qualitatively compared with those obtained experimentally. These results evidence that an intrinsic dissipation is not necessary to describe the pseudoelastic behaviour. In general, this example emphasizes once again that thermal and mechanical analyses should be carried out jointly to characterize properties of materials.
The effect of different parameters on the rupture properties of leather in a tensile test
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Makho K (1998) The effect of different parameters on the rupture properties of leather in a tensile test. Master thesis dissertation, Faculty of science, Physics and Electronics, Rhodes University (South Africa)
The effect of atmospheric moisture on the physical properties of vegetable and chrome tanned calf leathers
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Ewans WD, Critchfield CL (1933) The effect of atmospheric moisture on the physical properties of vegetable and chrome tanned calf leathers. Bur Stand J Res 11:147-162
Standard test method for elongation of leather
ASTM D2211-00 (2015) Standard test method for elongation of leather. ASTM International, West Conshohocken, p 201542