An original method to estimate local thermophysical properties and latent heat from Thermal Field Measurement (TFM)

International audienceThis paper presents an original method: (i) to estimate thermophysical properties (heat capacity C and thermal condictivity K) and (ii) to experimentally validate heat source estimations. The method, called Thermal Field Measurements (TFM), is based on infrared thermal observations during cooling experiments in a same experimental setup. Results obtained with this method are in good agreement with other resutls published in literature. Only the homogeneous case is presented in this paper but a 1D heterogeneous case will also be presented in the conference

Extension of the optimised virtual fields method to estimate viscoelastic material parameters from 3D dynamic displacement fields

In vivo measurement of the mechanical properties of soft tissues is essential to provide necessary data in biomechanics and medicine (early cancer diagnosis, study of traumatic brain injuries, etc.). Imaging techniques such as magnetic resonance elastography can provide 3D displacement maps in the bulk and in vivo, from which, using inverse methods, it is then possible to identify some mechanical parameters of the tissues (stiffness, damping, etc.). The main difficulties in these inverse identification procedures consist in dealing with the pressure waves contained in the data and with the experimental noise perturbing the spatial derivatives required during the processing. The optimised virtual fields method (OVFM) (Comput. Mech. 34, 2004, 439), designed to be robust to noise, presents natural and rigorous solution to deal with these problems. The OVFM has been adapted to identify material parameter maps from magnetic resonance elastography data consisting of 3D displacement fields in harmonically loaded soft materials. In this work, the method has been developed to identify elastic and viscoelastic models.The OVFM sensitivity to spatial resolution and to noise has been studied by analysing 3D analytically simulated displacement data. This study evaluates and describes the OVFM identification performances: Different biases on the identified parameters are induced by the spatial resolution and experimental noise. The well-known identification problems in the case of quasi-incompressible materials also find a natural solution in the OVFM. Moreover, an a posteriori criterion to estimate the local identification quality is proposed. The identification results obtained on actual experiments are briefly presente

Dissipated energy measurements as a marker of microstructural evolution: 316L and DP600

The thermomechanical characteristics and, more specifically, the dissipative behavior of two steels (a DP600 and a 316L stainless steel) have been studied using infrared measurement methods. All dissipated energy measurements have been performed during traction–traction uniaxial tests in the elastic domain. It has been shown that the dissipated energy of these materials is dependent on the material plastic strain and could be used as a non-destructive criterion to monitor the material evolution during loading sequences. Different kinds of loading sequences have been tested, including uniaxial tensile tests, alternative traction–traction loadings and recovery periods to underline specific characteristics of the material

Dissipative energy as an indicator of material microstructural evolution

In this study, the material microstructure evolution has been studied thanks to two indicators: the cumulated plastic strain and the energy dissipation due to internal friction under cyclic loading. An experimental procedure has been designed to underline the variations of the dissipative energy due to cold work on a DP600 specimen. The results showed that the dissipative energy increases with the plastic strain and can be used as an indicator of material microstructural evolution

An original method to estimate local thermophysical properties and latent heat from Thermal Field Measurement (TFM)

International audienceThis paper presents an original method: (i) to estimate thermophysical properties (heat capacity C and thermal condictivity K) and (ii) to experimentally validate heat source estimations. The method, called Thermal Field Measurements (TFM), is based on infrared thermal observations during cooling experiments in a same experimental setup. Results obtained with this method are in good agreement with other resutls published in literature. Only the homogeneous case is presented in this paper but a 1D heterogeneous case will also be presented in the conference

Bi-layer stiffness identification of soft tissues by suction

BackgroundThe in vivo and non-invasive mechanical characterisation of biological soft tissue is a challenge even under moderate quasi-static loading. Suction-based devices represent a promising technique However, the underlying tissues are often assumed to be homogeneous and the heavy and time-consuming postprocessing times hinders any clinical application. ObjectivesThe aim is to improve suction-based mechanical characterization of soft tissues considered as bilayered structures. The whole method shall be practical and unexpensive. Inverse identification of the Young’s moduli of the bilayers should be performed in almost real-time for any patient.MethodsAn original suction system is proposed based on volume measurements. Cyclic partial vacuum is applied under small deformation using suction cups of aperture diameters ranging from 4 to 30~mm. An inverse methodology is implemented to estimate both of the bilayer elastic stiffness, and optionally the upper layer thickness, based on the interpolation of an off-line finite element database. The setup is validated on silicone bilayer phantoms, then tested in vivo on the abdomen skin of one healthy volunteer. ResultsOn bilayer silicone phantoms with superficial upper layer thickness of 3mm, both Young's moduli identified by suction or uniaxial tension presented a relative difference lower than 10%. Preliminary tests on in vivo abdomen tissue provided the skin and underlying adipose tissue Young's Moduli at 54kPa and 4.8kPa respectively. Once the experimental data were acquired, inverse identification was performed in less than one minute.ConclusionsThis approach is promising to evaluate elastic moduli in vivo at small strain of bilayered tissues

Finite Element study of strains around sacral and heel pressure ulcers with a new bi-layer dressing

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In-vivo tongue stiffness measured by aspiration: Resting vs general anesthesia

International audienceTongue cancer treatment often results in impaired speech, swallowing, or mastication. Simulating the effect of treatments can help the patient and the treating physician to understand the effects and impact of the intervention. To simulate deformations of the tongue, identifying accurate mechanical properties of tissue is essential. However, not many succeeded in characterizing in-vivo tongue stiffness. Those who did, measured the tongue At Rest (AR), in which muscle tone subsides even if muscles are not willingly activated. We expected to find an absolute rest state in participants ‘under General Anesthesia’ (GA). We elaborated on previous work by measuring the mechanical behavior of the in-vivo tongue under aspiration using an improved volume-based method. Using this technique, 5 to 7 measurements were performed on 10 participants both AR and under GA. The obtained Pressure-Shape curves were first analyzed using the initial slope and its variations. Hereafter, an inverse Finite Element Analysis (FEA) was applied to identify the mechanical parameters using the Yeoh, Gent, and Ogden hyperelastic models. The measurements AR provided a mean Young’s Modulus of 1638 Pa (min 1035 – max 2019) using the Yeoh constitutive model, which is in line with previous ex-vivo measurements. However, while hoping to find a rest state under GA, the tongue unexpectedly appeared to be approximately 2 to 2.5 times stiffer under GA than AR. Explanations for this were sought by examining drugs administered during GA, blood flow, perfusion, and upper airway reflexes, but neither of these explanations could be confirmed