INFRARED TECHNIQUE FOR MEASURING ULTRASONIC ABSORPTION IN POLYCRYSTALLINE METALS

Dans la plupart des matériaux polycrystallins, l'atténuation due à la diffusion par les grains est dominante et a pour conséquence de ne pas permettre la mesure de l'absorption (ou frottement interne). Nous avons mis au point une technique qui permet de mesurer directement le frottement interne aux fréquences ultrasonores, même en présence de la diffusion. Cette technique est basée sur la détection infrarouge de la chaleur produite par un faisceau ultrasonore modulé à basse fréquence. Nous présentons des résultats expérimentaux pour un acier commercial qui montre clairement les contributions dues à la diffusion, aux mouvements des parois des domaines magnétiques et à d'autres phénomènes.In most polycrystalline materials, attenuation at high frequencies is dominated by grain boundary scattering making it impossible to measure absorption (or internal friction) with any accuracy. A technique was devised to measure internal friction directly at ultrasonic frequencies even in the presence of substantial scattering. The method is based on the infrared detection of the heat produced by slowly modulated ultrasound. Results, presented for a commercial steel sample, clearly separate contributions due to scattering, magnetic domain wall motion and other sources

ANOMALOUS ELASTIC SOFTENING OF POLYCRYSTALLINE Nb3Sn BELOW THE MARTENSITIC TRANSFORMATION TEMPERATURE

Nb3Sn exhibits an elastic shear-mode softening associated with a structural transformation from cubic to tetragonal symmetry at a temperature of Tm∼ 45K. Ultrasonic propagation measurements in single crystals indicate that the shear modulus ½(C11-C12) softens dramatically as the temperature is decreased towards 45K, but increases rapidly below Tm. We have measured Young's modulus and internal friction of polycrystalline Nb3Sn between 300 and 4.2K. Our results indicate that transforming polycrystals soften monotonically down to 18K with no discontinuity in the slope at Tm. These results are found to be incompatible with polycrystalline averages of single crystal elastic constants, even when the possibility of the presence of a fraction of non-transforming material is taken into account. A tentative explanation of this anomalous softening below Tm is given in terms of stress-induced domain wall motion below Tm

Conservation de la semence congelée de verrat. Résultats in vitro et après insémination.

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INTERNAL FRICTION AND YOUNG'S MODULUS OF Nb3Sn BETWEEN 6 AND 300 K

Young's modulus and internal friction were measured in polycrystalline NB3Sn between 6 and 300K. Below the martensitic transformation temperature a large rise in internal friction is observed, in agreement with recent observations. In this temperature range, the internal friction is found to be nearly independent of frequency, a behavior consistent with stress-induced wall motion between domains of the tetragonal phase as the main source of relaxation. Internal friction peaks observed at 140 and 260K in Nb/Nb3Sn composite tapes are tentatively attributed respectively to an α peak in Nb and hydrogen in the Nb3Sn layer. Annealing at 750 °C for 1/2h is found to decrease the transformation temperature from 49 to 40K, and to modify the softening of Young's modulus in this temperature range. Young's modulus at 6K is 46% of room temperature value and is unchanged by the annealing treatment

Cholesterol/phospholipid ratio in sperm of several domestic species does not directly predict sperm fitness for cryopreservation

Chapitre : Pourquoi et comment utiliser la technique de congélation pour les animaux ?International audienc

Analyse de facteurs de variation de la production spermatique de verrats d'insémination artificielle

National audienc

La cryoconservation des races porcines menacées de disparition. La situation en France, en Allemagne, en Italie et en Espagne.

National audienc