172 research outputs found

    Vibrational behaviour of bcc Cu-based shape memory alloys close to the martensitic transition

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    Experimental data from ultrasonic and inelastic neutron scattering measurements are analyzed for different families of Cu-based shape-memory alloys. It is shown that the transition occurs at a value, independent of composition and alloy family, of the ratio between the elastic constants associated with the two shears necessary to accomplish the lattice distortion from the bcc to the close-packed structure. The zone boundary frequency of the TA2[110] branch evaluated at the transition point (TM), weakly depends, for each family, on composition. A linear relationship between this frequency and the inverse of the elastic constant C', both quantities evaluated at TM, has been found, in agreement with the prediction of a Landau model proposed for martensitic transformations

    Tracking the dynamics of power sources and sinks during the martensitic transformation of a Cu-Al-Ni single crystal

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    We have tracked the dynamics of the martensitic transformation in a Cu-Al-Ni single crystal by means of acoustic emission and infrared imaging techniques. A Fourier equation-based post-processing of temperature maps has enabled us to reveal the inhomogeneous and discontinuous character of heat power sources and sinks during the transiton. A good correlation between the dynamics of thermal and mechanical energy release has been evidenced. It has also been shown that the merging of martensitic interfaces results in an enhanced heat absorptio

    Demagnetizing field-induced magnetocaloric effect in Gd

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    We have studied the impact of demagnetizing fields on the magnetocaloric effect of commercial-grade gadolinium plates. Adiabatic temperature changes (⁠ ⁠) were measured for magnetic fields applied along the parallel and perpendicular directions of the plates. The differences in the obtained values were accounted for by differences in the internal field due to demagnetizing effects. A combination of calorimetric measurements under a magnetic field and thermometric measurements has enabled us to obtain Brayton cycles for the two different magnetic field orientations. It has been found that the refrigerant capacity for a Brayton cycle working at 1.6 T around room temperature reduces from to  J kg when the demagnetizing factor changes from  = 0.035 to for the parallel and perpendicular configurations, respectively. It has been shown that it is possible to obtain significant demagnetizing field-induced magnetocaloric effects by rotating the sample in a region of a constant applied magnetic field. The refrigerant capacity of a Brayton cycle around room temperature for a T constant applied magnetic field is  J kg ⁠. The feasibility of these demagnetizing field-induced effects has been confirmed by direct thermometric measurements, which reveal adiabatic temperature changes of 1 K when the sample is rotated between the perpendicular and parallel configurations

    Experimental Evidence of Accelerated Seismic Release without Critical Failure in Acoustic Emissions of Compressed Nanoporous Materials

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    The total energy of acoustic emission (AE) events in externally stressed materials diverges when approaching macroscopic failure. Numerical and conceptual models explain this accelerated seismic release (ASR) as the approach to a critical point that coincides with ultimate failure. Here, we report ASR during soft uniaxial compression of three silica-based ( SiO2) nanoporous materials. Instead of a singular critical point, the distribution of AE energies is stationary, and variations in the activity rate are sufficient to explain the presence of multiple periods of ASR leading to distinct brittle failure events. We propose that critical failure is suppressed in the AE statistics by mechanisms of transient hardening. Some of the critical exponents estimated from the experiments are compatible with mean field models, while others are still open to interpretation in terms of the solution of frictional and fracture avalanche models

    Avalanches in compressed porous SiO2-based materials

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    The failure dynamics in SiO2-based porous materials under compression, namely the synthetic glass Gelsil and three natural sandstones, has been studied for slowly increasing compressive uniaxial stress with rates between 0.2 and 2.8 kPa/s. The measured collapsed dynamics is similar to Vycor, which is another synthetic porous SiO2 glass similar to Gelsil but with a different porous mesostructure. Compression occurs by jerks of strain release and a major collapse at the failure point. The acoustic emission and shrinking of the samples during jerks are measured and analyzed. The energy of acoustic emission events, its duration, and waiting times between events show that the failure process follows avalanche criticality with power law statistics over ca. 4 decades with a power law exponent ε 1.4 for the energy distribution. This exponent is consistent with the mean-field value for the collapse of granular media. Besides the absence of length, energy, and time scales, we demonstrate the existence of aftershock correlations during the failure process

    Scale-invariant avalanche dynamics in the temperature-driven martensitic transition of a Cu-Al-Be single crystal

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    We have combined high sensitivity, extra-low differential temperature scanning rate calorimetry, and acoustic emission (AE) measurements to study avalanches during the cubic ↔ 18R martensitic transition of a Cu-Al-Be single crystalline shape memory alloy. Both AE and calorimetry corroborate a good power-law behavior for cooling with an exponent É≃1.6. For heating, a slope is observed in the maximum likelihood curves, which confirms that our data are affected by an exponential cutoff. An effective energy exponent, É∼1.85, and a cutoff, λ-1=0.115(38)×10-3aJ, were determined by fits of power-laws with exponential damping. The long tail observed in the lowerature region by calorimetric measurements suggests the existence of significant elastic effects that constrain the progress of the transformation at low temperatures. While thermodynamic features such as transformation enthalpy and entropy are those expected for Cu-based shape-memory alloys undergoing a cubic ↔ 18R transition, the critical behavior deviates from the corresponding behavior expected from this symmetry change. These deviations are a consequence of the elastic hardening induced by the interplay of the transformation with dislocation jamming, which has the effect of effectively reducing the number of pathways connecting the parent and martensitic phas

    Statistical Similarity between the Compression of a Porous Material and Earthquakes

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    It has long been stated that there are profound analogies between fracture experiments and earthquakes; however, few works attempt a complete characterization of the parallels between these so separate phenomena. We study the acoustic emission events produced during the compression of Vycor (SiO2). The Gutenberg-Richter law, the modified Omori's law, and the law of aftershock productivity hold for a minimum of 5 decades, are independent of the compression rate, and keep stationary for all the duration of the experiments. The waiting-time distribution fulfills a unified scaling law with a power-law exponent close to 2.45 for long times, which is explained in terms of the temporal variations of the activity rate

    Meaning of reflective practice in the acquisition and transfer of communication skills in nursing

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    La práctica reflexiva constituye un método educativo de cambio destinado a desarrollar las competencias profesionales del estudiante de enfermería. El proceso parte de la capacidad autocrítica, del reconocimiento de las potencialidades y limitaciones, del fomento de la responsabilidad y del crecimiento personal. Sin embargo, los cambios obtenidos tras ese proceso formativo pueden no mantenerse en la etapa profesional. El objetivo principal de este estudio fue explorar el significado que las enfermeras atribuían al proceso educativo, a lo largo de su formación universitaria, de adquisición de competencias relacionales mediante la práctica reflexiva y a su mantenimiento posterior. Desde una aproximación fenomenológica se entrevistó a dieciséis enfermeras que habían adquirido competencias relacionales a través de la práctica reflexiva. Del análisis de los datos resultó un dominio central que se denominó «espíritu del prácticum» y cinco núcleos temáticos: aprendizaje profesional, prejuicios y dificultades personales, características y desarrollo de la profesión, crecimiento personal y profesional, y evolución hacia la profesionalidad. La transformación y el cambio experimentados durante el prácticum se mantuvieron en el tiempo y se transfirieron a la práctica profesional en ámbitos de enfermería distintos a la salud mental. Junto con la voluntad de transferir las competencias adquiridas, los resultados revelaron dificultades asociadas a las condiciones laborales y la necesidad de un mayor soporte externo y formación continuada. Si bien este estudio evidencia la importancia de la práctica reflexiva como método educativo para la adquisición de competencias profesionales, se considera necesaria una mayor indagación sobre todo respecto a su uso en etapas profesionales de postgrado

    Statistical similarity between the compression of a porous material and earthquakes

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    It has been long stated that there are profound analogies between fracture experiments and earthquakes; however, few works attempt a complete characterization of the parallelisms between these so separate phenomena. We study the Acoustic Emission events produced during the compression of Vycor (SiO&sub&2&/sub&). The Gutenberg-Richter law, the modified Omori's law, and the law of aftershock productivity hold for a minimum of 5 decades, are independent of the compression rate, and keep stationary for all the duration of the experiments. The waiting-time distribution fulfills a unified scaling law with a power-law exponent close to 2.45 for long times, which is explained in terms of the temporal variations of the activity rate
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