Self-Heating Measurements for a Dual-Phase Steel under Ultrasonic Fatigue Loading for stress amplitudes below the conventional fatigue limit

AbstractThe aim of the present research was to study the self-heating behavior of a dual-phase steel under ultrasonic fatigue loading for stress amplitudes lower than the conventional fatigue limit. The steel studied in this research was DP600 commercial dual phase steel. Fatigue tests were conducted for different values of stress amplitudes up to 107 cycles using an ultrasonic fatigue machine at a testing frequency of 20 kHz with flat specimens. An infrared camera was used to measure the mean temperature evolution during the tests. A specific form of heat diffusion equation was adopted in this work to calculate the intrinsic dissipation from temperature measurements. The variation of the dissipated energy versus stress amplitude under cyclic loading was also studied

Thermal response of DP600 dual-phase steel under ultrasonic fatigue loading

The present work employed in situ infrared thermography to investigate the thermal response and dissipative mechanisms of a dual-phase steel under ultrasonic tension-compression fatigue testing. A classical thermal response occurred for stress amplitudes below 247 MPa but an abnormal thermal response was observed for stress amplitudes above 247 MPa, in that the temperature stabilized after a steep increase of up to 350 °C. The mean dissipated energy per cycle was estimated based on temperature measurements using the heat diffusion equation. The relationship between the mean dissipated energy per cycle and the stress amplitude was studied, and mechanisms related to the observed thermal response were discussed

Correlation of the high and very high cycle fatigue response of ferrite based steels with strain rate-temperature conditions

The discrepancies observed between conventional and ultrasonic fatigue testing are assessed through the mechanisms of dislocation mobility in BCC metals. The existence of a transition condition between thermally-activated and athermal regimes for screw dislocation mobility is studied under fatigue loading based on infrared thermography and microstructural characterization, here in the case of DP600 dual-phase steel. Evidence is obtained regarding the microstructural sources of crack initiation, which is found to be consistent with the existence of a transition in the modes of deformation. From the analysis of the experimental data gathered in this work, guidelines are given regarding the comparison and interpretation of S-N curves obtained from conventional and ultrasonic fatigue testing. The inevitable temperature increases under ultrasonic fatigue at high stress amplitudes along with the rate dependent deformation behavior of ferrite, as a BCC structure, were found as the key parameters explaining the observed fatigue behavior and thermal response under low and ultrasonic frequencies. A transition map was produced using the experimental results for DP600 steel as well as data available in the literature for other ferrite based steels, showing the correlation between thermally-activated screw dislocation movement and the absence of failure in very high cycle fatigue

Calorimetric Studies and Self-Heating Measurements for a Dual-Phase Steel Under Ultrasonic Fatigue Loading

The objective of the present research is to study the self-heating behavior of a dual-phase (DP) steel under ultrasonic fatigue loading and to investigate the effect of frequency on intrinsic heat dissipation of the material. The steel studied in this work is DP600 commercial DP steel. Fatigue tests were conducted using an ultrasonic fatigue machine at a testing frequency of 20 kHz with flat specimens. An infrared camera was used to measure the mean temperature evolution during the tests. A specific form of heat diffusion equation was adopted in this work to calculate the heat dissipation per cycle from temperature measurements. The variation of this dissipation versus stress amplitude in cyclic loading was also studied

Optimization and comparison of porosity rate measurement methods of Selective Laser Melted metallic parts

The systematic occurrence of porosities inside selective laser melted (SLM) parts is a well-known phenomenon. In order to improve the density of SLM parts, it is important not only to assess the physical origin of the different types of porosities, but also to be able to measure as precisely as possible the porosity rate so that one may select the optimum manufacturing parameters. Considering 316 L steel parts built with different input energies, the current paper aims to (1) present the different types of porosities generated by SLM and their origins, (2) compare different methods for measuring parts density and (3) propose optimal procedures. After a preliminary optimization step, three methods were used for quantifying porosity rate: the Archimedes method, the helium pycnometry and micrographic observations. The Archimedes method shows that results depend on the nature and temperature of the fluid, but also on the sample volume and its surface roughness. During the micrographic observations, it has been shown that the results depend on the magnification used and the number of micrographs considered. A comparison of the three methods showed that the optimized Archimedes method and the helium pycnometry technique gave similar results, whereas optimized micrographic observations systematically underestimated the porosity rate. In a second step, samples were analyzed to illustrate the physical phenomena involved in the generation of porosities. It was confirmed that: (1) low Volume Energy Density (VED) causes non-spherical porosities due to insufficient fusion, (2) in intermediary VED the small amount of remaining blowhole porosities come from gas occlusion in the melt-pool and (3) in excessive VED, cavities are formed due to the key-hole welding mode

Première étude archéométrique des vestiges de l’atelier gallo-romain de production de grands bronzes du Grand Hôtel de Bordeaux

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Un atelier de production de grands bronzes au Ier siècle à Burdigala

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Première étude archéométrique des vestiges de l’atelier gallo-romain de production de grands bronzes du Grand Hôtel de Bordeaux

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Reconstitution en 3D de l'organisation d'un atelier gallo-romain de fonderie de grands bronzes à Bordeaux (Gironde, France)

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