Fretting-Fatigue Analysis of Shot-Peened Al 7075-T651 Test Specimens

Shot peening is a mechanical treatment that induces several changes in the material: surface roughness, increased hardness close to the surface, and, the most important, compressive residual stresses. This paper analyzes the effect of this treatment on alloy Al 7075-T651 in the case of fretting fatigue with cylindrical contact through the results of 114 fretting fatigue tests. There are three independent loads applied in this type of test: a constant normal load N, pressing the contact pad against the specimen; a cyclic bulk stress σ in the specimen; and a cyclic tangential load Q through the contact. Four specimens at each of 23 different combinations of these three parameters were tested—two specimens without any treatment and two treated with shot peening. The fatigue lives, contact surface, fracture surface, and residual stresses and hardness were studied. Improvement in fatigue life ranged from 3 to 22, depending on fatigue life. The relaxation of residual-stress distribution related to the number of applied cycles was also measured. Finally, another group of specimens treated with shot peening was polished and tested, obtaining similar lives as in the tests with specimens that were shot-peened but not polished.Junta de Andalucía P12-TEP-263

A Molecular Counterpart to the Herbig-Haro 1-2 Flow

We present high angular resolution (12"-24") and high sensitivity 12CO and 13CO J=2-1 and J=1-0 observations of the HH 1-2 outflow. The observations show the molecular counterpart, moving with a velocity of approx. 30 km/s, of the optical bipolar system driven by the VLA 1 embedded source. Along the optical jet there are certain regions where the molecular gas reaches deprojected velocities of 100-200 km/s, and that we interpret as the molecular jet. The bipolar CO outflow has a length of approx. 260" with a curved morphology towards the North where it extends beyond the HH 1 object (approx. 120") . Two new molecular outflows have been detected, one arising from IRAS 05339-0647 which excites the HH 147 optical flow and another powered by VLA 2 which drives the HH 144 optical outflow. The molecular outflow driven by the VLA 3 source is also clearly detected and spatially resolved from the VLA 1 main outflow.Comment: 14 pages, 4 figures, accepted ApJLet

Rolling effect in fretting fatigue test at the crack initiation stage

The aim of this work is to perform a detailed analysis of the cracks paths obtained in fretting fatigue tests with cylindrical contact but taking into account the presence of an unavoidable (due to the fretting device’s stiffness) small oscillatory rolling. In order to obtain the crack paths crack surfaces were measured with a confocal microscope after the tests. Besides, the contact area and the surface crack initiation location were obtained by means of an optical microscope. The measurements indicate that the contact area is substantially larger than the theoretical one according to Hertz´s theory, contrary to tests done with only static normal load, in which both theoretical and experimental areas match perfectly. This observation means, that, due to the surface contact pad’s geometry (cylindrical) and the stiffness of the test setup, rolling is occurring during tests when tangential loading is developed. To reproduce this phenomenon, a 2D FEM model is developed. Stress/strain fields along the fretting cycle are analysed, noticing a substantial change of the contact surface hot-spot point and surface contact stress distribution, when compared with the non-rolling case. To predict the initial crack path, a previously developed model, based on a critical plane parameter, is applied using FEM stress/strain results. The results obtained show a better prediction of the surface hot-spot point and initial crack orientation estimation, when compared with the non-rolling case, and considering as a reference the experimental crack paths measured via confocal microscope.Ministerio de Ciencia, Innovación y Universidades - Agencia Estatal de Investigación (AEI) - Fondo Europeo de Desarrollo Regional (FEDER) RTI2018-09659-B-10

Influence of the rolling of contact pads on crack initiation in fretting fatigue tests

This work performs a detailed analysis of the cracks obtained in fretting fatigue tests with cylindrical contact in the presence of a small oscillatory rolling of the contact pad. To do so, fretting fatigue tests have been carried out. Preliminary observations indicate that the contact area is larger than the theoretical one according to Hertz’s theory. This could mean that, due to the contact geometry and the stiffness of the test setup, rolling of the contact pad is occurring during tests. To reproduce this phenomenon, a 2D numerical model is developed. The results are compared with actual crack profile measurements

Lambda = 3 mm line survey of nearby active galaxies

We used the IRAM 30m telescope to observe the frequency range [86-116]GHz towards the central regions of the starburst galaxies M83, M82, and NGC253, the AGNs M51, NGC1068, and NGC7469, and the ULIRGs Arp220 and Mrk231. Assuming LTE conditions, we calculated the column densities of 27 molecules and 10 isotopologues. Among others, we report the first tentative detections of CH3CHO, HNCO, and NS in M82 and, for the first time in the extragalactic medium, HC5N in NGC253. Halpha recombination lines were only found in M82 and NGC253. Vibrationally excited lines of HC3N were only detected in Arp220. CH3CCH emission is only seen in the starburst-dominated galaxies. By comparison of the fractional abundances among the galaxies, we looked for the molecules that are best suited to characterise the chemistry of starbursts, AGNs and ULIRGs, as well as the differences among galaxies within the same group.Comment: 24 pages, 6 figures, 12 tables. Accepted for publication in Astronomy and Astrophysic

Origin of the ionized wind in MWC 349A

The UC-HII region of MWC 349A is the prototype of an ionized wind driven by a massive star surrounded by a disk. Recent high angular resolution observations of the millimeter recombination lines have shown that the disk rotates with a Keplerian law in its outer parts. However, the kinematics of innermost regions in the UC-HII region of MWC 349A is still unknown, in particular the radius where the wind is launched from the disk. We performed hydrogen recombination line observations with the Heterodyne Instrument for the Far Infrared (HIFI) onboard the Herschel Space Observatory to study the kinematics of its innermost regions by studying their spectral features. In addition to the two laser peaks, we report the first detection of two new components that are blueshifted with respect to the laser peaks for all the recombination lines with principal quantum number n<22. These new spectral features originate from the region where the wind is ejected from the disk. We used our 3D non-LTE radiative transfer model for recombination lines (MORELI) to show that these features are consistent with the wind being ejected at a radius of about 24 AU from the star, which supports magnetohydrodynamic wind models.Comment: 5 pages, 5 figure

Análisis del comportamiento de membranas semitransparentes frente a incidencia oblicua de haces gaussianos con modos superiores

There are two main possibilities that are being considered for the ALMA calibration system. The first one consists in a polarizing grid used as beam splitter. The second one consists in a semitransparent vane, a more feasible system but less accurate, which is now under test. This paper introduces an analysis of this kind of devices with the intention of advancing into the knowledge of the vane features and improving its accuracy in this manner. Therefore, a study about its response to oblique incidence is developed. Together with a beam mode expansion into gaussian higher order modes over the fields that impinge on the foam, observing its behaviour in contrast with fundamental mode analysis, taking into account the finite dimensions of the foam. By last, a study about effects of the incidence angle into power transmitted by the foam is presented

The first detections of the key prebiotic molecule PO in star-forming regions

Phosphorus is a crucial element in prebiotic chemistry, especially the P$-$O bond, which is key for the formation of the backbone of the deoxyribonucleic acid. So far, PO had only been detected towards the envelope of evolved stars, and never towards star-forming regions. We report the first detection of PO towards two massive star-forming regions, W51 e1/e2 and W3(OH), using data from the IRAM 30m telescope. PN has also been detected towards the two regions. The abundance ratio PO/PN is 1.8 and 3 for W51 and W3(OH), respectively. Our chemical model indicates that the two molecules are chemically related and are formed via gas-phase ion-molecule and neutral-neutral reactions during the cold collapse. The molecules freeze out onto grains at the end of the collapse and desorb during the warm-up phase once the temperature reaches $\sim$35 K. The observed molecular abundances of 10$^{-10}$ are predicted by the model if a relatively high initial abundance of phosphorus, 5$\times$10$^{-9}$, is assumed.Comment: To appear in "Astrochemistry VII -- Through the Cosmos from Galaxies to Planets", proceedings of the IAU Symposium No. 332, 2017, Puerto Varas, Chile. M. Cunningham, T. Millar and Y. Aikawa, eds. (6 pages

Influencia del shot peening sobre la vida en ensayos de fatiga por fretting con contacto cilíndrico

En este trabajo se caracteriza la aleación de AL7075-T651 ante la fatiga por fretting. En concreto se estudia la influencia sobre la vida a fatiga por fretting del coeficiente de rozamiento, la rugosidad superficial y las tensiones residuales introducidas por un tratamiento de shot-peening. Este tratamiento, además de introducir tensiones residuales de compresión en el material, también modifica la rugosidad superficial debida a la deformación plástica producida por el tratamiento. De las probetas ensayadas, un grupo no ha sido tratado superficialmente. Otro grupo de probetas están tratadas con shot-peening para su comparación con ensayos con las mismas cargas aplicadas. En otro grupo de probetas se induce la rugosidad superficial que causa el tratamiento pero sin la deformación plástica y la consecuente tensión residual de compresión. En un último grupo de probetas tratadas con shot peening se altera la rugosidad superficial sin modificar la tensión residual de compresión. Para analizar cómo afectan a la vida en ensayos de fatiga por fretting las diferentes modificaciones producidas por el shot peening, se ha medido la rugosidad superficial, coeficiente de rozamiento y tensiones residuales para correlacionarlas con la vida a fatiga y lugar de iniciación de las grietas.Junta de Andalucía P12-TEP-263