Magnetic Trapping of Metastable Calcium Atoms

Metastable calcium atoms, produced in a magneto-optic trap (MOT) operating within the singlet system, are continuously loaded into a magnetic trap formed by the magnetic quadrupole field of the MOT. At MOT temperatures of 3 mK and 240 ms loading time we observe 1.1 x 10^8 magnetically trapped 3P2 atoms at densities of 2.4 x 10^8 cm^-3 and temperatures of 0.61 mK. In a modified scheme we first load a MOT for metastable atoms at a temperature of 0.18 mK and subsequently release these atoms into the magnetic trap. In this case 240 ms of loading yields 2.4 x 10^8 trapped 3P2 atoms at a peak density of 8.7 x 10^10 cm^-3 and a temperature of 0.13 mK. The temperature decrease observed in the magnetic trap for both loading schemes can be explained only in part by trap size effects.Comment: 4 figure

Plasticity and fracture of martensitic boron steel under plane stress conditions

Two series of multiaxial experiments are performed to characterize the mechanical behavior of a hot formed martensitic 22MnB5 boron steel. In the first series, flat specimens of uniform cross-section are subjected to various combinations of tensile and shear loading to characterize the elasto-plastic response. Butterfly-shaped specimens of non-uniform cross-section are used for the second series to study the onset of fracture in the martensitic steel. It is found from the analysis of the experimental results that the planar isotropic Hill’48 yield function along with an associated flow rule provides good estimates of the stress–strain response over a wide range of loading paths. The fracture experiments demonstrate that the crack initiation depends strongly on the loading state. A simple stress triaxiality dependent phenomenological fracture model is calibrated to describe the onset of fracture. Using the proposed plasticity and fracture model, numerical simulations of the fracture of tensile specimens of different notch radii are performed and compared with experiments.Centre National de la Recherche Scientifique (France)Massachusetts Institute of Technology (Joint MIT/Industry AHSS Fracture Consortium

Modified Kolsky Formulas for an Increased Measurement Duration of SHPB Systems

International audienceThe so-called incident, reflected and transmitted strain histories are typically recorded during standard Split Hopkinson Pressure Bar (SHPB) experiments. Subsequently, the stress-strain curve for the specimen material is determined based on these recordings. Unless wave deconvolution techniques are employed, the reliable measurement of the reflected wave requires an input bar which is at least twice as long as the striker bar (of equal impedance). The present brief technical note elucidates the advantages of a simple alternative configuration which has only been seldom used in the past. Based on the assumption of quasi-static equilibrium at the specimen level, we present a modification of Kolsky's formulas such that the stress-strain curve for the specimen material can be obtained from the measurement of the incident and transmitted strain histories only. As a result, the measurement of the reflected wave may be omitted and a much shorter input bar can be chosen. Conversely, a much longer striker bar may be used for a given input bar length, thereby increasing the valid duration of standard SHPB experiments by up to 100% through the use of the modified Kolky formulas. An example experiment is shown where the duration of valid measurements has been increased by more than 70%

Critical hardening rate model for predicting path-dependent ductile fracture

A new phenomenological framework for predicting ductile fracture after non-proportional loading paths is proposed, implemented into FE software and validated experimentally for a limited set of monotonic and reverse loading conditions. Assuming that ductile fracture initiation is imminent with the formation of a shear band, a shear localization criterion in terms of the elastoplastic tangent matrix is sufficient from a theoretical point of view to predict ductile fracture after proportional and non-proportional loading. As a computationally efficient alternative to analyzing the acoustic tensor, a phenomenological criterion is proposed which expresses the equivalent hardening rate at the onset of fracture as a function of the stress triaxiality and the Lode angle parameter. The mathematical form of the criterion is chosen such that it reduces to the Hosford–Coulomb criterion for proportional loading. The proposed framework implies that the plasticity model is responsible for the effect of loading history on ductile fracture. Important non-isotropic hardening features such as the Bauschinger effect, transient softening and hardening stagnation must be taken into account by the plasticity model formulation to obtain reasonable fracture predictions after non-proportional loading histories. A new comprehensive plasticity model taking the above effects into account is thus an important byproduct of this work. In addition, compression–tension and reverse-shear experiments are performed on specimens extracted from dual-phase steel sheets to validate the proposed plasticity and fracture model.MIT/Industrial Fracture Consortiu

Experimental investigation and constitutive modeling of metallic honeycombs in sandwich structures

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2003.Includes bibliographical references (p. 207-214).Traditionally, honeycomb sandwich structures are designed in the elastic range, but recent studies on the crushing of sandwich profiles have shown their potential in crashworthiness applications. Thin sandwich sheets also hold a promise for widespread use in automotive industry because standard sheet metal forming technology could be used to produce double-curvature sandwich shell structures. The crashworthiness and formability of sandwich structures are critically dependent on the behavior of the sandwich core under large plastic deformation. In this thesis, a new biaxial testing device has been developed for the reliable characterization of the mechanical behavior of cellular materials. Using this device, the macroscopic phenomenology and the underlying microstructural deformation mechanisms of thin-walled aluminum honeycomb have been studied experimentally for combined out-of-plane shear and normal loading. Furthermore, numerical simulations of the experiments have been performed where the cell walls of the specimen microstructure have been discretized with fine shell element meshes. Based on the experimental and numerical results, a finite-strain rate-independent orthotropic constitutive model for metallic honeycombs has been formulated and implemented into commercial finite element software. The good agreement of the model predictions with the experimental results encourages the use of this constitutive model for applications involving large plastic out-of-plane deformation. On the structural scale, the model has been used to predict the crushing response of a thin-walled sandwich profile with a micro-cell stainless steel honeycomb core.by Dirk Mohr.Ph.D

Modeling of core properties of ultralight sandwich plates

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2000.Includes bibliographical references (leaf 51).by Dirk Mohr.S.M

Fabrication of highly resistive NiO thin films for nanoelectronic applications

Thin films of the prototypical charge transfer insulator NiO appear to be a promising material for novel nanoelectronic devices. The fabrication of the material is challenging however, and mostly a p-type semiconducting phase is reported. Here, the results of a factorial experiment are presented that allow optimization of the film properties of thin films deposited using sputtering. A cluster analysis is performed, and four main types of films are found. Among them, the desired insulating phase is identified. From this material, nanoscale devices are fabricated, which demonstrate that the results carry over to relevant length scales. Initial switching results are reported