An analytical insight into the buckling paradox for circular cylindrical shells under axial and lateral loading

A large number of authors in the past have concluded that the flow theory of plasticity tends to overestimate significantly the buckling load for many problems of plates and shells in the plastic range, while the deformation theory generally provides much more accurate predictions and is consequently used in practical applications. Following previous numerical studies by the same authors focused on axially compressed cylinders, the present work presents an analytical investigation which comprises the broader and different case of nonproportional loading. The analytical results are discussed and compared with experimental and numerical findings and the reason for the apparent discrepancy on the basis of the so-called “buckling paradox” appears once again to lay in the overconstrained kinematics on the basis of the analytical and numerical approaches present in the literature

Variational analysis of self-focusing of intense ultrashort pulses in gases

By using perturbation theory we derive an expression for the electrical field of a Gaussian laser pulse propagating in a gas medium. This expression is used as a trial solution in a variational method to get quasianalytical solutions for the width, intensity and self-focusing distance of ultrashort pulse. The approximation gives an improved agreement with results of numerical simulations for a broad range of values of the input power of the pulse than previous analytical results available in the literature.Comment: 19 pages, 8 figure

Reduction of gear pair transmission error with tooth profile modification

The gear noise problem that widely occurs in power transmission systems is typically characterised by one or more high amplitude acoustic signals. The noise originates from the vibration of the gear pair system caused by transmission error excitation that arises from tooth profile errors, misalignment and tooth deflections. This paper aims to further research the effect of tooth profile modifications on the transmission error of gear pairs. A spur gear pair was modelled using finite elements, and the gear mesh was simulated and analysed under static conditions. The results obtained were used to study the effect of intentional tooth profile modifications on the transmission error of the gear pair. A detailed parametric study, involving development of an optimisation algorithm to design the tooth modifications, was performed to quantify the changes in the transmission error as a function of tooth profile modification parameters as compared to an unmodified gear pair baseline

A thermodynamically consistent derivation of a frictional-damage cohesive-zone model with different mode i and mode II fracture energies

The present paper deals with the derivation of an interface model characterized by macroscopic fracture energies which are different in modes I and II, the macroscopic fracture energy being the total energy dissipated per unit of fracture area. It is first shown that thermo-dynamical consistency for a model governed by a single damage variable, combined with the choice of employing an equivalent relative displacement and of a linear softening in the stress-relative displacement law, leads to the coincidence of fracture energies in modes I and II. To retrieve the experimental evidence of a greater fracture energy in mode II, a micro-structured geometry is considered at the typical point of the interface where a Representative Interface Element (RIE) characterized by a periodic arrangement of distinct inclined planes is introduced. The interaction within each of these surfaces is governed by a coupled damage-friction law. A sensitivity analysis of the correlation between micromechanical parameters and the numerically computed single-point microstructural response in mode II is reported. An assessment of the capability of the model in predicting different mixed mode fracture energies is carried out both at the single microstructural interface point level and with a structural example. For the latter a double cantilever beam with uneven bending moments has been analyzed and numerical results are compared with experimental data reported in the literature for different values of mode mixity. © 2014 Elsevier Masson SAS. All rights reserved

Advances in ultrafast time resolved fluorescence physics for cancer detection in optical biopsy

We discuss the use of time resolved fluorescence spectroscopy to extract fundamental kinetic information on molecular species in tissues. The temporal profiles reveal the lifetime and amplitudes associated with key active molecules distinguishing the local spectral environment of tissues. The femtosecond laser pulses at 310 nm excite the tissue. The emission profile at 340 nm from tryptophan is non-exponential due to the micro-environment. The slow and fast amplitudes and lifetimes of emission profiles reveal that cancer and normal states can be distinguished. Time resolved optical methods offer a new cancer diagnostic modality for the medical community

Nonradiative relaxation and laser action in tunable solid state laser crystals

Room-temperature pulsed laser action was obtained in chromium-activated forsterite (Cr:Mg2SiO4) for both 532 and 1064 nm pumping. Free running laser emission in both cases is centered at 1235 nm and has a bandwidth of approximately 30 nm. Slope efficiency as high as 22 percent was measured. Using different sets of output mirrors and a single birefrigent plate as the intracavity wavelength selecting element tunability over the 1167 to 1268 nm spectral range was demonstrated. Continuous wave laser operation at room temperature was obtained for 1064 nm pumping from a CW Nd:YAG laser. The output power slope efficiency is 6.8 percent. The gain cross section is estimated to be 1.1 x 10 to the 19th sq cm. Spectroscopic studies suggest that the laser action is due to a center other than the trivalent chromium (Cr 3+), presumably the tetravalent chromium (Cr 4+) in a tetrahedrally coordinated site

Bond-slip analysis via a cohesive-zone model simulating damage, friction and interlocking

A recently proposed cohesive-zone model which effectively combines damage, friction and mechanical interlocking has been revisited and further validated by numerically simulating the pull-out test, from a concrete block, of a ribbed steel bar in the post-yield deformation range. The simulated response is in good agreement with experimental measurements of the bond slip characteristics in the post-yield range of deformed bars reported in the literature. This study highlights the main features of the model: with physically justified and relatively simple arguments, and within the sound framework of thermodynamics with internal variables, the model effectively separates the three main sources of energy dissipation, i.e. loss of adhesion, friction along flat interfaces and mechanical interlocking. This study provides further evidence that the proposed approach allows easier and physically clearer procedures for the determination of the model parameters of such three elementary mechanical behaviours, and makes possible their interpretation and measurement as separate material property, as a viable alternative to lumping these parameters into single values of the fracture energy. In particular, the proposed approach allows to consider a single value of the adhesion energy for modes I and II

Photon Entanglement Through Brain Tissue.

Photon entanglement, the cornerstone of quantum correlations, provides a level of coherence that is not present in classical correlations. Harnessing it by study of its passage through organic matter may offer new possibilities for medical diagnosis technique. In this work, we study the preservation of photon entanglement in polarization, created by spontaneous parametric down-conversion, after one entangled photon propagates through multiphoton-scattering brain tissue slices with different thickness. The Tangle-Entropy (TS) plots show the strong preservation of entanglement of photons propagating in brain tissue. By spatially filtering the ballistic scattering of an entangled photon, we find that its polarization entanglement is preserved and non-locally correlated with its twin in the TS plots. The degree of entanglement correlates better with structure and water content than with sample thickness