Emission of stress waves during fracture

Emission of both longitudinal and surface (Rayleigh) waves during fracture of plates under conditions of plane stress and plane strain were studied experimentally. The non-equilibrated tensile stress in the fractured section of the plate creates an elastic wave, which travels radially along the plate at the sound speed. Moreover, the high surface deformation around the crack tip, due to the high stress concentration there, propagates as a surface wave following fracture of this zone, at the respective Rayleigh wave speed with a circular wavefront. The influence of the thickness of the plate and the type of fracture (brittle or ductile) was examined and interesting results were derived, by utilizing a high speed photography technique.     

Micromechanistic origin of irradiation-assisted stress corrosion cracking

A multi-scale study of the micromechanics of dislocation–grain boundary interactions in proton and ion-irradiated stainless steels is presented. Interactions of dislocation channels with grain boundaries result in slip transfer, discontinuous slip without or with slip along the grain boundary. The presence of the irradiation damage enhances the importance of the magnitude of the resolved shear stress on the slip system activated by the grain boundary to transfer slip across it. However, the selected slip system is still determined by the minimization of the grain boundary strain energy density condition. These findings have implications for modelling the mechanical properties of irradiated metals as well as in establishing the mechanism for disrupting the grain boundary oxide, which is a necessary prerequisite for irradiation-assisted stress corrosion cracking.     

An experimental study on the keyhole shapes in laser deep penetration welding

In this paper, the laser spot diameter and its intensity distribution are measured with a scanning pinhole, and the keyhole shapes are observed using a specially designed setup in laser deep penetration welding of glass GG17. Based on the above experimental results, the effects of the following factors on the keyhole shapes are studied: the laser spot diameter and its intensity distribution, defocus, welding speed and inverse Bremsstrahlung absorption of the plasma.     

Evidence for a deep acceptor level in p-InSb from the variation of hole density with uniaxial stress

The hole density of Cd doped p-InSb has been studied as a function of uniaxial compressive stress along [001] at T = 77 K. Analysis implies the presence of a deep acceptor level whose activation energy decreases with compressive stress, dE_A/dχ = ?3.9 meV/kbar.     

Studies of water penetration into LDPE--calcium lactate composite

Penetration of water into low-density polyethylene-calcium lactate composite is studied with NMR techniques. The presence of filler speeds up the water uptake by the polymer matrix and facilitates polyethylene degradation. Spatial distribution of absorbed water molecules within the composite visualised with MRI corroborates differences in dynamical behaviour of the absorbed water molecules revealed by T2 measurements.     

Evidence for nodal quasiparticles in electron-doped cuprates from penetration depth measurements

The in-plane magnetic penetration depth, lambda(T), was measured down to 0.4 K in single crystals of electron-doped superconductors, Pr(1.85)Ce(0.15)CuO(4-delta) (PCCO) and Nd(1.85)Ce(0.15)CuO(4-delta) (NCCO). In PCCO, the superfluid density varies as T2 from 0.025 up to roughly 0.3T/T(c) suggestive of a d-wave state with impurities. In NCCO, lambda(T) shows a pronounced upturn for T<4 K due to the paramagnetic contribution of Nd3+ ions. Fits to an s-wave order parameter over the standard BCS range (T/T(c) = 0.32) limit any gap to less than Delta(min)(0)/T(c) = 0.57 in NCCO. For PCCO, the absence of paramagnetism permits a lower temperature fit and yields an upper limit of Delta(min)(0)/T(c) = 0.2.     

Modulated periodic stokes waves in deep water

Modulated deep-water 1D Stokes waves are considered experimentally and theoretically. Wave trains are modulated in a controlled fashion and their evolution is recorded. Data from repeated laboratory experiments are reproducible near the wave maker, but diverge away from the wave maker. Numerical integration of a perturbed nonlinear Schrodinger equation and an associated linear spectral problem indicate that under suitable conditions modulated periodic Stokes waves evolve chaotically. Sensitive spectral evolution in the neighborhood of homoclinic manifolds of the unperturbed nonlinear Schrodinger equation is found.     

Fresnel absorption and inverse bremsstrahlung absorption in an actual 3D keyhole during deep penetration CO2 laser welding of aluminum 6016

An actual keyhole is captured by a high-speed camera during deep penetration laser welding of aluminum alloy 6016. With the help of spectrograph, plasma spectra are acquired, and then after Abel transformation, electron temperature is calculated. Through Lorenz nonlinear fitting, the FWHM of Stark broadening lines is obtained to compute electron density. To know more about the mechanism of deep penetration laser welding, both the effect of Fresnel absorption and inverse bremsstrahlung absorption of plasma on the laser power distribution is considered. Results indicate that electron temperature is very unstable in the keyhole which has a declining tendency in the radius direction, electron density increases in the depth direction while it does not change too much along radius. Laser intensity absorbed on the keyhole wall through Fresnel absorption is hardly uniform and distributes mainly on the front wall and the bottom of keyhole wall, and inverse bremsstrahlung absorption of keyhole plasma plays a dominant role in absorbing laser power compared with Fresnel absorption.     

Focusing of nonlinear wave groups in deep water

The freak wave phenomenon in the ocean is explained by the nonlinear dynamics of phase-modulated wave trains. It is shown that the preliminary quadratic phase modulation of wave packets leads to a significant amplification of the usual modulation (Benjamin-Feir) instability. Physically, the phase modulation of water waves may be due to a variable wind in storm areas. The well-known breather solutions of the cubic Schrödinger equation appear on the final stage of the nonlinear dynamics of wave packets when the phase modulation becomes more uniform.     

Wideband deep penetration of photon-number fluctuations into the quantum regime in series-coupled light-emitting diodes

Experimental results on the squeezing of photon-number fluctuations of series-coupled LED's driven by a constant-voltage source are presented that illustrate the advantage of series-coupled LED's, namely, a greater squeezing capability than that of a single LED driven through an equivalent series resistor. We discuss the microscopic origin of the deep squeezing on the basis of quantum-mechanical Langevin equations.     

Evidence for shape deformations of fragments in deep inelastic collisions

The number and energy spectra of neutrons evaporated by the heavy and light fragments in deep inelastic collisions have been obtained for the system 240 MeV⁴⁰Ar +¹⁹⁷Au. They indicate that at the scission point the deformation energy is a large part of the total excitation energy espe cially for the light fragment. In the last two years, neutron emission has been studied in deep inelastic reactions^1?6. All the results are similar: the incident energy which is lost during the process is transformed into excitation energy of the final fragments and is shared between the two products proportionnally to their masses. This means that the nuclear temperature was uniform in the composite system. The excitation energy equilibration time is really very short since equilibration is achieved after reaction times as small as 10^?22 s^2,6. Moreover, no preequilibrium emission (direct neutrons-) has been observed.     

Temporal coherence of sound transmissions in deep water revisited

This paper examines the signal coherence loss due to internal waves in deep water in terms of the signal coherence time and compare to data reported in the literature over the past 35 years. The coherence time of the early raylike arrivals was previously modeled by Munk and Zachariasen ["Sound propagation through a fluctuating stratified ocean: Theory and observation," J. Acoust. Soc. Am. 59, 818-838 (1976)] using the supereikonal approximation and by Dashen et al. ["Path-integral treatment of acoustic mutual coherence functions for arrays in a sound channel," J. Acoust. Soc. Am. 77, 1716-1722 (1985)] using the path integral approach; a -1 [corrected] power frequency dependence and a -1/2 [corrected] power range dependence were predicted. Recent data in shallow water in downward refractive environments with internal waves suggested that the signal coherence time of the mode arrivals follows a -3/2 power frequency dependence and a -1/2 power range dependence. Since the temporal coherence of the acoustic signal is related to the temporal coherence of the internal waves, based on the observation that the (linear) internal waves in deep and shallow waters have a similar frequency spectrum, it is argued that the modelike arrivals in deep water should exhibit a similar frequency dependence in deep and shallow waters. This argument is supported by a brute-force application of the path integral to mode arrivals based on the WKB relation between the ray and mode. It is found that the data are consistent with the -3/2 power frequency dependence but more data are needed to further test the hypothesis.     

The dynamics of water in nanoporous silica studied by dielectric spectroscopy

Broad-band dielectric spectroscopy is used to investigate the dynamics of hydration water on the surface of the cylindrical pores of a nanostructured silica material (MCM-41, with pore diameter of 3.2 nm) at various hydrations, in the temperature range 250-150 K. We focus our attention on orientational relaxations that shift from 0.5 MHz at 250 K to less than 1 Hz at 150 K. The measurements distinguish the relaxation of the hydroxyl groups at the surface of silica from the orientational dynamics of hydration water which strongly depends on the degree of hydration. Although it is significantly faster than the dynamics of water in ice, the orientational relaxation of non-freezing water has an activation energy comparable to that in ice when the hydration layer is complete and approximately two-molecule thick.     

Terahertz time-domain spectroscopy of submonolayer water adsorption in hydrophilic silica aerogel

We report a terahertz time-domain spectroscopy study of the adsorption of water in hydrophilic silica aerogel. The adsorbed water is in submonolayer form and shows properties of index of refraction similar to those of bulk water but different absorption properties.     

Evidence for surface Andreev bound states in cuprate superconductors from penetration depth measurements

Tunneling and theoretical studies have suggested that Andreev bound states form at certain surfaces of unconventional superconductors. Through studies of the temperature and field dependence of the in-plane magnetic penetration depth lambda(ab) at low temperature, we have found strong evidence for the presence of these states in clean single crystal YBCO and BSCCO. Crystals cut to expose (110) surfaces show a strong upturn in lambda(ab) at around 7 K, when the field is oriented along the c axis. In YBCO this upturn is completely suppressed by a field of approximately 0.1 T.     

Dynamics of supercooled water in mesoporous silica matrix MCM-48-S

Using three different quasielastic neutron spectrometers with widely different resolutions, we have been able to study the microscopic translational and rotational dynamics of water, in a mesoporous silica matrix MCM-48-S, from T=300 K to 220 K, with a single consistent model. We formulated our fitting routine using the relaxing cage model. Thus, from the fit of the experimental data, we extracted the fraction of water bound to the surface of the pore, the characteristic relaxation times of the long-time translational and rotational decays, the stretch exponent describing the shape of the relaxation processes, and the power exponent determining the Q-dependence of the translational relaxation time. A tremendous slowing down of the rotational relaxation time, as compared to the translational one, has been observed.     

Density profile of water confined in cylindrical pores in MCM-41 silica

Recently, water absorbed in the porous silica material MCM-41-S15 has been used to demonstrate an apparent fragile to strong dynamical crossover on cooling below ～220 K, and also to claim that the density of confined water reaches a minimum at a temperature around 200 K. Both of these behaviours are purported to arise from the crossing of a Widom line above a conjectured liquid-liquid critical point in bulk water. Here it is shown that traditional estimates of the pore diameter in this porous silica material (of order 15 ?) are too small to allow the amount of water that is observed to be absorbed by these materials (around 0.5 g H(2)O/g substrate) to be absorbed only inside the pore. Either the additional water is absorbed on the surface of the silica particles and outside the pores, or else the pores are larger than the traditional estimates. In addition the low Q Bragg intensities from a sample of MCM-41-S15 porous silica under different dry and wet conditions and with different hydrogen isotopes are simulated using a simple model of the water and silica density profile across the pore. It is found the best agreement of these intensities with experimental data is shown by assuming the much larger pore diameter of 25 ? (radius 12.5 ?). Qualitative agreement is found between these simulated density profiles and those found in recent empirical potential structure refinement simulations of the same data, even though the latter data did not specifically include the Bragg peaks in the structure refinement. It is shown that the change in the (100) peak intensity on cooling from 300 to 210 K, which previously has been ascribed to a change in density of the confined water on cooling, can equally be ascribed to a change in density profile at constant average density. It is further pointed out that, independent of whether the pore diameter really is as large as 25 ? or whether a significant amount of water is absorbed outside the pore, the earlier reports of a dynamic crossover in supercooled confined water could in fact be a crystallization transition in the larger pore or surface water.     

Diversity combining for long-range acoustic communication in deep water

A recent experiment showed that coherent long-range acoustic communication is feasible in deep water over a ～550?km range between a source towed slowly at ～75?m depth and a horizontal line array towed at 3.5 knots at ～200?m depth. This letter further demonstrates that diversity combining mitigates channel fading and increases the output signal-to-noise ratio. Using sparse channel-estimate-based equalization, three transmissions are combined successfully to decode a 40?Hz bandwidth (230-270?Hz) 8 phase-shift-keying communication signal, achieving an effective data rate of 17 bits/s at ～550?km range.     

Dependence of fracture stress upon diameter in strong polymeric fibers

The axial and radial dimensions of a fiber are known to be key factors with respect to the mechanical stress necessary to promote failure, this being known as the size effect. Usually different methods are used to quantify the two types of size effects: Linear elastic fracture mechanics (lefm) and related schemes provide the theoretical basis for the effect of diameter variability upon strength whereas statistical theories, generally based upon the Weibull probability distribution combined with the weakest-link theorem, describe length effects. Here we show that simple modifications of the classical Poisson/Weibull form yield a new failure probability function which provides a more adequate explanation for diameter effects on strength in polydiacetylene fibers, and also resolves in a satisfactory way a current problematic issue inherent to the Weibull/weakest-link model. A maximum likelihood estimation procedure is presented for the evaluation of the most appropriate parameters of the proposed failure probability function.