Endotracheal tube colonization in patients undergoing mechanical ventilation was investigated. In the first part of this prospective study, airway access examined for presence secretions, obstruction and bacterial colonization, cases extubation or change. second sequence oropharyngeal, gastric, respiratory tract endotracheal investigated by sequential swabbing at each site twice daily 5 days consecutive noninfected patients. part, it noted that all tubes had secretions lining interior distal...

Organized substructural arrangements of dislocations formed in wavy slip, face–centred–cubic metals during cyclic stress–induced fatigue are shown analytically to engender a substantial nonlinearity the microelastic–plastic deformation resulting from an impressed stress perturbation. The non–Hookean stress–strain relationship is quantified by material parameter βthat for given state highly sensitive volume fractions veins and persistent slip bands (PSBs), PSB internal stresses, dislocation...

Abstract A model is presented of the interaction an acoustic wave with dislocation dipoles and dipole-array approximations to veins persistent slip bands (substructures) formed during metal fatigue. The predicts generation a substantial second harmonic that depends on distance between glide planes dipole pair, density, particular arrangement volume fraction in given substructure fatigued solid. Experimental evidence which strongly supports essential features for aluminium alloy 2024-T4.

A model is presented of the dependence acoustic harmonic generation in polycrystalline solids on coherency strains resulting from lattice mismatch at interface between matrix material and a precipitated second phase material. The nonlinearity parameter (a quantitative measure nonlinearity) shown to depend second, third, fourth order elastic constants material, precipitate-matrix misfit parameter, volume fraction precipitates. applied artificial aging aluminum alloy 2024 T4 T6 temper....

Abstract A comprehensive, analytical treatment is presented of the microelastic–plastic nonlinearities resulting from interaction a stress perturbation with dislocation substructures and cracks that evolve during cyclic fatigue wavy slip metals. The quantified by material nonlinearity parameter β extracted acoustic harmonic generation measurements. contribution to obtained Cantrell model. applying Paris law Nazarov–Sutin crack equation. two contributions predicted increase monotonically...

An analytical model based on precipitate-dislocation interactions is presented that predicts a decrease in ultrasonic harmonic generation during the nucleation of precipitates and an increase precipitate growth up to point precipitate-matrix coherency loss. The opposing effects allow independent, indirect assessments rates from curve fit equation experimental data. Application assessment S′ precipitation artificial aging AA2024 yields constant rate 5.4×1015 m−3 s−1 increasing as 1.2 power...

AbstractThe increasing demand for new, more reliable materials, which are often used in hostile environments, has led to the necessity of establishing equally reliable, quantitative techniques non-destructive evaluation (NDE) and characterisation such materials. Non-destructive methods commonly applications ranging from materials processing control monitoring effects environmental degradation estimation remaining useful life Although linear ultrasonic have long been among most popular NDE...

The ultrasonic harmonic generation technique has been used to extend measurement of the nonlinearity parameters copper 3 °K. Comparison these data and combinations truly adiabatic third-order elastic (TOE) constants with predictions simplified models show that a central force, nearest neighbor model accounts reasonably well for behavior in regions 45 200 °K less at other regions. also gives good qualitative explanation temperature dependence TOE are measured this investigation.

A scanning probe microscope methodology, called resonant difference-frequency atomic force ultrasonic microscopy (RDF-AFUM), has been developed. It employs an wave launched from the bottom of a sample while cantilever microscope, driven at frequency differing by one contact resonance frequencies cantilever, engages top surface. The nonlinear mixing oscillating and in region defined tip–sample surface interaction generates oscillations resonance. resonance-enhanced signals are used to create...

The Boussinesq radiation stress associated with an acoustic wave propagating in infinite and semi-infinite lossless solids of arbitrary crystalline symmetry is examined use two independent approaches. In both approaches the found to depend directly on nonlinearity parameter which characterizes a spatially extended acoustic-radiation-induced static strain, stress-generalized solid, energy density wave. Application Boltzmann-Ehrenfest principle adiabatic invariance self-constrained system...

A quantitative measure of elastic wave nonlinearity in crystals is provided by the acoustic parameters. The parameters are defined for arbitrary propagation modes solids crystalline symmetry and determined along pure mode directions 33 cubic from data reported literature. magnitudes found to exhibit a strong dependence on structure symmetries associated with modal direction solid. Calculations based Born–Mayer potential having dominant repulsive contribution constants interatomic pair...

The first direct experimental evidence for the radiation-stress-induced static strain or displacement associated with propagation of acoustic waves in solids is reported. Ultrasonic tonebursts launched into samples single-crystal silicon along [110] direction and vitreous silica (Suprasil W1) are seen to generate pulses having shape a right-angled triangle. This agreement acoustic-radiation-stress theory which also predicts that slope pulse depends directly on magnitude sign nonlinearity...

Anharmonic effects in solids are often described terms of generalized Gr\"uneisen parameters which measure the strain dependence lattice vibrational frequencies. The relationship between these and solid nonlinearity measured directly ultrasonic harmonic generation experiments is derived using an approach valid for normal-mode elastic wave propagation any crystalline direction. resulting purely isentropic contrast to Brugger-Gr\"uneisen a mixed thermodynamic state. Experimental data comparing...

An analytical model of the effects Peierls–Nabarro barrier stress on nonlinear dynamics dislocation motion in crystalline solids resulting from a perturbative ultrasonic wave is derived. The nonlinearity quantified by material parameter β extracted measurements amplitudes fundamental and harmonically generated waveforms. found to be functionally dependent magnitude stress, loop length, shear modulus, Burgers vector crystal. shown exhibit Bessel function oscillatory dependence amplitude...

An analytical model is presented for the effects on acoustic nonlinearity of interaction matrix dislocations with precipitate coherency strains in metallic alloys. The parameters are shown to depend linearly dislocation density, volume fraction precipitates and precipitate-matrix misfit parameters, fourth power loop length. predicts changes parameter associated growth during artificial aging aluminum alloy 2024 that good agreement experimental measurements reported literature.

Self-organized substructural arrangements of dislocations formed during cyclic stress-induced fatigue metals produce substantial changes in the material microelastic-plastic nonlinearity, a quantitative measure which is nonlinearity parameter β extracted from acoustic harmonic generation measurements. The contributions to evolution and crack growth for fatigued martensitic 410Cb stainless steel are calculated Cantrell model [Proc. R. Soc. London, Ser. A 460, 757 (2004)] as function percent...

The harmonic-generation technique was used to measure the third-order elastic constants ${C}_{111}$ of four types fused silica. samples differed in amount OH content and directional homogeneity. results indicate that a relatively large silica may give rise greater value is weak function temperature regardless content. relationship between experimentally defined ultrasonic nonlinearity parameter $\ensuremath{\beta}=\ensuremath{-}\frac{(3{C}_{11}+{C}_{111})}{3{C}_{11}}$ Gr\"uneisen...

An analytical model is presented of the microelastic-plastic nonlinearities resulting from interactions a stress perturbation with dislocation substructures and cracks that evolve during cyclic fatigue planar slip metals. The are quantified by material nonlinearity parameter β extracted acoustic (ultrasonic) harmonic generation measurements. for given state highly sensitive to volume fractions active persistent Luders bands (PLBs) PLB internal stresses, as well densities, loop lengths,...

Burns of all kinds represent a serious class traumatic injury, affecting over two million people annually in the United States alone. Of this number, 70 000 are enough to require extended hospitalization. Currently, there is no sensitive, quantitative technique for measuring depth thermal damage such injuries. An ultrasonic pulse‐echo described here whereby information can be obtained within few minutes postburn. Results from model porcine experiments given illustrate usefulness method, and...

The success of the ultrasonic pulse-echo technique for measurement depth burn necrosis in porcine skin [Med. Phys. 4, 259–263 (1977)] has led to present investigation specific acoustic impedance difference between necrotic (burn) tissue and underlying viable (nonburn) tissue. Experimental results show that group velocities mass densities are approximately same these tissues yielding average values (1.72±0.12) ×105 cm/s velocity (1.093×0.009) g/cm3 density. characteristic (density times...

An analytical model is developed of the interaction cantilever tip an atomic force microscope with sample surface that treats and as independent systems coupled by a nonlinear acting between volume element surface. To maintain equilibrium, subjected to restoring from remainder sample. The accounts for positions on tip, laser probe, excitation (if any). leads pair differential equations are solved analytically using matrix iteration procedure. Solutions obtained phase amplitude signals...