196 research outputs found
Toward an efficient inverse characterization of the viscoelastic properties of anisotropic media based on the ultrasonic polar scan
Composite materials (e.g., carbon fiber reinforced plastics (CFRP)) are increasingly used for critical components in several industrial sectors (e.g. aerospace, automotive). Their anisotropic nature makes it difficult to accurately determine material properties or to assess internal damages. To resolve these challenges, the Ultrasonic Polar Scan (UPS) technique has been introduced. In a UPS experiment, a fixed material spot is insonified at a multitude of incidence angles Psi(theta,phi) for which the transmission amplitude as well as the associated arrival time (time-of-flight) are measured. Mapping these quantities on a polar diagram represents a fingerprint of the local viscoelasticity of the investigated material. In the present study, we propose a novel two-stage inversion scheme that is able to infer both the elastic and the viscous properties. In the first step, we solve the inverse problem of determining the elastic constants from time-of-flight UPS recordings. The second stage handles a similar inverse problem, but now operates on the amplitude landscape of a UPS experiment for determining the viscous part of the viscoelastic tensor. This two-stage procedure thus yields the viscoelastic tensor of the insonified material spot. The developed characterization scheme has been employed on both virtual (numerical) UPS recordings, to test the effectiveness of the method, and experimental UPS recordings of unidirectional C/E plates
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Application of nonlinear wave modulation spectroscopy to discern material damage
Materials containing structural damage have a far greater nonlinear elastic response than materials with no structural damage. This is the basis for nonlinear wave diagnostics of damage, methods which are remarkably sensitive to the detection and progression of damage in materials. Here the authors describe one nonlinear method, the application of harmonics and sum and difference frequency to discern damage in materials. The method is termed Nonlinear Wave Modulation Spectroscopy (NWMS). It consists of exciting a sample with continuous waves of two separate frequencies simultaneously, and inspecting the harmonics of the two waves, and their sum and difference frequencies (sidebands). Undamaged materials are essentially linear in their response to the two waves, while the same material, when damaged, becomes highly nonlinear, manifested by harmonics and sideband generation. The authors illustrate the method by experiments on uncracked and cracked plexiglass and sandstone samples, and by applying it to intact and damaged engine components
Prevalence, Risk Factors, and Clinical Relevance of Fluoroquinolone-Resistant Organisms in Rectal Cultures: Should We Target Antibiotic Prophylaxis Prior to Prostate Biopsy?
The rise of infectious complications after prostate biopsy has been linked to the growing resistance of enterobacteria to fluoroquinolone (FQ) antibiotics. In this review, we investigated the potential benefit of targeted antibiotic prophylaxis based on rectal cultures prior to prostate biopsy. An electronic search for all related literature published in English was performed from April until June 2015 using the MEDLINE and EMBASE databases. Data were obtained regarding the true prevalence of FQ-resistant bacteria in the rectum of patients, the identification of those patients at risk of harbouring FQ-resistant bacteria, the risk of infectious complications after transrectal prostate biopsy in patients with FQ-resistant bacteria, and the effect of targeted prophylaxis. Although there is limited evidence that a targeted approach might be beneficial, we conclude that current studies on the use of rectal cultures in the prebiopsy setting have too many limitations and confounding variables to definitely accept this approach in clinical practice. Whether this methodology is useful in a certain region will greatly depend on local fluoroquinolone-resistance rates
Prevalence, Risk Factors, and Clinical Relevance of Fluoroquinolone-Resistant Organisms in Rectal Cultures: Should We Target Antibiotic Prophylaxis Prior to Prostate Biopsy?
The rise of infectious complications after prostate biopsy has been linked to the growing resistance of enterobacteria to fluoroquinolone (FQ) antibiotics. In this review, we investigated the potential benefit of targeted antibiotic prophylaxis based on rectal cultures prior to prostate biopsy. An electronic search for all related literature published in English was performed from April until June 2015 using the MEDLINE and EMBASE databases. Data were obtained regarding the true prevalence of FQresistant bacteria in the rectum of patients, the identification of those patients at risk of harbouring FQ-resistant bacteria, the risk of infectious complications after transrectal prostate biopsy in patients with FQ-resistant bacteria, and the effect of targeted prophylaxis. Although there is limited evidence that a targeted approach might be beneficial, we conclude that current studies on the use of rectal cultures in the prebiopsy setting have too many limitations and confounding variables to definitely accept this approach in clinical practice. Whether this methodology is useful in a certain region will greatly depend on local fluoroquinolone-resistance rates
Strain-induced kinetics of intergrain defects as the mechanism of slow dynamics in the nonlinear resonant response of humid sandstone bars
A closed-form description is proposed to explain nonlinear and slow dynamics
effects exhibited by sandstone bars in longitudinal resonance experiments.
Along with the fast subsystem of longitudinal nonlinear displacements we
examine the strain-dependent slow subsystem of broken intergrain and
interlamina cohesive bonds. We show that even the simplest but
phenomenologically correct modelling of their mutual feedback elucidates the
main experimental findings typical for forced longitudinal oscillations of
sandstone bars, namely, (i) hysteretic behavior of a resonance curve on both
its up- and down-slopes, (ii) linear softening of resonant frequency with
increase of driving level, and (iii) gradual recovery (increase) of resonant
frequency at low dynamical strains after the sample was conditioned by high
strains. In order to reproduce the highly nonlinear elastic features of
sandstone grained structure a realistic non-perturbative form of strain
potential energy was adopted. In our theory slow dynamics associated with the
experimentally observed memory of peak strain history is attributed to
strain-induced kinetic changes in concentration of ruptured inter-grain and
inter-lamina cohesive bonds causing a net hysteretic effect on the elastic
Young's modulus. Finally, we explain how enhancement of hysteretic phenomena
originates from an increase in equilibrium concentration of ruptured cohesive
bonds that are due to water saturation.Comment: 5 pages, 3 figure
Nonequilibrium and Nonlinear Dynamics in Geomaterials I : The Low Strain Regime
Members of a wide class of geomaterials are known to display complex and
fascinating nonlinear and nonequilibrium dynamical behaviors over a wide range
of bulk strains, down to surprisingly low values, e.g., 10^{-7}. In this paper
we investigate two sandstones, Berea and Fontainebleau, and characterize their
behavior under the influence of very small external forces via carefully
controlled resonant bar experiments. By reducing environmental effects due to
temperature and humidity variations, we are able to systematically and
reproducibly study dynamical behavior at strains as low as 10^{-9}. Our study
establishes the existence of two strain thresholds, the first, epsilon_L, below
which the material is essentially linear, and the second, epsilon_M, below
which the material is nonlinear but where quasiequilibrium thermodynamics still
applies as evidenced by the success of Landau theory and a simple macroscopic
description based on the Duffing oscillator. At strains above epsilon_M the
behavior becomes truly nonequilibrium -- as demonstrated by the existence of
material conditioning -- and Landau theory no longer applies. The main focus of
this paper is the study of the region below the second threshold, but we also
comment on how our work clarifies and resolves previous experimental conflicts,
as well as suggest new directions of research.Comment: 14 pages, 15 figure
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Nonlinear elastic wave interaction in a sandstone bar: A summary of recent pulse-mode experiments
We have performed nonlinear pulse propagation experiments in a 3.8 cm diameter rod of Berea sandstone 1.8 m long at ambient conditions. Unlike earlier studies, we measured acceleration and not displacement. Moreover, we detected 2nd and 3rd harmonic growth at smaller strain amplitudes than were observed previously (10{sup {minus}7}). Harmonic growth at identical strain amplitudes has also been noted in resonance studies using the same rock type. Current measurements are underway with the rod in vacuum where the wave attenuation is less and the conditions can be carefully controlled. Ultimately, we wish to test the validity of current analytic and numerical models for nonlinear propagation in microcracked materials
Damage signature of fatigued fabric reinforced plastics in the pulsed ultrasonic polar scan
This study investigates the use of both the amplitude and time-of-flight based pulsed ultrasonic polar scan (P-UPS) for the nondestructive detection and evaluation of fatigue damage in fiber reinforced composites. Several thermoplastic carbon fabric reinforced PPS specimens (CETEX), loaded under various fatigue conditions, have been scanned at multiple material spots according to the P-UPS technique in order to extract material degradation in a quantitative way.
The P-UPS results indicate that shear dominated fatigued carbon/PPS goes with a reduction of shear properties combined with large fiber distortions. The P-UPS results of the tension-tension fatigued carbon/PPS samples on the other hand reveal a directional degradation of the stiffness properties, reaching a maximum reduction of -12.8% along the loading direction. The P-UPS extracted damage characteristics are fully supported by simulations, conventional destructive tests as well as visual inspection. The results demonstrate the excellent capability of the P-UPS method for nondestructively assessing and quantifying both shear-dominated and tension-tension fatigue damage in fabric reinforced plastics
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