A Geometrical Interpretation of the Echo Formation of Short Pulses on Simple Shaped Targets

As a necessary step to turn into industrial a method to discriminate between crack-like and small volumic defects by means of ultrasonic NDE [1], we are studying the characterization of broad-band transducers by pulse-echo technique, particularly the influence of the shape and size of the target on the shape of the echo

A Theoretical Approach for the Discrimination of Crack Tip and Small Defect Echoes

Due to diffraction of US by crack tips, a misinterpretation of C-Scans can be made by mistaking detection of a large misoriented crack for a small flaw. As the latter is often tolerable, the former jeopardizes the life of the piece. Fig. 1. shows C-Scans at various amplification levels. The two spots due to diffraction by the near and far tips of a crack can be interpreted as arising from two small defects, even at high amplification level (+18 dB) for which S/N ratio becomes unacceptable

Application of Ultrasonic Beam Modeling to Phased Array Testing of Complex Geometry Components

For several years, the French Atomic Energy Commission (CEA) has developed phased array techniques to improve defect characterization and adaptability to various inspection configurations [1]. Such techniques allow to steer and focus the ultrasonic beam radiated by a transducer split into a set of individually addressed elements, using amplitude and delay laws. For most conventional systems, those delay laws are extracted from geometric ultrasonic paths between each element of the array and a geometric focusing applied to perform beam-forming abilities [2] for simple geometry components (e.g. beam- steering over a plane specimen), whereas experimental delays can be supplied to the array at transmission and reception to optimally adapt the ultrasonic beam to the detected defect, in a so-called self-focusing process [3,4]. This method, relevant for complex material or geometry leading to phase distortion or complex paths that cannot be predicted by simple geometrical calculations, obviously requires the existence of a reflector and the ultrasonic beam radiated by the experimental delay law cannot be known. Therefore this technique is used to improve defect detection (optimal sensibility) rather than defect characterization. To assess complex geometry components inspection with an adaptive system, the CEA has developed new modeling devoted to predict the ultrasonic field radiated by arbitrary transducers through complex geometry and material specimen [5]. A model allows to compute optimized delay laws to preserve the characteristics of the beam through the complex surface, as well as the actual radiated field using those delays. This paper presents two applications of this model : the inspection of a misaligned specimen, and the inspection of an irregular surface

Ignition of pulverized coal with hot air burners

Focusing on a new kind of pulverized coal burners in power plant boilers for directly ignition with hot air, based on on-site experiments and mathematical models, the effect of the coal powder's concentration, the pulverized coal entraining air's velocity and temperature, on ignition of a coal powder stream is being discussed in detail to serve as a guidance for burner design and also for the proper choice of operational parameters.link_to_subscribed_fulltex

Un modèle efficace pour prédire le champ transitoire rayonné dans un milieu élastique par les traducteurs ultrasonores

A model is proposed for the transient radiation by an arbitrary loading in an elastic half-space. The assumptions and approximations made in the derivation are explicitely presented. Their consequences on model's accuracy are discussed. A radiation integral for elastic waves is derived and shown to be related to the Rayleigh integral for acoustic waves. As an example, analytic expression for the field radiated by a disc thickness-mode transducer is given, leading to a solution to displacement impulse-responses closely related to the well-known solution to transient acoustic radiation. Results predicted by the present model are compared with exact results. The accuracy is very satisfactory both qualitatively (shape of waveforms) and quantitatively (amplitude)

Transformation of body force generated by non-contact sources of ultrasound in an isotropic solid of complex shape into equivalent surface stresses

International audienceNon-contact techniques in ultrasonic nondestructive evaluation use external non-mechanical excitation (electromagnetic, heat) which interacts with the mechanical part to be tested. The part itself becomes the source of ultrasounds by transforming the non-mechanical energy into a mechanical one. This process involves the generation of dynamic body forces or of an eigenstrain that can be modeled as equivalent body forces, these forces being confined in the vicinity of the part surface. Many models developed for predicting ultrasonic field radiation in solids assume source terms given as surface distributions of stress. In order to predict ultrasonic fields radiated by non-contact sources by means of these radiation models, we developed a method to transform dynamic body forces into equivalent surface stress distributions, irrespective of the nature of the excitation. The approximate transformation relies on a second order expansion of Green's integral formulation of the elastic wave equation. To make this transformation applicable broadly, the geometry of the surface considered herein is of complex shape, implying thorough differential and tensorial analyses to achieve our aim. Some assumptions, notably isotropic elasticity, are made in deriving the transformation method, which are discussed in detail to clearly define its applicability. Numerical examples of radiated fields are given for illustration and validation

A model to predict modal radiation by finite-sized sources in composite plates with account of caustics

cited By 0; Conference of ICU International Congress on Ultrasonics, ICU 2015 ; Conference Date: 11 May 2015 Through 14 May 2015International audienceThe guided wave field generated by finite size transducers in a composite plate is studied. For isotropic plates, Fraunhofer-like approximations can be found in the literature. Similar approximations fail when the plate is anisotropic. A new calculation method is proposed. Based on a modal decomposition, its principle is to integrate over the transducer surface as seen in energy directions from the calculation point. Special care is taken when dealing with energy paths in direction of caustics. To validate this method, some comparisons are made between our results and those obtained using a full integration over the surface of the source

RÉPONSES ÉCHOGRAPHIQUES DE CIBLES INSONIFIÉES PAR DES TRADUCTEURS LARGE BANDE À PROFIL NON-UNIFORME

Les traducteurs classiquement utilisés en contrôle non-destructif des matériaux (CND) vibrent approximativement en piston avec un profil uniforme de source. En échographie bande étroite, la modification de ce profil (apodisation) est un moyen connu pour diminuer l'amplitude des lobes secondaires de diffraction. Nous montrons ici l'intérêt de telles modifications dans le cas d'imagerie large bande en étudiant la réponse échographique de cibles immergées de forme simple. La simplification de la structure spatio-temporelle du champ insonifiant la cible entraîne une simplification de cette réponse. Un modèle récent est utilisé pour faire une étude paramétrique de cette influence.Commonly used transducers in nondestructive testing (CND in french initials) approximately vibrate as piston with an uniform amplitude distribution. In narrowband imaging, a well-known mean of avoiding diffraction secondary lobes consists in changing the shape of this distribution. Here we show the advantages of such a change in broadband imaging. For this, we study echo-responses from immersed simple shaphed targets for different amplitude distributions. Simplification of time and space dependence of the incident field leads to this of echo-responses. A recently developed modeling is used for a parametric study of this effect