Theory of Ultrasonic Backscatter From Multiphase Polycrystalline Solids

Ultrasound scatters from the microscopic single crystals that constitute polycrystalline solids. The scattering originates from crystallite-crystallite variations in the density and elastic constants. For single-phase materials, each crystallite has the same density and the same crystalline symmetry. Hence, in single-phase materials scattering arises from the variation in velocity, which in turn is due to the anisotropy of the elastic constants and the more or less random orientation of the crystallites [1,2]. The situation is considerably more complicated in multiphase alloys where the density, the crystal symmetry and the elastic constants vary from crystallite to crystallite

Nondestructive Testing System to Assess Lack-Of-Bond in Brazed Generator Coils by Ultrasonic Retro-Reflection

Margetan et al. investigated the problem of assessing the integrity of diffusion bonds using reflected ultrasound at oblique incidence [1,2]. They presented a quasi-static distributed spring model to derive the ultrasonic reflectivity of an imperfectly-bonded interface as a function of frequency and angle of incidence. The results were then incorporated in a model for the corner reflection from a diffusion-bonded joint between two butting plates. Rose also studied the ultrasonic reflectivity of diffusion bonds and utilized it for quantitatively characterizing defective joints [3, 4]. Angel and Achenbach investigated the reflection of ultrasonic waves by an array of microcracks [5]

Crack Signal Saturation in High Sensitivity ACFM Technique

Recently, we proposed the high sensitivity ac field measurement (ACFM) technique for detecting and sizing surface cracks in metals [1–2]. This non-destructive evaluation (NDE) technique requires an inducer which possesses a field region with odd symmetry for positioning the probe. When properly orientated in this field region, a single linear probe acts as a differential probe with the added advantage of a phase contribution due to the crack. Both a rectangular coil and a rhombic wire loop can be used as inducers to provide the necessary field. In Fig.1, the positions of the probe with respect to these inducers are shown. As can be inferred from this figure, the probe is coupled to the magnetic field tangent to the metal surface

The Digital Ultrasonic Instrument

In order to provide a capability for performing advanced signal processing on ultrasonic and acoustic emission signals at speeds that are sufficient for practical applications, a high speed Digital Ultrasonic Instrument (DUI) has been developed. The DUI performs its processing entirely digitally and therefore can do the phase- and frequency-sensitive processing which is necessary in many advanced NDE techniques. Its speed of computations is sufficient to handle pulse repetition frequencies (PRFs) of several hundred Hertz. Three applications of the DUI are described, one each in the areas of flaw detection, flaw characterization and acoustic emission source characterization. The first application is improved near surface flaw detection by the use of subtraction of front surface echoes. The second application is a real-time operator-interactive method for correcting a flaw signal to remove system response and interface signals and thereby prepare the flaw signal for flaw characterization techniques such as the Born Inversion. The third application is the automatic identification of sources of acoustic emission in a fastener-hole geometry

The Inverse Born Approximation: Exact Determination of Shape of Convex Voids

The Inverse Born Approximation (IBA) to the elastic wave inverse scattering problem is known to give highly accurate results for the shape of complex voids. In this paper we present an argument demonstrating that the IBA is, in fact, exact for determining the size, shape and orientation of a wide class of these scatterers given infinite bandwidth and unlimited aperture information. Essentially, our argument demonstrates how the IBA algorithm picks out the singular contribution to the impulse response function and correctly relates it to the shape of the scatterer. Some specific examples will be used to illustrate the more intuitive aspects of the discussion

Ultrasonic Sizing of Voids Using Area Functions

We present a simple technique for determining the size of voids by the inversion of backscattered ultrasonic signals using the area function formula. The formulation of this method is based on the Born approximation, which is a weak scattering approximation, but the method works well for voids. The area function has been widely used as a method for determining the position of the flaw centroid to assist implementation of some inversion algorithms. The method has been reported in [6]. Here, we report some further studies, and more experimental results in detail

A Generalized Model of the Effects of Microstructure on Ultrasonic Backscattering and Flaw Detection

The influence of microstructure on ultrasonic inspection is well known. Familiar examples include the attenuation of ultrasound due to scattering from grain boundaries and the anisotropies in velocity that are associated with preferred grain orientation. Less commonly discussed are the creation of backscattered noise, which can mask flaw signals, and the modification of transducer radiation patterns, e.g. the modulation of the phase fronts in a beam, which can cause fluctuations in signals reflected from surfaces [1]. The latter influence the measurement of attenuation as well as the strength of signals reflected from flaws. The goal of this work is to develop a unified basis for understanding these phenomena, as can be used in the analysis of the performance of ultrasonic flaw detection systems. Of interest are correlations of noise in time as well as the variance of noise signals (about their mean of zero) and reflected signals (about a non-zero mean).</p

Relationships Between Ultrasonic Noise and Macrostructure of Titanium Alloys

The complex microstructure of two-phase titanium alloys can produce considerable ultrasonic backscattering noise. The noise introduces problems in detecting small flaws, such as hard-alpha inclusions, by forming a background which can mask the flaw signals. Therefore better understanding of grain noise is required to quantify and increase the detectability of the small flaws. As an aid to understanding the grain noise, an independent scattering model was constructed and studied during last two years by Margetan and Thompson. In that model for the backscattered noise generated by a tone burst, the grain noise is described by following equation (1) N(t)=FOM×M(t) where N(t) is the rms grain noise, FOM is a material characteristic parameter and M is a factor that depends on the detailed description of the experimental configuration as well as the ultrasonic attenuation. The argument, t, is the time delay at which the noise is observed and can be related to a spatial position within the material. Since the model gives an explicit functional form for M, it is possible to use Eq. (1) to infer the FOM from a measurement of N(t).1 Figure 1 presents the results of such a measurement in which the noise was observed, through each of three orthogonal sides of a set of four Ti-6246 specimens, whose history of heat treatment is summarized in Table 1.2 The FOM’s of each of specimens A1, A2 and B2 varied by an order of magnitude, depending on the side of the measurement. However, on specimen C1, which was annealed above the beta transus of 1775 °F, the noise was nearly isotropic. The purpose of this paper is to understand the origin of this anisotropy

A Perturbation Method for Inverse Scattering in Three-Dimensions Based on the Exact Inverse Scattering Equations

The detection and characterization of macroscopic flaws, such as cracks in solids are fundamental goals of nondestructive evaluation. Many inspection methods use scattered electromagnetic or ultrasonic waves. These methods rely explicitly on the development of inverse scattering theory. This theory seeks to determine the geometrical and material properties of flaws from scattering data

Thickness and Conductivity of Metallic Layers from Pulsed Eddy Current Measurements

Coatings and surface treatments find a wide range of technological applications; they can provide wear resistance, oxidation and corrosion protection, electrical contact or isolation and thermal insulation. Consequently, the ability to determine the thickness of coated metals is important for both process control and in-service inspection of parts. Presently ultrasonic, thermal, and eddy current inspection methods are used, depending on the circumstances. A number of commercial instruments for determining the thickness of nonconducting coatings on metal substrates are based on the fact that the impedance change of the coil decreases exponentially with the distance of the coil from the metal (the lift-off effect). However, these instruments are not suitable for determining the thickness of metal layers on conducting substrates