A New Technique for Time-Domain Ultrasonic NDE of Extremely Thin Plates

The strength of a joint is significantly affected by the thickness of the bond. Over the past quarter-century, a wide variety of ultrasonic techniques have been reported for the measurement of the thickness and wave velocity (or modulus) of a single layer. Among them are pulse-echo [1], resonance testing [2] and pulse interference [3] methods. Evidently as the plate thickness decreases the time interval between two successive echoes, At, decreases and finally the echoes become inseparable. All of the classical methods break down when the successive reflections from the two faces of the layer cannot be separated in the time-domain. In this paper a specimen will be called “thin” if h\u3c3 λ, where λ is the wavelength in the interrogated material at the transducer center-frequency; conversely, it will be called “thick” if h\u3e3 λ. In much of the aerospace applications, the typical adhesive thickness is of the order of 10-2 mm or 10−3 inch. In order to use any of the aforementioned methods, the transducer frequency would have to be larger than 150 MHz. By combining the theory of Fourier transforms with conventional ultrasonic hardware Kinra and Dayal developed a new ultrasonic NDE technique which removed this limitation [4]. Subsequently, this technique has been used for NDE of the properties of an extremely thin plate [5] as well as a three-layered medium (adherend-adhesive-adherend) where the combined thickness of the joint (h) qualifies as thin [6, 7]

Identification of Viscoelastic Moduli of Composite Materials from the Plate Transmission Coefficients

A quick and accurate method of measuring elastic and viscoelastic constants of a material is the essential first step for characterizing the material. This is more challenging for composite materials because unlike homogeneous metals and ceramics the material properties change from specimen to specimen for composite materials as the volume fraction of fibers and their orientations change. Anisotropic properties of composite materials add another difficulty in the measurement technique, since anisotropy increases the number of independent material constants. Polymer composites exhibit a high degree of attenuation in the matrix material; as a result, these composite materials cannot be assumed to be pure elastic material, so they should be modeled as viscoelastic materials by making the material constants complex. The real part is associated with the elastic behavior and the imaginary part is associated with the viscoelastic or attenuative behavior of the material. The number of independent material constants for a unidirectional (UD) composite, which is transversely isotropic, is ten (five real and five imaginary). Naturally, it is not practical and almost impossible to measure all these material constants by the traditional engineering method of applying stresses and measuring strains in different directions. Because of the measurement difficulty the imaginary parts of the material constants are often ignored. However, it should be mentioned here that it is important to measure the imaginary components of material constants because porosity and microcracking in the matrix due to material fatigue and aging affect the attenuation more than the elastic properties. In other words, the imaginary components of the material constants are a better indicator of material aging compared to the real components. Hence, an efficient technique to measure both real and imaginary components of the material constants is warranted and developed in this paper.</p

Acoustic Response of a Layer of Spherical Inclusions with a Random or Periodic Arrangement

Starting with the classic work of Ying and Truell [1], the scattering of a plane elastic wave by an isolated elastic sphere embedded in an unbounded medium has been studied in great detail. Similarly, the propagation of an effective elastic wave in an elastic matrix containing a random or periodic distribution of inclusions has received considerable attention. By comparison, an intermediate level of microstructure — a single layer of inclusions in an elastic matrix — has received very little attention. Apart from the fact that this problem is worth studying in its own right because of its inherent value as a canonical problem in elastodynamics of materials with a microstructure, it has applications in geophysics and quantitative nondestructive evaluation

Ultrasonic NDE of Adhesive Bonds: The Inverse Problem

Over the past quarter century, a wide variety of ultrasonic techniques have been developed to determine the phase velocity and thickness of elastic plates. Techniques to measure the phase velocity include toneburst [1–4], separable pulse methods [5–7], and spectroscopy [8–11]. These classical methods require that the specimen be thick enough such that two successive echoes from the front and the back faces of the specimen, respectively, be separable in the time domain. Kinra and Dayal [12], developed a through transmission technique which removes this particular limitation of the classical methods. This technique works satisfactorily for the measurement of the phase velocity for specimens whose thickness is greater than one-half of the wavelength; for thinner specimens, however, their numerical algorithm runs into convergence problems. Moreover, their numerical algorithm cannot be used to determine thickness at any wavelength. The reasons for their convergence problems are discussed in detail by Iyer, Hanneman and Kinra [13]. They demonstrated that a detailed sensitivity analysis is a necessary pre-requisite for the development of a robust inversion algorithm. Accordingly, a new inversion scheme based on the method of least squares was developed by Iyer and Kinra to determine thickness from the measurements of phase, magnitude and complex spectrum, respectively, [14–17]. In all of the above ultrasonic methods only one parameter can be determined i.e., an accurate knowledge of thickness is required to determine the wavespeed and vice versa. This defines the central objective of the present work: In this paper we present a technique for determining, simultaneously, the thickness and wavespeed of a thin layer

Interface Effects on Attenuation and Phase Velocities in Metal-Matrix Composites

One often determines the effective elastic moduli and damping of a heterogeneous material by using elastic waves (propagating or standing). Several theoretical studies show that for long wavelengths one can calculate the effective wave speeds of plane longitudinal and shear waves through a composite material. At long wavelengths the wave speeds thus calculated are nondispersive and hence provide the values for the static effective elastic properties. References to some of the recent theoretical and experimental studies can be found in [1–12]. The scattering formulations developed in [1–8] provide a means to obtain both the effective wave speeds and the damping caused by scattering

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

Denaturing gradient gel electrophoresis identifies genomic DNA polymorphism with high frequency in maize

We have used denaturing gradient gel electrophoresis (DGGE) to identify genomic DNA polymorphism in maize (Zea mays L.). DGGE probes detect polymorphism in maize at a frequency comparable to the incidence of restriction fragment length polymorphism (RFLP). Probes identifying polymorphism were mapped to maize chromosome arms by utilizing DGGE and maize lines carrying B-A chromosomal translocations. The methods for library construction, probe screening, and genome analysis, described here for maize, can also be applied to the genomic analysis of other organisms. © 1990 Springer-Verlag.link_to_subscribed_fulltex

A Comparative Study of Multi-loop Edgewise Arch Wire and Nickel-titanium Wire in vivo and in vitro

Wave propagation through particulate composites has received considerable attention in recent years. The dispersive wave propagation through particulate composites with both random and periodic distributions has been studied theoretically [1] and experimentally [2–7]. The response of a layered composite with a finite number of layers can be predicted using the acoustic complex-valued transfer functions for a single layer [8]. The effect of the in-plane structure of an inclusion layer and resonance of individual particles on the wave propagation phenomena have been studied [9]. For a single layer of inclusions, it was shown that the arrangement of the inclusions has a significant effect on a wave propagating normal to the layer. The objective of this work is to study further the effect of the in-plane structure of an inclusion layer and acoustical properties of individual particles on the wave propagation phenomena

Simultaneous Reconstruction of the Acoustic Properties of a Layered Medium: The Inverse Problem

In recent years, multi-layered metal materials have found increasing use in various disciplines, particularly in aerospace, electrical, automotive and pressure vessel industries. These applications utilize certain unique physical properties of the layered metals such as heat conductivity, electric conductivity or corrosion resistance. Thus, a nondestructive evaluation technique is essential for these advanced materials. In this work, we present a frequency domain nondestructive evaluation technique for these layered media and the results obtained when the technique is applied to the three-layer clad metal materials