N. K. Batra

Use of a transient wave propagation code for 3D simulation of cw radiated transducer fields

In order to compute continuous wave (cw) ultrasonic wave fields in complex media, where analytical approaches are extremely difficult, numerical simulations on large computational grids must be employed. The use of a time domain code, normally used for transient wave propagation in heterogeneous media, is used here as a tool to simulate continuous wave fields. As a starting point, the case of the three-dimensional pressure field radiated from a circular aperture in water is computed. These numerical simulations are performed on a massively parallel computer and compared with experiment and known theory. The computations are performed on very large three-dimensional grids that span the near to the far field. As a trial case, a numerical computation of the radiated field from a continuous-wave excited transducer in a baffle is compared with an analytical evaluation using the Rayleigh surface integral and experiment. In addition, results are presented that show the effect of a small defect placed in the beam. To do this, a small cylindrical copper scatterer was placed in the near field, in both the computation and an accompanying experiment. These cases are done in preparation for using the same approach for computing cw fields radiated from a transducer into complex heterogeneous media.

Determination of Elastic Constants of Anisotropic Materials from Oblique Angle Ultrasonic Wave Measurements II: Experimental

There has been a renewed interest in the study of elastic properties of anisotropic materials in recent years, particularly with the wide usage of custom made advanced composites in many aspects of aerospace structures. These composites are heterogeneous and anisotropic materials. Nondestructive evaluation of composites is highly desirable, both for defects characterization and mechanical properties. Ultrasonic methods are especially well suited for determination of the elastic properties of materials.

Mapping of Three-Dimensional Radiation Field of Ultrasonic Transducers

Piezoelectric transducers convert radio-frequency (rf) electrical signals into mechanical ultrasonic vibrations and are the key elements in all medical and industrial ultrasound. These are used for ultrasonic imaging, NDE, determination of material property and detection/sizing of flaws[l]. In all such measurements complete knowledge of the radiation source, receiver and associated electronics as well as the field inside the immersion fluid or the material can be useful in the understanding of the material properties. The standard information that is provided for these transducers by the vendors has limited value for imaging purposes. The manufacturers provide data in the form of rf reflection from a small target, such as a ball and its frequency spectra to indicate bandwidth. Advanced precision measurements with piezoelectric transducers will require temporal and spatial distribution of the radiation field in the propagation medium. For example, in the measurement of material properties using Lamb Waves or oblique angle time-of-flight[2] measurements, we measure the phase velocity which is dependent upon the angle of incidence. If the transducer element is misoriented inside the enclosed case, any measurements using the radiation field of such a transducer would require either alignment of the transducer field such that the spatial coordinates of maxima in the amplitude and minima in the phase at all the axial distances from the transducer coincide or else a priori knowledge of three-dimensional (3-D) mapping of fields from the transducer. In the latter case, any perturbation to these fields due to the material property variation can be measured precisely. In this paper, we show a method to map three-dimensional and volumetric radiation fields for piezoelectric transducers.

Ultrasonic Imaging and Finite Element Analysis of Adhesively Bonded Cylinders

Adhesively bonded structures are increasingly being used for marine applications. One such application involves the use of adhesively bonded cylinders of polyethylene—rubber—steel. Steel forms the inner-most cylindrical lamina and polyethylene forms the outermost layer of this component. The adhesively sandwiched lamina, rubber, is subjected to axial shear loads—tangential to the curved surfaces. For this component to perform well under load it is necessary that the adhesive bonds at the polyethylene-rubber and steel-rubber interfaces be strong and free of any deleterious delaminations. Any surface areas which are devoid of adhesive, have trapped gas, are not chemically bonded by the adhesive or are simply in mechanical contact, form areas of delaminations detrimental to the performance of these components.

Ultrasonic method for monitoring corrosion damage in petroleum metal storage tanks using spectral tracking

Within the oil industry there is a need to monitor and accurately characterize corrosion damage in large metal storage tanks. These structures most often will contain highly irregular (non-specular) surfaces with sediment, water and layers of suspended particles at or near the bottom surfaces. Inspection procedures for these tanks almost always, require de-inventorying and cleaning the tank bottom surfaces for visual and ultrasonic non-destructive examination of damage, as a result of corrosion. As known by the oil industry, de-inventorying oil storage tanks are a costly expense. In this paper, we discuss some results of an actual section of a corroded mild steel tank bottom, using ultrasonic spectral tracking. Initial research using this method, has shown promising results in the possibility of mapping corrosion damage in the presence of sedimentation.

PARALLEL COMPUTATION OF ULTRASONIC WAVE PROPAGATION IN ORTHOTROPIC COMPOSITE MATERIALS

There has been much effort devoted to modeling wave propagation and scattering in isotropic materials [1]. Most of these efforts have employed finite difference techniques to compute the time evolution of the wave field in materials with scatterers. The forward problem is important in improving and understanding ultrasonic testing methods. It is also important for solving the inverse problem of determining the material and flaw properties from the scattering data [2].

Simulation of Ultrasound Propagation across Interfaces with Imperfect Contact

In elastic materials the propagation of ultrasonic waves is governed by the Christoffers equation, which relates the displacement vector as a function of time and position to the stiffness tensor at that point. If the material is inhomogeneous, an analytical solution of the partial differential equation becomes exceedingly difficult or impossible, especially in the presence of non-trivial boundary or initial conditions. Finite Difference Equations (FDE) provide a very convenient tool for the solution of partial differential equations (PDE’s) in media, in which the physical properties are homogeneous or vary continuously, such as Epstein Layers. Otherwise the use of FDE’s may be justified only as an approximation. In fact, for the conversion of derivatives into finite differences, a “smoothing” of the variables across the interfaces is required and, if the discontinuity is sharp, severe errors or ambiguities may result [1,2].

In-situ ultrasonic inspection of submarine shaft seal housing for corrosion damage

The interior of the housings of primary and backup shaft seals of 637 class submarines are exposed to sea water during service and become corroded during service. Corrosion damage evaluation requires disassembly of the housing and visual inspection. In this paper, we present quantitative results of in situ nondestructive ultrasonic technique developed for the inspection of the seal housings. Due to vast variations in velocity in the seal material, the velocity was determined at suitable sites not subjected to corrosion and of known thickness from the blueprints. Using this normalized velocity and measured time-of-flight, we determined the thickness of the seal housing at various locations on the circumference. Subsequent mechanical thickness measurements, made when the housings were removed from service, agreed within the predicted uncertainty of 1.5% of ultrasonic measurements. This technique for the assessment of corrosion damage saves time and money, by preventing premature disassembly and downtime for the submarine.

Nondestructive ultrasonic determination of disbonds in adhesively bonded submarine shaft vibration reducers

Good adhesive bond between the laminates is extremely important for the shear strength of adhesively bonded poly-rubber-steel concetric cylinders used as marine shaft vibration reducers (SVR). The SVR are inspected nondestructively by using a high-speed PC-based transmission ultrasonic C-scan system capable of scanning and producing images. Transmission of ultrasound at frequencies between 5-20 MHz and time-of-flight variations of these signals through these multilayered structures is used to determine the disbonds at the interfaces. Two- and three-dimensional images are generated and analyzed statistically to quantify the areas of disbond. Mathematical models for the propagation of ultrasound through such a layered structure are applied for the interpretation of these images.

Measurement of Anisotropic Elastic Moduli and Comparison with Equivalent Media Theories

Anisotropic elastic moduli were determined for a layered glass plate-epoxy composite from oblique angle time-of-flight measurements using an immersion system. The composite consisted of thin glass plates bonded with a UV curing epoxy. The glass plates were 5cm square and 0.04cm thick. The epoxy bonds were very thin, comprising only 1.99% of the composite. Both the glass and the epoxy behave as isotropic materials in bulk form. However, once assembled to produce a heterogeneous structure (composite material) and probed with sufficiently long wavelengths (approximately ten times the thickness of the glass plates at 1 MHz), the stack of thin glass-epoxy layers appeared homogeneous but with anisotropic properties. The anisotropy exhibited by the glass-epoxy stack was transverse isotropy. Equivalent media theories are designed to predict behavior of homogeneous material combined in a heterogeneous structure. Consequently, with the measured properties of the constitutive bulk glass and epoxy and knowledge of their ratio in the layered stack, forward calculations can be made to determine the elastic moduli of the composite. Of the equivalent media theories available for calculating expected behavior in composite media, we consider a thickness-weighted averaging model as well as a theory which models the thin compliant epoxy bonds as a set of fractures. Anisotropic elastic moduli predicted using these two theories will be compared to the measured elastic moduli of the glass-epoxy stack.