C. M. Fortunko

NDE applications of air-coupled ultrasonic transducers

In recent years, several new NDE applications of air-coupled, as opposed to liquid-coupled, ultrasonics have been seriously pursued. The applications range from through transmission testing of aerospace laminates and various honeycomb or foam filled materials, and solar cells, to pulse-echo flaw detection in plastic electronic packaging materials and in-line inspection of large-diameter gas pipelines. Most of the new applications have been made possible by major advances in component technologies: transducers, preamplifiers, pulsers, signal-conditioning, and impedance matching and noise-matching circuits. In this paper, we describe the transducer types and circuit topologies that are especially suitable for applications in the 20 kHz to 20 MHz frequency region. We also address the important key differences between air-coupled and liquid-coupled inspection configurations. In particular, we focus on the practical advantages and limitations that arise from the fact that gases and liquids exhibit substantially different specific densities and sound speeds.

BROADBAND ELECTROSTATIC TRANSDUCERS : MODELING AND EXPERIMENTS

A model for broadband electrostatic transducers capable of generating and detecting ultrasound in air at megahertz frequencies has been developed. This model uses a lumped parameter approximation to describe a transducer with a grooved backplate and a stretched diaphragm. The mechanical stiffness effects included in the model are compressibility of the air gap separating the diaphragm and the backplate, flexure bending stiffness of the diaphragm, and in‐plane tension forces applied to the diaphragm. A prototype transducer with backplate grooves 200 μm wide and 3.75 μm deep was constructed using micromachining techniques. Measurements of the electrical admittance and transmit sensitivity were made at various polarization voltages and diaphragm tensions. Model predictions of transducer electrical admittance and transmit sensitivity compare well with experimental data. The resonance frequency is predicted within 50 kHz of the nominal measured value near 500 kHz, and model predictions of transmit sensitivity ...

Determination of in‐plane residual stress states in plates using horizontally polarized shear waves

In this paper a new approach for using acoustic measurements to evaluate residual stresses in the presence of unknown material property variation is presented. Procedures previously applied to the evaluation of stress with acoustic measurements are reviewed, and it is shown that these involve using measurements with bulk waves propagating along the normal to the surface of a plate and do not provide sufficient information to separate the influences of stress and material property variations. To overcome this fundamental limitation, an alternative theory is developed that governs the propagation of shear waves polarized horizontally with respect to the surface of a plate (SH waves), but propagating at oblique angles with respect to the surface normal. The question of separating the effects of residual stress and material properties on acoustic velocity is addressed in detail. A practical experimental procedure is developed that permits the evaluation of the in‐plane components of the principal stresses in a plate exhibiting an unknown inhomogeneous initial anisotropy caused by material texture or microstructure. The procedure is then verified experimentally using an aluminum specimen with a known residual stress state, but unknown initial anisotropy.

Orthotropic elastic constants of a boron‐aluminum fiber‐reinforced composite: An acoustic‐resonance‐spectroscopy study

Acoustic‐resonance spectroscopy was used to determine the complete elastic constants of a uniaxial‐fiber‐reinforced metal‐matrix composite: boron‐aluminum. This material exhibits orthotropic macroscopic symmetry and, therefore, nine independent elastic stiffnesses Cij. The off‐diagonal elastic constants (C12,C13,C23), which contain large errors when measured by conventional methods, were especially focused on. Good agreement emerged among present results, a previous pulse‐echo study, and theoretical predictions using a plane‐scattered‐wave ensemble‐average model. Attempts to measure the internal‐friction ‘‘tensor’’ failed.

Nondestructive evaluation of planar defects in plates using low‐frequency shear horizontal waves

An ultrasonic technique is described that allows the determination of the through‐thickness dimension and limited localization of planar defects (cracks) in an isotropic metal plate. The scattering of horizontally polarized shear (SH) plate waves by edge and buried planar defects is investigated using a variational integral expression. Numerical results are presented that allow the calculation of the SH plate wave signal amplitudes as a function of defect through‐thickness dimension and location within a plate for two‐dimensional cracks. It is shown that SH waves are particularly useful for detecting and sizing of crack‐like defects. In addition, it is demonstrated that in plates, which can support a number of propagating SH plate waves, it is also possible to determine the relative position of a defect from interference phenomena. The numerical results are confirmed experimentally using an electromagnetic‐acoustic transducer system to generate and detect 454‐kHz SH wave signals along the normal to the ci...

DIFFRACTION OF MEDIUM AND LONG WAVELENGTH HORIZONTALLY POLARIZED SHEAR WAVES BY EDGE CRACKS

Scattering of horizontally polarized shear (SH) waves by edge cracks of length l and of different orientations relative to the free surface of the half‐space is studied. A combined finite element and analytical technique is presented that is suitable for analyzing scattering by cracks or inhomogeneities in a semi‐infinite elastic medium. Attention is focused in the range of wavelengths λ, when the ratio l/λ is ⩽1/2. Numerical results are presented for crack opening displacements and scattered fields on the surface when the medium is homogeneous or when the crack is located in an insert with different material properties. Comparison of the theoretical solution is also made with available experimental results. The experimental results are obtained using electromagnetic‐acoustic transducers that can efficiently generate and detect low‐frequency SH waves in metals. The results are applicable to sizing and characterization of weld defects. Recent experimental evidence suggests that this can be accomplished whe...

ELASTIC CONSTANTS AND INTERNAL FRICTION OF POLYCRYSTALLINE COPPER

Using ultrasonic-resonance spectroscopy (URS), we measured the elastic constant C and companion internal friction Q −1 of isotropic polycrystalline copper. The annealed material was 0.9999 pure with equiaxed heavily twinned grains averaging about 75 μm diameter. The URS method offers the principal advantage of point contact or loose coupling, thus there was no contribution from a transducer-specimen bond and only small contributions from transducers and fixture. A second advantage is one measurement for all elastic constants and all associated internal frictions. The C s agree with established values. The Q −1 s are much lower than pulse-echo-method values. Comparison of measured Q −1 with the Koehler-Granato-Lucke model permits estimating an effective dislocation-loop length. Q −1 (shear) exceeds Q −1 (longitudinal) by a factor of about 1.5.

Development of practical wideband high-fidelity acoustic emission sensors

The development of a series of wideband acoustic emission sensor/preamplifier systems is described. Key design factors are discussed along with the actual design and characterization of the sensors. These new sensors with integral amplification are out-of-plane, displacement response sensors nearly independent of frequency over a range from a slow as 30 to 50 kHz up to 1.2 MHz. The sensor design includes electromagnetic shielding and mechanical protection of the sensitive elements. More importantly, these practical sensors have signal-to-noise sensitivity that is equivalent to typical commercial resonant sensor/preamplifier systems operating from 100 kHz to 300 kHz. A mounting fixture for the sensors has also been developed.

Ultrasonic Detection and Sizing of Two-Dimensional Weld Defects in the Long-Wavelength Limit

hkW A,ultrasonic inspection technique is described for detecting elongated defects in butt-weldnents. The technique can be used to detect and size two-dimensional defects which can potentially impair the fitness-for-purp ose condition of pipeline girth welds. The defect sizing is accomplished by inverting long-wavelength scattering data o btained with shear-horizontal (SH) waves. NoncourJling, electromagneticacoustic transducers (EMATs) are used to excite and detect t he 450 kHz SH-wave probing signals. The operation of a prototype system is demonstrated in conjunction with a weld fatigue i nvestigation. The technique is used to detect an incomplete fusion d efect with a through-wall depth of 0.5 mm (0.020 inch) and to monitor crack growth in the heat affected zone (HAZ) under tension-tension fatigue loading. In addition, an experimental calibration curve is given for sizing two-dimensional surface d efects. The experimental results are explained in terms of a reciprocity r elationship evaluated in the elastostatic limit.

Resonant transmission of a three‐dimensional acoustic sound beam through a solid plate in air: Theory and measurement

A quantitative model has been developed to explain the prominent features of the resonantly enhanced transmission of a three‐dimensional airborne sound beam through a solid elastic plate. A quantitative model of this phenomenon is important in order that air coupling can be exploited for applications, such as the determination of elastic properties of plate‐shaped materials. In our model, the beam axis is aligned to exploit resonant coupling with the A0 Lamb mode of the plate, and a finite‐sized transducer is used to measure transmission of the beam through the plate. The interaction of airborne and liquid beams with a solid plate differs significantly. Because of the difference in densities between air and the solid plate material, the resonant coupling mechanism for airborne beams is very sensitive to the incidence angle of individual plane waves in the angular beam spectrum, approximately 1000 times more sensitive than if the coupling fluid is water. It was found that the elastic waves excited in the p...