Peter B. Nagy

Fatigue damage assessment by nonlinear ultrasonic materials characterization

The ultimate strength of most structural materials is mainly limited by the presence of microscopic imperfections serving as nuclei of the fracture process. Since these nuclei are considerably shorter than the acoustic wavelength at the frequencies normally used in ultrasonic nondestructive evaluation (NDE), linear acoustic characteristics are not sufficiently sensitive to this kind of microscopic degradation of the materials integrity. On the other hand, even very small imperfections can produce very significant excess nonlinearity which can be orders of magnitude higher than the intrinsic nonlinearity of the intact material. The excess nonlinearity is produced mainly by the strong local nonlinearity of microcracks whose opening is smaller than the particle displacement. Parametric modulation via crack-closure significantly increases the stress-dependence of fatigued materials. A special experimental technique was introduced to measure the second-order acousto-elastic coefficient in a great variety of materials including plastics, metals, composites and adhesives. Experimental results are presented to illustrate that the nonlinear acoustic parameters are earlier and more sensitive indicators of fatigue damage than their linear counterparts.

The use of non-collinear mixing for nonlinear ultrasonic detection of plasticity and fatigue

This letter reports on the application of the non-collinear mixing technique to the ultrasonic measurement of material nonlinearity to assess plasticity and fatigue damage. Non-collinear mixing is potentially more attractive for assessing material state than other nonlinear ultrasonic techniques because system nonlinearities can be both independently measured and largely eliminated. Here, measurements made on a sample after plastic deformation and on a sample subjected to low-cycle fatigue show that the non-collinear technique is indeed capable of measuring changes in both, and is therefore a viable inspection technique for these types of material degradation.

Revolutionizing biodegradable metals

Development of biodegradable metal implants is a complex problem because it combines engineering and medical requirements for a material. This article discusses the development of sensing and corrosion control techniques that can help in the design of biodegradable metallic implants. Biodegradable metallic implants dissolve as new tissue is formed. One of the most important factors in the design of biodegradable implants is to study the active interface, which should be monitored and controlled to address the medical concern of biocompatibility. Thus miniaturized and nanotechnology-based sensors that measure the activities of the degradation process and the formation of tissue are discussed for use with in vitro and in vivo experiments. These sensors can monitor chemical components and also cell activity and can provide new knowledge about biodegradable interfaces and how to actively control the interface to provide the best bioactivity to regenerate new tissue in a short time. Development of new alloys, nano-materials, miniature sensors, corrosion control coatings, and auxiliary applications such as biodegradable drug delivery capsules is expected to open up a new era in the engineering of materials for medicine.

Nondestructive evaluation of adhesive joints by guided waves

Guided waves in an adhesive layer between two adherend half‐spaces are shown to be uniquely sensitive to most types of bond defects. A simple experimental technique based on ultrasonic transmission mesurement is introduced to detect leaky guided modes in the interface layer. Experimental results for these dispersive guided modes are shown to be in good quantitative agreement with theoretical predictions. The suggested technique might find numerous applications in nondestructive evaluation of different bonds of layered structure such as adhesive and brazed joints.

Excess attenuation of leaky Lamb waves due to viscous fluid loading

In two recent papers [J. Acoust. Soc. Am. 97, 3191–3193 (1995) and 98, 1057–1064 (1995)], Zhu and Wu presented an analytical technique to assess the effect of viscous fluid loading on the propagation properties of Rayleigh and Lamb waves in fluid-loaded solids. They modeled the viscous fluid as a hypothetical isotropic solid having rigidity c55=−iωη, where η denotes the viscosity of the fluid and ω is the angular frequency. In this way, the vorticity mode associated with the viscosity of the fluid is formally described as the shear-mode in the fictitious solid. In this paper this technique is further developed by removing certain inconsistencies that unnecessarily reduce the accuracy and the range of validity of Zhu and Wu’s results. By properly accounting for viscous effects on the bulk compressional wave in the fluid and applying a rigorous treatment of the field equations and boundary conditions, the exact dispersion equations that are not limited to low frequencies and viscosities are derived. Examples of these results are presented to illustrate the effect of fluid viscosity on the lowest-order symmetric and antisymmetric Lamb modes. One interesting feature revealed by these calculations is the presence of a sharp minimum in the viscosity induced attenuation of the lowest-order symmetric mode of thin plates either immersed in or coated with a viscous fluid. This minimum occurs at a particular frequency where the otherwise elliptical polarization of the surface vibration becomes linearly polarized in the normal direction.In two recent papers [J. Acoust. Soc. Am. 97, 3191–3193 (1995) and 98, 1057–1064 (1995)], Zhu and Wu presented an analytical technique to assess the effect of viscous fluid loading on the propagation properties of Rayleigh and Lamb waves in fluid-loaded solids. They modeled the viscous fluid as a hypothetical isotropic solid having rigidity c55=−iωη, where η denotes the viscosity of the fluid and ω is the angular frequency. In this way, the vorticity mode associated with the viscosity of the fluid is formally described as the shear-mode in the fictitious solid. In this paper this technique is further developed by removing certain inconsistencies that unnecessarily reduce the accuracy and the range of validity of Zhu and Wu’s results. By properly accounting for viscous effects on the bulk compressional wave in the fluid and applying a rigorous treatment of the field equations and boundary conditions, the exact dispersion equations that are not limited to low frequencies and viscosities are derived. Example...

Study and comparison of different EMAT configurations for SH wave inspection

Guided wave inspection has proven to be a very effective method for the rapid inspection of large structures. The fundamental shear horizontal (SH) wave mode in plates and the torsional mode in pipe-like structures are especially useful because of their non-dispersive character. Guided waves can be generated by either piezoelectric transducers or electro- magnetic acoustic transducers (EMATs), and EMATs can be based on either the Lorentz force or magnetostriction. Several EMAT configurations can be used to produce SH waves, the most common being Lorentz-force periodic permanent magnet and magnetostrictive EMATs, the latter being directly applied on the sample or with a bonded strip of highly magnetostrictive material on the plate. This paper compares the performance of these solutions on steel structures. To quantitatively assess the wave amplitude produced by different probes, a finite element model of the elementary transducers has been developed. The results of the model are experimentally validated and the simulations are further used to study the dependence of ultrasonic wave amplitude on key design parameters. The analysis shows that magnetostrictive EMATs directly applied on mild steel plates have comparatively poor performance that is dependent on the precise magneto-mechanical properties of the test object. Periodic permanent magnet EMATs generate intermediate wave amplitudes and are noncontact and insensitive to the variations in properties seen across typical steels. Large signal amplitudes can be achieved with magnetostrictive EMATs with a layer of highly magnetostrictive material attached between the transducer and the plate, but this compromises the noncontact nature of the transducer.

LONGITUDINAL GUIDED WAVE PROPAGATION IN A TRANSVERSELY ISOTROPIC ROD IMMERSED IN FLUID

The propagation of longitudinal guided waves in fluid‐loaded transversely isotropic rods has been investigated based on the superposition of partial waves. The numerical results indicate that fluid loading causes not only significant attenuation via energy leakage into the fluid but also strongly affects the phase velocity of the modes in certain frequency ranges. There is an apparent ‘‘mode switching’’ between the two lowest‐order modes when the rod is loaded by a relatively high‐density fluid. This phenomenon is analogous to the anomalous topology previously observed in the Lamb wave spectra of low‐density water‐loaded plates.

Slow wave propagation in air‐filled porous materials and natural rocks

Slow compressional waves in fluid‐saturated porous solids offer a unique acoustical means to study certain material properties, such as tortuosity and permeability. We present a novel experimental technique based on the transmission of airborne ultrasound through air‐filled porous samples. The suggested method can be used to measure the velocity and attenuation of the slow compressional wave in a wide frequency range from 30 to 500 kHz. More important, the technique is so sensitive that it provides irrefutable evidence of slow wave propagation in air‐saturated natural rocks and lends itself quite easily to tortuosity measurements in such materials, too.

Surface roughness induced attenuation of reflected and transmitted ultrasonic waves

The problem of ultrasonic transmission and reflection at a randomly rough interface is considered in connection with ultrasonic NDE of rough surface samples by immersion method. A simple first‐order phase perturbation technique is used to calculate both transmitted and reflected components for comparison with experimental results. The transmitted wave is shown to be attenuated in a similar way to the reflected one, and their attenuation ratio is found to be independent of frequency in the considered cases of slight surface roughness. For instance, the surface roughness induced attenuation of the wave reflected from a water–aluminum interface is about seven times higher than that of the transmitted component. Experimental results are presented to show good agreement with calculated predictions of the suggested simple technique.

Corrosion and erosion monitoring in plates and pipes using constant group velocity Lamb wave inspection

Recent improvements in tomographic reconstruction techniques generated a renewed interest in short-range ultrasonic guided wave inspection for real-time monitoring of internal corrosion and erosion in pipes and other plate-like structures. Emerging evidence suggests that in most cases the fundamental asymmetric A0 mode holds a distinct advantage over the earlier market leader fundamental symmetric S0 mode. Most existing A0 mode inspections operate at relatively low inspection frequencies where the mode is highly dispersive therefore very sensitive to variations in wall thickness. This paper examines the potential advantages of increasing the inspection frequency to the so-called constant group velocity (CGV) point where the group velocity remains essentially constant over a wide range of wall thickness variation, but the phase velocity is still dispersive enough to allow accurate wall thickness assessment from phase angle measurements. This paper shows that in the CGV region the crucial issue of temperature correction becomes especially simple, which is particularly beneficial when higher-order helical modes are also exploited for tomography. One disadvantage of working at such relatively high inspection frequency is that, as the slower A0 mode becomes faster and less dispersive, the competing faster S0 mode becomes slower and more dispersive. At higher inspection frequencies these modes cannot be separated any longer based on their vibration polarization only, which is mostly tangential for the S0 mode while mostly normal for the A0 at low frequencies, as the two modes become more similar as the frequency increases. Therefore, we propose a novel method for suppressing the unwanted S0 mode based on the Poisson effect of the material by optimizing the angle of inclination of the equivalent transduction force of the Electromagnetic Acoustic Transducers (EMATs) used for generation and detection purposes.