Frank J. Margetan

Bio: Frank J. Margetan is an academic researcher from Iowa State University. The author has contributed to research in topics: Ultrasonic sensor & Ultrasonic testing. The author has an hindex of 18, co-authored 98 publications receiving 1079 citations.

Interfacial Spring Model for Ultrasonic Interactions with Imperfect Interfaces: Theory of Oblique Incidence and Application to Diffusion-Bonded Butt Joints

Abstract: The quasi-static distributed spring model is used to derive the ultrasonic reflectivity of an imperfectly-bonded interface as a function of frequency and angle of incidence. The results are then incorporated in a model for the corner reflection from a diffusion-bonded joint between two abutting plates, the corner being defined by the bond plane and the common lower surface plane of the plates. An immersion-inspection geometry is assumed, and seven categories of corner reflections are identified and examined in detail. These fall into two classes: those having parallel incident and exiting rays in water (φ′=φ), and those having nonparallel water rays (φ′ ≠ φ). The φ′ = φ categories are suitable for single probe (pulse-echo) inspections of the joint. Based on the amplitude of the outgoing corner-reflected signal, two φ = φ′ geometries appear promising. These employ, respectively, a corner reflection involving only longitudinal waves with the interface illuminated at near-grazing incidence (LLL), and a corner reflection involving only transverse waves with the interface illuminated at near 45° incidence (TTT). In addition, two practical φ′ ≠ φ geometries are indicated; these both involve mode conversion upon reflection from the interface, with the incident or outgoing longitudinal wave traveling nearly parallel to the interface. Model predictions for LLL and TTT reflections are compared to measurements on diffusion-bonded Inconel specimens, and techniques for applying the model results to more complicated bond geometries are discussed.

Backscattered microstructural noise in ultrasonic toneburst inspections

Abstract: A model is presented which relates the absolute backscattered noise level observed in an ultrasonic immersion inspection to details of the measurement system and properties of the metal specimen under study. The model assumes that the backscattered noise signal observed for a given transducer position is an incoherent superposition of echoes from many grains. The model applies to normal-incidence, pulse-echo inspections of weakly-scattering materials using toneburst pulses from either a planar or focused transducer. The model can be used in two distinct ways. Measured noise echoes can be analyzed to deduce a “Figure-of-Merit” (FOM) which is a property of the specimen alone, and which parameterizes the contribution of the microstructure to the observed noise. If the FOM is known, the model can be used to predict the absolute noise levels that would be observed under various inspection scenarios. Tests of the model are reported, using both synthetic noise echoes, and measured noise echoes from metal specimens having simple and complicated microstructures.

A Technique for Quantitatively Measuring Microstructurally Induced Ultrasonic Noise

Abstract: In ultrasonic inspections of aircraft engine components, the detectability of critical defects can be limited by grain noise. This is likely to be the case for subtle defects, such as hard-alpha-phase inclusions in titanium alloys, where the difference between the acoustic impedances of the defect and host is small. A sound quantitative description of grain noise in such alloys is essential for accurate estimates of flaw detection reliability. In this work we present a method for quantifying backscattered grain noise by using positional averaging to determine the root-mean-squared (rms) noise level. The measured noise level will depend on details of the measurement system, as well as on inherent material properties of the alloy. We present a preliminary model of the noise measurement process which accounts for system effects, and we compare its predictions with experiment. We then indicate how the rms noise data can be processed to extract a factor which parameterizes the inherent noise severity independent of the measurement process.

The interaction of ultrasound with imperfect interfaces: Experimental studies of model structures

Abstract: Model specimens are prepared, each of which may be viewed as two sections of similar material joined imperfectly at a planar interface. Measurements of the ultrasonic reflection from, mode conversion at, and/or transmission through these imperfect interfaces, are reported. The interface structures include distributions of pores, contacts, and inclusions. Included are both near-periodic and random cases. As the frequency is increased, the measured reflection coefficients generally show an initially linear increase from zero, followed by a maximum which may exhibit multiple peaks, and a subsequent decay. These results are interpreted in terms of a quasi-static model and an independent scattering model for ultrasonic interactions with imperfect interfaces.

A new method for estimation of velocity vectors

Abstract: The paper describes a new method for determining the velocity vector of a remotely sensed object using either sound or electromagnetic radiation. The movement of the object is determined from a field with spatial oscillations in both the axial direction of the transducer and in one or two directions transverse to the axial direction. By using a number of pulse emissions, the inter-pulse movement can be estimated and the velocity found from the estimated movement and the time between pulses. The method is based on the principle of using transverse spatial modulation for making the received signal influenced by transverse motion. Such a transverse modulation can be generated by using apodization on individual transducer array elements together with a special focusing scheme. A method for making such a field is presented along with a suitable two-dimensional velocity estimator. An implementation usable in medical ultrasound is described, and simulated results are presented. Simulation results for a flow of 1 m/s in a tube rotated in the image plane at specific angles (0, 15, 35, 55, 75, and 90 degrees) are made and characterized by the estimated mean value, estimated angle, and the standard deviation in the lateral and longitudinal direction. The average performance of the estimates for all angles is: mean velocity 0.99 m/s, longitudinal S.D. 0.015 m/s, and lateral S.D. 0.196 m/s. For flow parallel to the transducer the results are: mean velocity 0.95 m/s, angle 0.10, longitudinal S.D. 0.020 m/s, and lateral S.D. 0.172 m/s.

Ultrasonic classification of imperfect interfaces

Abstract: Ultrasonic reflection measurements from material interfaces are commonly used to detect and quantitatively characterize boundary imperfections of different kinds. Either shear or longitudinal waves can be used to assess the degree of the interface imperfection in acoustical terms. On the other hand, the evaluation of this data in terms of strength-related mechanical properties requiresa priori knowledge of the physical nature of the imperfection. It is shown in this paper that the ratio between the normal and transverse interfacial stiffnesses can be used to classify the interface imperfection. This ratio is readily measured, e.g., by comparing the longitudinal and shear reflection coefficients at normal incidence. Both theoretical and experimental results indicate that different types of imperfections, such as kissing, partial, and slip bonds, can be distinguished by this simple technique.

A Study of the Interaction between Ultrasound and a Partially Contacting Solid--Solid Interface

Abstract: The measurement of the reflection of ultrasonic waves from a partially contacting solid--solid interface can be used to study the contact conditions at that interface. This paper describes measurements and predictions of the reflection of ultrasonic waves from partially contacting aluminium--aluminium interfaces, performed in the low frequency regime where the wavelength of the ultrasound is large compared to the size of the gaps. The proportion of the incident wave which is reflected at the interface (the reflection coefficient) was measured as a function of frequency with a single wideband ultrasonic transducer. When load was applied across the interface three regions of contact can be seen; no contact, partial contact and perfect contact. In the no contact region the measured reflection coefficient was unity at all frequencies. In the partial contact region the measured reflection coefficient increased with frequency. No measurements were taken in the perfect contact region in which the reflection coefficient is known to be zero at all frequencies as this state is the same as a continuous piece of aluminium. The reflection coefficient variation with frequency was modelled using a spring model, good agreement between experiments and predictions being achieved. Reflection coefficient measurements were then used to study the contact between two aluminium surfaces under repeated loading and unloading cycles. Plastic flow on first loading was evident while subsequent loading cycles revealed largely elastic behaviour. Both elastic and plastic statistical contact models, as well as a numerical contact model, were used to predict the variation of interfacial stiffness with pressure. These models agreed qualitatively with the experimentally determined stiffness variations and the predicted stiffness was within an order of magnitude of the measured value in all cases.