Ultrasonic Characterization of Material Interfaces and Flaws

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Ultrasonic Characterization of Material Interfaces and Flaws Ultrasonic inspection has become the primary tool for industry to characterize flaws in engineering structures, and to evaluate material properties; this mirrors a parallel development of ultrasonic diagnostic techniques in the biomedical field. In its simplest form, a short mechanical impulse, with a central frequency in the MHz range is sent into an engineering component, where it interacts with defects, interfaces and other types of material discontinuities. Diffracted and reflected waves are captured, and then analyzed to determine the internal structure of the component under examination. Numerical modeling can then be used to assess the stress levels and the probability of component failure before the next inspection takes place. More advanced testing modes include resonance tests, evaluation of material anisotropy, grain size distribution assessments, and measurements of non-linear material properties. This technology is being challenged by factors such as (a) an increasing focus on product safety, (b) the use of brittle materials such as ceramics with low fracture toughness, (c) the need for inspection of very hot or radioactive components, (d) new composite materials that introduce complexities to wave propagation, and (e) a growing tendency to push engineering materials closer to their theoretical limit where even the smallest flaw can be critical. In this research program, we are developing several key elements of ultrasonic nondestructive evaluation technology to meet these challenges. Each project has active involvement from industry, with financial support coming from government granting agencies and private sponsors: (1) Development of new ultrasonic transducers for use in harsh environments of high temperature and/or gamma radiation. The key unique aspect of this project is the attenuative backing element for the piezoleectric element – we are developing a new form of porous ceramic material that has the high-temperature stability, acoustic impedance, and attenuation required for this special role. (2) New digital signal processing strategies to enhance the sharpness of ultrasonic images, such that defect size or crack depth can be measured to a fraction of a millimeter. This project takes signal processing techniques commonly used in one field of study such as biomedical imaging or geophysics, and develops them for new applications in the industrial nondestructive evaluation world. (3) Ultrasonic characterization of material interfaces, in particular those featuring partially degraded adhesion –e.g., an environmentally degraded adhesive bond; a cold shut in a turbine blade; a lapping defect in an extruded tube. In this class of problems, we are confronted with an interfacial region whose thickness is substantially less than the typical ultrasonic wavelength of ~1 mm produced by conventional transducers. A combination of techniques is required to achieve adequate imaging resolution for this class of problems: very high frequency transducers; spectral extrapolation techniques to extract high frequency components; use of angled shear waves that have a shorter wavelength and more sensitivity to weak interfaces than compression waves, frequency analysis of wave interfacial reflection and transmission. (4) Finite element modeling of wave propagation in non-homogeneous materials. This issue is concerned not just with functionally graded materials, but materials that have a strong temperature gradient such as encountered in on-line industrial inspections. Not only does the speed of sound change with temperature, but waves will “skew” onto a curved trajectory when they encounter a temperature gradient.