This study develops underwater surface profiling utilizing a hybrid technique with laser excitation and ultrasonic detection, in which a laser beam is scanned over the object surface and ultrasonic waves radiating from the surface are measured by an array of transducers. Using a measurement system employing a 532 nm wavelength pulsed laser, which transmits well through water, and a 1-MHz, 64-element ultrasonic linear array probe, we conducted experiments of surface profiling on an aluminum extrusion frame. The results demonstrated that this measurement technique can reconstruct several geometrical features of the cross section of the frame. However, the accuracy of the reconstruction can be affected by uncertainty in the position of the array relative to the path of the laser beam. This issue is addressed by observing that to form an image of the medium it is sufficient to use the waveforms recorded by a single array element. Moreover, the images from different elements align with each other only if the correct array position is used when forming the images. Therefore, we proposed an optimization method that finds the true position of the array by imposing that all the element-generated images align with each other.

The transmission of compressional ultrasonic waves into a rigid and dense solid with a doubly-curved surface is impeded when the solid is placed in a liquid medium and its surface is irradiated with waves traveling through the liquid. Measurable power transmission is only possible when the incident ultrasonic beam is close to normal to the surface. This condition is difficult to realize when the waves are excited and detected by a linear array of transducers and limits the possibility of forming cross-sectional images of the solid from the array data. Here, it is shown that the interior of the solid can be imaged with enhanced fidelity if the water is frozen. The high speed of compressional waves in polycrystalline ice (approximately 4000 ms-1) along with its rigid behavior ensure that ultrasonic waves can be transmitted through the surface over a broad range of angles of incidence. However, due to the double curvature, the rays that form the ultrasonic beam can be deflected outside the array azimuthal plane after entering the solid. Therefore, the two-dimensional images obtained from the linear array data may not be consistent with the fully three-dimensional structure of the ray paths. The analysis of this phenomenon for the special case of solid spheres reveals that the image, to a good approximation, corresponds to a section of the sphere that is parallel to the azimuthal plane and at a standoff distance from it. The distance increases with the angle that the normal to the surface forms relative to the azimuthal plane while it decreases as the velocity contrast between ice and the material of the sphere decreases. While this property is not expected to hold for more complex surfaces, the ray-based framework used in this study is applicable to more general surface configurations and can be used to correlate the images to the structure of the solid. These findings are relevant to the inspection of metallic components with complex geometry which represents a long-standing challenge in the field of nondestructive testing.

Cryo-ultrasonic testing utilizes polycrystalline ice coupling to enable the inspection of metallic components with complex shape. The relatively high velocity of compressional waves in ice (approximately 4000 m s-1) -1 ) and its ability to support the propagation of shear waves, significantly strengthen the ultrasonic transmission through curved interfaces over conventional water coupling. This paper explores the possibility of further enhancing the ultrasonic properties of ice by dispersing solid particles in water before it is frozen. Complex physicochemical phenomena occur when aqueous dispersions freeze which can lead to a solid material with microstructural characteristics that may be unfavorable to the propagation of ultrasonic waves. Here, these effects are controlled to produce a composite material consisting of alumina nanoparticles in an ice matrix. The composite exhibits compressional and shear wave velocities of approximately 4800 m s -1 and 2700 m s -1 , respectively. Importantly, the mass density of the material is more than twice as large as the density of water. Finally, it is shown that a phenomenon similar to a glass transition occurs during freezing which results in low ultrasonic attenuation when the temperature approaches - 100 degrees C.

The thermal expansion occurring when a laser beam is incident on the surface of a solid can be used to excite ultrasonic waves in the solid medium, without causing damage to the material. The resulting wavefield is characterized by a dominant shear wave and a weak compressional wave. This Letter demonstrates the possibility of generating a dominant compressional wavefield by coating the surface with a layer of clear ice. This is achieved by exploiting a minimum in the attenuation spectrum of light in ice, which occurs at around 500 nm and renders the coating transparent to green laser radiation. Ice coatings could, therefore, provide a path for the development of more sensitive laser-based nondestructive testing methods that have traditionally been affected by the poor excitability of compressional waves, especially along the direction orthogonal to the surface.

Polycrystalline solids are composed of many small grains of varying sizes and crystallographic orientations. An elastic wave that propagates through such a material experiences distortion and attenuation. While the influence on propagation in random configurations can be captured with conventional statistical descriptors, the role of second-order features such as the hierarchical gradient in material properties has not been explored. In this paper, we optimize a numerical strategy based on Finite Elements and Local Max-Entropy approximants to characterize the role of grain density gradients on ultrasonic attenuation. We focus on ice as a model for mesoscale ordered configurations due to its relevance to the emerging technology of cryoultrasonics. Our simulations in one-and two-dimensional settings indicate that second-order descriptors are required to predict attenuation in polycrystalline ice. Furthermore, we define a novel parameter, based on the standard deviation of the speed of sound gradient distribution, which shows a quadratic relationship with the ultrasonic attenuation. The model results can be understood as a phase diagram for the design of metamaterials with specific ultrasonic scattering properties.

Tomographic methods are used with ultrasonic guided wave wall thickness monitors to provide two-dimensional maps of the wall thickness throughout a monitored area. Tomographic monitoring systems are typically configured with 16 to 32 non-intrusive ultrasound transducers positioned in predetermined patterns for optimal coverage and resolution. Other monitoring systems have much fewer transducers, positioned according to the pipe geometry and wall area of main concern. Although these systems are not optimized for wall thickness mapping, the same or similar tomographic methods can improve analysis and interpretation of the datasets they produce. Algorithms are sought for implementation in responsive, interactive software for browsing and analysis of recorded wall thickness monitoring data. Simple tomographic algorithms are also desirable for use in embedded software. A "light weight" tomographic algorithm is outlined, and a prototype implementation is tested against a proven full-scale tomography software package. The two approaches produce similar results on small datasets corresponding to wall thickness monitoring systems with few transducers installed. For the larger datasets generated for high resolution wall thickness mapping, the reference software is superior in terms of both computational load and accuracy.

Cryo-ultrasonic testing (Cryo-UT) is an emerging nondestructive testing technique that utilizes polycrystalline ice as a solid couplant to enable the ultrasonic inspection of complex-shape metallic components. After encasing the component in a block of ice, the inspection is performed by scanning an ultrasonic probe over the surface of the block. The high velocity of compressional and shear waves in ice, combined with its excellent adhesion properties to metals, facilitate the transmission of ultrasonic waves through the contours of the component and provide an effective means to probe its volume. This paper presents the first experimental investigation into the performance of Cryo-UT in the presence of curved components and demonstrates the benefits of Cryo-UT over conventional immersion testing. The study considers the canonical cases of convex and concave interfaces and proves the possibility of detecting side drilled holes inside curved objects placed behind curved walls. It is shown that the material region below a convex interface can be insonified with high degree of uniformity and with limited dependence on the radius of curvature. Moreover, the zones of ultrasonic shadow, which occur when a concave interface is tested in immersion, are eliminated with ice coupling thanks to the broader ray coverage offered by transmitted compressional waves and the presence of mode converted shear waves that do not experience the total reflection phenomenon. Finally, the good agreement achieved between experiments and simulations suggests that two-dimensional elastic wave simulations performed with the finite difference time domain method provide a robust modeling platform to continue the development of Cryo-UT and explore its applications.

Elastic waves traveling through polycrystalline ice Ih can experience distortion due to the transversely isotropic structure of its crystals. This can pose a challenge to nondestructive ultrasonic testing when ice is used as a coupling medium to transfer energy between an ultrasonic source and the solid to be inspected for the presence of damage. Here, it is shown that when ice is grown on a metallic solid using directional freezing techniques, it develops a coarse columnar grain structure, which causes the ultrasonic signals to vary greatly depending on which region of the ice volume is interrogated. On the other hand, if small ice particles are compacted around the solid and saturated with degassed liquid water before directional freezing, an equiaxed grain structure is obtained, which behaves as a homo-geneous and isotropic medium. Furthermore, it is demonstrated that ultrasonic transmission through an ice-metal interface can be achieved regardless of the angle formed between the ultrasonic beam and the interface. Therefore, it is possible to overcome one of the fundamental limitations of conventional water coupling, which prevents transmission when the beam is not close to orthogonal to the interface due to the total reflection phenomenon. This property is useful to expand the scope of the application of ultrasonic testing to solids with complex geometries such as those obtained with additive manufacturing methods.

Detection of damage in safety-critical components with complex geometry represents a long-standing challenge in NDE. The ultrasonic inspection of these parts in immersion is often not feasible due to the large impedance and velocity contrast between water and metals which can severely limit the penetration of the ultrasonic signal inside the part. However, this contrast can be significantly reduced if the water is frozen prior to the inspection leading to what has been referred to as Cryoultrasonic NDE. Since the speed of longitudinal waves in ice is more than 2.5 times greater than the speed in water, ice can be an ideal solid couplant provided that it is devoid of bubbles and cracks and that it is fully bonded to the surface of the part. This paper introduces new experimental methods to encase complex parts in blocks of crystal clear ice and presents the first low temperature scanner for ultrasonic array testing of ice-encapsulated parts. It is shown that by controlling the propagation of the solidification front while water is freezing, it is possible to prevent the formation of cracks inside the ice volume. On the other hand, bubble nucleation can be avoided by continuously forcing water circulation on the solidification front. An analytical model is provided to describe the propagation of the front and predict freezing times. Moreover, high-frequency ultrasonic monitoring experiments confirm the excellent adhesion strength properties of ice to metals. Finally, experiments performed with an additively manufactured Ti6Al4V impeller demonstrate the feasibility of performing low temperature array contact scans on the surface of the ice block encasing the part.

Continuous monitoring of corrosion damage in pipelines requires sensor systems that can achieve high precision (repeatability) to detect and size subtle wall thickness (WT) losses. In fact, corrosion typically progresses at a rate of 1 mm per year or less which poses very demanding performance requirements if the state of the pipe has to be assessed on a weekly or even monthly basis. Guided ultrasonic wave tomography (GUWT) has emerged as an attractive approach for continuous monitoring owing to its ability to map WT losses over an extended pipe section. This is possible because GUWT employs ultrasonic waves that are guided by the pipe wall to travel a large distance from the source transducer. Defects along the path of the guided wave cause a perturbation of the signal which is then interpreted by model-based inversion schemes to determine the WT loss. In addition to damage, there is a vast number of time-dependent operational and environmental (O & E) factors that can perturb the signal also when damage is not present. To achieve high precision, it is therefore essential that GUWT can detect the changes in the signal due to damage and suppress those caused by other benign factors such as temperature variations. Although numerous studies have been conducted to estimate the accuracy of various implementations of GUWT, precision has received less attention due to the challenges associated with reproducing realistic O & E conditions in the lab. This work focuses on the effect of temperature and presents the results of a continuous monitoring experiment conducted on a pipe bend kept outdoors and exposed to weather conditions for a period 21 months. It is shown that the pipe undergoes a relatively severe temperature cycling with typical daily peak-to-trough variations of 20 degrees C that cover the range from -15 degrees C to +51 degrees C as the seasons alternate. The effect of temperature is suppressed by exploiting the spatial diversity of array measurements and combining multiple datasets measured during the monitoring period. The maximum WT loss is estimated with high precision exhibiting a standard deviation not exceeding 0.4% of the nominal WT.

Recent advances in computational methods, materials science, and new manufacturing processes are resulting in an unprecedented design flexibility which is driving the geometrical complexity of the components found in modern structures and machines. For safety-critical components, the geometrical complexity poses a significant challenge to the sensitivity of the existing nondestructive evaluation (NDE) methods available for the detection of manufacturing defects or damage that develops while a component is in service. Although X-ray computed tomography is the primary NDE method used to test these parts in current industrial practice, it is widely recognized that it has limited sensitivity to critical defects, such as cracks, especially in the presence of large size parts made of dense materials. The lack of sensitive NDE methods represents a major technology gap that could impede the acceptance of rapidly developing technologies, such as 3-D printing, for the production of safety-critical components. This paper attempts to bridge this gap by exploring the possibility of inspecting a complex-shaped part with ultrasonic waves after it has been encapsulated in ice, under the paradigm of what can be defined as cryo-ultrasonic NDE. The underpinning hypothesis is that through ice encapsulation a complex-shaped part can be transformed into a simple-shaped solid whose volume can be probed with ultrasonic waves, which are known to be highly sensitive to both pores and crack-like defects and over a wide range of material properties. Damage detection is then performed by analyzing cross-sectional images of the ice-encapsulated part obtained by applying migration methods to the ultrasonic signals measured by an array of transducers. This paper lays the foundation for cryo-ultrasonic NDE and presents the first experimental results demonstrating the possibility of imaging defects through multiple ice-metal interfaces. This paves the way to the detection of defects in complex-shaped parts containing internal vanes which have so far limited the use of conventional NDE methods.

Detection and monitoring of corrosion and erosion damage in pipe bends are open challenges due to the curvature of the elbow, the complex morphology of these defects, and their unpredictable location. Combining model-based inversion with guided ultrasonic waves propagating along the elbow and inside its walls offers the possibility of mapping wall-thickness losses over the entire bend and from a few permanently installed transducers under the realm of guided wave tomography (GWT). This paper provides the experimental demonstration of GWT of pipe bends based on a novel curved ray tomography algorithm and an optimal transducer configuration consisting of two ring arrays mounted at the ends of the elbow and a line of transducers fixed to the outer side of the elbow (extrados). Using realistic, localized corrosion defects, it is shown that detection of both the presence and progression of damage can be achieved with 100% sensitivity regardless of damage position around the bend. Importantly, this is possible for defects as shallow as 0.50% of wall thickness (WT) and for maximum depth increments of just 0.25% WT. However, due to the highly irregular profile of corrosion defects, GWT generally underestimates maximum depth relative to the values obtained from 3-D laser scans of the same defects, leading in many cases to errors between 3% WT and 8% WT.

Ultrasonic guided wave tomography (GWT) methods for the detection of corrosion and erosion damage in straight pipe sections are now well advanced. However, successful application of GWT to pipe bends has not yet been demonstrated due to the computational burden associated with the complex forward model required to simulate guided wave propagation through the bend. In a previous paper [Brath et al., IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 61, pp. 815-829, 2014], we have shown that the speed of the forward model can be increased by replacing the 3-D pipe bend with a 2-D rectangular domain in which guided wave propagation is formulated based on an artificially inhomogeneous and elliptically anisotropic (INELAN) acoustic model. This paper provides further experimental validation of the INLEAN model by studying the traveltime shifts caused by the introduction of shallow defects on the elbow of a pipe bend. Comparison between experiments and simulations confirms that a defect can be modeled as a phase velocity perturbation to the INLEAN velocity field with accuracy that is within the experimental error of the measurements. In addition, it is found that the sensitivity of traveltime measurements to the presence of damage decreases as the damage position moves from the interior side of the bend (intrados) to the exterior one (extrados). This effect is due to the nonuniform ray coverage obtainable when transmitting the guided wave signals with one ring array of sources on one side of the elbow and receiving with a second array on the other side.

The effective life management of pipelines in the Oil and Gas industry requires monitoring techniques that can probe an extended section of a pipe and can sample its state with relatively high frequency so as to implement corrosion mitigation in a timely fashion. Due to the typically low corrosion rates, 1 mm/yr or less, the monitoring technique must be sensitive to wall-thickness changes that are in the order of a few tens of micrometers. Ultrasonic guided wave tomography (GWT) with permanently installed sensors offers an attractive compromise between sensitivity and area coverage by combining the long range propagation characteristics of guided ultrasonic waves with the principles of model-based inversion. Here, the pipe section to be monitored is delimited by two ring arrays of ultrasonic transducers that encircle the pipe and measure guided wave signals after traveling inside the pipe wall from one array to the other. The signals are interpreted by advanced GWT algorithms to form a wall thickness map of the entire pipe section. In this paper we review the principle of operation of GWT, discuss its performance, and present the first long term field results obtained for a pipe in a deepwater rig in the Gulf of Mexico. The field results show that GWT based on EMAT transduction provides highly stable wall thickness estimations even in the presence of multiphase flow inside the pipe undergoing a wide range of pressure and temperature fluctuations.

Progress in computational fluid dynamics and the availability of new composite materials are driving major advances in the design of aerospace engine components which now have highly complex geometries optimized to maximize system performance. However, shape complexity poses significant challenges to traditional nondestructive evaluation methods whose sensitivity and selectivity rapidly decrease as surface curvature increases. In addition, new aerospace materials typically exhibit an intricate microstructure that further complicates the inspection. In this context, an attractive solution is offered by combining ultrasonic phased array (PA) technology with immersion testing. Here, the water column formed between the complex surface of the component and the flat face of a linear or matrix array probe ensures ideal acoustic coupling between the array and the component as the probe is continuously scanned to form a volumetric rendering of the part. While the immersion configuration is desirable for practical testing, the interpretation of the measured ultrasonic signals for image formation is complicated by reflection and refraction effects that occur at the water-component interface. To account for refraction, the geometry of the interface must first be reconstructed from the reflected signals and subsequently used to compute suitable delay laws to focus inside the component. These calculations are based on ray theory and can be computationally intensive. Moreover, strong reflections from the interface can lead to a thick dead zone beneath the surface of the component which limits sensitivity to shallow subsurface defects. This paper presents a general approach that combines advanced computing for rapid ray tracing in anisotropic media with a 256-channel parallel array architecture. The full-volume inspection of complex-shape components is enabled through the combination of both reflected and transmitted signals through the part using a pair of arrays held in a yoke configuration. Experimental results are provided for specimens of increasing complexity relevant to aerospace applications such as fan blades. It is shown that PA technology can provide a robust solution to detect a variety of defects including porosity and waviness in composite parts.

Ceramic matrix composites (CMCs) are poised to revolutionize jet engine technology by enabling operation temperatures well beyond those possible with current superalloys, while reducing active cooling requirements and engine weight. Manufacturing of parts formed by silicon-carbide (SiC) fibers in a SiC matrix is now well advanced, with the first non-structural static components entering service in 2017 with the CFM Leap® engine that uses SiC/SiC turbine shrouds. In order to expand the scope of application of CMCs to rotating parts, such as turbine blades, much work is being conducted to understand and characterize the modes of failure of these materials at temperatures beyond ∼1100°C. In this context, the ability of nondestructively monitoring the formation and progression of damage in CMCs specimens during high-temperature mechanical testing is critical. However, the elevated temperature precludes the possibility of using sensors placed in direct contact with the specimen and therefore severely restricts the range of available NDE techniques. This paper provides the first experimental assessment of the feasibility of noncontact laser ultrasonic inspection of SiC/SiC flat coupons. An Nd: Yag laser is used to excite ultrasonic waves on one side of the specimen while a Michelson interferometer detects the signals emerging on the other side at the epicenter position. The lasers are mounted on synchronized linear stages to form C-scans as in conventional immersion ultrasonics while ablation damage to the surface of the specimen is prevented by operating the lasers at low power density. Despite the complex microstructure of the SiC/SiC material it is found that the measured waveforms are remarkably similar to those observed when conducting the same tests in aluminum specimens. Moreover, it is shown that it is possible to image interlaminar defects caused by impacts, and monitor crack opening under tensile load. Finally, very good signal stability is observed when temperature is increased from 25 to 1250°C which confirms the feasibility of laser monitoring at high temperature and is consistent with the good thermal stability of ceramic materials.

Mapping the speed of mechanical waves traveling inside a medium is a topic of great interest across many fields from geoscience to medical diagnostics. Much work has been done to characterize the fidelity with which the geometrical features of the medium can be reconstructed and multiple resolution criteria have been proposed depending on the wave-matter interaction model used to decode the wave speed map from scattering measurements. However, these criteria do not define the accuracy with which the wave speed values can be reconstructed. Using two-dimensional simulations, it is shown that the first-arrival traveltime predicted by ray theory can be an accurate representation of the arrival of a pulse first break even in the presence of diffraction and other phenomena that are not accounted for by ray theory. As a result, ray-based tomographic inversions can yield accurate wave speed estimations also when the size of a sound speed anomaly is smaller than the resolution length of the inversion method provided that traveltimes are estimated from the signal first break. This increased sensitivity however renders the inversion more susceptible to noise since the amplitude of the signal around the first break is typically low especially when three-dimensional anomalies are considered.

Throughout the oil and gas industry corrosion and erosion damage monitoring plays a central role in managing asset integrity. This paper introduces a novel technology for continuous monitoring of wall-loss rates in pipelines. A pair of permanently installed ring arrays of ultrasonic transducers encircles the pipe and delimits the section to be monitored. The arrays excite and receive guided ultrasonic waves that travel inside the pipe wall and insonify the entire pipe section. The received signals are then processed by advanced tomographic algorithms to produce a point-by-point map of wall thickness loss between the arrays. The algorithms are designed to detect changes between two material states of the pipe and use differential measurements to eliminate time-independent experimental uncertainties. As a result, wall loss can be estimated with accuracy beyond the pipe manufacturing tolerances. Moreover, a strategy that combines a robust temperature compensation scheme with the intrinsic thermal stability of electromagnetic acoustic transducers (EMATs) is used to address signal instabilities caused by typical thermal fluctuations experienced in field applications. Full-scale experiments are presented demonstrating maximum-depth estimation accuracy better that 0.5% of wall thickness with excellent thermal stability up to 175 °C.