Cryo-Ultrasonic NDE: Ice-Cold Ultrasonic Waves for the Detection of Damage in Complex-Shaped Engineering Components.

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.

Phased Array Imaging of Complex-Geometry Composite Components.

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.

Guided Wave Tomography of Pipe Bends.

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.

Experimental Validation of a Fast Forward Model for Guided Wave Tomography of Pipe Elbows.

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.

Acoustic formulation of elastic guided wave propagation and scattering in curved tubular structures.

Recently, the use of guided wave technology in conjunction with tomographic techniques has provided the possibility of obtaining point-by-point maps of corrosion or erosion depth over the entire volume of a pipeline section between two ring arrays of ultrasonic transducers. However, current research has focused on straight pipes and little work has been done on pipe bends and other curved tubular structures which are also the most susceptible to developing damage. Tomography of curved tubes is challenging because of the complexity and computational cost of the 3-D elastic model required to accurately describe guided wave propagation. Based on the definition of travel-time-preserving orthogonal parametric representations of curved tubes, this paper demonstrates that guided wave propagation and scattering can be approximated by an equivalent 2-D acoustic model which is inhomogeneous and elliptically anisotropic. Numerical methods to solve the full wave equation and predict ray paths and travel times are introduced and applied to the case of a bend. Particular emphasis is given to the shortest-path ray tracing method, which is applied to the 2-D model to compute ray paths and predict travel times of the fundamental flexural mode, A0, propagating across a curved pipe. Good agreement is found between predictions and experiments performed on a 220-mm-diameter (8-in-diameter) (D) pipe with 1.5D bend radius. The 2-D model also reveals the existence of an acoustic lensing effect which leads to a focusing phenomenon also confirmed by the experiments. The computational efficiency of the 2-D model makes it ideally suited for tomographic algorithms.