Phased Array Imaging of Complex-Geometry Composite Components

Abstract

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.

An Inverse Approach for Ultrasonic Imaging From Full Matrix Capture Data: Application to Resolution Enhancement in NDT

“…For a layered isotropic medium, times of flight can be computed using an optimization algorithm [36] or by analytical results for flat surfaces [37]. For anisotropic materials, they can be computed using the Shortest Path Raytracing method [38] or the Fast Marching Method [39], [40]. The Excitelet algorithm [20] is also a post-processing algorithm that focuses at each point of the reconstruction grid.…”

Locally Optimal Subsampling Strategies for Full Matrix Capture Measurements in Pipe Inspection

“…In many publications, iterative schemes for finding a propagation path that fulfils Fermats principle given one or multiple surfaces are used [11,12,19,20]. On the other hand, e.g., in [21], the Eikonal equations are solved directly to determine the propagation times.…”

Ultrasonic synthetic aperture imaging with interposed transducer–medium coupling path

“…As stated in the previous section, due to wave refraction, these rays do not have straight paths since they bend at the wedge/ medium interface, thus following the paths of least travel time described by Fermat's principle. 12 The point at the interface where a ray connecting the focus point to a transducer element bends is the ''virtual'' position of that transducer. The ''virtual'' array can then be created by finding all the rays connecting a specific focus point to the physical array.…”

“…In various applications of ultrasonic imaging, phased arrays are used to inspect different portions of the medium by dynamic beam focusing and beam steering. [7][8][9][10][11][12] This is achieved by well-known focus laws that require multiple simultaneous active channels to the array transmitters, which also need to be controlled individually, in order to physically focus the transmitted beam. Hence, the transmission hardware of phased-array (PA) probes can be fairly complex.…”

“…12,[38][39][40] Ray tracing allows computing the only physically possible wave propagation paths and allows to track changes along these paths. The rays are computed following Fermat's principle, 12 which allows to identify the paths of least travel time between two points. This technique represents a simplification of the actual wavefield and propagation behavior, which can be complex to model.…”