✕

The aerospace industry mandates the highest standards of safety and reliability, making nondestructive testing (NDT) a critical process in the inspection of aircraft structures and components. The evolution of automated NDT technologies, particularly phased arrays, advanced automated 3D scanning and C-Scan mapping techniques, can significantly enhance ultrasonic testing precision and efficiency and reduces the likelihood of missing critical flaws due to any human error. This article explores the benefits of combining these advanced scanning techniques, focusing on their application in detecting flaws in complex aerospace structures.

ADAPTIVE PHASED ARRAY SCANNING

Since its introduction to the NDT field, phased array technology has proven to be a valuable enhancement to single-element ultrasonic testing, providing various focusing capabilities and enabling rapid inspection coverage. While both NDT techniques are exploited in automated testing, it is the reduced scanning time resulting from electronic indexing across the array that makes phased array scanning an interesting option. When performing a raster automated C-scan on a flat sample with a single probe, only one single line of pixels can be recorded at a time, followed with a mechanical index between each line to complete the overall scan.

When scanning a structure possessing relatively flat surfaces with phased arrays, focal laws (delays applied on a group of elements to form beam focusing) are used and indexed electronically across the array, resulting in multiple index lines being scanned simultaneously from a single scan line movement. However, to do such phased array scans on structures with complex surfaces, more advanced and automated NDT techniques are required in order to achieve accurate, repeatable, and efficient scanning inspections.

To overcome these challenges, an adaptive phased array scanning method was developed using a master array element concept (single element in the array with ultrasound beam perpendicular to the inspected surface) ensuring that the ultrasonic beam formed by a given group of the array elements is respecting predefined angular tolerances obtained from scans of reference parts. Hence, when performing automated scans on curved parts, the phased array adapts to the curvature and uses only the elements near the master element with acceptable angular tolerances.

Fig. 2: Focal law aiming at 0° on the part surface

This adaptive scanning method ensures that the generated focal laws by the phased array probe do not fire at the inspected surface at angles outside the given tolerances. When the array reaches surfaces with minimal curvature, all the elements can be used simultaneously to perform the required scan.

ADVANCED 3D SCANNING IN ULTRASONIC TESTING

When performing automated phased array testing, the scanner moves the array probe along the test part while recording the reflected/transmitted signals at specific locations. The resulting data grid of points forms a C-Scan, which can then be analyzed to determine the presence of defects and measure their size and location. Thus, for improved inspection of aerospace structures with complex and curved surfaces, automated ultrasonic testing systems with advanced 3D motion control capabilities are required.  These systems, combined with the adaptive phased array scanning solution, allow the probe to follow the complex contours of the inspected part, ensuring precise defect detection and improved inspection efficiency.

The developed advanced 3D scanning technique enhances this process by extracting the part geometry from CAD drawings, enabling precise C-Scan mapping that provides detailed information on both external and internal features of the inspected components. The motion control software associates the surfaces of the inspected part with the appropriate axes and creates virtual orthogonal scanning paths in a parametric space. This forms a rectangular grid where C-Scan pixels are recorded. The software then generates accurate trajectories for 3D phased array positioning along the calculated scanning paths, maintaining probe/array alignment with the calculated grid.

The following sequence illustrates the inspection of an aero-engine blade, starting with the sample surface extraction from the CAD model, followed by calculating the scanning trajectories to fully cover the part’s surface and produce the C-Scan inspection results.

Fig. 3: Surface extraction & creation of orthogonal axes trajectories from CAD sample

The final 3D scanning results provide detailed visualization of components and structure geometries, leading to more accurate flaw detection and characterization. This capability is particularly essential when inspecting complex composite materials and bonded structures, which are increasingly used in modern aircraft designs.

ADAPTIVE PHASED ARRAY SCANNING: INSPECTION RESULTS

To demonstrate the developed adaptive phased array scanning technique, experimental automated 3D UT and phased array tests were conducted on an aero-engine fan blade made of Carbon Fiber Reinforced Plastic composite (CFRP) with titanium leading edge. Teflon tapes were applied to the external surfaces to simulate disbonds between the leading edge and the composite part of the blade. A 10-axis immersion tank equipped with phased array unit was used to perform the automated scans , utilizing two independently controlled X and Y carriages, two Z axes, and two fully automated and submersible Gimbal/Gimbal manipulators.

Fig. 4: Automated 10-axis ultrasonic immersion system

To ensure optimal sound transmission and coverage throughout the blade, the scanning trajectories were calculated and validated against the extracted blade surface. Validating the scanner’s trajectories is a critical step in 3D scanning and involves comparing the deviation of the water path and probe incidence angle against predetermined acceptable errors.

Fig. 4: C-Scan results (a) Through transmission UT (b) Adaptive Through Transmission Phased Array scanning

The above figure 4 (a) shows C-Scan results of conventional through transmission UT using a pair of 2.25MHz transducers, while figure 4 (b) was obtained with a pair of 2.25MHz, 64 elements phased array probes using adaptive scanning with 1-degree angular deviation tolerance of the elements in the array probe. The adaptive phased array results showed similar defect detectability compared to conventional UT but with better inspection time for the case of phased array.

The results showed that the adaptive phased array technique offers comparable defect detectability to conventional UT but with significantly improved inspection time. A series of experiments with varying array angular tolerances further demonstrated that differences in defect detection were not perceptible within a 0.5° to 1° angular deviation. Hence, optimizing the adaptive angular tolerance can done to increase scanning speed while maintain defect detectability.

Fig. 5: Evaluation of the Optimal Angular Tolerance for Adaptive PA Scanning

CONCLUSION

Automated 3D nondestructive testing, enhanced by adaptive phased array scanning technique, provides important tools for advanced automated ultrasonic testing. These technologies enhance the accuracy, speed, and thoroughness of flaw detection in critical components, contributing to improved safety and reliability in aerospace operations. As the aerospace industry continues to evolve, the integration of automated NDT methods will be essential in meeting the increasing demands for higher inspection quality and operational efficiency.