New Possibilities in Aerospace Applications
Surface-Adaptive Ultrasound allows for inspection of complex composite parts. Composite materials offer significant advantages for aerospace applications, including resistance to corrosion and highly favorable strength-to-weight ratios compared to alternatives. Advanced composites also offer a wide range of choices for fiber and matrix materials, and evolving manufacturing technology, including new machining, drilling and fastening techniques, are making new aerospace applications possible. The wide range of material choices along with the ability to create complex shapes allows engineers tremendous flexibility in designing parts and structures that are optimized for performance and cost.
These advantages have resulted in far greater use of composite materials in the latest generation of planes than in earlier models. For example, composites comprise more of the primary structure of the Boeing 787 than any of the company’s previous commercial planes, accounting for nearly half of the materials that make up the airframe. For the Airbus A380, the world’s largest passenger airliner, more than 20% of the airframe is comprised of composite materials, including the central wing box. Composites on the F-35 Joint Strike Fighter are used primarily for skin applications, including the fuselage and tail sections, and account for approximately 35% of the structural weight of the aircraft. Use of composites in these planes delivers weight savings that are typically 20% compared to aluminum.
Although the flexibility that allows engineers to optimize composite parts for weight, strength and cost is a tremendous advantage, it often results in complex shapes that present significant inspection challenges. These shapes include highly contoured surfaces and tight radii, such as those found on blades, stiffeners and hollow-core structures. Not only do the complex shapes complicate inspection, but composite parts also tend to exhibit substantially more part-to-part variation than components made from conventional materials.
Inspection plays an integral role in the manufacturing of composite components. To achieve target production rates requires automated inspections whenever possible. Complex shapes and the part-to-part variation that is common with composites add to the automation challenge. Repeatable component positioning and accurate part following can be difficult, impacting flaw detection and sizing capabilities. In addition to flaw detection and characterization, metrology is an essential component of quality. Composite components for modern aircraft often come from several domestic and international suppliers, requiring tight tolerances on thickness and final shape to assure that pieces fit together during assembly.
There are several phased array technologies that address the issues created by complex composite parts. Curved and shaped arrays offer a solution for radii and contoured geometries, but accurate probe positioning is usually essential and multiple custom probes are often required for a single part. Flexible arrays including “smart” probes that adapt focal laws on the fly are another option, but these probes are expensive. High-resolution imaging techniques can also be employed, but the amount of data collected and the analysis time required can be problematic for high-speed industrial applications.
Surface-Adaptive ULtrasound (SAUL) is a promising new measurement technique that has been developed to help address inspection challenges that include complex geometries, irregular surfaces such as those created by ply dropoff, part-to-part variation, and variability introduced by probe misalignment. Because the technique is applied in real time, it is well suited to automated inspections. For the surface-adaptive technique, all elements of the probe are fired in transmission and the specimen shape is measured from the front-surface echoes. An iterative process is used to adapt the time delays applied to the probe elements in real time to obtain shape-corrected scans on the fly. The following examples illustrate how SAUL is being used to overcome many of the challenges presented by complex composite parts.
Results of Laboratory Experiments
A series of laboratory experiments have been conducted on a variety of composite specimens to test and validate the surface adaptive technique. The photographs included here show curved and tubular composite test specimens that are all sections of actual parts. The samples are shown along with a phased array probe indicating how linear arrays are used together with SAUL to detect and image embedded defects in curved parts.
SAUL has allowed detection and imaging of the embedded defects even for those cases where the strong curvature of the part prevented defect detection using a linear probe alone (without the surface-adaptive algorithm). Defects (release film) as small as 1/8 inch (3.2 millimeters) have been successfully imaged in tubular structures with radii as small as ¼ inch (6.4 millimeters) using SAUL with a linear probe. In another experiment, holes drilled into the radius of a composite U-shaped stringer were successfully detected and imaged using SAUL with a linear probe. These promising results indicate that SAUL stands to greatly improve the functionality of linear arrays, while also reducing inspection costs, by increasing the range of parts that can be inspected with linear probes.
In addition to the experiments performed with flat linear arrays, measurements have also been made using a curved array and a matrix phased array probe. Curved arrays can be optimized for any specific curvature, but as mentioned earlier, accurate probe positioning is essential. Complex shapes and the part-to-part variation that are typical for composites greatly increase the probe-positioning challenge. The role of SAUL in these cases is to reduce the need for multiple custom probes and to lessen the sensitivity to probe positioning. In experiments designed to test SAUL’s ability to overcome loss of sensitivity arising from misalignment of probes, results using a curved array show that even when misalignment resulted in complete loss of signal, excellent results could still be achieved with the same probe and position using the surface-adaptive SAUL technique. For highly contoured parts and components with varying curvature, use of a matrix array together with SAUL is particularly advantageous because the matrix probe makes it possible to compensate for changes in geometry in more than one direction.
These initial results demonstrate that the surface-adaptive SAUL technique shows tremendous promise for helping to overcome challenges associated with inspection of composite structures including curved and highly contoured surfaces, changes in thickness and geometry, and part-to-part variation. Ongoing testing of SAUL is aimed at determining detection and sizing capabilities, as well as quantifying resolution for various geometries and conditions. Since SAUL is applied on the fly it is well suited to automated inspection, providing a cost-effective solution for addressing complex geometry and the variability in parts and probe alignment that are typically encountered in production environments.