Composite materials require a specific set of nondestructive testing (NDT) techniques that can adapt to complex structures. Major aircraft mishaps can occur regardless of enhanced technologies and routine maintenance schedules if there are flaws in these components. Vigilant inspection is needed to discover and remedy underlying flaws that can compromise an aircraft’s integrity.
Composites are—outside of rare metal matrix composites—generally blends of fibers and polymer resins. Carbon fiber variants are the most widespread example used presently in aerospace. The absence of magnetically reactive and electrically conductive materials in these composites means ultrasonic waves are the best way to analyze composite structures. As such, an ideal NDT inspection method for composites is phased array ultrasonic testing (PAUT). PAUT offers real-time visualizations and excellent speed in flaw detection.
PAUT for Composite Inspection
With standard ultrasonic testing (UT) technology, technicians are stuck with rigid beaming angles that may pass over or conceal flaws during testing. Instead, PAUT is the preferred choice for composites since it can achieve multiple angles within a single probe, helping to locate flaws or defects in a joint. Conventional UT doesn’t provide beam customization options to further scrutinize flaws more closely and derive information as to their severity. PAUT, on the other hand, permits in-depth focusing features and beam-shape tailoring for closer inspection of the shape, depth, and breadth of flaws that allow analysts to record reflected sound from multiple points.
In spite of these advantages, PAUT inspection methods for composites are not absolutely foolproof. Depending on how the probe is oriented, cracks and other flaws can be missed. In real-world environments, inspecting the composite elements of an aircraft can be extremely challenging due to the variable and complex geometries of those elements including the fuselage, the wing skin, stringers and spars. To solve this problem, technicians can complement PAUT inspection methods for composites with time-reversal techniques.
Time-Reversal NDT Inspection Methods for Composites
As effective as PAUT may be, additional UT testing techniques can ensure a more accurate inspection of composite structures during and after the manufacturing process. A form of PAUT testing uses time-reversal techniques to achieve complete coverage and enhanced inspection quality needed for high-performance composites. It can do so at a scanning speed similar to standard PAUT.
Time-reversal uses front wall echoes and time-of-flight (TOF) measurements to create multi-dimensional images. An algorithm adjusts and reconciles the TOF of these reflected waves to account for misalignment of the probe or part and surface geometry changes. It uses sound waves bouncing back from the front wall to conduct a scan from the direction opposite the probe array. The time-reversal PAUT process uses two steps:
- Comprehensive Coverage: A full profile of the part is achieved using the reflected waves from the probe. These waves are altered by the shape of the component, with differing flight times for each element of the probe. These differing flight times create an accurate profile of the object being scanned.
- Data Capture: The delay data is gathered and analyzed by an algorithm to normalize beam incidences on the inspection surface. Time-of-flight for all probing elements is calculated using software to assess the delays, achieving a complete surface rendering and proper alignment regardless of geometric patterns.
When it comes to composite inspections, time-reversal PAUT augments conventional PAUT because it can help improve signal quality when probe alignment is not kept optimal with the inspected component surface. This, in turn, improves coverage and detection rates on these complex structures. Best of all, the time-reversal technique can be used without impacting inspection speed negatively.
NDT Inspection Methods for Composites: The Bottom-Line
One of the most important NDT inspection methods for composites is phased array ultrasound testing. However, PAUT on its own isn’t enough to ensure accurate NDT inspection methods for composites used in critical roles in aerospace and other industries. Rather, technicians should apply time-reversal techniques to obtain a complete surface profile and accurate image data.
NDT inspection methods for composites require instruments with strong signal quality, support for the time-reversal technique and homing gates. Analysts should use quality equipment from reputable NDT manufacturers that invest in advanced technologies and conduct rigorous quality control testing. With the right technology, technicians can more easily detect, image, and remedy dangerous aberrations that could hamper plane functionality and cause public safety hazards.