NASA’s Langley Research Center has developed a new Non-Destructive Testing (NDT) method for identifying and characterizing hidden damage in composite materials. The new technique requires only single-sided access to the test specimen, and uses trapped energy analysis to detect and characterize damage that was previously ob - scured. Current methods, usually ultrasound or laser ultrasound, cannot characterize imperfections below or hidden by near surface damage. The new method uses 3D custom ultrasonic simulation tools to study ultrasonic guided wave behavior and energy trapping due to multilayer delamination damage (Figure 1).
Understanding the extent of composite damage is essential for repair and replacement decisions for aerospace composites. Better understanding of composite damage could eliminate unnecessary repairs and aid in preventing catastrophic in-service failure.
Guided wavefield techniques require excitation of guided waves in a specimen through contact or noncontact methods (such as attached piezoelectric transducers or laser generation). The resulting wavefield is recorded via noncontact methods such as laser Doppler vibrometry or air-coupled ultrasound. If the specimen contains damage, the waves will interact with that damage, resulting in an altered wavefield (compared to the pristine case). When guided wave modes enter into a delaminated region of a composite, the energy is split above/below delaminations and travels through the material between delaminations. Some of the energy propagates beyond the delamination and reemerges as the original guided wave modes. However, a portion of the wave energy is trapped as standing waves between delaminations. The trapped waves slowly leak from the delaminated region, but energy remains trapped for some time after the incident waves have propagated beyond the damage region.
Simulation results show changes in the trapped energy at the composite surface when additional delaminations exist through the composite thickness (Figure 2). The results are a preliminary proof-of-concept for utilizing trapped energy measurements to identify the presence of hidden delaminations when only single-sided access is available to a component/vehicle. Currently, no other single-sided field-applicable NDT techniques exist for identifying hidden delamination damage.
This technology can be used in aerospace applications for in-service damage assessment (e.g., fuselage collisions with ramps or baggage carts), automotive applications for in-service inspection of composite structures, and wind turbines to reduce inspection time for large-scale inspection of turbine blades.