ADVANCED MATERIALS & PROCESSES | MAY 2026 25 components, with fatigue cracks often appearing at fastener holes, welds, and geometric transitions. Similarly, disbonds and thin-skin defects may develop within only a few millimeters of the surface. Detecting such flaws can be challenging because ultrasonic inspections often exhibit reduced sensitivity near the probe surface due to near-field effects and dead zones. To address this limitation, specialized probe designs and wedge configurations have been developed to improve near-surface performance. Combined with impedance-matched delay lines, these probes and wedges can provide dramatically improved sensitivity for shallow defects. When paired with portable PAUT flaw detectors, near-wall probes and impedance-matched wedges utilize UT surface waves to effectively detect near-surface flaws, enhancing manual and automated inspections of: • Thin aluminum skins • Bonded composite structures • Surface-breaking fatigue cracks • Adhesive joints and layered assemblies Near-surface probes may also be integrated with automated scanning systems, such as XY scanners, to enable repeatable inspections over large areas. This enables the collection of dense datasets, which allow inspectors to identify subtle indications that might otherwise be missed during manual inspection. ADVANCED PAUT IMAGING TECHNIQUES Recent advances in PAUT imaging are significantly improving the characterization of defects in aerospace structures, enhancing image resolution and providing inspectors with new flaw morphology data. Total Focusing Method. Total Focusing Method (TFM) is an advanced imaging approach based on full matrix capture (FMC) data acquisition. Instead of focusing the ultrasonic beam during transmission, FMC records signals from all transmitter-receiver element combinations. The resulting dataset can then be processed to synthetically focus the image at every point within the region of interest. TFM produces highly detailed ultrasonic images that reveal small defects and complex flaw shapes. In aerospace inspections, TFM is used to evaluate crack growth, characterize defects in welded structures, and examine components with complicated geometries. Phase Coherence Imaging. Traditional PAUT images rely primarily on the amplitude of reflected signals. Phase coherence imaging (PCI), by contrast, uses phase information from ultrasonic waveforms to construct images. By evaluating the phase consistency of reflected signals, PCI enhances the visibility of crack tips and branching features. This is particularly useful when evaluating fatigue cracks in aluminum structures, where subtle crack morphology may be difficult to distinguish using amplitude-based imaging alone. TFM and PCI can both be found in portable PAUT platforms in preset configurations, reducing setup complexity and making sophisticated inspection methods more accessible. offer several advantages for composite inspection: • Improved acoustic coupling during scanning • Reduced water consumption • Enhanced near-surface resolution • Efficient coverage of large composite panels • Reduced setup requirements compared to full immersion In typical local immersion setups, 64- or 128-element probes operating in the 1 to 5 MHz range are used for composite panel inspection. The large aperture supports broad coverage, while the water column minimizes variability in coupling across large surfaces. Arrays containing dozens of elements can provide wide inspection coverage while maintaining the spatial resolution required to detect delaminations and disbonds. Improved near-surface resolution is particularly important for detecting shallow delaminations or impact damage just beneath the skin. NEAR-SURFACE INSPECTION IN THIN SKINS AND BONDED STRUCTURES Many critical aerospace defects occur close to the surface of structural Local immersion using Evident’s OmniScan X4 phased array flaw detector and RollerFORM scanner is a practical, efficient alternative to full immersion setups in aerospace manufacturing and maintenance environments.
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