Air-coupled ultrasonic detection of microholes in flexible envelope materials

Flexible envelope materials are widely used in the manufacturing of parachutes, hot air balloons, and other products requiring high air tightness, owing to their lightweight, high strength, and low leakage properties. However, the presence of microholes in envelope materials can significantly compromise their air tightness, posing a critical challenge for quality safety. Although accurate detection of microholes is critical for ensuring product safety, there is limited research in the open literature on the application of nondestructive testing (NDT) methods for detecting microholes in envelope materials. In this study, the air-coupled ultrasonic (ACU) method was applied for the first time to detect microholes in envelope materials. The influence of ultrasonic focusing and attenuation effects (along the propagation path) on the detection results was investigated by theoretical analysis and numerical simulations. Experiments were conducted to evaluate the microhole detection performance of transducers with different frequencies and focal lengths. Spatial filtering and contrast enhancement were used to process C-scan images. The experimental results demonstrate that the proposed method enables high-resolution detection of sub-wavelength microholes (30 μm), showing its potential for NDT of flexible envelope materials.