Air-coupled ultrasound for damage detection in CFRP using Lamb waves and ultrasonic verification

摘要

Modern aircraft consist of Carbon Fiber Reinforced Polymer (CFRP) structures and require advanced nondestructive testing (NDT) techniques to detect barely visible impact damage (BVID). Conventional NDT methods, based on manual, or automated, scanning with ultrasonic transducers with water or gel couplant are time consuming for larger areas. Ultrasonic Lamb waves are of interest for structural health monitoring (SHM) of aircraft and have been investigated previously for this application by the authors and others. Lamb waves are guided by the plate-like aircraft structure and for SHM applications an array of permanently fixed embedded or surface-mounted piezoelectric transducers (PZTs) can be used to monitor large surface areas by using Lamb waves. This paper describes the novel combination of air-coupled ultrasound transducers (ACTs) and Lamb wave sensing using ultrasonic verification (USV). USV is an analysis procedure of monitoring the structural health of CFRP, developed jointly by the University of Amsterdam and the University of Applied Sciences, Amsterdam. For the scanning of larger structures, fixed transducers are not desirable to avoid damage to the surface by contact between the probe head and the couplant. ACTs can overcome these problems by using air as a couplant. Developments in ACT technology now allow much higher energy transfer between the transducer and the structure, making the technology more accessible for aerospace applications. The objectives of this research were to determine the optimum transducer parameters for damage detection in CFRP and to determine if ACTs can be used as alternative for PZTs when applying the USV procedure. A Design Of Experiments (DOE) with the variables frequency, type of transducer, distance between the transducer and the material, wave mode and angle of incidence determined the parameters with the highest sensitivity to detect damage. A damage test comparing the fidelity and pulse energy methods showed that damage can be detected best with the energy method based on the variances between the measurements, difference in percentage between undamaged and damaged signals and time windows in which damage can be detected. Because damage can be detected in CFRP, ACTs can be used as alternative for PZTs in USV.