Air coupled ultrasonic inspection with Lamb waves in plates showing mode conversion

Abstract

In this paper we demonstrate a non-destructive, non-contact detection method for small defects in thin polymer plates using an air coupled ultrasonic (ACUS) setup. There exist many applications for such methods, e.g. quality control in the manufacturing process or failure prevention by periodical inspections during the lifetime of a product. We demonstrate a setup for the inspection of plates together with signal analysis algorithms to process the measured data, meeting the challenges to handle the dispersive signals and establishing a robust failure criterion. Pressure waves from the transmitter excite different modes of Lamb waves inside the plate. These Lamb waves propagate in the plate and reradiate pressure waves into the air that are then detected by the receiver. Lamb mode conversion is used for defect detection. A numerical model allows the visualization of the propagating waves in the air as well as the Lamb waves inside the plate.Four key parameters of the setup are identified, two angles and two distances. The transmitter and the receiver angles are used to select which Lamb mode (anti-symmetric A 0 or symmetric S 0 ) is mainly excited and detected, respectively. For the acquisition of the Lamb wave signal the distance from the transmitter to the receiver should be as large as possible but is limited by the attenuation of the signal. Measurements for different values of this distance are essential for signal analysis. The distance between transducer and plate surface should be as small as possible even if it may introduce secondary Lamb waves due to reflections of the pressure wave between transmitter and plate surface. Two algorithms, a model based one and a data driven one, are presented to separate Lamb modes that overlap in time. In these separated signals, the Lamb mode conversion from A 0 to S 0 is shown, allowing a localization of the defect. We conclude that defect detection and localization with Lamb mode conversion is possible with an air coupled ultrasonic setup. Multiple measurements along the propagation direction of the Lamb waves are necessary to allow a thorough signal analysis and visualize the mode conversion.

Ultrasonic Methods

“…Non-contact inspection methods like air-coupled or laser ultrasonics provide an ability to adjust to the complex surface of the structure. Such methods can be adjusted to excite different guided wave modes as the angle between the transducer and the sample can be easily adjusted (Panda et al 2018;Römmeler et al 2020). Laser and conventional air-coupled monitoring are usually combined, using conventional piezoelectric transducers for transmission and laser vibrometer for the reception (Jurek et al 2018).…”

Excitation and Reception of Higher-Order Guided Lamb Wave’s A1 and S1 Modes in Plastic and Composite Materials

“…When the ultrasound velocity in air is lower than the phase speed of the guided waves in the object, the optimal incident angle of the ultrasonic wave propagating from the air onto the plate exists at which the biggest amplitude of the excited guided wave is obtained. Then, the propagating guided wave in the plate material radiates a leaky wave into air which may be used for air-coupled reception [ 8 ]. This mode is commonly applied for detecting defects inside composite materials as the leakage is increasing at the defective zone.…”

Optimal slope angles of the air-coupled transducer from viewpoint of the effective Lamb waves propagation in different materials

“…The generation of these waves can be done in either contact [1], [2], [3] or non-contact [4], [5] manner. In turn, the waves detection can also be done in contact [1], [2], [3] or non-contact way [4], [6], [7]. Although contact methods are characterized by relatively high amplitudes of the generated signals, they have important drawback, i.e.…”

“…Hence, to perform the inspection on a machine while it operates, it has to be turned off first, what generates economic losses. An interesting way to excite Lamb waves in structures based on non-contact methods is the usage of laser source [8], [9] or air-coupled transducer (ACT) [4], [7], [10]. The important issue while using ACT to generate the waves is the excitation frequency at which the waves are generated.…”