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Quality Classification of Adhesive Bonds in Composite Structures by Single-sided Air-coupled Ultrasonic Testing Using Linear Phased Array Probes

Adhesive bonding is widely used in composite material technology. The mechanical performance and durability of such composite structures strongly depends on the quality of the bond. Air-coupled ultrasonic testing is a promising nondestructive method for the characterization of these bonds. A main advantage is the contactless measurement without the use of oil or water using high-power ultrasonic pulses in transmission mode. The absence of glue acts as an additional interface within the testing object which strongly reduces the transmitted ultrasound wave. Glue application errors can be detected easily. Additional characterizations such as size determination are possible as well. In contrast to common transmission mode testing we present a single-sided pitch-catch approach for testing adhesive bonds between metal sheets and carbon fiber reinforced plastics (CFRP). New approaches of data analysis are possible by using broadband transducers. A direct characterization of different types of flaw indications becomes possible. Broadband transducers can also be used for additional characterizations in transmission mode. Due to its wavelength dispersive properties the quality of the adhesive bonding can be determined within one single measurement. This method will be demonstrated by a weak bond test within a CFRP structure. The sound velocity of Lamb waves strongly depends on the material and on the sample thickness. To transmit these waves efficiently the angle of incidence (AOI) of the ultrasonic wave has to be adjusted. We present a new generation of ultrasonic linear phased array probes allowing the AOI to be adjusted electronically in order to achieve maximum signal intensity.

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