Airborne non-contact and contact broadband ultrasounds for frequency attenuation profile estimation of cementitious materials

Comparative Study of Coupling Techniques in Lamb Wave Testing of Metallic and Cementitious Plates

“…Two pair of piezoelectric air-coupled transducers with a 32 mm diameter mm and central frequencies of 250 kHz and 500 kHz were employed for the excitation and detection of Lamb waves (information about these transducers can be found in [95,96]). The longitudinal wave velocity in air is 343 m/s [53,58]. The actuator was excited by a five-cycle tone burst using a programmable signal generator (33120A, Agilent Technologies, Loveland, CO, USA) and amplified by a factor of50 (WMA-300, Falco Systems, Amsterdam, The Netherlands).…”

“…In this study, the three distances, d, dc1 and dc2, are measured using a ruler. Wherever possible, it is recommended that these distances have a value equal to or greater than the near field distance to work in the far-field region [14,53,68,82,83]. The near field distance, N, is given as [1,53]:N=D2f4c, where D is the transducer diameter.…”

“…This recommendation is due to the near field or Fresnel region, where great ultrasound pressure fluctuations occur, hindering defect detection; while in the far-field or Fraunhofer region, the ultrasound pressure gradually tends toward zero [48,53]. If the transducers are placed close to the plate, undesirable reflections can appear between the transducer face and the specimen [59,84].…”

“…However, immersion is not practical for testing large structures, hot structures or specimens whose surface cannot be contaminated [40,51]. In these cases, air coupling is a better solution, since it maintains the advantages from immersion testing, such as constant coupling, provide fast scanning measurements [52] and almost solves the drawbacks from the undesired contributions, except for the direct acoustic wave through the air, which can be attenuated by an acoustic barrier placed between the transducers [53,54]. Two types of air-coupled transducers can be found, capacitive and piezoelectric.…”

“…The capacitive design generally offers more bandwidth than the piezoelectric ceramic elements [55,56]. Nevertheless, the received waveforms show a low signal-to-noise ratio (SNR) because of the high attenuation present in air [52,53]. Therefore, amplification in transmission/reception and averaging is needed [43,52,57,58].…”