Use of parabolic reflector to amplify in-air signals generated during impact-echo testing

Abstract

The impact-echo method is a commonly used nondestructive testing technique for elastic plates in civil engineering. The impact-echo mode corresponds to the frequency at zero group velocity of S1 Lamb mode. Recent development of the air-coupled impact-echo (ACIE) method introduces the possibility for rapid scanning of large structures and increases the practicality of in situ measurements. However, sensors used in ACIE are susceptible to ambient noise, which complicates in situ ACIE measurements. This letter presents the results of ACIE measurements taken using a parabolic reflector together with standard measurement microphones to increase the signal to noise ratio for ACIE measurements. The signal gain and effects of sensor location with respect to impact location are discussed.

A focused electric spark source for non-contact stress wave excitation in solids

“…However, the S 1 ZGV mode is a non-propagating Lamb wave mode, with a mode shape of Bessel function (Gibson and Popovics, 2005). The S 1 ZGV frequency can be measured even at a large source-receiver offset distance (up to 90 cm) with a large lift-off height (up to 50 cm) (Dai et al, 2011). Therefore, the resonance frequency measurement is not affected by small misalignments or variation of the liftoff distance between the source and the sensor.…”

“…The contact requirement significantly limits test speed and is one of the major challenges in NDT of large infrastructure. Development in air-coupled sensing for concrete (Zhu and Popovics, 2007;Dai et al, 2011) provides a partial solution for rapid scanning of concrete structures and more consistent signals by employing aircoupled sensors (microphones) and a contact impact source for wave excitation. However, to make a true breakthrough in rapid scanning, a non-contact air-coupled source is required to realize a fully air-coupled testing system.…”

Transient axial solution for plane and axisymmetric waves focused by a paraboloidal reflector

“…In nondestructive evaluation, the airborne plane waves generated by a planar air-coupled transducer can be focused by a paraboloidal reflector. 1 Recently Dai et al 2 have used a paraboloidal reflector to amplify the airborne zero-group-velocity Lamb wave signals radiated from a concrete plate and to improve the signal to noise ratio. The S1 zero-group-velocity (S1ZGV) Lamb wave mode has been found to show high excitability and allow high transmission efficiency of acoustic waves through plates.…”

Numerical simulation and analytical solution for plane waves focused by a parabolic reflector

“…In the air-coupled impact-echo test, the airborne zero-group-velocity (ZGV) Lamb waves radiated from a resonant plate structure are quasi-planar, and the signal amplitude can effectively be enhanced by using parabolic reflectors [1][2][3]. Moreover, the use of parabolic reflectors further increases the signal to noise ratio (SNR) because the ambient noises are not normally incident and are not amplified equally as ZGV waves at the focal point [2].…”