The fundamental shear horizontal (SH<sub>0</sub>) wave has been regarded to be specially useful in guided wave-based detection techniques since it is the only non-dispersive wave mode in plate-like structures. In structural health monitoring (SHM) applications, it is required that the transducers can generate and receive guided wave omni-directionally to detect defects in all directions. However, developing omnidirectional SH wave piezoelectric transducer (OSH-PT) has always been a challenge. In this work, we firstly proposed an OSH-PT based on a thickness-poled piezoelectric ring. By dividing the ring into twelve sectors and applying the electric field circumferentially, a new thickness shear (d<sub>15</sub>) mode was formed. Both finite element simulations and experiments indicated that the proposed OSH-PT can excite and receive single-mode SH<sub>0</sub> wave in a wide frequency range with uniform sensitivities. Then the performance of the proposed OSH-PT is demonstrated by defects localization in a 1000 mm×1000 mm×2 mm aluminum plate. Both single defect and multiple defects were detected and the defects localization algorithm was also presented. Results indicated that the proposed SH<sub>0</sub> wave-based SHM scheme can locate defects in high resolution with the position error less than 10 mm. Considering its excellent performances, low cost and easy fabrication, the proposed OSH-PT is expected to be widely used in SHM of plate-like structures in near future.
The non-dispersive fundamental shear horizontal (SH0) and torsional [T(0,1)] waves are extremely useful in guidedwave-based inspection techniques. However, excitation of SH0 and T(0,1) waves using piezoelectrics is always a challenge. In this work, firstly, a newly defined face-shear d24 PZT wafer is proposed to excite and receive SH0 wave mode. The d24 wafer is in-plane poled and its working electric field is applied along another orthogonal in-plane direction. Both finite element simulations and experiments show that single SH0 mode can be excited by using the d24 wafer along two orthogonal directions (0° and 90°). Then an omnidirectional SH0 wave piezoelectric transducer (OSHPT) is developed which consists of a circular array of twelve face-shear d24 trapezoidal PZT elements. Results show that the proposed OSH-PT exhibits good omnidirectional properties, no matter it is used as a SH0 wave transmitter or receiver. Finally, the development of a T(0,1) wave transducer for pipes based on a ring array of d24 PZT elements is described. Both finite element simulations and experiments show that the d24 elements ring can excite single T(0,1) mode and suppress all the unwanted non-axisymmetric modes. This work may greatly promote the applications of SH0 and T(0,1) waves in nondestructive testing (NDT) and structural health monitoring (SHM).
A suitable defect identification parameter is very important in the field of nondestructive testing (NDT). In this work, we proposed a NDT method which detects the sample’s local contact stiffness (LCS) based on the contact resonance of a piezoelectric cantilever. Firstly, through finite element analysis we showed that LCS is quite sensitive to typical defects including debonding, voids, cracks and inclusions, making it a good identification parameter. Secondly, a homemade NDT system containing a piezoelectric unimorph cantilever was assembled to detect the sample’s LCS by tracking the contact resonance frequency (CRF) of the cantilever-sample system based on strain signals. Testing results indicated that this NDT system could detect the above mentioned defects efficiently. The cantilever-stiffness dependent detection sensitivity was specially investigated and the stiffer cantilevers were found to be more sensitive to small defects. Then, a piezoelectric bimorph cantilever was fabricated and the electromechanical impedance, other than the strain signals, was measured to track the CRF of the cantilever-system. The LCS is then derived by using the equivalent-circuit model. The electromechanical impedance based NDT system is more compact and can be further developed to be a portable device. Finally, a Vicker indenter is fabricated onto the bimorph tip and the contact area is derived from the measured LCS. Thus the NDT system turns to be a hardness tester without any optical devices. It is very useful for in-situ testing or testing on inner surfaces where conventional hardness tester is not applicable.