Acoustic shearography for crack detection in metallic plates

Abstract

Shearography is an optical non-destructive testing (NDT) technique which is suitable for large area and fast inspections. However, it is difficult to detect cracks in metals by using conventional stress loading methods for shearography, including stress loadings by vacuum, thermal and vibrational excitations, due to the high rigidity and/or the high thermal conductivity of metals. In this work, we propose and demonstrate an acoustic shearography method for detection of cracks in metallic plates. Piezoelectric transducers bonded on the metallic structures are used to generate acoustic waves, which interact with crack defects and act as the stress loading for shearography testing. Systematic investigations on the wave-defect interaction based shearography indicate that the optimal testing frequencies are determined by the resonant frequencies of the piezoelectric transducers, which are independent of defect type and sample dimensions of the metallic plates. By using the optimal testing frequency, we successfully detected surface fatigue cracks as short as 3 mm and subsurface cracks at 10 mm depth in aluminum plates. We also demonstrate the feasibility of simultaneous detection of multiple notches in multiple fastener holes with one piezoelectric transducer. This wave-defect interaction based shearography developed here allows fast and effective detection of deep subsurface fatigue cracks, which is promising for practical NDT of defects in metallic structures.

Wave-based acoustic shearography for defect imaging

“…The various acoustic wave modes have their own unique purposes, where dispersive waves are useful for localization of defects over larger area and focused waves are better at penetrating deep into the test object. The optimal frequencies to introduce the higher stress loads for enhanced shearography detection are typically the resonant frequencies of the piezoelectric transducers, where the electrical impedance is at its lowest and has the highest efficiency in converting electrical energy to mechanical energy [4].…”

Digital Shearography for NDT: Phase Measurement Technique and Recent Developments

“…In 2010, Kurtz proposed the concept of directed acoustic shearography [53]. In 2017, Liu et al demonstrated that it was feasible to use acoustic waves as stress loading for shearography testing with a piezoelectric disc transducer [54,55], thus demonstrating the feasibility of such a system. The directed acoustic shearography system consists of a phased array transducer, laser light source, and shearography sensor as shown in Figure 13.…”

A robust phase extraction method for overcoming spectrum overlapping in shearography

“…Shearography is a highly sensitive optical interferometry method 1 . Because of its large measurement field of view and strong vibration-resistant capability, which most optical interference techniques do not feature, shearography has been increasingly studied and applied in the fields of three-dimensional non-destructive testing, material property analysis, and object vibration pattern recognition [2][3][4] . It is a two-step imaging technique that cannot directly obtain the measured information: it must first extract the phase information from the speckle pattern obtained by a camera at different moments during object deformation, and then the corresponding object deformation information can be obtained 5 .…”