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In this study, we applied the reverse time migration (RTM) method to ultrasonic defect imaging in anisotropic materials. RTM offered that the defect shape could be uniquely determined by calculating a cross correlation of the incident and the reverse propagated waves from the array transducer. 2D simulations demonstrated that defect imaging by the RTM method requires an accurate numerical setup. We validated our technique using measured scattered waves from a slit in unidirectional solidified 316L stainless steel. By using the elastic constants determined from the ultrasonic wavefield data, the slit shape was correctly reconstructed. This provides a proof of principle that the RTM method is effective in nondestructive imaging of composite structures containing anisotropic materials.

A simulation-aided defect reconstruction method using an ultrasonic time-reversal approach that uses elastic constants determined by ultrasonic field data is proposed. Scattered waves from a defect are recorded using an array probe on a polystyrene wedge and the time-reversed waves are re-emitted in a finite element simulation. The target defect in these experiments is an electrical discharge machined slit in unidirectionally solidified SUS316L stainless steel. For the time-reversal analysis, two kinds of elastic constant of SUS316L derived from an electromagnetic acoustic resonance (EMAR) method and a laser scanning ultrasonic visualisation method are used. The through-wall extent of the electrical discharge machined slit can be visually estimated from the focal point of the ultrasonic wave in the numerical model using the elastic constants determined by the laser ultrasonic method. This provides a proof of principle that the method is working.

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