Nonlinear Ultrasonic Evaluation for Corrosion Assessment of Steel Plates Embedded in Concrete

Ultrasonic testing uses sound waves above the human audible range to image internal structures. However, in concrete, high-frequency sound waves are quickly absorbed, necessitating the use of low-frequency waves, which reduces the ability to resolve small defects. Nonlinear ultrasonic evaluation offers a solution. Defects like cracks cause elastic nonlinearity, leading to waveform distortion. This distortion creates new frequency components in the sound wave, allowing for the detection of defects regardless of the wave's frequency. The strength of these new components relative to the original frequency provides a measure of damage in the material. The research, conducted on small-scale laboratory specimens, primarily focused on pre-corroded steel plates. The results demonstrate that nonlinear evaluation can locate and assess corrosion in embedded plates, with more severe corrosion yielding stronger nonlinear indications. Additionally, the method shows potential for detecting embedded foreign objects, such as wood, and separations between the plate and concrete. The findings suggest that combining conventional and nonlinear ultrasonic imaging techniques can provide a better understanding of a structure's damage state. However, further experiments on more realistic specimens are necessary to fully evaluate the method's detection capabilities. These experiments should consider the challenges posed by intrinsic concrete micro-cracking, coarse aggregates, reinforcement, and large dimensions typical in nuclear power plants.