Abstract: A piezoelectric sensor for sensing an ovalling mode in a cylindrical structure, a non-destructive testing system for determining the strength condition of the cylindrical structure, and a method for non-destructive testing of the cylindrical structure are provided. The piezoelectric sensor includes an H-shaped caliper with a first and second arm, connected by a crossbar, and has a caliper connector near first end of each arm and a wire connector near second end of each arm. A piezoelectric wire is connected and stretched between the wire connectors and is connected to an electrical terminal to provide electrical signal in response to expansion and contraction as a result of vibrations induced in the cylindrical structure. The system includes a measurement unit connected to the electrical terminal, to receive and perform a frequency analysis of the electrical signal based on the ovalling mode and output a stiffness value of the cylindrical structure.