Combined ultrasonic and eddy-current method and apparatus for non-destructive testing of tubular objects to determine thickness of metallic linings or coatings

摘要

A combined ultrasonic and eddy-current method and apparatus for non-destructively determining the liner thickness of a zirconium liner provided at the inner surface of a zirconium alloy nuclear fuel rod tube utilizes dimensional data acquired via conventional ultrasonic measurement techniques and impedance data acquired through electromagnetic techniques to calculate liner thickness in accordance with a specific formula. The apparatus utilizes a computer or programmable arithmetic unit with associated memory and I/O devices connected to electromagnetic and ultrasonic measurement subsystems. A particular eddy-current probe arrangement consisting of a differential coil pair is employed to obtain impedance measurements from the outside of the cladding tube. Calibrated reference impedance values for various different inner and outer tube diameters having a constant liner thickness are measured and retained in a memory. Calibrated reference impedance values for tubes of various liner thicknesses with the same inside and outside diameter dimensions are also measured and stored. A specimen cladding tube is tested and an inside diameter is computed using conventional electromagnetic techniques. The specimen tube inside diameter is also measured ultrasonically and a specific calculation of liner thickness is performed based on the difference between the inside diameter as determined by ultrasonic technique and the inside diameter as measured by eddy-current technique. The specific method of calculation utilized corrects for the erroneous effect that variations in cladding tube liner thickness can have on dimensions computed via conventional electromagnetic eddy-current techniques alone.