Testing for Discontinuities in Metals Using Film Flux Detectors

Abstract

Results of studying the use of film flux detectors for monitoring artificial continuity defects are presented. Photographs are provided for a film flux detector with images of magnetic fields due to holes 2.0 × 10^–3 m in diameter in an iron plate with a thickness of 6 × 10^–4 m and a hole 1.0 × 10^–2 m in diameter in an aluminum plate with a thickness of 2.0 × 10^–3 after exposure to magnetic field pulses with a rise time of approximately 2.0 × 10^–5–5.0 × 10^–5 s, as well as of fields that passed through gaps 1 × 10^–4 and 1 × 10^–5 m wide between joined aluminum plates with a thickness of 3.0 × 10^–3 m for different applied-field directions. Results of measuring the maximum value of the tangential H_τm (3.9 × 10⁴ A/m) and normal H_τm (3.3 × 10⁴ A/m) components of the strength of the total magnetic field near the surface of the aluminum plate (in the immediate vicinity to the hole in it) using a discrete magnetic-field sensor are presented. Based on the research, magnetic pulsed methods of searching for continuity defects in objects made of diamagnetic, paramagnetic, and ferromagnetic metals have been developed. These methods consist in exposing an object, with a flux detector atatched to it, to a magnetic field pulse and determining the defectiveness of the object based on the analysis of the optical image of magnetic field by studying the shape of the image and the distribution of gray levels (chromaticity) in it, while taking account of anisotropy parameters and the calibration characteristics of the sensor. The amplitude of the field pulse, its rise time, the shape of the leading and trailing edges, the number of pulses, the parameters of magnetic field spikes were preliminarily determined and the uniform illumination of the detector surface was ensured. Before use, the detector was demagnetized or polarized; the type and parameters of the magnetic field inductor were determined. It has been proposed to increase the image contrast by exposing the object–detector assembly to a series of pulses.