An optical technique for the detection of surface defects in ferromagnetic samples

Abstract

We discuss the details of a new optical technique developed for the detection of defects in ferromagnetic materials or components, using a ferrofluid emulsion. This new flux leakage probe consists of a monodispersed ferrofluid either confined between two thin transparent sheets or in a cuvette, and a white light source for illumination. By employing ferrofluid droplets of suitable size and surfactant concentration, and mounting the cell on the test specimen surface, one can detect the region where the defect is located in the test specimen by visually observing a colour change in the ferrofluid cell, in the vicinity of the defect. The origin of this colour change in the backscattering direction is Bragg scattering from the droplet chains, which are formed by the leaked magnetic flux in the presence of a defect. The details of the defect detection procedure and typical experimental results obtained in a ferromagnetic specimen with a rectangular slot (i.e. an artificial defect) are presented.