Application of magneto-optical method for detection of material structure changes

Abstract

The possibilities of magneto-optical sensors to control the damage of ferromagnetic and para-magnetic materials and products are considered. In the introduction it is shown that modern magneto-optical materials used in creating sensors have a high sensitivity and spatial resolution. So, on their basis it is possible to develop sensitive and informative means of non-destructive testing for a wide range of applications. For example, it is used to detect microcracks, corrosion damage, degradation changes in the material structure, surface deformations, and subsurface defects. The method ability to detect appearance of magnetic phases in paramagnetic materials, that are precursors of fracture, is of a special importance. The advantage of magneto-optic sensors is a large observation area and high spatial resolution. Resolution of the sensor is determined by the period and size of the domain structure, which averages 13...50 micrometres. High sensitivity of the sensor is due to a small saturation field of the magneto-optic material from 0.1 mT to 0.7 mT. In addition, these parameters are controlled by changing the temperature of the sensor, direction and intensity of the magnetic field. In this paper an optical scheme based on magneto-optical garnet film for visualization of fatigue cracks, which are formed in compact samples during their experimental investigation on fatigue failure is described. The developed scheme allowed us to visualize and fix position of the crack and determine its actual length, considering the closed part of the crack. A further direction of research will be to increase the sensitivity of the developed scheme and reduce the noise of magneto-optical images to identify the initial stages of the degradation process of ferromagnetic and paramagnetic materials and products.