Damage detection of surface cracks in metallic parts by pulsed Eddy-Current probe

Abstract

Eddy currents are curls of electrical current induced in conductors by a changing magnetic field. Eddy currents can be induced within conductors by a time-varying magnetic field which can be created by the circulation of some transient current in coil, electromagnet or transformer. The magnitude of the induced current is proportional to the strength of the magnetic field and the rate of change of flux. Harmonic current excitation is known to produce eddy currents patterns that cannot enter deep in the material. Transient pulsed current excitation technique is being developed for detection of flaws located at depth within metallic structures. The obtained eddy currents interact with flaws in a conducting structure to produce an output signal that provides information about the flaw. The technique involves a transmission/reception probe to produce an output signal centered on a given frequency. In this work, surface crack detection is considered through modeling of eddy-current pattern which are induced by the presence of small flaws. This was undertaken by means of the finite element method under transient inductive excitation having the form of short time pulses. The proposed model included the excitation coil, the sensing coil, the tested defected plate and ambient air. Different types of probes were simulated by changing some of the design parameters in an attempt to generate an output signal of optimum magnitude and shape.