Nondestructive Evaluation of Contact Damage of Ferromagnetic Materials Based on Metal Magnetic Memory Method

A technique based on the metal magnetic memory (MMM) is developed to evaluate the contact damage of ferromagnetic materials under nonferromagnetic and ferromagnetic indenters by measuring the magnetic flux leakage (MFL) signals. Great difference between the MFL signals for the ferromagnetic indenter and non-ferromagnetic indenter is experimentally observed. The normal signal shows a dip in the contact region for the ferromagnetic indenter but an increase for the non-ferromagnetic indenter; the tangential signal experiences a peak-peak change through zero in the contact region for the ferromagnetic indenter but a peak for the non-ferromagnetic indenter. Furthermore, the amplitude of MFL signals for the ferromagnetic indenter is considerably larger than that for the non-ferromagnetic indenter. The mechanism of the signal variation is analyzed by considering the magnetic-stress coupling effect. Criteria of the early contact damage are developed based on the variations of the MFL signals and their gradients; and the evaluation parameters are extracted.