Detection of fatigue in structural steels by magnetic property measurements

Abstract

The magnetic properties of ferromagnetic materials are sensitive to the mechanical and microstructural condition of the material. Fatigue can affect the magnetic properties due to microstructural changes, primarily dislocation production. Magnetic hysteresis measurements have been used to monitor the changes in the parameters due to low cycle fatigue, with the overall objective of developing a new tool to enhance the present NDE techniques for detecting failure. The magnetic measurements were performed using the Magnescope, a portable magnetic inspection system. Materials for fatiguing included plain low carbon steel and samples of quenched and tempered AISI 4340. The coercivity and remanence of the low carbon steel samples increased during the early stages of fatigue, reflecting strain hardening. As cycling progressed, the magnetic parameters leveled off and dropped sharply shortly before failure. The coercivity and remanence of the 4340 samples decreased during the initial stages of cycling, reflecting fatigue softening. The parameters plateaued, then decreased shortly before failure. The amount of change in the magnetic parameters was found to depend on the strain amplitude of the cycling.