Investigation of Change Mechanism of Magnetic Flux Density Distribution around Fatigue Cracks Hirotaka TANABE 1 , Yui IZUMI 1 , Tohru TAKAMATSU 1 , Jun SHIMADA 1 , Katsuyuki KIDA 2 and Edson Costa Santos 2 1 Dept. of Mechanical Systems Engineering, The University of Shiga Prefecture; Hikone, Japan; Phone: +81 749 28 8380, Fax: +81 749 28 8523; e-mail: [email protected], [email protected], [email protected], [email protected]2 Dept. of Mechanical Engineering, Kyushu University; Fukuoka, Japan; E-mail: [email protected], [email protected]Abstract In order to clarify the changing mechanisms in the distribution of magnetic flux density around fatigue cracks during crack propagation, JIS SCM440 specimens were tested according to the following procedure: after fatigue testing to a crack length of 9 mm, the specimen was annealed. The fatigue test was then continued until the final crack length of 10 mm. The magnetic flux density was measured at 3 stages: once the crack reached 9 mm length, after the annealing process and once the crack reached 10 mm length. From the changes of the magnetic flux density distribution occurring at “the annealing” and “the fatigue testing after annealing” stages, it was concluded that these changes could be explained by the inverse magnetostrictive effect due to the residual stress caused by plastic deformation generated around the fatigue crack tip and accumulated along the crack path. Keywords: NDT, Magnetic flux density, Fatigue Crack, Crack propagation, MI sensor 1. Introduction In our previous studies[1][2], four-point bending fatigue tests under various conditions were carried out on JIS SCM440 steel specimens with artificial slits in order to develop a new non- destructive technique of fatigue evaluation based on magnetic information. In these studies the magnetic flux density distribution around the fatigue cracks was observed using a newly developed apparatus consisting of an MI (Magneto-Impedance) sensor and an x-y stage. Fig. 1 shows an example of the magnetic flux density distribution measurement results. It was found that the magnetic flux density distribution changed and moved along with crack propagation (as shown in Fig.1). A strong correlation between the movement of the magnetic flux density distribution and stress intensity factor was established, regardless of the loading conditions. Creating a non-destructive evaluation of the stress intensity factor for fatigue cracks by observing the relation between the two factors seems possible. However, the mechanism of the magnetic flux density distribution changes remains unclear. When a magnetic material is subjected to mechanical load, the spontaneous magnetization of the material will change. This change is called “inverse magnetostrictive effect” [3] or “Villari effect”. This effect was closely studied in order to reveal the mechanism of the changes in the magnetic flux density distribution around fatigue cracks. Near the fatigue crack tip, plastic deformation occurs, and consequently the residual stress accumulates. This is known to influence the changes of the magnetic flux density distribution and in order to investigate this matter, specimens were annealed after the fatigue tests and then additional fatigue testing was performed on annealed material. The magnetic flux density distribution was observed at each step, and based on the observation results,1 the mechanism of the changes in the magnetic flux density distribution is discussed. 13th International Symposium on Nondestructive Characterization of Materials (NDCM-XIII) , 20-24 May 2013, Le Mans, France www.ndt.net/?id=15502 More Info at Open Access Database www.ndt.net/?id=15502
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Investigation of Change Mechanism of
Magnetic Flux Density Distribution around Fatigue Cracks
Hirotaka TANABE 1, Yui IZUMI
1, Tohru TAKAMATSU
1,
Jun SHIMADA 1, Katsuyuki KIDA
2 and Edson Costa Santos
2
1 Dept. of Mechanical Systems Engineering, The University of Shiga Prefecture; Hikone, Japan;