Investigation of Change Mechanism of Magnetic Flux Density Distribution around Fatigue Cracks H.Tanabe, Yui Izumi, T.Takamatsu, J.Shimada The University of Shiga Prefecture, JAPAN K.Kida, E.C.Santos Kyushu University, JAPAN

Introduction Specimen Artificial slit Crack tip 500  m Our previous study To evaluate the stress intensity factor of fatigue crack non-destructively using a Magnetic Flux Density (MFD) distribution around fatigue crack, ・ Four-point bending fatigue test ・ Observation of the MFD distribution around the fatigue crack

Changes in MFD distribution around fatigue crack (P max =15.8 kN, R=0.1) Center of gravity of the MFD distribution (B x ) Crack propagation

Relationship between Y c 1/2 and K max Non-destructive evaluation of the stress intensity factor of fatigue crack Changing mechanism of magnetic flux density distribution remains unclear

● To examine the effects of “Annealing after fatigue testing” and “Additional fatigue testing after the annealing” on the MFD distribution ● To discuss the changing mechanisms of MFD distribution based on these results Objective To clarify changing mechanisms of MFD distribution around fatigue cracks “Inverse magnetostrictive effect” Mechanical stress (Residual stress) Change of Spontaneous magnetization Crack propagationPlastic deformationResidual stress

Flowchart of a series of experiments Experimental methods ① Fatigue test ③ Annealing ⑤ Additional fatigue test ⑥ Measurement of MFD and FWHM ④ Measurement of MFD and FWHM ② Measurement of MFD and FWHM Crack length 9mm Crack length 10mm 873K, 5hours in N 2 atmosphere

Specimen Material: Chromium molybdenum steel JIS SCM440 Hardness:HV200 Artificial slit:Wire electric discharge machining Length = 2.0 mm Tip radius  = 100  m Shape and dimensions of specimen

Crack growth tests Four-point bending fatigue test Electro-hydraulic servo fatigue testing machine Load span: 50 mm Support span: 100 mm Frequency : 20Hz Stress ratio R : 0.1 Maximum load P max : 15.8kN Specimen Artificial slit 50mm 100mm Four-point bending loading jigs

Measurement of MFD (Magnetic Flux Density) MI sensor (M agneto- I mpedance sensor ) AMI201 (Aichi Micro Intelligent Corporation) MI sensor XY stage Specimen 3mm Apparatus for measurement of Magnetic flux density distribution Small, Extremely high sensitivity

Measuring region of MFD A B C C B A D D A B C 3D Displaying of B x 2D Displaying of B x

Measurement of FWHM Measurement of FWHM h/2 FWHM X-ray diffraction profile FWHM (Full Width at Half Maximum) Measurement conditions Indicator of Dislocation density or Plastic deformation Measurement position

Experimental results Effects of Annealing on FWHM distribution Before Annealing (After Fatigue test) After Annealing Plastic deformation generated in crack propagation Relaxation of plastic deformation Measurement results of FWHM before and after the annealing

[  T] Magnetic flux density  T [  T] After Annealing Before Annealing Effects of Annealing on MFD distribution Annealing Decrease of MFD

Effects of Additional fatigue test on FWHM FWHM increased in the range where the fatigue crack propagated in the additional fatigue test Measurement results of FWHM before and after the additional fatigue test After additional fatigue test Before additional fatigue test (After Annealing)

N =3.4×10 5 a =9.00 mm N =3.5×10 5 a =10.1 mm [  T] [  T] After additional fatigue test Before additional fatigue test Magnetic flux density  T Effects of Additional fatigue test on MFD Measurement results of MFD before and after the additional fatigue test Increase of MFD in the part where the fatigue crack propagated in the additional fatigue test

FWHMMFDAnnealingDecreaseDecrease Additional Fatigue Test IncreaseIncrease Crack propagation Plastic deformation FWHM (Change of FWHM) Residual stress Inverse magnetostrictive effect Change of MFD Changing mechanism of MFD distribution Experimental results Consideration of the changing mechanism of MFD Good correspondence

Conclusions 1. The magnetic flux density around the fatigue crack decreased through annealing, and a clear correlation between the change of the magnetic flux density and FWHM by the annealing was observed. 2. An obvious increase of the magnetic flux density was observed around the part where the crack propagated during the additional fatigue test after the annealing. In this region, FWHM also increased, and a good correlation was observed between the change of the magnetic flux density and FWHM by the additional fatigue test, as well as by annealing. 3. It was concluded that the change of the magnetic flux density around the fatigue crack in its propagation process was a result of the inverse magnetostrictive effect caused by the plastic deformation generated around the crack tip and accumulated along the crack path.

Plastic zone caused by crack propagation （ a ） Before loading ss （ b ） Loading （ c ） Only elastic deformation （ d ） With plastic deformation Magnetic domain Plastic zone caused by crack propagation Plastic zone Crack 19 Unloading

Experimental result Relationship between Y c and a Ｇ ood correlation between Y c and a

Relationship between Y c 1/2 and K max