Non-destructive Evaluation of Fatigue Damage in the Stainless Steel by Using Electromagnetic Method

Transcription

1 E-Journal of Advanced Maintenance Vol.1(9) 77- Japan Society of Maintenology Non-destructive Evaluation of Fatigue Damage in the Stainless Steel by Using Electromagnetic Method Mohachiro Oka 1,*, Terutoshi Yakushiji 1, Yuji Tsuchida, and Masato Enokizono 1 Oita National College of Technology, 1 Maki, Oita, 7-15, Japan Oita University, 7 Dannoharu, Oita, 7-119, Japan (Received; April 1 st, 9) Abstract. The magnetic property of a semi-stable austenitic stainless steel (SUS34) is changed greatly by martensitic transformation caused by fatigue. It is well known that there is a high correlation between the amount of fatigue damage and the residual leakage magnetic flux density caused by remanent magnetization of it. On the other hand, because a micro structure of metallic materials such as austenitic stainless steels changes by fatigue, it is thought that the resistivity of them changes as well as the change of remanent magnetization as magnetic properties by fatigue. Therefore, we paid attention to the relationship between the amount of fatigue damage and the resistivity of austenitic stainless steels, and did experiments that examined this relation. From our experiments, the change of the resistivity clearly depends on the amount of fatigue damage in austenitic stainless steels. This paper shows the relationship between electromagnetic properties and the amount of fatigue damage of SUS34 and SUS31. KEYWORDS: resistivity, 4-point probe, SUS34, SUS31, fatigue, non-destructive testing 1. Introduction Many structural components used in chemical plants and power stations and etc. are made of austenitic stainless steels such as SUS34 (ANSI34) or SUS31 (ANSI31). They show excellent corrosion resistance. From a viewpoint of economy and safety, it is necessary to prevent accidents caused by the degradation of structural components made of them. To evaluate fatigue damage of them, the remanent magnetization method [1, ] and the eddy current testing method [3] and etc. have been already proposed by some research gropes. These methods use the change of magnetism caused by fatigue. In SUS34, transformation from the austenitic phase to the martensitic phase happens easily by fatigue. On the other hand, fatigue increases the dislocation of metallic materials such as austenitic stainless steels. It is thought that an increase of martensitic transformation and the dislocation changes not only magnetization bat also the resistivity (ρ) of metallic materials. This method is called the resistivity method. Accordingly, to propose the new fatigue evaluation method for austenitic stainless steels (SUS34 and SUS31) using the change of the resistivity, we investigated the relationship between fatigue damage and ρ using the 4-poit probe method. In this paper, the result of this investigation is reported. And, the remanent magnetization method used for the fatigue evaluation has already been shown in SUS34 and SUS31 [, 4, 5]. Therefore, to show the utility of the resistivity method, the results of the residual magnetization method are shown and compared.. The 4-point Probe Method Figure 1 shows the schematic diagram of the 4-point probe method. A specimen used in our experiment is thin. Then, we calculated ρ by the thin film approximation method because the current flows as shown in Fig.. Resistance R between x 1 and x is shown by equation (1). Therefore, ρ is written as shown equation (). * Corresponding author, oka@oita-ct.ac.jp ISSN /9 9 JSM and the authors. All rights reserved. 77

2 R d x = x 1 d π dx d ρ dx ρ ρ V ρ = = ln( x) = ln = xt d t x t (1) π π π t I π t V ρ = () ln I Where, x, x 1, x, d, t, V, and I are defined as shown in Fig. 1. When ρ is calculated, the following two approximations were done. The shape of the specimen was assumed to be an infinite plate. And, thickness (t) of a specimen and the probe spacing (d) were assumed to be t << d. These approximations are not strictly correct. But, these assumptions are effective in case of this experiment because the shape of a specimen doesn't change while experimenting. From the results of our experiment, ρ of the specimen of this shape as shown in Fig. 3 is about 4 % larger than ρ of a plate of 3 mm x 3 mm by 1 mm in thickness. The standard values of the resistivity of SUS34 and SUS31 are about.7 μωm, and.74 μωm respectively. These two our measurement values are corresponding to these standard values well (See Fig. 1 and Fig.11.). Specimen Fig. 1 The 4-point probe method. Fig. The thin film approximation. 3. Experimental Method Figure 3 shows the shape and dimensions of the specimen, the measurement area of the residual leakage magnetic flux density (B z ), and the measurement point of ρ. All specimens are made of SUS34 or SUS31. Thickness of a specimen is a 1. mm. Specimens are cut out by the electrical discharge machining method as shown in Fig. 1. The applied stress was amended by using the measured width and the measured thickness of the specimen of the stress concentration zone. In case of SUS34, the experiment was done at about 3 C to avoid the influence of temperature effect. We experimented similarly for SUS31 in the vicinity of the room temperature of about 5 C. Figure 4 shows a block diagram of a measurement system for the 4-point probe method used in our experiments. In this case, d which is each space of probes and t which is the thickness of a specimen are 5 mm and 1 mm respectively. To make the good contact between a specimen and four probes, special measurement apparatus was made. The probes have the spring internally, and they are plated by gold. To know the influence of the resistivity of surface state on the measurement, the resistivity of the specimen that did electro-polish and the resistivity of the specimen that did not do electro-polish were compared. As a result, there was no significant difference. Then, electro-polish of the specimen was not done in this experiment. To make clear the relationship between fatigue damage and ρ or the relationship between fatigue damage and B z, experiments were carefully carried out in the following procedure. This procedure is shown in Fig. 5. At first, specimens are demagnetized using a marketed demagnetizer. Secondly, specimens are magnetized by the static magnetic field (.3 T) in the Z direction (See Fig. 3.). Thirdly, the distribution of B z caused by remanent magnetization is measured within the area of 3 x 4 mm 7

3 every 1 mm step as shown in Fig. 3 using the thin-film flux-gate magnetic sensor (the FG magnetic sensor, SHIMADZU CORPORATION) []. Fourthly, ρ is measured by the 4-point probe method at the center of a specimen as shown in Fig. 3. The applied current is 1 A. Lastly, cyclic bending stress is added by a plane bending fatigue testing machine (UF15, SHIMADZU CORPORATION) with a capacity of. Nm operating at 3 Hz. This procedure is repeated until the specimen is destroyed. Table 1 shows chemical composition of SUS34 and SUS31 used in our experiments. In the case of SUS34, the applied bending stress (σ a ) were 9, 3, 3MPa in our experiment. Moreover, the applied σ a in SUS31 were 9, 3, 31, 3MPa. Fig. 3 The shape of specimen. Fig. 4 The measurement system. Table 1 Chemical composition in wt%. Ni Cr C Si P Mn S Mo SUS SUS Experimental Results and Discussions Fig. 5 Measurement procedure The distributions of B z The distributions of B z are shown in Fig., Fig. 7, Fig., and Fig. 9. In Fig. and Fig. 7, the specimens were made of SUS34 and σ a is 3 MPa. In Fig., the number of stress cycles (N) is zero. N is 1.4 x 1 5 in Fig. 7. In Fig. and Fig. 9, specimens were made of SUS31 and σ a is 3 MPa. In 79

4 Fig., the number of stress cycles (N) is zero. N is 3 x 1 5 in Fig. 9. From these figures, B z measured by the FG magnetic sensor clearly catches the distribution of fatigue damage in SUS34 and SUS31. In Fig., B z is large in the vicinity of both ends of Y-axis and in the center of X-axis as shown in two circles. The value of B z originates in the rust that remained at an electrical discharge machining. The value of db z as shown in Fig. 7 and Fig. 9 is a parameter to evaluate the amount of fatigue damage in austenitic stainless steels. 4.. The relationship among ρ and σ a, and N Figure 1 shows the relationship among ρ and σ a, and N when the specimen is SUS34. Figure 11 shows the same relationship as Fig. 1 for SUS31. From these figures, ρ increases clearly according to an increase in N and σ a. In Fig. 1 and Fig. 11, there are good correlations in N and σ a and an increase in ρ in these figures though the measurement varies somewhat. Moreover, the good correlation between σ a and ρ exists as N similarly. The value of ρ immediately before the rupture increases only by 1.5% in SUS34 and 1.7% in SUS31. It is thought that not only an increase in the dislocation and the micro-crack by fatigue damage but also the martensitic transformation by fatigue damage is a cause in the change in ρ in SUS34. On the other hand, it is thought that only an increase in the dislocation by fatigue damage is a cause in the change in ρ in SUS Conclusions In this paper, we have elucidated the following the results for the evaluation of fatigue damage in SUS34 or SUS31 using ρ measured by the 4-point probe method. From our experiments, the tendency to the change of ρ by fatigue damage and the change of db z by fatigue damage looks like well. And, the change of ρ and db z clearly depends on the amount of fatigue damage of both SUS34 and SUS31. Therefore, there is a possibility that ρ can use to evaluate the amount of fatigue damage in SUS34 and SUS31. However, the change of ρ and db z is little different when N is small. Therefore the cause of the change of ρ and db z by fatigue damage might be different. It is thought that the resistivity method has detected the dislocation and an increase in the micro-crack. Therefore, this method has the possibility that steel and aluminum, etc. can be applied, too. In addition, it is necessary to investigate the cause of an increase of db z for SUS31 in detail. Acknowledgments This work was supported in part by Grant-in-Aid for Scientific Research (C) No B z (μt) Y-position(mm) 15 1 B z (μt) X- position(m -1 m) db z Y-position(mm) X- 3 position(m - m) Fig. The distribution of B z. Fig. 7 The distribution of B z. (N =, σ a = 3 MPa, SUS34) (N = 1.4 x 1 5, σ a = 3 MPa, SUS34)

5 Bz (μt) X-position (mm) B z (μ T) X-position (mm) Y-position (mm) db z Y-position (mm) Fig. The distribution of B z. Fig. 9 The distribution of B z. (N =, σ a = 3 MPa, SUS31) (N = 3 x 1 5, σ a = 3 MPa, SUS31) Resistivity ρ (μω m) WNo.14(3MPa) WNo.13(3MPa) WNo.19(31MPa) WNo.(31MPa) WNo.1(9MPa) WNo.(9MPa) Resistivity ρ (μω m) No15(3MPa) No13(3MPa) No9(31MPa) No19(31MPa) No(3MPa) No7(3MPa) No1(9MPa) No(9MPa) Number of stress cycles (N ) Number of stress cycles (N) Fig. 1 The relationship among ρ and σ a and N. Fig. 11 The relationship among ρ and σ a and N. (SUS34) (SUS31) dbz (μt) WNo.14(3MPa) WNo.13(3MPa) WNo.19(31MPa) WNo.(31MPa) WNo.1(9MPa) WNo.(9MPa) dbz (μt) No13(3MPa) No15(3MPa) No9(31MPa) No19(31MPa) No(3MPa) No7(3Mpa) No1(9MPa) No(9MPa) Number of stress cycles (N ) Number of stress cycles (N ) Fig. 1 The relationship among db z and σ a and N. Fig. 13 The relationship among db z and σ a and N. (SUS34) (SUS31) 1

6 References [1] Y. Nakasone, Y. Iwasaki, T. Shimizu and S. Kasumi; Electromagnetic non-destructive detection of damage in an austenitic stainless steel by the use of martensitic transformation, Journal of the Japan society of Applied Electromagetics and Mechanics, Vol. 9, No., pp , 1 [] M. Oka, T. Yakushiji and M. Enokizono, Fatigue Dependence of Residual Magnetization in Austenitic Stainless Steel Plates, IEEE TRANSACTIONS ON MAGNETICS, Vol. 37, No. 5, pp , 1 [3] T. Uchimoto, T. Takagi, M. Kurosawa and S. Konoplyuk, Material Evaluation of Cast Iron Using the Eddy Current Method, Studies in Applied Electromagnetics and Mechanics, ELECTRICMAGNETIC NONDESTRUCTIVE EVALUATION (VIII), IOS Press, Vol. 4, pp , 4 [4] M. Oka, T. Yakushiji, Y. Tsuchida and M. Enokizono, Evaluation of the Martensite Induced by Fatigue in SUS34 Using the Perpendicular Magnetization Method for NDE, Studies in Applied Electromagnetics and Mechanics, ELECTRICMAGNETIC NONDESTRUCTIVE EVALUATION (VII), IOS Press, Vol., pp. -, [5] M. Oka, E. Wada, T. Yakushiji, Y. Tsuchida and M. Enokizono, Evaluation of Fatigue Damage in SUS31 and SUS31L Using the FG Magnetic Sensor, Studies in Applied Electromagnetics and Mechanics 5, ELECTRICMAGNETIC NONDESTRUCTIVE EVALUATION (IX), IOS Press, pp , 5 [] Y. Yamda, K. Yoshimi, T. Munaka and R. Yoshida, Application of Thin-Film Fluxgate Magnetic Sensor, New Ceramics, No., pp. 15-, 199