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Effect of pulsed electric current on the growth behavior of fatigue crack in Al alloy
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Available online www.sciencedirect.com Available online at at www.sciencedirect.com Structural Structural Integrity Integrity Procedia Procedia 00 00 (2016) (2016) 000–000 000–000
ScienceDirect ScienceDirect
Procedia Structural 2 (2016) 2989–2993 Structural IntegrityIntegrity Procedia 00 (2016) 000–000
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
www.elsevier.com/locate/procedia
21st European Conference on Fracture, ECF21, 20-24 June 2016, Catania, Italy
Effect of pulsed electric current on the growth behavior of fatigue crack in Al alloy Thermo-mechanical a high Morita pressure turbine aa bb Jaewoong Jungmodeling , Yang Jubb*,ofYasuyuki , Yuki Tokubb blade of an airplane gas turbine engine Graduate Graduate student, student, Nagoya Nagoya University, University, Furo-cho, Furo-cho, Chikusa-ku, Chikusa-ku, Nagoya, Nagoya, 464-8603, 464-8603, Japan Japan
XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal
a a
bb
Faculty Faculty of of Engineering, Engineering, Nagoya Nagoya University, University, Furo-cho, Furo-cho, Chikusa-ku, Chikusa-ku, Nagoya, Nagoya, 464-8603, 464-8603, Japan Japan
P. Brandãoa, V. Infanteb, A.M. Deusc*
a
Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal The of in alloy, 6061-T6, under the of pulsed electric current was studied. To c growth The growth Department of fatigue fatigue ofcrack crack in aluminum aluminum alloy, 6061-T6, under the application application current was studied. To CeFEMA, Mechanical Engineering, Instituto Superior Técnico, Universidadeofdepulsed Lisboa, electric Av. Rovisco Pais, 1, 1049-001 Lisboa, examine the effect of pulsed electric current, different electric current examine the effect of pulsed electric current, different electricPortugal current densities densities were were used used at at different different fatigue fatigue cycles, cycles, where where the the
Abstract Abstract b
2 2 2 electric and 150 150 A/mm A/mm2.. With With the the electric electric current current density density of of 90 90 A/mm A/mm2,, the the fatigue fatigue electric current current densities densities were were subjected subjected as as 90 90 A/mm A/mm2 and 22, the fatigue life was decreased. life life was was increased. increased. However, However, when when the the current current density density levels levels became became higher, higher, that that is, is, 150 150 A/mm A/mm , the fatigue life was decreased. From the Abstract From the fracture fracture surface surface observations, observations, the the local local melting melting was was found found on on crack crack surface. surface. It It is is concluded concluded that that the the increase increase of of fatigue fatigue life life is is attributed attributed to to the the crack crack shielding shielding effect effect resulted resulted from from the the local local melting melting induced induced by by the the pulsed pulsed electric electric current. current. In In addition, addition, the effect pulsed electric current more remarkable in region. their operation, aircraft components are subjected theDuring effect of of pulsed electric modern current was was moreengine remarkable in small small crack crack region. to increasingly demanding operating conditions, especially the high pressure turbine (HPT)B.V. blades. Such conditions cause these parts to undergo different types of time-dependent © 2016 The Authors. Published by Elsevier © 2016 The Authors. Published by Elsevier B.V. © 2016, PROSTR (Procedia Structural Integrity) Hosting by Elsevier Ltd. All rights reserved. degradation, one of which is creep. model using the finite Peer-review under responsibility of the Committee ofelement ECF21. Peer-review under responsibility theAScientific Scientific Committee ECF21.method (FEM) was developed, in order to be able to predict Peer-review under responsibility of theof Scientific Committee ECF21.of(FDR) the creep behaviour of HPT blades. Flight dataofrecords for a specific aircraft, provided by a commercial aviation company, were Pulsed used to obtain thermal and mechanical for three different flight cycles. In order to create the 3D model Keywords: Fatigue; electric current, Aluminum alloy, propagation Keywords: Fatigue; Pulsed electric current, Aluminum alloy, crack crack data propagation needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The 1. Introduction 1. overall Introduction expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
There There are are many many studies studies to to improve improve the the long long term term duration duration and and reliability reliability of of machine machine parts parts under under cyclic cyclic loads. loads. Especially, the effect of pulsed electric current has been found to be able to improve the fatigue properties © 2016 Thethe Authors. Elseviercurrent B.V. has been found to be able to improve the fatigue properties and Especially, effect Published of pulsedbyelectric and the the mechanical of et showed Peer-reviewproperties under responsibility of the Karpenko Scientific Committee of PCF 2016. that mechanical properties of materials. materials. Karpenko et al. al. (1976) (1976) showed that the the low low cycle cycle fatigue fatigue life life of of steel steel was was prolonged prolonged by by the the application application of of pulsed pulsed electric electric current. current. Conrad Conrad et et al. al. (1991) (1991) demonstrated demonstrated that that the the fatigue fatigue life life of of Keywords: High Pressureby Turbine Blade; of Creep; Finite Element Method; 3D Model; Simulation. copper was increased the effect the pulsed electric current. They concluded that the electric current influenced copper was increased by the effect of the pulsed electric current. They concluded that the electric current influenced
* * Corresponding Corresponding author. author. Tel.: Tel.: +81-52-789-4672; +81-52-789-4672; fax: fax: +81-52-789-3109. +81-52-789-3109. E-mail E-mail address: address: [email protected] [email protected] 2452-3216 2452-3216 © © 2016 2016 The The Authors. Authors. Published Published by by Elsevier Elsevier B.V. B.V.
Peer-review under responsibility the * Corresponding Tel.: +351of Peer-review underauthor. responsibility of218419991. the Scientific Scientific Committee Committee of of ECF21. ECF21. E-mail address: [email protected]
2452-3216 © 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 © 2016, PROSTR (Procedia Structural Integrity) Hosting by Elsevier Ltd. All rights reserved. Peer-review under responsibility of the Scientific Committee of ECF21. 10.1016/j.prostr.2016.06.374
Jaewoong Jung et al. / Procedia Structural Integrity 2 (2016) 2989–2993 Author name / Structural Integrity Procedia 00 (2016) 000–000
2990 2
inducing slip homogenization and encouraging dislocation mobility in the material. Furthermore, Salandro et al. (2010) and Roh et al. (2014) showed that the pulsed electric current had an effect on increasing elongation of aluminum alloy. Recently, crack healing in metallic materials were investigated. Hosoi et al. (2012) succeeded crack healing by the application of pulsed electric current. Zhou et al. (2001) showed that the pre-crack in carbon steel was healed. Although several researches related with fatigue strength and electric current have been reported, the mechanisms have not been fully understood. In this study, aluminum alloy was chosen for determining the effect of pulsed electric current. Aluminum alloy is widely used in engineering field like automotive and chemical industries for structural parts. The purpose of this work is to study the growth behavior of fatigue crack affected by the application of pulsed electric current in aluminum alloy. To investigate the effect of electric current density, the current was subjected with the density of 90 A/mm2 and 150 A/mm2. The effect on fatigue crack was evaluated by comparing before and after applying electric current. Besides, the fracture surfaces were observed in order to analyze the fracture mechanism using scanning electron microscopy (SEM). 2. Material and experimental details Aluminum alloy A6061-T6 was used as the experimental material. The chemical composition and mechanical properties of the material are presented in Tables 1 and 2, respectively. Fig.1 shows the schematic of the fatigue specimen. The fatigue specimens were machined to shallow notched dumbbell shape whose minimum thickness and width were 4.5 mm and 8 mm. The surface of the specimen was polished using emery papers of grain numbers from #180 to #2000. Hereafter, it was finished up into a mirror plane using buffing with alumina powder with grain diameter of 0.05 µm. Fatigue tests were performed at room temperature in laboratory air under a controlled load condition with an electro-hydraulic fatigue testing machine. The tests were operated at a stress ratio of -1 and a frequency of 15 Hz. To investigate the effect of pulsed electric current on the growth behavior of fatigue crack, the electric currents were given during the fatigue test, at each 10% fatigue cycle ratio after a fatigue cycle ratio of 70% until failure. The fatigue cycle ratio was calculated from the result of the untreated specimen, and the fatigue crack initiation was observed at 60% fatigue cycle ratio. The applied current densities were 90 and 150 A/mm2, respectively, and the pulsing duration was 0.5 ms. Table 1. Chemical composition of the material (mass%). Material
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Al
A6061
0.66
0.30
0.30
0.06
1.00
0.17
0.02
0.021
Bal.
Table 2. Mechanical properties of the material. 0.2% proof stress
Tensile strength
Elongation
Young’s Modulus
Vickers hardness
(MPa)
(MPa)
(%)
E (GPa)
HV
292
325
12
68
102
Fig. 1. Configuration of fatigue specimen.
Jaewoong Jung et al. / Procedia Structural Integrity 2 (2016) 2989–2993 Author name / Structural Integrity Procedia 00 (2016) 000–000
(a)
(b) 180 Stress amplitude (MPa)
180 Stress amplitude (MPa)
2991 3
160
140
A6061−T6
2
Current level of 90 A/mm Without pulsed electric current With pulsed electric current
120 4 3×10
10
5
Number of cycles to failure N f
10
6
160
140
A6061−T6
2
Current level of 150 A/mm Without pulsed electric current With pulsed electric current
120 4 3×10
10
5
10
6
Number of cycles to failure N f
Fig. 2 S-N diagram of A6061-T6 obtained by applying electric current with the density of: (a) 90 A/mm2; (b) 150 A/mm2.
Fatigue crack growth tests were carried out by the plastic replication method. The stress amplitude for fatigue crack growth tests was 160 MPa. To calculate the stress intensity factor, Newman-Raju solution (Raju and Newman, 1979; Newman and Raju, 1981) was used, assuming an aspect ratio of a/C = 1, where a is crack depth and 2C is crack length. After the fatigue tests, the fracture surfaces near the crack initiation site were observed using SEM. 3. Results and discussions 3.1. Fatigue strength The S-N diagrams (alternating stress, S, versus the number of cycles to failure, N) are shown Fig. 2(a) and (b), obtained from fatigue tests under the electric current density level of 90 A/mm2 and 150 A/mm2, respectively. To compare with the results, circle marks were introduced for in both figures, where each point corresponds to the result of one specimen. As seen in Fig. 2(a), the fatigue life was prolonged by the application of electric current (90 A/mm2), significantly. The ratio of maximum increment in the fatigue life was 55% compared to untreated specimen. On the other hand, at current density level of 150 A/mm2 (Fig. 2(b)), the fatigue life was decreased by the application of electric current. It was lower than the untreated specimen. The decreasing ratio of fatigue life was 10% compared to untreated specimen. 3.2. Fracture surface Fig. 3 shows fracture surfaces near the crack initiation site after the fatigue tests. Fig. 3(a) is the SEM micrograph of the fracture surface in untreated specimen. The fatigue crack was occurred at the surface of specimen and propagated with cyclic slip deformation. Fig. 3(b) shows the fracture surface of the specimen by applying electric current with the density of 90 A/mm2. The local melting site can be seen in the micrograph. It is well known that electric current can heal the fatigue crack (Qin et al., 2002; Zhou et al. 2004) by concentrating at the crack tip. The healing process was as follows: (a) the electric current field formed at the fatigue crack tip; (b) the local melting occurred at the crack tip by Joule heating. Therefore, it is considered that the fatigue life at the current density level of 90 A/mm2 was increased by the healing effect. For the fatigue test with 90 A/mm2 current treating, it could be considered that the crack shielding (Ritchie et al., 1989) was induced by the local melting. In the case of electric current density level of 150 A/mm2 as shown Fig. 3(c), the micrograph also shows the local melting site, but the brittleness fracture surface was observed. It is considered that the electric current density becomes too high, the thermal damage occurred at the crack tip.
Jaewoong Jung et al. / Procedia Structural Integrity 2 (2016) 2989–2993 Author name / Structural Integrity Procedia 00 (2016) 000–000
2992 4
(c)
(b)
(a)
Fig. 3. SEM micrographs of the fracture surfaces near crack initiation site: (a) untreated; (b) treated by 90 A/mm2; (c) treated by 150 A/mm2. The arrow marks indicate the local melting site.
3.3. Fatigue crack growth behavior Fig. 4 (a) and (b) show the relationship between the fatigue crack growth rates, da/dN, and maximum stress intensity factor, Kmax, obtained by applying electric current with the density level of 90 A/mm2 and 150 A/mm2, respectively. In the figures, the arrow marks indicate the application points of the electric current. The fatigue crack growth behavior of the specimen applied electric current was compared to the result of untreated specimen. Every fatigue crack was initiated on the surface of the specimen. In the result with the electric current level of 90 A/mm2, the growth rates of fatigue crack were slower than those of untreated one. The significant delay effect just shows after the application of electric current. Especially, in low Kmax region, the rates were decreased due to the effort of 10
(b)
−3
A6061−T6
2
Current level of 90 A/mm
10
−4
10
−5
10
−6
10
−7
10
−8
0.1
Without pulsed electric current With pulsed electric current Current applied point
1
Crack growth rate d a / dN (mm/cycle)
Crack growth rate d a / dN (mm/cycle)
(a)
−3
10
1/2
Maximum stress intensity factor Kmax (MPam )
2
−4
10
−5
10
−6
10
−7
Without pulsed electric current With pulsed electric current Current applied point
10
−8
10
10
A6061−T6
Current level of 150 A/mm
0.1
1
10 1/2
Maximum stress intensity factor Kmax (MPam )
Fig. 4. Fatigue crack growth rates as a function of the maximum stress intensity factor for the specimens with and without electric current: (a) 90 A/mm2; (b) 150 A/mm2.
Jaewoong Jung et al. / Procedia Structural Integrity 2 (2016) 2989–2993 Author name / Structural Integrity Procedia 00 (2016) 000–000
2993 5
pulsed electric current, significantly. However, it became almost the same as the behavior of untreated specimen in high Kmax. From the result, it is considered that when the stress intensity becomes larger than about 0.91 MPam1/2, the concentration of electric current is decreased at the fatigue crack tip. It means that the electric current is not enough to melt the fatigue crack. On the other hand, the crack growth rates at the electric current density of 150 A/mm2 were faster than that of untreated one as shown Fig. 3(b). From the SEM micrograph, it is considered that the rapid crack propagation was caused by the thermal damage. As a result, it was clarified that the fatigue crack growth behavior was influenced by the application of electric current. Furthermore, it could be considered that the fatigue life was increased by the delay effect of the fatigue crack propagation. 4. Conclusions The effect of pulsed electric current on fatigue crack was examined in this study. The fatigue life was increased by the application of electric current with the density level of 90 A/mm2. However, when the density becomes too high, that is 150 A/mm2, the fatigue life was decreased compared to the untreated specimen. The fracture surfaces and the fatigue crack behaviors were influenced by the application of electric current. With the electric current density of 90 A/mm2, the crack growth rates were decreased by inducing local melting. On the other hand, with 150 A/mm2, the crack growth rates were increased due to thermal damage. Consequently, it is considered that the fatigue life was increased by the delay effect of fatigue crack growth. References Kapenko, G., Kuzin, O, Tkachev, V., Rudenko, V., 1976. Influence of an electric current upon the low-cycle fatigue of steel. Doklady Physics 21, 159-160. Conrad, H., White, J., Cao, W., Lu, X., Sprecher, A, 1991. Effect of electric current pulses on fatigue characteristics of polycrystalline copper. Materials Science and Engineering, A 145, 1-12. Salandro, W., Jones, J., McNeal, T., Roth, J., Hong, S., Smith, M., 2010. Formability of Al 5xxx sheet metals using pulsed current for various heat treatments. Journal of Manufacturing Science and Engineering 132, 051016-1-11. Roh, J., Seo, J., Hong, S., Kim, M., Han, H., Roth, J., 2014. The mechanical behavior of 5052-H32 aluminum alloys under a pulsed electric current. International Journal of Plasticity 58, 84-99. Hosoi, A., Nagahama, T., Ju, Y., 2012. Fatigue crack healing by a controlled high density electric current field. Material Science and Engineering, A 533, 38-42. Zhou, Y., Zeng, Y., He, G., Zhou, B., 2001. The healing of quenched crack in 1045 steel under electropulsing. Journal of Material Research 16, 17-19. Raju, I., Newman, J., 1979. Stress-intensity factors for a wide range of semi-elliptical surface cracks in finite-thickness plates. Engineering Fracture Mechanics 11, 817-829. Newman, J., Raju, I., 1981. An empirical stress-intensity factor equation for the surface crack. Engineering Fracture Mechanics 15, 185-192. Qin, R., Su, S., 2002. Thermodynamics of crack healing under electropulsing. Journal of Materials Research 17, 2048-2052. Zhou, Y., Guo, J., Gao, M., He, G., 2004. Crack healing in a steel by using electropulsing technique. Materials Letters 58, 1732-1736. Ritchie, R., Yu, W., Bucci, R., 1989. Fatigue crack propagation in ARALL LAMINATES: Measurement of the effect of crack-tip shielding from crack bridging. Engineering Fracture Mechanics 32, 361–377.
ScienceDirect
Available online www.sciencedirect.com Available online at at www.sciencedirect.com Structural Structural Integrity Integrity Procedia Procedia 00 00 (2016) (2016) 000–000 000–000
ScienceDirect ScienceDirect
Procedia Structural 2 (2016) 2989–2993 Structural IntegrityIntegrity Procedia 00 (2016) 000–000
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
www.elsevier.com/locate/procedia
21st European Conference on Fracture, ECF21, 20-24 June 2016, Catania, Italy
Effect of pulsed electric current on the growth behavior of fatigue crack in Al alloy Thermo-mechanical a high Morita pressure turbine aa bb Jaewoong Jungmodeling , Yang Jubb*,ofYasuyuki , Yuki Tokubb blade of an airplane gas turbine engine Graduate Graduate student, student, Nagoya Nagoya University, University, Furo-cho, Furo-cho, Chikusa-ku, Chikusa-ku, Nagoya, Nagoya, 464-8603, 464-8603, Japan Japan
XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal
a a
bb
Faculty Faculty of of Engineering, Engineering, Nagoya Nagoya University, University, Furo-cho, Furo-cho, Chikusa-ku, Chikusa-ku, Nagoya, Nagoya, 464-8603, 464-8603, Japan Japan
P. Brandãoa, V. Infanteb, A.M. Deusc*
a
Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal The of in alloy, 6061-T6, under the of pulsed electric current was studied. To c growth The growth Department of fatigue fatigue ofcrack crack in aluminum aluminum alloy, 6061-T6, under the application application current was studied. To CeFEMA, Mechanical Engineering, Instituto Superior Técnico, Universidadeofdepulsed Lisboa, electric Av. Rovisco Pais, 1, 1049-001 Lisboa, examine the effect of pulsed electric current, different electric current examine the effect of pulsed electric current, different electricPortugal current densities densities were were used used at at different different fatigue fatigue cycles, cycles, where where the the
Abstract Abstract b
2 2 2 electric and 150 150 A/mm A/mm2.. With With the the electric electric current current density density of of 90 90 A/mm A/mm2,, the the fatigue fatigue electric current current densities densities were were subjected subjected as as 90 90 A/mm A/mm2 and 22, the fatigue life was decreased. life life was was increased. increased. However, However, when when the the current current density density levels levels became became higher, higher, that that is, is, 150 150 A/mm A/mm , the fatigue life was decreased. From the Abstract From the fracture fracture surface surface observations, observations, the the local local melting melting was was found found on on crack crack surface. surface. It It is is concluded concluded that that the the increase increase of of fatigue fatigue life life is is attributed attributed to to the the crack crack shielding shielding effect effect resulted resulted from from the the local local melting melting induced induced by by the the pulsed pulsed electric electric current. current. In In addition, addition, the effect pulsed electric current more remarkable in region. their operation, aircraft components are subjected theDuring effect of of pulsed electric modern current was was moreengine remarkable in small small crack crack region. to increasingly demanding operating conditions, especially the high pressure turbine (HPT)B.V. blades. Such conditions cause these parts to undergo different types of time-dependent © 2016 The Authors. Published by Elsevier © 2016 The Authors. Published by Elsevier B.V. © 2016, PROSTR (Procedia Structural Integrity) Hosting by Elsevier Ltd. All rights reserved. degradation, one of which is creep. model using the finite Peer-review under responsibility of the Committee ofelement ECF21. Peer-review under responsibility theAScientific Scientific Committee ECF21.method (FEM) was developed, in order to be able to predict Peer-review under responsibility of theof Scientific Committee ECF21.of(FDR) the creep behaviour of HPT blades. Flight dataofrecords for a specific aircraft, provided by a commercial aviation company, were Pulsed used to obtain thermal and mechanical for three different flight cycles. In order to create the 3D model Keywords: Fatigue; electric current, Aluminum alloy, propagation Keywords: Fatigue; Pulsed electric current, Aluminum alloy, crack crack data propagation needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The 1. Introduction 1. overall Introduction expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
There There are are many many studies studies to to improve improve the the long long term term duration duration and and reliability reliability of of machine machine parts parts under under cyclic cyclic loads. loads. Especially, the effect of pulsed electric current has been found to be able to improve the fatigue properties © 2016 Thethe Authors. Elseviercurrent B.V. has been found to be able to improve the fatigue properties and Especially, effect Published of pulsedbyelectric and the the mechanical of et showed Peer-reviewproperties under responsibility of the Karpenko Scientific Committee of PCF 2016. that mechanical properties of materials. materials. Karpenko et al. al. (1976) (1976) showed that the the low low cycle cycle fatigue fatigue life life of of steel steel was was prolonged prolonged by by the the application application of of pulsed pulsed electric electric current. current. Conrad Conrad et et al. al. (1991) (1991) demonstrated demonstrated that that the the fatigue fatigue life life of of Keywords: High Pressureby Turbine Blade; of Creep; Finite Element Method; 3D Model; Simulation. copper was increased the effect the pulsed electric current. They concluded that the electric current influenced copper was increased by the effect of the pulsed electric current. They concluded that the electric current influenced
* * Corresponding Corresponding author. author. Tel.: Tel.: +81-52-789-4672; +81-52-789-4672; fax: fax: +81-52-789-3109. +81-52-789-3109. E-mail E-mail address: address: [email protected] [email protected] 2452-3216 2452-3216 © © 2016 2016 The The Authors. Authors. Published Published by by Elsevier Elsevier B.V. B.V.
Peer-review under responsibility the * Corresponding Tel.: +351of Peer-review underauthor. responsibility of218419991. the Scientific Scientific Committee Committee of of ECF21. ECF21. E-mail address: [email protected]
2452-3216 © 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 © 2016, PROSTR (Procedia Structural Integrity) Hosting by Elsevier Ltd. All rights reserved. Peer-review under responsibility of the Scientific Committee of ECF21. 10.1016/j.prostr.2016.06.374
Jaewoong Jung et al. / Procedia Structural Integrity 2 (2016) 2989–2993 Author name / Structural Integrity Procedia 00 (2016) 000–000
2990 2
inducing slip homogenization and encouraging dislocation mobility in the material. Furthermore, Salandro et al. (2010) and Roh et al. (2014) showed that the pulsed electric current had an effect on increasing elongation of aluminum alloy. Recently, crack healing in metallic materials were investigated. Hosoi et al. (2012) succeeded crack healing by the application of pulsed electric current. Zhou et al. (2001) showed that the pre-crack in carbon steel was healed. Although several researches related with fatigue strength and electric current have been reported, the mechanisms have not been fully understood. In this study, aluminum alloy was chosen for determining the effect of pulsed electric current. Aluminum alloy is widely used in engineering field like automotive and chemical industries for structural parts. The purpose of this work is to study the growth behavior of fatigue crack affected by the application of pulsed electric current in aluminum alloy. To investigate the effect of electric current density, the current was subjected with the density of 90 A/mm2 and 150 A/mm2. The effect on fatigue crack was evaluated by comparing before and after applying electric current. Besides, the fracture surfaces were observed in order to analyze the fracture mechanism using scanning electron microscopy (SEM). 2. Material and experimental details Aluminum alloy A6061-T6 was used as the experimental material. The chemical composition and mechanical properties of the material are presented in Tables 1 and 2, respectively. Fig.1 shows the schematic of the fatigue specimen. The fatigue specimens were machined to shallow notched dumbbell shape whose minimum thickness and width were 4.5 mm and 8 mm. The surface of the specimen was polished using emery papers of grain numbers from #180 to #2000. Hereafter, it was finished up into a mirror plane using buffing with alumina powder with grain diameter of 0.05 µm. Fatigue tests were performed at room temperature in laboratory air under a controlled load condition with an electro-hydraulic fatigue testing machine. The tests were operated at a stress ratio of -1 and a frequency of 15 Hz. To investigate the effect of pulsed electric current on the growth behavior of fatigue crack, the electric currents were given during the fatigue test, at each 10% fatigue cycle ratio after a fatigue cycle ratio of 70% until failure. The fatigue cycle ratio was calculated from the result of the untreated specimen, and the fatigue crack initiation was observed at 60% fatigue cycle ratio. The applied current densities were 90 and 150 A/mm2, respectively, and the pulsing duration was 0.5 ms. Table 1. Chemical composition of the material (mass%). Material
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Al
A6061
0.66
0.30
0.30
0.06
1.00
0.17
0.02
0.021
Bal.
Table 2. Mechanical properties of the material. 0.2% proof stress
Tensile strength
Elongation
Young’s Modulus
Vickers hardness
(MPa)
(MPa)
(%)
E (GPa)
HV
292
325
12
68
102
Fig. 1. Configuration of fatigue specimen.
Jaewoong Jung et al. / Procedia Structural Integrity 2 (2016) 2989–2993 Author name / Structural Integrity Procedia 00 (2016) 000–000
(a)
(b) 180 Stress amplitude (MPa)
180 Stress amplitude (MPa)
2991 3
160
140
A6061−T6
2
Current level of 90 A/mm Without pulsed electric current With pulsed electric current
120 4 3×10
10
5
Number of cycles to failure N f
10
6
160
140
A6061−T6
2
Current level of 150 A/mm Without pulsed electric current With pulsed electric current
120 4 3×10
10
5
10
6
Number of cycles to failure N f
Fig. 2 S-N diagram of A6061-T6 obtained by applying electric current with the density of: (a) 90 A/mm2; (b) 150 A/mm2.
Fatigue crack growth tests were carried out by the plastic replication method. The stress amplitude for fatigue crack growth tests was 160 MPa. To calculate the stress intensity factor, Newman-Raju solution (Raju and Newman, 1979; Newman and Raju, 1981) was used, assuming an aspect ratio of a/C = 1, where a is crack depth and 2C is crack length. After the fatigue tests, the fracture surfaces near the crack initiation site were observed using SEM. 3. Results and discussions 3.1. Fatigue strength The S-N diagrams (alternating stress, S, versus the number of cycles to failure, N) are shown Fig. 2(a) and (b), obtained from fatigue tests under the electric current density level of 90 A/mm2 and 150 A/mm2, respectively. To compare with the results, circle marks were introduced for in both figures, where each point corresponds to the result of one specimen. As seen in Fig. 2(a), the fatigue life was prolonged by the application of electric current (90 A/mm2), significantly. The ratio of maximum increment in the fatigue life was 55% compared to untreated specimen. On the other hand, at current density level of 150 A/mm2 (Fig. 2(b)), the fatigue life was decreased by the application of electric current. It was lower than the untreated specimen. The decreasing ratio of fatigue life was 10% compared to untreated specimen. 3.2. Fracture surface Fig. 3 shows fracture surfaces near the crack initiation site after the fatigue tests. Fig. 3(a) is the SEM micrograph of the fracture surface in untreated specimen. The fatigue crack was occurred at the surface of specimen and propagated with cyclic slip deformation. Fig. 3(b) shows the fracture surface of the specimen by applying electric current with the density of 90 A/mm2. The local melting site can be seen in the micrograph. It is well known that electric current can heal the fatigue crack (Qin et al., 2002; Zhou et al. 2004) by concentrating at the crack tip. The healing process was as follows: (a) the electric current field formed at the fatigue crack tip; (b) the local melting occurred at the crack tip by Joule heating. Therefore, it is considered that the fatigue life at the current density level of 90 A/mm2 was increased by the healing effect. For the fatigue test with 90 A/mm2 current treating, it could be considered that the crack shielding (Ritchie et al., 1989) was induced by the local melting. In the case of electric current density level of 150 A/mm2 as shown Fig. 3(c), the micrograph also shows the local melting site, but the brittleness fracture surface was observed. It is considered that the electric current density becomes too high, the thermal damage occurred at the crack tip.
Jaewoong Jung et al. / Procedia Structural Integrity 2 (2016) 2989–2993 Author name / Structural Integrity Procedia 00 (2016) 000–000
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Fig. 3. SEM micrographs of the fracture surfaces near crack initiation site: (a) untreated; (b) treated by 90 A/mm2; (c) treated by 150 A/mm2. The arrow marks indicate the local melting site.
3.3. Fatigue crack growth behavior Fig. 4 (a) and (b) show the relationship between the fatigue crack growth rates, da/dN, and maximum stress intensity factor, Kmax, obtained by applying electric current with the density level of 90 A/mm2 and 150 A/mm2, respectively. In the figures, the arrow marks indicate the application points of the electric current. The fatigue crack growth behavior of the specimen applied electric current was compared to the result of untreated specimen. Every fatigue crack was initiated on the surface of the specimen. In the result with the electric current level of 90 A/mm2, the growth rates of fatigue crack were slower than those of untreated one. The significant delay effect just shows after the application of electric current. Especially, in low Kmax region, the rates were decreased due to the effort of 10
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Fig. 4. Fatigue crack growth rates as a function of the maximum stress intensity factor for the specimens with and without electric current: (a) 90 A/mm2; (b) 150 A/mm2.
Jaewoong Jung et al. / Procedia Structural Integrity 2 (2016) 2989–2993 Author name / Structural Integrity Procedia 00 (2016) 000–000
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pulsed electric current, significantly. However, it became almost the same as the behavior of untreated specimen in high Kmax. From the result, it is considered that when the stress intensity becomes larger than about 0.91 MPam1/2, the concentration of electric current is decreased at the fatigue crack tip. It means that the electric current is not enough to melt the fatigue crack. On the other hand, the crack growth rates at the electric current density of 150 A/mm2 were faster than that of untreated one as shown Fig. 3(b). From the SEM micrograph, it is considered that the rapid crack propagation was caused by the thermal damage. As a result, it was clarified that the fatigue crack growth behavior was influenced by the application of electric current. Furthermore, it could be considered that the fatigue life was increased by the delay effect of the fatigue crack propagation. 4. Conclusions The effect of pulsed electric current on fatigue crack was examined in this study. The fatigue life was increased by the application of electric current with the density level of 90 A/mm2. However, when the density becomes too high, that is 150 A/mm2, the fatigue life was decreased compared to the untreated specimen. The fracture surfaces and the fatigue crack behaviors were influenced by the application of electric current. With the electric current density of 90 A/mm2, the crack growth rates were decreased by inducing local melting. On the other hand, with 150 A/mm2, the crack growth rates were increased due to thermal damage. Consequently, it is considered that the fatigue life was increased by the delay effect of fatigue crack growth. References Kapenko, G., Kuzin, O, Tkachev, V., Rudenko, V., 1976. Influence of an electric current upon the low-cycle fatigue of steel. Doklady Physics 21, 159-160. Conrad, H., White, J., Cao, W., Lu, X., Sprecher, A, 1991. Effect of electric current pulses on fatigue characteristics of polycrystalline copper. Materials Science and Engineering, A 145, 1-12. Salandro, W., Jones, J., McNeal, T., Roth, J., Hong, S., Smith, M., 2010. Formability of Al 5xxx sheet metals using pulsed current for various heat treatments. Journal of Manufacturing Science and Engineering 132, 051016-1-11. Roh, J., Seo, J., Hong, S., Kim, M., Han, H., Roth, J., 2014. The mechanical behavior of 5052-H32 aluminum alloys under a pulsed electric current. International Journal of Plasticity 58, 84-99. Hosoi, A., Nagahama, T., Ju, Y., 2012. Fatigue crack healing by a controlled high density electric current field. Material Science and Engineering, A 533, 38-42. Zhou, Y., Zeng, Y., He, G., Zhou, B., 2001. The healing of quenched crack in 1045 steel under electropulsing. Journal of Material Research 16, 17-19. Raju, I., Newman, J., 1979. Stress-intensity factors for a wide range of semi-elliptical surface cracks in finite-thickness plates. Engineering Fracture Mechanics 11, 817-829. Newman, J., Raju, I., 1981. An empirical stress-intensity factor equation for the surface crack. Engineering Fracture Mechanics 15, 185-192. Qin, R., Su, S., 2002. Thermodynamics of crack healing under electropulsing. Journal of Materials Research 17, 2048-2052. Zhou, Y., Guo, J., Gao, M., He, G., 2004. Crack healing in a steel by using electropulsing technique. Materials Letters 58, 1732-1736. Ritchie, R., Yu, W., Bucci, R., 1989. Fatigue crack propagation in ARALL LAMINATES: Measurement of the effect of crack-tip shielding from crack bridging. Engineering Fracture Mechanics 32, 361–377.