Materials Science and Engineering A 533 (2012) 38–42

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Materials Science and Engineering A
journal homepage: www.elsevier.com/locate/msea

Fatigue crack healing by a controlled high density electric current ﬁeld
A. Hosoi, T. Nagahama, Y. Ju ∗Department of Mechanical Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japana r t i c l e

i n f oArticle history:
Received 18 June 2011
Received in revised form 28 October 2011
Accepted 7 November 2011
Available online 26 November 2011
Keywords:
Fatigue
Crack healing
Crack propagation
Electric current
Stainless steel

a b s t r a c t
A technique was developed to heal a fatigue crack in stainless steel by controlling a high-density electric
current ﬁeld. The high density electric current ﬁeld was applied at the tip of the crack by using closely
spaced electrodes. From the experimental results, it was observed the crack closure and the bridging
between the surfaces of a crack were caused around the vicinity of the crack tip after a high density pulse
current was applied to a specimen. It was shown that crack propagation was delayed temporarily in thehealed specimen.
© 2011 Elsevier B.V. All rights reserved.

1. Introduction
Global warming and other environmental concerns continue
increasing as industrialization rapidly grows. Reducing energy consumption to preserve the environment is a critical issue. Improving
the long-term durability and reliability of structures helps to preserve the environment by reducing the resources and energy
consumed during a structure’s life cycle. Speciﬁcally, the maincause of failure accident is fatigue, thus, preventing fatigue fractures in structures is one way to save energy. Techniques that
improve fatigue strength, such as high-frequency quenching, carburizing, nitriding and shot peening, have been developed. These
techniques can prolong the fatigue life of materials by suppressing
crack initiation at the material’s surface. However, the advantagesof these methods are not effective in materials with a pre-existing
crack.
Various studies have investigated crack healing techniques andtheir mechanisms. Chen et al. [1] suggested a method that uses
thermal reversibility to heal polymer materials. White et al. [2]investigated a microencapsulation method for similar materials.
For ceramic materials, the method to heal a surface crack by oxidation was studied [3]. On the other hand, the technique to heal a crack
in metallic materials has also been studied. Lumley [4] showedthat creep resistance was enhanced and a fatigue crack was healed
by dynamic precipitation in aluminum alloys. Kyono and Shinya[5] succeeded to control the growth of creep voids by promoting∗ Corresponding author. Tel.: +81 52 789 4672; fax: +81 52 789 3109.
E-mail address: ju@mech.nagoya-u.ac.jp (Y. Ju).
0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2011.11.024

precipitation in the stainless steel added boron, nickel and titan
instead of sulfur. However, these healing techniques are limited
to particular materials and circumstances in which precipitation
was caused. Wang et al. [6] also evaluated analytically the healing
processes of defects in metallic materials by ﬁnite element modeling. However, the modeling assumed ideal conditions and did notcompare to the actual phenomenon.
It is known that electric currents inﬂuence the material properties of metals. Karpenko et al. [7] demonstrated that the fatigue
life of steel was prolonged by applying an electric current during
fatigue loading, whereas Abdellatif [8] showed that the fatigue limit
and fatigue life of mild steel were reduced by pre-application of analternating current. Conrad and coauthors [9–13] investigated indetail the inﬂuence of a high density pulse current on materials,
and showed that dislocation mobility was encouraged due to theaction of drift electrons. Also, the following phenomena and theireffects have been reported: the generation of Joule heating [14], thecause of compressive stress due to Joule heating [15–17], the induction of a Lorentz force [18], and the cause of an electron wind force
due to the ﬂow of the electric current [19]. In recent years, studies
on crack healing in metallic materials have been conducted by utilizing the effects mentioned above. Yizhou et al. [20] was successful
in healing a quenched crack in 1045 steel by applying a high density pulse current. They concluded that crack healing was causedby the temperature and transient thermal compressive stress during electropulsing. It was reported that microcracks formed during
plastic deformation closed, and local recrystallization occurred dueto the electropulsing treatment [21]. Suhong et al. [22] and Conrad
et al. [11] showed that the persistent slip bands in fatigued copper
vanished and homogenized locally by treated with high current

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39

Fig. 1. Schematic of the specimen.

pulsing. Moreover, Lin et al. [23] showed that the propagation ofthermal fatigue crack in cast hot working die steel delayed due tothe reﬁnement of the grains around the crack tip by pulse current
stimulation.
However, the essential healing of fatigue cracks in metallic
materials by high density pulse currents has not been achieved.
Moreover, the healing effect from high density pulse currentson the fatigue crack has not been evaluated quantitatively. In
this paper, a technique to heal a fatigue crack by controlling high density electric current ﬁelds was demonstrated, andthe healing effect was evaluated quantitatively with the Parislaw.

Fig. 2. Specimen and electrodes used for applying electric current.

(SEM) before and after an electrical stimulation. There is no additional treatment being carried out after the electrical stimulation.
After observing the crack, the fatigue test was again carried out withthe same specimen. The crack length under cyclic loading was measured by in situ observations with an OM. The change in the crack
growth behavior due to the electrical stimulation was evaluatedquantitatively with the Paris law.

2. Experiments
2.3. Conditions of the fatigue test and electrical stimulation
2.1. Specimen
Tensile fatigue tests were conducted at room temperature in an
atmosphere under load control conditions with a hydraulic driven
testing machine. All of the tests were run at a stress ratio of R = 0.05,
a maximum stress of  max = 150 MPa and a frequency of f = 10 Hz.
The details of the fatigue test conditions are shown in Table 3. The
two specimens which were treated with current healing process
were named “Specimen A” and “Specimen B”. For comparison, thespecimen which was examined with a similar fatigue test without
applying electric current was named “Specimen C”. It was used to
conﬁrm the inﬂuence on crack propagation which may be induced
by removing the specimen from the testing machine and resetting
it again. The pre-crack length in Specimens A, B and C was 5.51,
4.07 and 4.17 mm, respectively.
The electric current was supplied by a transistor power sourcein the range of 0.5–10 kA. It was applied into the specimen throughtwo electrodes with the pulse duration in the range of 0.5–10 ms.
Chromium copper electrodes with 5 mm diameter were used. The
two electrodes were connected to the side of the specimen andstraddling the notch as shown in Fig. 2. The distance between
the two electrodes was 1.3 mm. The conditions of the electricalstimulation for specimens A and B are shown in Table 4. Whenan instantaneous pulse current was applied, only the crack area
can be electrically stimulated without inﬂuencing the rest of thespecimen.

Austenite stainless steel SUS316NG was used in this study. The
chemical compositions and the mechanical properties of it are
shown in Tables 1 and 2, respectively. Dumbbell-shaped specimen
was used (see Fig. 1), and a notch was created at the center of the
one edge of the specimen. The length and the root radius of thenotch were 2 mm and 0.18 mm, respectively. The specimen was
treated with a stress relief annealing treatment to remove residual
stresses that were caused by the machining process. The heat treatment process was as follows. The specimens were heated to 1173 K
for 4 h, and then were kept the temperature of 1173 K for 10 min.
Afterwards, the specimens were cooled slowly to room temperature in a furnace. After the heat treatment, the surface was polishedwith emery papers of the grain number from #180 to #2000,
and was ﬁnished up into a mirror plane by bufﬁng with alumina
powder of 0.05 ␮m grain diameter in order to observe a fatigue
crack.
2.2. Experimental procedure
After a fatigue crack with an arbitrary length was introduced
into the specimen, a high density pulse current was applied to thespecimen. The crack state on the specimen’s surface was observedby an optical microscope (OM) and a scanning electron microscope

Table 1
Chemical composition of the SUS316NG stainless steel (wt.%).
C

Si

Mn

P

S

Cu

Ni

Cr

Mo

N

C+N

0.016

0.43

1.54

0.028

0.0006

0.26

11.94

17.13

2.15

0.1

0.12

Table 2Mechanical properties of SUS316NG.
Yield stress [MPa]Tensile strength [MPa]Young’s modulus [GPa]Poisson’s ratio

Elongation [%]Reduction of area [%]

253

581

197

0.295

49

84

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A. Hosoi et al. / Materials Science and Engineering A 533 (2012) 38–42

Table 3
Fatigue test conditions for specimens A, B and C.

Maximum stress,  max [MPa]Stress ratio, R
Frequency, f [Hz]Number of cycles for pre-crack introduction, N [cycle]
Crack length before current application, a [mm]Stress intensity factor range, K [MPa·mm1/2 ]Specimen A

Specimen B

Specimen C

150
0.05
10
6.5 × 104
5.51
38.6

150
0.05
10
7.4 × 104
4.07
26.4

150
0.05
10
7.9 × 104
4.17
26.9

3. Experimental results

Table 4
Electric current applied in specimens A and B.

3.1. Observation of the fatigue crack
Figs. 3–6 are the SEM photographs, which show changes in
the condition of the fatigue crack before and after the application of the electric current. Fig. 3 shows a picture of Specimen A.
It was observed that the crack closed due to the electrical stimulation. Fig. 4 is a close-up picture of Specimen A at the crack
tip. The crack healed around the vicinity of the crack tip after
applying the electric current. Also, microparticles were observed
around the crack tip on the specimen’s surface after the electricalstimulation.
Fig. 5 shows a picture of Specimen B, while Fig. 6 is anenlarged picture of the area, which is approximately 300 ␮maway from the crack tip in Specimen B. Formation of the bridges
between the crack surfaces around the vicinity of the crack tip wasobserved.

Application current [A]Pulse duration [ms]Specimen A

Specimen B

5560
0.5

970
0.5

3.2. Evaluation of the crack growth behavior
The crack growth behavior was evaluated quantitatively with
the Paris law to observe the electrical stimulation’s effect on crack
growth. Fig. 7 shows the test result of Specimen C by removing itfrom the testing machine and resetting it again after setting it to
the current inducing system without applying electric current. The
open symbols show the behavior of the crack growth before removing the specimen from a testing machine, and the solid symbols
show the behavior of the crack growth after setting the specimen
to the current applying system without applying electric current.

Fig. 3. Images of the fatigue cracks in specimen A: (a) before and (b) after the application of the electric current.

Fig. 4. Magniﬁed images of the crack tip in Fig. 3: (a) before and (b) after the application of the electric current.

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41

Fig. 5. Images of the fatigue cracks in specimen B: (a) before and (b) after the application of the electric current.

Fig. 6. Magniﬁed images of the bridging in Fig. 5: (a) before and (b) after the application of the electric current.

It was found that there is no effect on crack propagation by removing a specimen from the testing machine and resetting it again.
Fig. 8 shows the crack growth behavior of Specimen B before andafter the application of the electric current. The solid line indicatesthe results obtained from Fig. 7. The open and solid symbols in
Specimen B show the behavior of the crack growth before andafter the application of the electric current, respectively. It wasobserved that the crack growth rate decreased from 7.65 × 10−5
to 2.40 × 10−5 mm/cycle just after the electrical stimulation. However, the crack growth rate returned to the standard crack growthrate after several thousand loading cycles were applied to the specimen.

Fig. 7. Fatigue crack growth rate as a function of the stress intensity factor range
before removing Specimen C from the testing machine and after resetting it again.

Fig. 8. Fatigue crack growth rate as a function of the stress intensity factor range
before and after the application of the electric current.

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A. Hosoi et al. / Materials Science and Engineering A 533 (2012) 38–42

4. Discussion
4.1. Closure and healing of a crack
One hypothesis to explain the crack closure and healing thatare shown in Figs. 3 and 4 is Joule heating generated by the highdensity electric current ﬁeld at the crack tip. When an electric current is applied across a crack, it ﬂows along the crack because of
the electrical resistance on the crack’s surface. Therefore, a highdensity electric current ﬁeld is formed at the crack tip. The area at
the tip and the vicinity of the crack is heated rapidly and expands
due to Joule heating because the high electric current is applied injust a few hundred microseconds. In contrast, the area outside of
the crack tip, where the high density electric ﬁeld is not applied,
remains intact. Because the heated area was constrained by thesurrounding unheated area, the thermal compressive stress wascaused. When the compressive stress exceeds the yield point, it isthought that the crack will then close. Moreover, part of the crack
will melt and heal when the material’s melting point is reached. It
is necessary to apply the appropriate electric current to close andheal a crack. When too high of a current ﬂow is applied, an open
hole forms around the vicinity of the crack tip, when too low of acurrent ﬂow is applied, the crack remains unchanged. In addition,
it is thought that the bridges were formed due to the discharge
caused by the high density electric current.
4.2. Change in the crack growth rate due to the electricalstimulation
The crack propagation rate decreased as shown in Fig. 8 in thisstudy. It is considered that crack closure, bridging between thecrack surfaces, annealing at the crack tip and blunting due to the
local melting at the crack tip have an inﬂuence on the fatigue crack
growth by the application of electric current. The crack closure
reduces the driving force for the crack propagation because of thedecrease of the crack opening displacement. The bridging between
the crack surfaces also makes the crack opening suppressive. These
results are in agreement with those of Ritchie’s studies [24,25]. It
is shown that the contact shielding through wedging and bridging
reduces driving force of fatigue crack growth. On the other hand,
the residual stress at the crack tip also has an inﬂuence of the crack
growth resistance. It is thought that the tensile residual stress may
once occur at the vicinity of the crack tip when the compressive
thermal stress applied at the crack tip exceeds the yield points andcloses the crack. The tensile residual stress has the effect to promote
the fatigue crack growth. However, it is reported that the annealingby heating can release the residual stress around the crack tip [26].
Therefore, it is thought that the tensile residual stress at the crack
tip was ﬁnally released by the local annealing effect. In addition,

since the blunting at the crack tip cannot be conﬁrmed by the SEM
observation, it is thought that the effect of the blunting is not a main
factor on the crack growth retardation in this study.
5. Conclusion
A technique to heal cracks by controlling a high density electric
current ﬁeld was studied. The closure of the crack and the formationof bridges between the crack surfaces were achieved by applying
a high density electric current ﬁeld at the crack tip. Moreover, by
the Paris law, it was observed that the crack growth rate decreased
just after applying the electric current. The results indicate that thistechnique has the potential to heal a fatigue crack.
Acknowledgement
This work was supported by the Japan Society for the Promotionof Science under Grant-in-Aid for Young Scientists (B) 21760072.
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