US 2006O163332A1(19) United States(12) Patent Application Publication (10) Pub. No.: US 2006/0163332 A1(43) Pub. Date:

Takahashi et al.

(54) METHOD OF SUPPRESSING EXTENSION
OF FATIGUE CRACK, METHOD IF
DETECTING FATIOUE CRACK AND PASTE
USED FOR THE METHODS(76) Inventors: Ichihiko Takahashi, Mitaka-shi (JP);Chiori Takahashi, Mitaka-shi (JP)Correspondence Address:

Jul. 27, 2006

Publication Classification(51) Int. Cl.
B23K 3L/00(2006.01)(52) U.S. Cl. ....................... 228/248.1; 228/119; 403/265(57)

ABSTRACTFRISHAUF, HOLTZ, GOODMAN & CHICK,PC
220 Fifth Avenue
16TH Floor

NEW YORK, NY 10001-7708 (US)(21) Appl. No.:

10/563,412(22) PCT Filed:

Jul. 1, 2004

(86). PCT No.:

PCT/UP04/09312(30)

Foreign Application Priority Data
Jul. 7, 2003

(JP)...................................... 2003-193051- 2 FINE GRAN PASTE(7 MICRO CRACK\-SURFACE

W 1 BASE MATERIAL

A method of detecting fatigue crack by which the fatigue
crack can be easily detected by visual observation even in a
relatively early stage, and a method of restraining propaga
tion thereof. The detection method includes the steps of: (a)preparing a paste in which particles having hardness not lessthan that of a base material and an oil having viscosity are
mixed with each other; (b) applying the paste to a desired
portion of the base material; and (c) detecting fatigue crack
based on a change in color generated by movement of basematerial powder to a Surface of the paste, and the basematerial powder is produced when the particles grind thebase material due to opening and closing of the fatigue crack
in the base material.

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Patent Application Publication Jul. 27, 2006 Sheet 1 of 8

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Patent Application Publication Jul. 27, 2006 Sheet 3 of 8

US 2006/0163332 A1

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Patent Application Publication Jul. 27, 2006 Sheet 5 of 8

FIG.5A

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Patent Application Publication Jul. 27, 2006 Sheet 6 of 8

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Patent Application Publication Jul. 27, 2006 Sheet 7 of 8

FIG. 7A

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Patent Application Publication Jul. 27, 2006 Sheet 8 of 8

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FIG.8A

SURFACE

FIG.8B

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FIG.8C

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FIG.8D

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US 2006/0163332 A1

METHOD OF SUPPRESSING EXTENSION OF

FATIGUE CRACK, METHOD IF DETECTING
FATIOUE CRACK AND PASTE USED FOR THE
METHODSTECHNICAL FIELD

0001. The present invention relates to a method of
restraining growth of fatigue crack initiating in a Surface of
metal or the like, and a method of detecting the fatigue
crack. Furthermore, the present invention relates to a paste
to be used for those methods.
BACKGROUND ART

0002. A fatigue phenomenon of metal is conventionallyknown that stresses are repeatedly applied to metal andmicro crack (fatigue crack) initiates, even if the stresses aremuch lower than ultimate tensile strength. By further apply
ing stresses repeatedly to the portion where the fatigue crack
has initiated, the fatigue crack grows and leads to failure ofmetal. On the other hand, in a Society at these days, metal isused everywhere. Accordingly, restraint of fatigue crack
growth and early detection of fatigue crack are very important issues.

0003 FIGS. 8A-8D are diagrams for explanation of
initiation and growth of fatigue crack. As shown in FIG. 8A,
micro crack initiates in a metal Surface due to accumulationof fatigue damage by repeated stresses. Further, as shown in
FIG. 8B, the crack opens by application of tensile load, and,as shown in FIG. 8C, the crack closes by removal of thetensile load. Furthermore, as shown in FIG. 8D, the crack

opens by reapplication of tensile load. Thus, the crack
propagates by repeating the opening and closing of thecrack. Here, there is a problem that it is difficult to visuallydetect crack because the crack closes when no tensile load

is applied as shown in FIGS. 8A and 8C.
0004 As a related technology, Japanese Patent Application Publication JP-A-8-294.10 (pp. 1-3 and FIG. 1) dis
closes a crack detection method of nondestructively detect
ing crack, which occurs randomly with respect to portions
and orientations, at a high speed with high sensitivity. The
method is usable in production lines for Solid objects having
black opaque appearance, rough surfaces and various
shapes.
0005 According to the crack detection method, the presence of crack can be easily detected by impregnating thecrack with a volatile solvent Such as acetone and benzene,

leaving the sample with a dried Surface in a sealed containerat rest, and detecting the concentration of the volatile solventthat has vaporized and mixed in a carrier gas such as helium
or nitrogen flowing along the Surface of the sample.
0006. However, in the crack detection method, even if thepresence of crack has been detected, the portion where thecrack of the sample is present cannot be detected. Further,since the sealed container is required, the detection isdifficult with respect to a large-scaled sample Such as a shipor aircraft. Therefore, in order to detect the portion where thecrack of the sample is present, it is necessary to employ Suchtechnology as an ultrasonic flaw detection method, eddy
current flaw detection method, magnetic flaw detection
method or staining flaw detection method as in the conventional detection.

Jul. 27, 2006

0007 Further, Japanese Patent Application Publication
JP-A-56-12552 (pp. 1-2 and FIG. 1) discloses a crack
detection method, by which even an inexperienced person
can easily determine the presence or absence of flaw, without requiring a person having experience like the staining
flaw detection method. After the application of the liquid anddetection, it does not require application and removal ofstaining penetrating agent and developing agent like thestaining flaw detection method, but requires only waiting fornatural evaporation of a liquid.
0008 According to the crack detection method, the presence of crack can be detected by impregnating a liquid Suchas water or ethyl alcohol in the crack, Scanning the Surfaceof the object with dried surfaces by using a temperature
sensor or an alcohol sensor, and detecting the concentrationof the vapor of the liquid remaining within the crack.
0009. However, what is sensed by the sensor is the vapor
in a range extremely near the sensor, and therefore, it isnecessary to scan the entire object along the object Surface
in the crack detection method. Consequently, the detection
takes much scanning time for a large-scaled object Such asa ship or aircraft.
DISCLOSURE OF THE INVENTION

0010. Accordingly, in view of the above-mentioned problems, a first object of the present invention is to provide amethod of detecting fatigue crack by which fatigue crack
can be easily detected by visual observation even in a
relatively early stage. Further, a second object of the present
invention is to provide a method of restraining fatigue crack
growth. Furthermore, a third object of the present invention
is to provide a paste to be used for those methods.
0011. In order to solve the above-mentioned problems, amethod of restraining fatigue crack growth according to one
aspect of the present invention includes the steps of: (a)preparing a paste in which particles having hardness not lessthan that of a base material and an oil having viscosity are
mixed with each other, and (b) applying the paste to adesired portion of the base material.
0012 Further, a method of detecting fatigue crack
according to one aspect of the present invention includes thesteps of: (a) preparing a paste in which particles having
hardness not less than that of the base material and an oil

having Viscosity are mixed with each other; (b) applying thepaste to a desired portion of the base material; and (c)detecting fatigue crack based on a change in color generated
by movement of base material powder to a surface of thepaste, and the base material powder is produced when theparticles grind the base material due to opening and closingof the fatigue crack in the base material.
0013 Furthermore, a paste according to the present
invention is a paste to be applied to a desired portion of abase material for restraining fatigue crack growth in the basematerial or detecting fatigue crack in the base material, andthe paste includes: particles having diameters of 2 um to 40
um; and an oil having viscosity of 5,000 cp (centipoises) to
15,000 cp; wherein the particles and the oil are mixed witheach other.

0014. According to the present invention, a method of
detecting fatigue crack by which fatigue crack can be easily
detected by visual observation even in a relatively earlyUS 2006/0163332 A1stage can be provided. Further, a method of restraining
fatigue crack growth and a paste to be used for thosemethods can be provided.
BEST MODE FOR CARRYING OUT THE
INVENTION

0015. Hereinafter, preferred embodiments of the present
invention will be described in detail by referring to thedrawings. First, a method of restraining fatigue crack growthaccording to one embodiment of the present invention will
be described. FIGS. 1A-1D are diagrams for explanation of
the method of restraining fatigue crack growth according to
one embodiment of the present invention.
0016 First, a fine grain paste in a paste form is created bymixing fine particles of metal, ceramics or the like into an oil
having appropriate viscosity. As shown in FIG. 1A, the finegrain paste 2 is applied to a Surface of at least one portion
where initiation and propagation of fatigue crack have beenpreviously expected in a base material 1 of metal, reinforced
plastic or the like.
0017. Here, as the fine particles, a material having hard
ness equal to or larger than that of the base material isdesirably used. For example, in the case where the basematerial is a metal having Vickers hardness of about 200 Hv,
a material having Vickers hardness equal to or more than
200Hv is used as the fine particles. The diameters of the fineparticles are desirably made uniform in a range from about
2 um to about 40 um (an average is about 15 um) by using
sieves.

0018 Further, as the oil into which the fine particles are
mixed, an oil having viscosity of about 5,000 cp (centi
poises) to about 15,000 cB is used. Here, 1000 cF=10P 20=1
Pas (pascal second). Especially, an oil having viscosity thatvaries little due to temperature change such as silicone
grease is desirably used.
0019. As shown in FIG. 1A, micro crack initiates in a
metal Surface due to accumulation of fatigue damage by
repeated stresses. Further, as shown in FIG. 1B, the crack
opens and grows by application of tensile load. The finegrain paste 2 enters the open crack because of the pumping
action accompanied by opening and closing of the crack andthe capillary phenomenon at the crack tip.
0020. Here, as shown in FIG. 1C, even when the tensile
load is removed, the fine grain paste 2 remains within thecrack as a wedge and prevents the closing of the crack.
Accordingly, as shown in FIG. 1D, when the tensile load isapplied again, the displacement of the crack Surface isrestrained, and thereby, the growth rate of the crack due toopening and closing of the crack can be reduced and thefatigue life can be extended.
0021 Next, an effect of crack growth restraint by the finegrain paste 2 will be described based on results of a fatigue
test. FIG. 2 shows a shape of a test piece used as a basematerial to which a fine grain paste is applied in the fatigue
test. As the base material, a test piece of aluminum-magne
sium alloy JIS A5083P-O (manufactured by Nippon Light
Metal Co., Ltd.) having a plate thickness of 5 mm has beenused. As shown in FIG. 2, a hole having a diameter of 2 mm
has been formed at the central part of the test piece and an
artificial notch having a length of 10 mm and a width of 0.3

Jul. 27, 2006

mm has been formed by electric discharge machining Suchthat the artificial notch passes through the hole.
0022. The fatigue test has been conducted by using anelectro-hydraulic servo system fatigue testing machine hav
ing a dynamic capacity of 98 kN under complete pulsatingload control with a load ratio R=0. The load waveform has

been set to a sine wave, and a personal computer with a
32-bit CPU has been used for generating control signals andsampling load data in which the sampling frequency has
been set to 200 HZ.

0023. As the fine particles, iron magnetic particles formagnetic particle flaw detection (dry and gray, manufac
tured by Taiyo products co., Ltd.) and two types of alumina
particles having different particle size distributions (manufactured by Powlex co., Ltd.) have been used. These fineparticles have been mixed into oils to form pastes in acondition in which they do not drip, and applied to the notch
portions and the expected crack propagation paths of the test
pieces, respectively.
0024 FIG. 3 shows relationships between nominal stressrange AO, in portions having no notch and failure life N in
results of the fatigue test. Here, the symbol O shows a test
result in the case where nothing is applied, and the symbol

O shows a test result in the case where only the oil isapplied. The symbol O shows a test result in the case wherea magnetic particle paste using magnetic particles (Fe) for
magnetic particle flaw detection is applied.
0025) Further, the symbol A shows a test result in the casewhere an alumina paste using alumina particles (Al2O)
having an average diameter of 47.3 um is applied, and thesymbol V shows a test result in the case where an alumina
paste using alumina particles (Al2O) having an average
diameter of 15.2 um is applied.
0026. As shown in FIG. 3, the failure life in the casewhere only the oil is applied is nearly equal to that in thecase where nothing is applied. In contrast, the failure life in
the case where the magnetic particle paste is applied is about
180,000 to 250,000 cycles longer than that in the case wherenothing is applied. Therefore, it is conceivable that thewedge effect by the oil itself is negligible and the failure life
extends because of the wedge effect by magnetic particles.
0027) Further, the failure life in the case where thealumina paste using alumina particles having an average
diameter of 47.3 um is applied is nearly equal to that in thecase where nothing is applied. The reason is conceivable asfollows. In the case where the diameter is large, the Suspen
sibility with oil is poor and alumina particles are not
Supplied to the crack tip.
0028. In contrast, the failure life in the case where thealumina paste using alumina particles having an average
diameter of 15.2 um is applied is about 400,000 to 700,000
cycles longer than that in the case where nothing is applied.
In this case, the most remarkable effect of crack growth
restraint is obtained among the test results.
0029 Next, a method of detecting fatigue crack accord
ing to one embodiment of the present invention will be
described. FIGS. 4A and 4B are diagrams for explanation of
the method of detecting fatigue crack according to the one
embodiment of the present invention. The method of detect
ing fatigue crack according to the embodiment can beUS 2006/0163332 A1

performed in conjunction with the above-mentioned methodof restraining fatigue crack growth.
0030. In the method of detecting fatigue crack according
to the embodiment, a fine grain paste in a light-colored paste
form is created by mixing light-colored fine particles likeceramics of alumina, Zirconia or the like into an oil. Other

conditions with respect to the fine particles and conditions
with respect to the oil are the same as those in the above
mentioned method of restraining fatigue crack growth. In
the embodiment, steel is used as the base material and whiteceramics is used as the fine particles. In this case, thehardness of the fine particles is larger than that of the basematerial.

0031. As described by referring to FIGS. 1A-1D, since
the fine grain paste 2 remaining within the crack forms awedge, the crack growth due to opening and closing of thecrack can be restrained. In the opening and closing of thecrack, as shown in FIG. 4A, when the tensile load isremoved, the crack Surface tends to close and a large
compressive force is generated between the wedge of fineparticles and the crack surface. Therefore, the fine particles
having relatively larger hardness than that of the basematerial apply high pressure to the crack Surface and par
tially grind the crack surface to pulverize the base materialof the crack surface. Further, the fine particles are also
pulverized by the compressive force into finer grains.
0032. Furthermore, as shown in FIG. 4B, the base material powder is mixed with the oil of the fine grain paste 2 andthe finer grains because of the pumping action accompaniedby the opening and closing of the crack, and exudes to theSurface of the fine grain paste 2. Since a metal powder
generally takes on a black color, the fine grain paste 3 inwhich the base material powder is mixed takes on a blackcolor. Here, since the fine grain paste 2 having no basematerial powder mixed therewith is white, a change in color
appears due to the contrast between white and black, and thelocation, length, etc. of the crack can be easily and visually
recognized.
0033 Next, test results of the method of detecting fatigue
crack according to the embodiment will be described. FIGS.
5A and 5B show a detection state and a propagation state of
fatigue crack in high tension steel JIS SM490A having ashape as shown in FIG. 2. Here, FIG. 5A shows a state after
tensile loading is repeated at 576,000 cycles, and FIG. 5B
shows a state after tensile loading is repeated at 672,000
cycles. The vertical direction in FIGS. 5A and 5B is theloading direction.
0034. As shown in FIG. 5A, since a black color repre
senting a cracking portion develops on the white fine grain
paste, the location, length, etc. of the crack can be easily andvisually recognized. Further, the propagation state of thecrack can be easily and visually recognized such that theblack part in FIG. 5B extends longer than that in FIG. 5A.
0035 FIGS. 6A and 6B show a shape of a test piece used
for another test of the method of detecting fatigue crack.
Here, FIG. 6A is a plan view of the test piece and FIG. 6B
is a side view of the test piece. As shown in FIGS. 6A and
6B, the test piece is a welded joint test piece formed bywelding two rib plates with one main plate and the material
thereof is TMCP (thermo-mechanical control process) steel
K32A.

Jul. 27, 2006

0.036 FIGS. 7A and 7B show a detection state of crack
in the welded joint test piece as shown in FIGS. 6A and 6B.
Here, FIG. 7A shows a state before a fine grain paste isapplied, and FIG. 7B shows a state after tensile loading isrepeated at 1,200,000 cycles in the vertical direction in
FIGS. 7A and 7B.

0037. It is easy to imagine that fatigue crack will initiate
in the welded portion as shown in FIG. 7A. Accordingly, thefine grain paste has been previously applied to the welded
portion where initiation of fatigue crack is expected. Then,as shown in FIG. 7B, a black color develops on the whitefine grain paste after tensile loading is repeated at 1,200,000
cycles, and thereby, the initiation and growth of crack can beeasily and visually recognized.
0038 According to the embodiment, fatigue crack can beeasily recognized by visual test even in a relatively earlystage, and the contribution to improvements in Safety ofvarious machines, structures, etc. can be made. Further,since the fine grain paste forms a wedge, the crack growth
can be automatically restrained and the fatigue lives ofvarious machines, structures, etc. can be extended. Further

more, the method only requires to previously apply the finegrain paste to the portions, where initiation and propagationof fatigue crack is expected, for the detection and restraintof fatigue crack, it can be far easier and cheaper to perform
compared with nondestructive detection methods such as anultrasonic flaw detection method, penetration flaw detection
method and magnetic flaw detection method.
INDUSTRIAL APPLICABILITY

0039 The present invention can be utilized in a methodof restraining fatigue crack growth in a Surface of metal or
the like, and a method of detecting the fatigue crack.
BRIEF DESCRIPTION OF THE DRAWINGS

0040 FIGS. 1A-1D are diagrams for explanation of amethod of restraining fatigue crack growth according to one
embodiment of the present invention;
0041 FIG. 2 is a diagram showing a shape of a test piece
used as a base material to which a fine grain paste is appliedin a fatigue test;
0042 FIG. 3 is a diagram showing relationships between
nominal stress range AO, and failure life Nf in test results ofthe fatigue test;
0043 FIGS. 4A and 4B are diagrams for explanation ofa method of detecting fatigue crack according to one
embodiment of the present invention;
0044 FIGS. 5A and 5B are diagrams showing a detec
tion state and a propagation state of fatigue crack in a test
piece of a high tension steel having a shape as shown in FIG.
2:0045 FIGS. 6A and 6B are diagrams showing a shape ofa welded joint test piece used for another test of the methodof detecting fatigue crack;
0046 FIGS. 7A and 7B are diagrams showing a detec
tion state of fatigue crack in a welded portion of the welded
joint test piece as shown in FIGS. 6A and 6B; and0047 FIGS. 8A-8D are diagrams for explanation of
initiation and propagation of fatigue crack.

US 2006/0163332 A1

1. A method of restraining fatigue crack growth in a basematerial, said method comprising the steps of(a) preparing a paste in which particles having hardness
not less than that of the base material and an oil having
Viscosity are mixed with each other, and(b) applying said paste to a desired portion of said basematerial.

2. A method of restraining fatigue crack growth according
to claim 1, wherein said particles have diameters of 2 um to
40 Lum.
3. A method of restraining fatigue crack growth according
to claim 1, wherein step (a) includes the steps of(a1) adjusting the oil having viscosity of 5,000 centipoises
to 15,000 centipoises; and(a2) mixing said particles into the oil adjusted at step (al).
4. A method of restraining fatigue crack growth according
to claim 2, wherein step (a) includes the steps of(a1) adjusting the oil having viscosity of 5,000 centipoises
to 15,000 centipoises; and(a2) mixing said particles into the oil adjusted at step (al).
5. A method of detecting fatigue crack in a base material,
said method comprising the steps of:(a) preparing a paste in which particles having hardness
not less than that of the base material and an oil having
viscosity are mixed with each other;(b) applying said paste to a desired portion of said basematerial; and(c) detecting fatigue crack based on a change in color
generated by movement of base material powder to aJul. 27, 2006

Surface of said paste, said base material powder being
produced when said particles grind the base material
due to opening and closing of the fatigue crack in saidbase material.

6. A method of detecting fatigue crack according to claim
5, wherein said particles include white ceramics.
7. A method of detecting fatigue crack according to claim
5, wherein step (a) includes the steps of:(a1) adjusting the oil having viscosity of 5,000 centipoises
to 15,000 centipoises; and(a2) mixing said particles into the oil adjusted at Step (al).
8. A method of detecting fatigue crack according to claim
6, wherein step (a) includes the steps of:(a1) adjusting the oil having viscosity of 5,000 centipoises
to 15,000 centipoises; and(a2) mixing said particles into the oil adjusted at Step (al).
9. A paste to be applied to a desired portion of a basematerial for at least one of restraining fatigue crack growthin said base material and detecting fatigue crack in said basematerial, said paste comprising:particles having diameters of 2 um to 40 um; andan oil having viscosity of 5,000 centipoises to 15,000
centipoises;
wherein said particles and said oil are mixed with eachother.

10. A paste according to claim 9, wherein said particles
include white ceramics.