US007963 015B2(12) United States Patent(10) Patent No.:

Takahashi et al.

US 7.963,015 B2(45) Date of Patent:(54) METHOD OF SUPPRESSING EXTENSION OF
FATIGUE CRACK, METHOD OF DETECTING

Jun. 21, 2011(52) U.S. Cl. ..................................... 29/402.18; 428/404(58) Field of Classification Search ............... 29/402.18,

FATGUE CRACK AND PASTE USED FOR

29/402.01, 460; 228/248.1, 118; 403/265;

THE METHODS

428/404

See application file for complete search history.
(75) Inventors: Ichihiko Takahashi, Mitaka (JP);Chiori Takahashi, Mitaka (JP)(56)

(73) Assignee: National Maritime Research Institute,Tokyo (JP)(*) Notice:

References Cited
FOREIGN PATENT DOCUMENTS

JP

75OO5599* 3, 1975

JP

56-0.12552 A

2, 1981

Subject to any disclaimer, the term of thispatent is extended or adjusted under 35

JP
JP
JP

62O65485* 3,3, 1993
1987
5-57532 A
5-1 19032. A
5, 1993

U.S.C. 154(b) by 1269 days.

JP

8-0294.10 A

2, 1996JP

11-123617 A

5, 1999(21) Appl. No.:

10/563,412* cited by examiner(22) PCT Filed:(86). PCT No.:S371 (c)(1),(2), (4) Date:

Jul. 1, 2004
PCT/UP2004/OO9312

Primary Examiner — John C Hong(74) Attorney, Agent, or Firm — Holtz, Holtz, Goodman &Chick, PC

Jan. 3, 2006

(57)
ABSTRACTA 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 base material powder is produced when the particles grind the basematerial due to opening and closing of the fatigue crack in the(87) PCT Pub. No.: WO2005/002782
PCT Pub. Date: Jan. 13, 2005
O O
(65)
Prior Publication Data
US 2006/O163332A1
Jul. 27, 2006(30)

Foreign Application Priority Data
Jul. 7, 2003

(JP) ................................. 2003-193051(51) Int. Cl.

base material.

B22D 9/10(2006.01)

B32B5/16(2006.01)- 2 FINE GRAN PASTE\suRFACE

MCRO CRACK

6 Claims, 8 Drawing Sheets~2
TENSE LOAD

U.S. PatentJun. 21, 2011

Sheet 1 of 8

FIGIA

US 7.963,015 B2-- 2 FINE GRAN PASTE(7 MCRO CRACK\-SURFACEV

FIG. IBBASE MATERIALTENSE LOADTENSILE LOAD
1/

U.S. PatentJun. 21, 2011

Sheet 2 of 8

US 7.963,015 B2

ASHLOB(NIQV£EOHTMS0IN)W

087

U.S. PatentJun. 21, 2011

Sheet 3 of 8

US 7.963,015 B2

L

H

CMO

<C

Cl

O

2.
C

s
O O)

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<s Dse

s
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O

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O

(ed W) "O73)NWSSELS WNIWONS.

U.S. PatentJun. 21, 2011

6SSYS

Sheet 4 of 8

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U.S. PatentJun. 21, 2011

Sheet 5 of 8

FIG.5A

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U.S. Patent

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U.S. PatentJun. 21, 2011

Sheet 7 of 8

FIG 7A

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U.S. PatentJun. 21, 2011

Sheet 8 of 8

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FIG.8A- SURFACE

2.
3.
2
3.3

MICRO CRACK

FIG.8B

FIG.8C

FIG.8D2.

TENSILE LOAD

US 7,963,015 B2
1.

2
determine the presence or absence of flaw, without requiringa person having experience like the staining flaw detection
FATIGUE CRACK, METHOD OF DETECTING
FATGUE CRACK AND PASTE USED FOR
method. After the application of the liquid and detection, it
THE METHODS
does not require application and removal of staining penetrat
ing agent and developing agent like the staining flaw detec
This application is a U.S. National Phase Application tion method, but requires only waiting for natural evaporation
under 35 USC 371 of International Application PCT/JP2004/ of a liquid.
According to the crack detection method, the presence ofO09312 filed Jul. 1, 2004.
crack can be detected by impregnating a liquid Such as water
10 or ethyl alcohol in the crack, Scanning the Surface of the object
TECHNICAL FIELD
with dried Surfaces by using a temperature sensor or an alco
The present invention relates to a method of restraining hol sensor, and detecting the concentration of the vapor of thegrowth of fatigue crack initiating in a Surface of metal or the liquid remaining within the crack.
like, and a method of detecting the fatigue crack. Further
However, what is sensed by the sensor is the vapor in a
more, the present invention relates to a paste to be used for 15 range extremely near the sensor, and therefore, it is necessary
those methods.
to Scan the entire object along the object Surface in the crack
detection method. Consequently, the detection takes muchBACKGROUND ART
scanning time for a large-scaled object such as a ship or
aircraft.
A fatigue phenomenon of metal is conventionally known
DISCLOSURE OF THE INVENTION
that stresses are repeatedly applied to metal and micro crack(fatigue crack) initiates, even if the stresses are much lower
than ultimate tensile strength. By further applying stresses
Accordingly, in view of the above-mentioned problems, a
repeatedly to the portion where the fatigue crack has initiated, first object of the present invention is to provide a method of
the fatigue crack grows and leads to failure of metal. On the 25 detecting fatigue crack by which fatigue crack can be easily
other hand, in a Society at these days, metal is used every detected by visual observation even in a relatively early stage.
where. Accordingly, restraint of fatigue crack growth and Further, a second object of the present invention is to provide
early detection of fatigue crack are very important issues.
a method of restraining fatigue crack growth. Furthermore, aFIGS. 8A-8D are diagrams for explanation of initiation and third object of the present invention is to provide a paste to be
growth of fatigue crack. As shown in FIG. 8A, micro crack 30 used for those methods.
initiates in a metal Surface due to accumulation of fatigue
In order to solve the above-mentioned problems, a methoddamage by repeated stresses. Further, as shown in FIG. 8B, of restraining fatigue crack growth according to one aspect of
the crack opens by application of tensile load, and, as shown the present invention includes the steps of: (a) preparing a
in FIG. 8C, the crack closes by removal of the tensile load. paste in which particles having hardness not less than that of
Furthermore, as shown in FIG.8D, the crack opens by reap 35 a base material and an oil having viscosity are mixed withplication oftensile load. Thus, the crack propagates by repeat each other, and (b) applying the paste to a desired portion of
ing the opening and closing of the crack. Here, there is a the base material.
problem that it is difficult to visually detect crack because the
Further, a method of detecting fatigue crack according tocrack closes when no tensile load is applied as shown in FIGS. one aspect of the present invention includes the steps of: (a)8A and 8C.
40 preparing a paste in which particles having hardness not less
As a related technology, Japanese Patent Application Pub than that of the base material and an oil having viscosity arelication JP-A-8-294.10 (pp. 1-3 and FIG. 1) discloses a crack mixed with each other; (b) applying the paste to a desired
detection method of nondestructively detecting crack, which portion of the base material; and (c) detecting fatigue crack
occurs randomly with respect to portions and orientations, at based on a change in color generated by movement of basea high speed with high sensitivity. The method is usable in 45 material powder to a Surface of the paste, and the base mateproduction lines for Solid objects having black opaque rial powder is produced when the particles grind the baseappearance, rough Surfaces and various shapes.
material due to opening and closing of the fatigue crack in the
According to the crack detection method, the presence of base material.
crack can be easily detected by impregnating the crack with a
Furthermore, a paste according to the present invention is a
Volatile solvent such as acetone and benzene, leaving the 50 paste to be applied to a desired portion of a base material forsample with a dried Surface in a sealed container at rest, and restraining fatigue crack growth in the base material or detect
detecting the concentration of the volatile solvent that has ing fatigue crack in the base material, and the paste includes:
vaporized and mixed in a carrier gas such as helium or nitro particles having diameters of 2 um to 40 um; and an oil having
gen flowing along the Surface of the sample.
viscosity of 5,000 cB (centipoises) to 15,000 cp; wherein the
However, in the crack detection method, even if the pres 55 particles and the oil are mixed with each other.
ence of crack has been detected, the portion where the crack
According to the present invention, a method of detectingof the sample is present cannot be detected. Further, since the fatigue crack by which fatigue crack can be easily detected bysealed container is required, the detection is difficult with visual observation even in a relatively early stage can berespect to a large-scaled sample such as a ship or aircraft. provided. Further, a method of restraining fatigue crack
Therefore, in order to detect the portion where the crack of the 60 growth and a paste to be used for those methods can be
sample is present, it is necessary to employ such technology provided.
as an ultrasonic flaw detection method, eddy-current flaw
BEST MODE FOR CARRYING OUT THE
detection method, magnetic flaw detection method or staining
METHOD OF SUPPRESSING EXTENSION OF

flaw detection method as in the conventional detection.

INVENTION

Further, Japanese Patent Application Publication.JP-A-56 65
Hereinafter, preferred embodiments of the present inven
12552 (pp. 1-2 and FIG. 1) discloses a crack detection
method, by which even an inexperienced person can easily tion will be described in detail by referring to the drawings.

US 7,963,015 B2
43First, a method of restraining fatigue crack growth according
FIG. 3 shows relationships between nominal stress range
to one embodiment of the present invention will be described. AO, in portions having no notch and failure life Nin results of
FIGS. 1A-1D are diagrams for explanation of the method of the fatigue test. Here, the symbol O shows a test result in the
restraining fatigue crack growth according to one embodi case where nothing is applied, and the symbol (9 shows a test
result in the case where only the oil is applied. The symbol O
ment of the present invention.
First, a fine grain paste in a paste form is created by mixing shows a test result in the case where a magnetic particle paste
fine particles of metal, ceramics or the like into an oil having using magnetic particles (Fe) for magnetic particle flaw
appropriate viscosity. As shown in FIG. 1A, the fine grain detection is applied.
paste 2 is applied to a surface of at least one portion where
Further, the symbol A shows a test result in the case where
initiation and propagation of fatigue crack have been previ 10 an alumina paste using alumina particles (Al2O) having an
ously expected in a base material 1 of metal, reinforced plastic average diameter of 47.3 um is applied, and the symbol Vor the like.
shows a test result in the case where an alumina paste using
Here, as the fine particles, a material having hardness equal alumina particles (Al2O) having an average diameter of 15.2
to or larger than that of the base material is desirably used. For um is applied.
example, in the case where the base material is a metal having 15 As shown in FIG. 3, the failure life in the case where only
Vickers hardness of about 200 Hv, a material having Vickers the oil is applied is nearly equal to that in the case where
hardness equal to or more than 200 Hv is used as the fine nothing is applied. In contrast, the failure life in the caseparticles. The diameters of the fine particles are desirably where the magnetic particle paste is applied is about 180,000
made uniform in a range from about 2 um to about 40 um (an to 250,000 cycles longer than that in the case where nothing
average is about 15um) by using sieves.
is applied. Therefore, it is conceivable that the wedge effectFurther, as the oil into which the fine particles are mixed, an by the oil itself is negligible and the failure life extends
oil having viscosity of about 5,000 cp (centipoises) to about because of the wedge effect by magnetic particles.
15,000 cB is used. Here, 1000 cF=10P 20=1 Pa's (pascal
Further, the failure life in the case where the alumina paste
second). Especially, an oil having viscosity that varies little using alumina particles having an average diameter of 47.3
due to temperature change Such as silicone grease is desirably 25 um is applied is nearly equal to that in the case where nothingused.
is applied. The reason is conceivable as follows. In the casewhere the diameter is large, the suspensibility with oil is poor
As shown in FIG. 1A, micro crack initiates in a metal
Surface due to accumulation of fatigue damage by repeated and alumina particles are not supplied to the crack tip.
In contrast, the failure life in the case where the alumina
stresses. Further, as shown in FIG. 1B, the crack opens andgrows by application of tensile load. The fine grain paste 2 30 paste using alumina particles having an average diameter of
15.2 um is applied is about 400,000 to 700,000 cycles longer
enters the open crack because of the pumping action accom
panied by opening and closing of the crack and the capillary than that in the case where nothing is applied. In this case, thephenomenon at the crack tip.
most remarkable effect of crack growth restraint is obtained
among the test results.
Here, as shown in FIG. 1C, even when the tensile load isremoved, the fine grain paste 2 remains within the crack as a 35 Next, a method of detecting fatigue crack according to one
wedge and prevents the closing of the crack. Accordingly, as embodiment of the present invention will be described. FIGS.
shown in FIG. 1D, when the tensile load is applied again, the 4A and 4B are diagrams for explanation of the method of
displacement of the crack Surface is restrained, and thereby, detecting fatigue crack according to the one embodiment of
the growth rate of the crack due to opening and closing of the the present invention. The method of detecting fatigue crack
crack can be reduced and the fatigue life can be extended. 40 according to the embodiment can be performed in conjunc
Next, an effect of crack growth restraint by the fine grain tion with the above-mentioned method of restraining fatigue
paste 2 will be described based on results of a fatigue test. crack growth.
In the method of detecting fatigue crack according to the
FIG. 2 shows a shape of a test piece used as a base material towhich a fine grain paste is applied in the fatigue test. As the embodiment, a fine grain paste in a light-colored paste form isbase material, a test piece of aluminum-magnesium alloy JIS 45 created by mixing light-colored fine particles like ceramics ofA5083P-O (manufactured by Nippon Light Metal Co., Ltd.) alumina, Zirconia or the like into an oil. Other conditions withhaving a plate thickness of 5 mm has been used. As shown in respect to the fine particles and conditions with respect to the
FIG. 2, a hole having a diameter of 2 mm has been formed at oil are the same as those in the above-mentioned method of
the central part of the test piece and an artificial notch having restraining fatigue crack growth. In the embodiment, steel isa length of 10 mm and a width of 0.3 mm has been formed by 50 used as the base material and white ceramics is used as theelectric discharge machining Such that the artificial notch fine particles. In this case, the hardness of the fine particles ispasses through the hole.
larger than that of the base material.
The fatigue test has been conducted by using an electro
As described by referring to FIGS. 1A-1D, since the fine
hydraulic servo system fatigue testing machine having a grain paste 2 remaining within the crack forms a wedge, thedynamic capacity of 98 kN under complete pulsating load 55 crack growth due to opening and closing of the crack can becontrol with a load ratio R=0. The load waveform has been set
restrained. In the opening and closing of the crack, as shown
to a sine wave, and a personal computer with a 32-bit CPU has in FIG. 4A, when the tensile load is removed, the crack
been used for generating control signals and sampling load Surface tends to close and a large compressive force is gen
data in which the sampling frequency has been set to 200 Hz. erated between the wedge of fine particles and the crack
As the fine particles, iron magnetic particles for magnetic 60 surface. Therefore, the fine particles having relatively larger
particle flaw detection (dry and gray, manufactured by Taiyo hardness than that of the base material apply high pressure to
products co., Ltd.) and two types of alumina particles having the crack Surface and partially grind the crack Surface todifferent particle size distributions (manufactured by Powlex pulverize the base material of the crack surface. Further, theco., Ltd.) have been used. These fine particles have been fine particles are also pulverized by the compressive force
mixed into oils to form pastes in a condition in which they do 65 into finer grains.
not drip, and applied to the notch portions and the expected
Furthermore, as shown in FIG. 4B, the base material pow
crack propagation paths of the test pieces, respectively.
der is mixed with the oil of the fine grain paste 2 and the finerUS 7,963,015 B2
5
6
grains because of the pumping action accompanied by the
FIG. 2 is a diagram showing a shape of a test piece used as
opening and closing of the crack, and exudes to the Surface of a base material to which a fine grain paste is applied in a
the fine grain paste 2. Since a metal powder generally takes on fatigue test;
a black color, the fine grain paste 3 in which the base material
FIG. 3 is a diagram showing relationships between nomi
powder is mixed takes on a black color. Here, since the fine nal stress range AO, and failure life N, in test results of thegrain paste 2 having no basematerial powder mixed therewith fatigue test;
is white, a change in colorappears due to the contrast between
FIGS. 4A and 4B are diagrams for explanation of a method
white and black, and the location, length, etc. of the crack can of detecting fatigue crack according to one embodiment of thebe easily and visually recognized.
invention;
Next, test results of the method of detecting fatigue crack 10 present
FIGS.
5A and 5B are diagrams showing a detection stateaccording to the embodiment will be described. FIGS.5A and and a propagation
state offatigue crackina test piece of a high
5B show a detection state and a propagation state of fatigue tension steel having
a shape as shown in FIG. 2;
crack in high tension steel JIS SM490A having a shape as
FIGS. 6A and 6B are diagrams showing a shape of ashown in FIG. 2. Here, FIG. 5A shows a state after tensileloading is repeated at 576,000 cycles, and FIG. 5B shows a 15 welded joint test piece used for another test of the method ofstate after tensile loading is repeated at 672,000 cycles. The detecting fatigue crack;
FIGS. 7A and 7B are diagrams showing a detection state ofvertical direction in FIGS.5A and 5B is the loading direction.
As shown in FIG. 5A, since a black color representing a fatigue crack in a welded portion of the welded joint test piececracking portion develops on the white fine grain paste, the as shown in FIGS. 6A and 6B; andFIGS. 8A-8D are diagrams for explanation of initiation andlocation, length, etc. of the crack can be easily and visually
recognized. Further, the propagation State of the crack can be propagation of fatigue crack.
easily and visually recognized such that the black part in FIG.
The invention claimed is:
5B extends longer than that in FIG. 5A.
1. A method of detecting fatigue crack in a base material,
FIGS. 6A and 6B show a shape of a test piece used forsaid method comprising: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 25 (a) preparing a paste in which particles having a hardness
not less than a hardness of the base material and an oilview of the test piece. As shown in FIGS. 6A and 6B, the test
piece is a welded joint test piece formed by welding two rib
having viscosity are mixed with each other;plates with one main plate and the material thereof is TMCP(b) applying said paste to a desired portion of said base(thermo-mechanical control process) steel K32A.
material; andFIGS. 7A and 7B show a detection state of crack in the 30(c) detecting fatigue crack based on a change in color
welded joint test piece as shown in FIGS. 6A and 6B. Here,generated by movement of base material powder to a
FIG. 7A shows a state before a fine grain paste is applied, andSurface of said paste, said base material powder being
FIG. 7B shows a state after tensile loading is repeated at
produced
when said base material at a Surface of the
1,200,000 cycles in the vertical direction in FIGS. 7A and 7B.
fatiguecrack
is ground in contact with said particles due
It is easy to imagine that fatigue crack will initiate in thetoopeningandclosing of the fatigue crack in said base35
welded portion as shown in FIG. 7A. Accordingly, the finematerial.
grain paste has been previously applied to the welded portion
A method of detecting fatigue crack according to claim
where initiation of fatigue crack is expected. Then, as shown 1, 2.whereinsaid particles comprise light-colored ceramics
in FIG. 7B, a black color develops on the white fine grain
paste after tensile loading is repeated at 1,200,000 cycles, and including white ceramics.
thereby, the initiation and growth of crack can be easily and 40 3. A method of detecting fatigue crack according to claim
1, wherein preparing the paste comprises:
visually recognized.
adjusting the oil to have a viscosity of 5,000 centipoises to
According to the embodiment, fatigue crack can be easily
15,000 centipoises; andrecognized by visual test even in a relatively early stage, andmixing said particles into the adjusted oil.
the contribution to improvements in safety of various
machines, structures, etc. can be made. Further, since the fine 45 4. A method of detecting fatigue crack according to claim
grain paste forms a wedge, the crack growth can be automati 2, wherein preparing the paste comprises:cally restrained and the fatigue lives of various machines,
adjusting the oil to have a viscosity of 5,000 centipoises tostructures, etc. can be extended. Furthermore, the method
15,000 centipoises; andonly requires to previously apply the fine grain paste to themixing said particles into the adjusted oil.
portions, where initiation and propagation of fatigue crack is 50 5. A paste to be applied to a desired portion of a baseexpected, for the detection and restraint of fatigue crack, it material for at least one of restraining fatigue crack growth incan be far easier and cheaper to perform compared with said base material and detecting fatigue crack in said basenondestructive detection methods such as an ultrasonic flaw
said paste comprising:detection method, penetration flaw detection method and material,
particles having diameters of 2 um to 40 um, said particles
magnetic flaw detection method.
comprising light-colored ceramics including white55
ceramics and having Vickers hardness of not less than
INDUSTRIAL APPLICABILITY
200 Hv, andThe present invention can be utilized in a method of
restraining fatigue crack growth in a Surface of metal or the
like, and a method of detecting the fatigue crack.
60

an oil having a viscosity of 5,000 centipoises to 15,000
centipoises;
wherein said particles and said oil are mixed with each

BRIEF DESCRIPTION OF THE DRAWINGS

6. A paste according to claim 5, wherein said oil comprises
silicone grease.

FIGS. 1A-1D are diagrams for explanation of a method of
restraining fatigue crack growth according to one embodi
ment of the present invention;other.