U.S. patent application number 11/337447 was filed with the patent office on 2007-02-15 for crack-propagation preventing structure, method for preventing crack propagation, crack-propagation preventing apparatus, and method for producing skin panel for aircraft.
Invention is credited to Toshihiko Nishimura.
Application Number | 20070033980 11/337447 |
Document ID | / |
Family ID | 37058933 |
Filed Date | 2007-02-15 |
United States Patent
Application |
20070033980 |
Kind Code |
A1 |
Nishimura; Toshihiko |
February 15, 2007 |
Crack-propagation preventing structure, method for preventing crack
propagation, crack-propagation preventing apparatus, and method for
producing skin panel for aircraft
Abstract
A crack-propagation preventing structure that can suppress or
prevent the propagation of cracks that would be generated in a
metal component or that have been generated in a metal component is
provided. The crack-propagation preventing structure includes a
point dimple formed by pressing a rigid sphere of a rigid body at
an appropriate position on the surface of a metal component, which
is subjected to crack-propagation prevention processing.
Inventors: |
Nishimura; Toshihiko;
(Aichi-ken, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
37058933 |
Appl. No.: |
11/337447 |
Filed: |
January 24, 2006 |
Current U.S.
Class: |
72/362 |
Current CPC
Class: |
B23P 6/04 20130101; B23P
9/00 20130101 |
Class at
Publication: |
072/362 |
International
Class: |
B21D 31/00 20060101
B21D031/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 9, 2005 |
JP |
2005-230586 |
Claims
1. A crack-propagation preventing structure comprising: a dimple
formed by pressing a rigid body at an appropriate position on the
surface of a metal component, which is subjected to
crack-propagation prevention processing.
2. The crack-propagation preventing structure according to claim 1,
wherein the dimple is disposed at at least one position in the form
of a point.
3. The crack-propagation preventing structure according to claim 1,
wherein the dimple is disposed in the form of a continuous
line.
4. A method for preventing crack propagation comprising forming a
dimple by pressing a rigid body at an appropriate position on the
surface of a metal component, which is subjected to
crack-propagation prevention processing.
5. The method for preventing crack propagation according to claim
4, wherein the dimple is formed in the form of a point or in the
form of a line.
6. The method for preventing crack propagation according to claim
4, wherein the dimple is formed near the tip of a crack generated
on the surface of the metal component.
7. The method for preventing crack propagation according to claim
6, wherein the dimple is formed on an extension of the crack or a
pair of the dimples are formed so as to sandwich the extension.
8. A crack-propagation preventing apparatus for forming a dimple by
pressing a rigid body at an appropriate position on the surface of
a metal component, which is subjected to crack-propagation
prevention processing, the apparatus comprising: an indenter part
serving as the rigid body that is pressed on the surface of the
metal component to apply a compressive load; a control part that
detects the magnitude of the compressive load and that adjusts the
compressive load; and a load-generating part for pressing the
indenter part on the surface of the metal component.
9. A method for producing a skin panel for an aircraft, comprising
forming a dimple by pressing a rigid body at an appropriate
position on the surface of an aircraft skin panel, which is
subjected to crack-propagation prevention processing.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a crack-propagation
preventing structure for suppressing or preventing the propagation
of cracks generated in metal components, a method for preventing
crack propagation, a crack-propagation preventing apparatus, and a
method for producing a skin panel for aircraft.
[0003] This application is based on Japanese Patent Application No.
2005-230586, the content of which is incorporated herein by
reference.
[0004] 2. Description of the Related Art
[0005] Hitherto, the following techniques are known for suppressing
or preventing the propagation of cracks generated in metal
components.
[0006] In a first technique, a drill hole that is called "a stop
hole" is formed at the leading tip of a crack generated in a metal
component. To form such a stop hole, a mandrel having a diameter
slightly larger than that of the stop hole may be inserted and
retracted, thereby performing cold working on the inner wall.
[0007] A second technique is called "burnishing". In this
technique, a steel ball is rolled on the surface of a metal
component while being lightly pressed with a machine tool. Thus,
any irregularities on the surface are planarized, and a compressive
residual stress is imparted to improve the fatigue strength (refer
to U.S. Pat. No. 5,826,453).
[0008] A third technique is called "a stress wave". A dimple is
formed on the surface of a metal component by strongly pressing an
indenter whose leading end has a flat or curved surface.
Subsequently, a hole having a diameter smaller than that of the
dimple is drilled at the center of the dimple. A compressive
residual stress is imparted by cold working, and thus the fatigue
strength of the periphery of the hole or a notch such as the bottom
surface of a gear is improved (refer to U.S. Pat. No. 6,389,865 B1
and U. S. Patent Application No. 2002/0148270 A1).
[0009] However, the above known techniques have the following
problems.
[0010] In the first stop hole technique, a new crack is generated
from the edge of the hole at an early stage and the length of a
crack increases immediately by an amount equal to the diameter of
the hole. Because of these problems, this technique does not
provide a significant advantage from the viewpoint of preventing
crack propagation. The cold working performed on the inner surface
of the hole is advantageous in suppressing or preventing crack
generation. However, the cold working does not sufficiently
suppress or prevent the propagation of cracks that have been
already generated. In addition, the cold working process is
complex.
[0011] The second burnishing technique is advantageous in
suppressing or preventing crack generation, but this technique also
does not prevent the propagation of cracks that have been already
generated. Also, this technique cannot be applied to the repair in
the field.
[0012] The third stress wave technique is also advantageous in
suppressing or preventing crack generation, but this technique is
not fundamentally aimed at preventing the propagation of cracks
that have been already generated. Furthermore, this technique can
be applied only to holes and notches, thus this cannot be applied
to other parts that do not include holes or notches.
[0013] As described above, the known techniques do not provide a
satisfactory effect to prevent crack propagation. Accordingly, for
example, in the fuselage of an aircraft, the condition of cracks
must be checked by periodic inspections. In other words, when crack
propagation is not satisfactorily prevented, the periodic
inspection must be performed more frequently, resulting in
increased maintenance costs. Therefore, it is desirable to develop
a technique that is effective in preventing crack propagation.
BRIEF SUMMARY OF THE INVENTION
[0014] The present invention has been made in view of the above
situation, and it is an object of the present invention to provide
a crack-propagation preventing structure that is capable of
suppressing or preventing the propagation of cracks that would be
generated in a metal component or that have been generated in a
metal component, a method for preventing crack propagation, a
crack-propagation preventing apparatus, and a method for producing
a skin panel for an aircraft.
[0015] In order to solve the above problems, the present invention
provides the following solutions.
[0016] A crack-propagation preventing structure of the present
invention includes a dimple formed by pressing a rigid body at an
appropriate position on the surface of a metal component, which is
subjected to crack-propagation prevention processing.
[0017] According to the crack-propagation preventing structure
described above, by forming a dimple at an appropriate position on
the surface of a metal component, plastic deformation is occurred
in the dimple and a compressive residual stress is induced in the
dimple and the vicinity area. Accordingly, when the dimple is
formed in the propagation path of a crack, the crack must pass
through the plastic deformation area, and is subjected to the
induced compressive residual stress. Thus, the crack propagation
can be suppressed or prevented by forming the dimple simply by
plastic deformation processing without increasing the weight. In
this case, preferred examples of the rigid body include a sphere
and a roller.
[0018] In the crack-propagation preventing structure, the dimple is
preferably disposed at at least one position in the form of a
point. In this case, the dimple is provided at an important
position so that the crack passes through a local plastic
deformation area and is subjected to an induced compressive
residual stress.
[0019] In the crack-propagation preventing structure, the dimple is
preferably disposed in the form of a continuous line. In this case,
the continuous linear dimple can form a plastic deformation area
and a compressive residual stress area that cover a wide area.
[0020] A method for preventing crack propagation according to the
present invention includes forming a dimple by pressing a rigid
body at an appropriate position on the surface of a metal
component, which is subjected to crack-propagation prevention
processing.
[0021] According to the method for preventing crack propagation, by
forming a dimple by pressing with a rigid body at an appropriate
position on the surface of a metal component, which is subjected to
crack-propagation prevention processing, a plastic deformation is
occurred in the dimple and a compressive residual stress is induced
in the dimple and the vicinity area. Accordingly, when the dimple
is formed in the propagation path of a crack, the crack must pass
through the plastic deformation area and is subjected to the
induced compressive residual stress. Thus, the crack propagation
can be suppressed or prevented without increasing the weight.
[0022] In the method for preventing crack propagation, the dimple
is preferably formed in the form of a point or in the form of a
line. In this case, a plastic deformation area and a compressive
residual stress area that are disposed locally or that cover a wide
area can be formed.
[0023] In the method for preventing crack propagation, the dimple
is preferably formed in the vicinity of crack tip generated on the
surface of the metal component. This is suitable for the case when
a crack is found during a periodic inspection or the like, and is
possible to minimize the number of dimples at important
positions.
[0024] In the method for preventing crack propagation, preferably,
the dimple is formed on an extension of the crack or a pair of the
dimples are formed so as to sandwich the extension of the crack. In
the former case, the crack passes through a plastic deformation
area, and is subjected to the compressive residual stress induced
by the dimple. In the later case, the crack passes through an
induced compressive residual stress area disposed between the
dimples.
[0025] According to a crack-propagation preventing apparatus of the
present invention, in a crack-propagation preventing apparatus for
forming a dimple by pressing a rigid body at an appropriate
position on the surface of a metal component, which is subjected to
crack-propagation prevention processing, the apparatus includes an
indenter part serving as the rigid body that is pressed on the
surface of the metal component to apply a compressive load, a
control part that detects the magnitude of the applied compressive
load and that adjusts the compressive load, and a load-generating
part for pressing the indenter part on the surface of the metal
component.
[0026] According to the crack-propagation preventing apparatus
described above, the apparatus includes the indenter part serving
as the rigid body that is pressed on the surface of the metal
component to apply a compressive load, the control part that
detects the magnitude of the applied compressive load and that
adjusts the compressive load, and the load-generating part for
pressing the indenter part on the surface of the metal component.
Accordingly, the load-generating part outputs a compressive load
that is adjusted by the control part and the indenter part is
pressed on the surface of the metal component with a desired
compressive force, thereby forming a dimple.
[0027] A method for producing a skin panel for an aircraft of the
present invention includes forming a dimple by pressing a rigid
body at an appropriate position on the surface of an aircraft skin
panel, which is subjected to crack-propagation prevention
processing.
[0028] According to the method for producing a skin panel for an
aircraft, by forming a dimple by pressing with a rigid body at an
appropriate position on the surface of an aircraft skin panel,
which is subjected to crack-propagation prevention processing, a
plastic deformation is occurred in the dimple and a compressive
residual stress is induced in the dimple and the vicinity area.
Accordingly, when the dimple is formed in the propagation path of a
crack, the crack must pass through the plastic deformation area and
is subjected to the induced compressive residual stress. Thus, the
crack propagation can be suppressed or prevented without increasing
the weight.
[0029] According to the present invention, by forming a dimple at
an appropriate position on the surface of a metal component, a
plastic deformation is occurred in the dimple and a compressive
residual stress is induced in the dimple and the vicinity area.
Therefore, when a dimple is provided near the propagation path of a
crack, the crack passes through the plastic deformation area or the
induced compressive residual stress area. In the plastic
deformation area, the material is dented and stretched material due
to dent induces the compressive residual stress, and decreases the
opening of the crack. In the compressive residual stress area, a
stress is applied in the direction that prevents crack propagation,
thus suppressing or preventing crack propagation. Therefore, crack
propagation can be suppressed or prevented by the dimple that is
formed simply by plastic deformation processing without increasing
the weight. Consequently, for example, when the present invention
is applied to a fuselage of an aircraft, the interval of periodic
inspections for cracks can be set longer than that used in the past
because the propagation of cracks can be prevented. Therefore, the
present invention can provide a significant advantage in reducing
maintenance costs.
[0030] When the present invention is applied to a newly produced
metal component or the like, in which the positions where cracks
are generated cannot be determined, dimples are disposed so as to
cover a wide area. In such a case, a plastic deformation area and a
compressive residual stress area are definitely present in the
propagation direction of a crack generated after commencing use,
and thus the propagation of undetermined crack in a newly produced
component can be prevented.
[0031] When the present invention is applied to an existing metal
component or the like, in which positions where cracks are
generated can be determined, dimples are disposed at appropriate
positions on the propagation path. Therefore, the propagation of
the cracks can be reliably prevented by forming the minimum number
of dimples.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0032] FIG. 1A is a plan view showing a crack-propagation
preventing structure and a method for preventing crack propagation
according to a first embodiment of the present invention.
[0033] FIG. 1B is a cross-sectional view of FIG. 1A.
[0034] FIG. 2A is a plan view showing a crack-propagation
preventing structure and a method for preventing crack propagation
according to a second embodiment of the present invention.
[0035] FIG. 2B is a cross-sectional view of FIG. 2A.
[0036] FIG. 3 is a perspective view showing apart of a body serving
as a structural example of an aircraft fuselage.
[0037] FIG. 4 is a plan view showing a crack-propagation preventing
structure and a method for preventing crack propagation according
to a third embodiment of the present invention.
[0038] FIG. 5 is a plan view showing a crack-propagation preventing
structure and a method for preventing crack propagation according
to a fourth embodiment of the present invention.
[0039] FIG. 6 is a plan view showing a crack-propagation preventing
structure and a method for preventing crack propagation according
to a fifth embodiment of the present invention.
[0040] FIG. 7 is a plan view showing a crack-propagation preventing
structure and a method for preventing crack propagation according
to a sixth embodiment of the present invention.
[0041] FIG. 8 is a graph showing the relationship between the
dimple diameter d and the load F applied to a rigid sphere.
[0042] FIG. 9A is a plan view showing an example in which a point
dimple is provided on the extension of a crack.
[0043] FIG. 9B is a cross-sectional view of FIG. 9A.
[0044] FIG. 10A is a plan view showing an example in which point
dimples are provided so as to sandwich the extension of a
crack.
[0045] FIG. 10B is a cross-sectional view of FIG. 10A.
[0046] FIG. 11 is a graph showing experimental results of the
comparison of propagation of a crack length a due to a repeated
stress when the dimple diameter is varied.
[0047] FIG. 12 is a graph showing experimental results of the
comparison of propagation of a crack length a due to a repeated
stress when point dimples are provided on the extension of a crack
and when point dimples are provided so as to sandwich the extension
of a crack.
[0048] FIG. 13 is a front view showing a crack-propagation
preventing apparatus according to an embodiment of the present
invention.
[0049] FIG. 14 is a front view showing a first modification of the
apparatus in FIG. 13.
[0050] FIG. 15A is a front view showing a second modification of
the apparatus in FIG. 13.
[0051] FIG. 15B is a side view of FIG. 15A.
[0052] FIG. 16A is a front view showing a third modification of the
apparatus in FIG. 13.
[0053] FIG. 16B is a top view of FIG. 16A.
[0054] FIG. 17A is a front view showing a fourth modification of
the apparatus in FIG. 13.
[0055] FIG. 17B is a top view of FIG. 17A.
DETAILED DESCRIPTION OF THE INVENTION
[0056] Embodiments of a crack-propagation preventing structure and
a method for preventing crack propagation according to the present
invention will now be described with reference to the drawings.
[0057] FIG. 3 shows a fuselage structure of an aircraft, which is
generally called "a skin-stringer structure", as an example of the
application of a crack-propagation preventing structure. The
example in the figure shows a body 10 of the aircraft. In the body
10, a skin panel 11 composed of an aluminum alloy is joined to the
outside of a skeleton component including frames 12 and stringers
13 with rivets 14. The skeleton component is formed by arranging
skeleton members, i.e., the frames 12 and the stringers 13,
composed of an aluminum alloy in a grid shape.
[0058] Cracks are generated in the body 10 having the above
structure by repeated load, etc. Periodic inspection is performed
at intervals that are determined so that the size of the cracks
does not exceed a predetermined value.
[0059] A first embodiment of a crack-propagation preventing
structure shown in FIGS. 1A and 1B suppresses or prevents the
propagation of a crack 2 generated in a metal component 1, such as
the above-described skin panel 11. In this structure, an
appropriate position on the surface of the metal component 1, which
is subjected to crack-propagation prevention processing, is pressed
under a load F with a rigid sphere 3 serving as a rigid body.
Thereby, a point dimple 4 is provided so that the surface of the
metal component 1 is dented substantially in the form of a
hemisphere. This point dimple 4, which is formed by a local plastic
deformation of the metal component 1, is appropriately provided
near the tip of the crack 2, since the crack usually linearly
propagates. FIG. 8 shows an example of the relationship between the
dimple diameter d and the load F applied to the rigid sphere 3. As
shown in FIG. 8, as the load F increases, the dimple diameter d due
to the plastic deformation also increases.
[0060] Regarding a preferred arrangement of the point dimple 4, for
example, as shown in FIGS. 9A and 9B, at least one point dimple 4
may be disposed on the extension of the propagation direction of
the crack 2. Alternatively, as shown in FIGS. 10A and 10B, a pair
of point dimples 4 may be disposed so as to sandwich the extension
of the crack 2. Although not shown in the figure, any combination
of one point dimple and a pair of point dimples 4 are possible, and
there is no restriction on the number of dimples.
[0061] According to this crack-propagation preventing structure,
the point dimples 4 are formed at appropriate positions on the
surface of the metal component 1; therefore, a plastic deformation
is occurred in the point dimples 4 and a compressive residual
stress is induced in the dimple and the vicinity area by
compression with the rigid sphere 3. Accordingly, when the point
dimples 4 are provided on the propagation path of the crack 2 or in
the vicinity of the leading tip of the crack 2, the crack 2 must
pass through the plastic deformation area or the compressive
residual stress area to propagate and increase in size. In the
plastic deformation area, the stretched material due to dent
induces a compressive residual stress and decreases the opening of
the crack 2. In addition, in a compressive residual stress area,
the crack is subjected to the negative stress which is opposite to
the tensile stress accelerating the propagation of the crack 2.
Consequently, the crack 2 does not easily propagate in these
areas.
[0062] Therefore, when the point dimples 4, which are formed simply
by plastic deformation processing without increasing the weight,
are provided at appropriate positions, a crack-propagation
preventing structure for suppressing or preventing the propagation
of the crack 2 can be provided. For example, by forming the minimum
number of point dimples 4 at appropriate positions in the vicinity
of the crack 2 that is found during a periodic inspection or the
like, the local plastic deformation area and the compressive
residual stress area are formed at important positions. Thus, a
crack-propagation preventing method that is effective in
suppressing or preventing the propagation of cracks is
provided.
[0063] In a second embodiment of a crack-propagation preventing
structure shown in FIGS. 2A and 2B, instead of the point dimple 4
formed using the rigid sphere 3, a continuous linear dimple 4A is
formed using a roller 3A as a rigid body. In this case, when the
roller 3A is pressed at an appropriate position in the vicinity of
a crack 2 and is moved in the direction shown by the arrow 5a6l ,
the linear dimple 4A is formed while the roller 3A is rotated as
shown by the arrow 5b. For example, as shown in FIG. 2A, the linear
dimple 4A is formed so as to intersect the extension of the
propagation direction of the crack 2.
[0064] In this crack-propagation preventing structure, the linear
dimple 4A, which is formed simply by plastic deformation processing
without increasing the weight, is provided on the extension of the
crack 2. Consequently, the plastic deformation is occurred in the
linear dimple, and the compressive residual stress is induced in
the dimple and the vicinity area, and thus the propagation of the
crack 2 can be suppressed or prevented. Therefore, for example, by
forming the linear dimple 4A on the extension of the crack 2 that
is found during a periodic inspection or the like, a
crack-propagation preventing method that is effective in
suppressing or preventing the propagation of cracks is provided in
which an area that intersects the linear dimple 4A is large.
[0065] The above crack-propagation preventing structures and the
methods for preventing crack propagation of the present invention
are effective not only in preventing crack propagation after the
crack 2 that has been generated in an existing body 10 is found but
also in preventing the propagation of a crack 2 that would be
generated in a newly produced body 10. In the case of such a newly
produced body 10, since the positions where cracks 2 are generated
or the propagation directions of the cracks 2 cannot be determined,
dimples that will be described below on the basis of FIGS. 4 to 7
should be formed.
[0066] According to a third embodiment shown in FIG. 4, in a newly
produced body 10 having a rivet structure, point dimples 4 are
formed in advance at appropriate positions of a skin panel 11. In
this case, on the skin panel 11, which is surrounded by frames. 12
and stringers 13, a large number of point dimples 4 are arranged
with a predetermined pitch in the directions parallel to the frames
12 and the stringers 13. In the example shown in the figure, the
point dimples 4 are linearly arranged from one end to the other end
so as to form two lines in each of the horizontal direction and the
vertical direction in a grid, but the arrangement is not limited
thereto.
[0067] By forming such point dimples 4 in advance, the propagation
of a crack 2 that is generated at an unpredictable position after
commencing use can be minimized.
[0068] Specifically, for example, when cracks 2a and 2c are
generated in the direction perpendicular to the frames 12 and the
cracks 2a and 2c propagate, since a large number of the point
dimples 4 are arranged on the extensions in the direction parallel
to the frames 12, when the cracks 2a and 2c reach a plastic
deformation area of the point dimples 4 or a compressive residual
stress area formed between the point dimples 4, the propagation of
the cracks 2a and 2c can be prevented.
[0069] On the other hand, when a crack 2b is generated in the
direction perpendicular to the stringers 13 and the crack 2b
propagates, since a large number of the point dimples 4 are
arranged on the extension in the direction parallel to the
stringers 13, when the crack 2b reaches a plastic deformation area
of the point dimples 4 or a compressive residual stress area formed
between the point dimples 4, the propagation of the crack 2b can be
prevented.
[0070] According to a fourth embodiment shown in FIG. 5, point
dimples 4 are formed in advance at appropriate positions of a skin
panel 11, as in the third embodiment. However, for example, as
shown in the right side of the figure, the point dimples 4 in this
case are arranged in the form of oblique lines connecting the
opposing corners of the grid formed by connecting the frames 12 and
the stringers 13. Alternatively, as shown in the left side of the
figure, on the skin panel 11 surrounded by the frames 12 and the
stringers 13, the point dimples 4 may be arranged in two or more
lines in directions substantially parallel to the diagonal lines of
the grid.
[0071] By forming such point dimples 4 in advance, the propagation
of a crack 2 that is generated at an unpredictable position after
commencing use can be minimized, as in the third embodiment.
[0072] Specifically, for example, when cracks 2a and 2c are
generated in the direction perpendicular to the frames 12 or when a
crack 2b is generated in the direction perpendicular to the
stringers 13, and the cracks 2a, 2b, and 2c propagate, since a
large number of the point dimples 4 are definitely present on the
extensions of those cracks, when the cracks 2a, 2b, and 2c reach a
plastic deformation area of the point dimples 4 or a compressive
residual stress area formed between the point dimples 4, the
propagation of any of the cracks 2a, 2b, and 2c can be
prevented.
[0073] A fifth embodiment in FIG. 6 shows an example in which the
present invention is applied to a newly produced body 10A formed by
joining with welded parts 15. In the body 10A, point dimples 4 are
formed in advance at appropriate positions on a skin panel 11, as
in the above third embodiment. In this case, on the skin panel 11
surrounded by frames 12 and stringers 13, a large number of point
dimples 4 are arranged with a predetermined pitch in the directions
parallel to the frames 12 and the stringers 13.
[0074] This structure can also minimize the propagation of a crack
2 that is generated at an unpredictable position after commencing
use, as in the third embodiment.
[0075] According to a sixth embodiment shown in FIG. 7, in a newly
produced body 10 having a rivet structure, linear dimples 4A are
formed in advance at appropriate positions of a skin panel 11. In
this case, on the skin panel 11 surrounded by frames 12 and
stringers 13, the linear dimples 4A are linearly formed in the
directions parallel to the frames 12 and the stringers 13. In the
example shown in the figure, the linear dimples 4A are arranged at
predetermined intervals from one end to the other end so as to form
two lines in each of the horizontal direction and the vertical
direction in a grid, but the arrangement is not limited
thereto.
[0076] By forming such linear dimples 4A in advance, the
propagation of a crack 2 that is generated at an unpredictable
position after commencing use can be minimized.
[0077] Specifically, for example, when cracks 2a and 2c are
generated in the direction perpendicular to the frames 12 and the
cracks 2a and 2c propagate, since the linear dimples 4A are
provided on the extensions in the direction parallel to the frames
12, when the cracks 2a and 2c reach a plastic deformation area or a
compressive residual stress area of the linear dimples 4A, the
propagation of the cracks 2a and 2c can be prevented.
[0078] On the other hand, when a crack 2b is generated in the
direction perpendicular to the stringers 13 and the crack 2b
propagates, since the linear dimple 4A is provided on the extension
of the crack 2b in the direction parallel to the stringers 13, when
the crack 2b reaches a plastic deformation area or a compressive
residual stress area of the linear dimple 4A, the propagation of
the crack 2b can be prevented.
[0079] The relationship between the diameter of the above-described
point dimple 4 (dimple diameter d) and the propagation of a crack
length a will now be described with reference to FIG. 11, which
shows experimental results for various dimple diameters d. As shown
in FIG. 8, the dimple diameter d can be adjusted by varying the
load F applied to the rigid sphere 3. In other words, when the load
F is applied to a metal component 1 composed of a predetermined
material (2024-T3 aluminum is used in the experiment) with the
rigid sphere 3 having a predetermined diameter (10 mm in the
experiment), as the value of the load F increases, the dimple
diameter d also increases.
[0080] According to the experimental results shown in FIG. 11, as
the dimple diameter d increases, the number of cycles required for
breaking the metal component 1 increases, and thus the propagation
of a crack 2 is prevented and the lifetime of the metal component 1
is extended. The reason for this is as follows: When a large load F
is applied in the formation of the point dimple 4, a large dent is
formed and a large stretched material induces the large compressive
residual stress, and decreases the opening of the crack. In FIG.
11, the breaking of the metal component 1 is defined as a point
where the curve showing the crack length a rises sharply in the
vertical direction.
[0081] Next, the arrangement of the above point dimples 4 will now
be described with reference to FIG. 12, which shows experimental
results on the relationship between the crack length a and the
number of cycles of the repeated load f. The figure shows a
comparison between the arrangement in FIGS. 9A and 9B, in which the
point dimples 4 are arranged on the extension of a crack 2, and the
arrangement in FIGS. 10A and 10B, in which the point dimples 4 are
arranged so as to sandwich the extension of a crack 2.
[0082] In the experiment shown in FIG. 12, the point dimple 4
having a dimple diameter d of 3 mm was used. The distance between
the point dimples 4 that sandwich the extension of the crack 2 was
4 mm. The result shows that the arrangement in which two point
dimples 4 are disposed so as to sandwich the extension of the crack
2 has a lifetime longer than that of the arrangement in which the
point dimple 4 is disposed on the extension of the crack 2. The
reason for this is that the propagation path of the crack 2
receives larger compression residual stress from both side dimples
of the path than that of the dimple on the crack path, because the
distance between the both side dimples 4 is sufficiently small.
[0083] Embodiments of a crack-propagation preventing apparatus for
forming the point dimple 4 or the linear dimple 4A on the surface
of a metal component 1 will now be described with reference to
FIGS. 13 to 17B.
[0084] A crack-propagation preventing apparatus 20 shown in FIG. 13
presses an appropriate position on the surface of a metal component
1, which is subjected to crack-propagation prevention processing,
with a rigid sphere 3 serving as a rigid body to form a point
dimple 4. The crack-propagation preventing apparatus 20 includes an
indenter part 23 serving as a rigid body, a load cell 24, and an
actuator 25. The rigid body is pressed onto the surface of the
metal component 1, which is disposed on a surface plate 6, to apply
a compressive load. The load cell 24 serves as a control part that
detects the magnitude of the compressive load to adjust the load.
The actuator 25 serves as a load-generating part for pressing the
indenter part 23 on the surface of the metal component 1. In this
case, the indenter part 23 includes a rigid sphere 3, a
rigid-sphere holding part 21 that prevents the rigid sphere 3 from
falling, and a holder 22. The holder 22 fixes and supports the
rigid-sphere holding part 21, and the upper end of the holder 22 is
connected to the load cell 24.
[0085] According to the crack-propagation preventing apparatus 20,
when the actuator 25 operates to output a compressive load F, the
compressive load F presses the rigid sphere 3 on the surface of the
metal component 1 via the load cell 24 and the indenter part 23. In
this step, the apparatus outputs to the actuator 25 a control
signal for adjusting the magnitude of the compressive load F
detected by the load cell 24 to a predetermined value. As a result,
the actuator 25 serving as the load-generating part outputs a
predetermined compressive load F that is adjusted by the control
part. The rigid sphere 3 provided on the indenter part 23 is then
pressed onto the surface of the metal component 1 with a desired
compressive force. Thus, the point dimple 4 having a desired
diameter d can be easily formed.
[0086] A crack-propagation preventing apparatus 30 shown in FIG.
14, which is a first modification, simultaneously presses
appropriate positions on the surface of a metal component 1
disposed on a surface plate 6 with a plurality of rigid spheres 3
to simultaneously form a plurality of point dimples 4 that are
arranged at predetermined intervals. The crack-propagation
preventing apparatus 30 includes an indenter part 33 serving as a
rigid body, a load cell 34, and an actuator 35. The rigid body is
pressed onto the surface of the metal component 1 to apply a
compressive load. The load cell 34 detects the magnitude of the
compressive load to adjust the load. The actuator 35 is used for
pressing the indenter part 33 on the surface of the metal component
1. In this case, the indenter part 33 includes a plurality of rigid
spheres 3, a rigid-sphere holding part 31 that prevents these rigid
spheres 3 from falling, and a holder 32. The holder 32 fixes and
supports the rigid-sphere holding part 31, and the upper end of the
holder 32 is connected to the load cell 34.
[0087] According to the crack-propagation preventing apparatus 30,
when the actuator 35 operates to output a compressive load F, the
compressive load F presses the plurality of rigid spheres 3 on the
surface of the metal component 1 via the load cell 34 and the
indenter part 33. As a result, the actuator 35 outputs a
predetermined compressive load F that is adjusted by the control
part. The plurality of rigid spheres 3 provided on the pressing
part 33 are then pressed onto the surface of the metal component 1
with a desired compressive force. Thus, the point dimples 4 having
a desired diameter d can be simultaneously formed at a
predetermined interval.
[0088] A crack-propagation preventing apparatus 40 shown in FIGS.
15A and 15B, which is the second modification, presses an
appropriate position on the surface of a metal component 1 with a
roller 3A to form a linear dimple 4A. The crack-propagation
preventing apparatus 40 includes an indenter part 41 serving as a
rigid body, a load cell 42, and an actuator 43. The rigid body is
pressed onto the surface of the metal component 1 to apply a
compressive load. The load cell 42 detects the magnitude of the
compressive load to adjust the load. The actuator 43 is used for
pressing the indenter part 41 on the surface of the metal component
1. In this case, the indenter part 41 allows free rotation of the
roller 3A and the upper end of the indenter part 41 is connected to
the load cell 42.
[0089] In the figure, the metal component 1 is disposed on a moving
stage 44. Roller members 45 are disposed between a surface plate 6
and the moving stage 44, and the metal component 1 moves together
with the moving stage 44.
[0090] According to the crack-propagation preventing apparatus 40,
when the actuator 43 operates to output a compressive load F, the
compressive load F presses the roller 3A on the surface of the
metal component 1 via the load cell 42 and the indenter part 41. As
a result, the actuator 43 outputs a predetermined compressive load
F that is adjusted by the control part. The roller 3A provided on
the indenter part 41 is then pressed onto the surface of the metal
component 1 with a desired compressive force. Thus, the linear
dimple 4A can be formed on the metal component 1, which moves on
the surface plate 6 together with the moving stage 44.
[0091] A crack-propagation preventing apparatus 50 shown in FIGS.
16A and 16B, which is a third modification, presses an appropriate
position on the surface of a metal component 1 with an indenter bar
53 having a substantially hemispherical indenter 52 at the leading
end to form a point dimple 4. This crack-propagation preventing
apparatus 50 is advantageous in that a crack 2 generated in an
existing body 10 can be easily repaired on site. The
crack-propagation preventing apparatus 50 includes only a support
block 51 disposed on the back-side of the metal component 1 and the
indenter bar 53. In this case, the dimensions of the protrusion of
the substantially hemispherical indenter 52 of the indenter bar 53
are determined by adding the depth of the point dimple 4 to a
margin corresponding to recovery of the plastic deformation.
[0092] According to the crack-propagation preventing apparatus 50,
the support block 51 is placed on the back-side of the metal
component 1 and the indenter bar 53 is then hit with a hammer or
the like. The resulting impact load F presses the indenter 52 via
the indenter bar 53 on the surface of the metal component 1. When
the indenter 52 reaches a predetermined depth, the entire surface
of the indenter bar 53 hits the surface of the metal component 1
and the indenter 52 is no longer pressed onto the surface of the
metal component 1. The diameter of the indenter bar 53 is set so as
to be sufficiently larger than the diameter of the indenter 52.
Thereby, the pressure applied on the surface of the metal component
1 is decreased according to the reciprocal of the area ratio.
Consequently, a desired point dimple 4 can be formed without
denting the surface of the metal component 1 with the indenter part
53.
[0093] In a crack-propagation preventing apparatus 60 shown in
FIGS. 17A and 17B, which is the fourth modification, a dimple is
formed by manually hitting a part of the apparatus with an impact
hammer or the like, as in the apparatus in FIGS. 16A and 16B. When
the indenter bar is gripped directly with the hand, a large impact
is applied to the hand. Therefore, in this apparatus, the indenter
bar is installed in a case 65 to reduce the impact. This
crack-propagation preventing apparatus 60 includes a support block
51 that is disposed on the back-side of a metal component 1, an
indenter bar 62, a holder 63, double nuts 64, and the case 65.
[0094] The size of the dimple is controlled by fixing the double
nuts 64 on the indenter bar 62 at a position that provides a
desired dimple depth. The double nuts 64 are used in order to vary
the dimple depth arbitrarily. Alternatively, when the depth is
fixed in use, the indenter bar 62 and the double nuts 64 may be
integrally formed.
[0095] When the depth of the point dimple 4 reaches a predetermined
value, the double nuts 64 and the holder 63 are hit, thus
dispersing the impact force F. Consequently, a point dimple 4
having a predetermined depth (size) can be formed without denting
the metal component 1. The outer diameter of the holder 63 is set
so as to be sufficiently larger than the diameter of the indenter
61. This structure prevents the surface of the metal component 1
from being dented by an excessive impact pressure.
[0096] In the examples shown in the figures, the leading end of the
indenter substantially has a hemispheric shape, but the shape is
not limited thereto.
[0097] As described above, according to the present invention, by
forming point or linear dimples at appropriate positions on the
surface of a metal component 1, plastic deformation is occurred in
the dimple, and compressive residual stress is induced in the
dimples and the vicinity area. When the dimple is disposed on the
propagation path of a crack 2, the crack 2 must pass through the
plastic deformation area and is subjected to the induced
compressive residual stress. In the plastic deformation area, the
metal is dented and stretched material induces the compressive
residual stress. The compressive residual stress is applied to the
metal in the direction that prevents the propagation, and decreases
the opening of the crack 2, thus the propagation of the crack 2 can
be suppressed or prevented.
[0098] As a result, the propagation of the crack 2 can be
suppressed or prevented by forming a point dimple 4 or a linear
dimple 4A that is formed simply by plastic deformation processing
without increasing the weight. For example, when the present
invention is applied to a fuselage of an aircraft, such as a skin
panel for an aircraft, the interval of periodic inspections for
cracks can be set longer than that used in the past because the
propagation of cracks 2 can be prevented.
[0099] In addition, when the present invention is applied to a
newly produced metal component 1, in which positions where cracks 2
are generated cannot be determined, an arrangement of point dimples
4 or linear dimples 4A is provided so as to cover a wide area. In
such a case, a plastic deformation area and a compressive residual
stress area are definitely present in the propagation direction of
a crack 2 generated after commencing use, and thus the propagation
of the crack 2 can be prevented.
[0100] When the present invention is applied to an existing metal
component 1, in which positions where cracks 2 are generated can be
determined, a local plastic deformation formed by point dimples 4
or linear dimples 4A disposed at appropriate positions induces a
compressive residual stress on the crack propagation path.
Therefore, the propagation of the cracks 2 can be reliably
prevented by forming a small number of dimples.
[0101] The point dimples 4 and the linear dimples 4A may be used in
combination depending on the conditions, such as the positions of
the dimples to be formed.
[0102] The present invention is not limited to the above
embodiments and may be appropriately modified without departing
from the spirit and the scope thereof. For example, the application
of the present invention is not limited to the fuselage structure
of an aircraft.
* * * * *