Metal deforming – By tool-couple embodying nonplanar tool-face
Reexamination Certificate
2002-03-18
2004-03-02
Crane, Daniel C. (Department: 3725)
Metal deforming
By tool-couple embodying nonplanar tool-face
C072S377000, C072S334000
Reexamination Certificate
active
06698268
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus and a method for cold working a material and, more particularly, to an apparatus and a method for cold working a material by separately applying various forces to the material in a single application.
In aerospace structures, materials such as structural members including spars, stringers, ribs and an outer skin, e.g., aluminum sheeting or planar material, may be joined together by the use of fasteners. For example, a stringer having an engaging or faying surface may be juxtaposed with a corresponding faying surface of a planar material. Thereafter, holes or apertures may be drilled into the materials and fasteners, such as rivets, inserted into the apertures to form a fastened joint.
The fastened joints are potentially subject to failure from fatigue by the formation of fissures or cracks in the materials after substantial use. In airplanes, for example, cyclical pressurization and depressurization during each flight causes various stress cycles on the fuselage skin. These stress cycles likely contribute to a formation of cracks, which typically start at the apertures in the material where fasteners join the airplane skin to the skeleton or rib structure of the airplane. This is because, e.g., varying loads caused by pressurization changes that are carried by the skin must be routed around the periphery of the apertures.
During assembly of the skin of an airplane, such as along the fuselage and along the wing structure, it is well known to first cold work the apertures prior to assembly with a fastener in order to create a residual compressive stress about the periphery of the aperture. The residual compressive stress functions to counteract a loading that generally includes, in the case of fuselage pressurization changes, tensile strain variations. In this way, the useful life of the fuselage skin is greatly enhanced.
Current cold working methods require completion of a substantial number of steps. For example, a prior art method for cold working a material (which may include a joint) is diagrammatically shown at
10
in FIG.
1
. In a first drill step
11
, a drill
12
is used to create an aperture
14
in a material
16
to be cold worked. In a subsequent inspect step
18
, an inspection device
20
may be employed to inspect the aperture
14
to determine if the diameter is within tolerance. Thereafter, in a cold work step
22
, a mandrel
24
having a diameter that is about 3% larger than that of the aperture
14
is forced through the aperture
14
from one side of the material
16
. Accordingly, the aperture
14
is thereby expanded which compresses the material surrounding the aperture
14
so as to create a residual stress field in the material
16
surrounding the aperture
14
. In a ream step
28
, the aperture
14
may be bored to a proper size for receiving a fastener (not shown) using a ream
30
. In a counter sink step
32
, the aperture
14
is counter sunk with a bit
34
to recess a head of a fastener (not shown). Thereafter, in a cleaning step
36
, a solvent may be used by a mechanic
40
to remove lubricating oil from the cold work step, prior to another inspection step
42
using the inspection device
20
.
Recently, it has been proposed that the material located, e.g., at a joint, be compressed or coined, prior to creating an aperture, so as to create a residual stress field which extends within a material to be fastened. One example of such a two step method is shown in
FIG. 2
a
where in a coining step
43
, a pair of indentors
44
and
46
are urged under a force in the direction of arrows
50
,
52
into contact with a material
48
.
As shown in greater detail in
FIG. 2
b
, the indentors
44
and
46
each include a blunt end
54
,
56
and a shoulder
58
,
60
in fixed relationship. The blunt ends
54
,
56
function to create a residual stress field, represented by arrows
62
, which extends deep within the material
48
to a joint
64
. The shoulders
58
,
60
function to create a larger diametrical residual stress field, represented by arrows
66
, but which extends generally closely to surfaces
68
,
70
of the material
48
. The use of shoulders
58
,
60
is to overcome the tensile stress created at the part surface by the blunt ends
54
,
56
. The depth and configuration of the residual stress field is dependent on, e.g., the force applied to the indentors
44
,
46
and a length between the blunt ends
54
,
56
and the shoulders
58
,
60
. Generally, it is desired that the residual stress field created by the shoulders
58
,
60
remain near the surface of the material while the stress field created by the blunt ends
54
,
56
extend to the joint
64
.
Thereafter and referring again to
FIG. 2
a
, in a drill and counter sink step
72
, an aperture
74
is created by a drill and a counter sink bit
76
.
While suitable for its intended purpose, a problem arises in that the apparatus and the method illustrated in
FIGS. 2
a
and
2
b
requires employing a different indentor with a different configuration depending on the desired depth and configurations of the residual stress fields. This is particularly cumbersome in the manufacture of airplanes as there are numerous joints to be fastened, most of which, vary in depth from the skin.
Accordingly, there is a need for an improved apparatus and method for cold working that does not require the changing of the indentors depending on depth of the joint.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, a passive-adaptive indentor is provided for cold working a material that includes a material surface and a depth extending in a direction that is perpendicular to a direction of the material surface. The passive-adaptive indentor comprises a first member that includes a first working end that is configured to create a first residual stress field that extends to a predetermined depth in the material. The indentor also comprises a second member that includes a second working end that is configured to create a second residual stress field that extends to a generally fixed second depth of the material regardless of the depth to which the first residual stress field extends.
In another aspect of the invention, a passive-adaptive indentor is provided for cold working a material that includes a material surface and a depth extending in a direction that is perpendicular to a direction of the material surface. The passive-adaptive indentor comprises a first member that has a first working end that is configured to create a first residual stress field and which extends to a predetermined depth in the material. The first member is subjected to a first force whereby the first working end strikes the material surface at a force sufficient to create the first residual stress field. A second member is disposed in slidable relationship with the first member and comprises a second working end. The second member is configured to create a second residual stress field that extends to a generally fixed second depth of the material regardless of the depth at which the first residual stress field extends. The second member also is subjected to a second force whereby the second working end strikes the material surface at a force sufficient to create the second residual stress field.
In a further aspect of the invention, a passive-adaptive indentor is provided for cold working a material that includes an aerospace structure. The aerospace structure includes a planar material having a planar material faying surface, a support material having a support material faying surface and a joint located at a contact portion of the planar material faying surface and the support material faying surface. The planar material includes a planar material surface and a depth extending in a direction that is perpendicular to a direction of the planar material surface. The passive-adaptive indentor comprises a first member that is configured to create a first residual stress field in the material that extends to the joint. The first me
Wilkerson Jeffrey A.
Woods Mark A.
Crane Daniel C.
Shimokaji Fritz LLP
The Boeing Company
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