Note: Descriptions are shown in the official language in which they were submitted.
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HOLE MANUFACTURE
The invention is in relation to a method of improving the fatigue life of
holes and areas which are to be cut out and in particular holes and cut-out
portions in structures which are subjected to fatigue loading for example
5 aerospace and aircraft applications. The present invention also provides an
apparatus for improving the fatigue life of holes and cut out areas.
In the production of aircraft, in order to fasten metal sheets together, holes
are made to house the fastening means, such as rivets or bolts. However, holes
are a source of localised weakness in the metal sheet or plate which can lead to10 failure of the particular sheet or plate due to the formation and propagation of
cracks from the hole.
It is generally known that the fatigue lifè of a sheet or plate having a hole
therethrough can be increased by generating compressive stresses within the
sheet about the hole. There have been numerous ways of creating comprassive
15 stresses around a hole, one of the most common being cold-working or cold-
expanding the hole using a split sleeve. In this method a hole is formed first
which is large enough to receive a split tubular sleeve, but which is undersizedin respect of the rivet or bolt. The split tubular sleeve which can be axially or
helically split, is placed into the hole. An expansion section of the cold-working
20 mandrel is drawn through the inside of the split tubular sleeve thus expanding
the tubular slesve and also the hole. After the expansion section of the mandrelhas passed through the pre-split tubular sleeve, the tubular sleeva may be
removed. The expanded tubular sleeve cannot be reused and is discarded after
forming one hole. Due to the high manufacturing tolerances required for tha
25 production of such slesves, their unit cost is high? and in cases where a large
number is required (as in the case of a typical aircraft), the process becomes
extremely costly to implement. Costs aside, there are further physical problems
with the cold-working method. Whilst the cold-working method provides the
desirable compressive stress around the hole, the compressive stress is not
30 evenîy distributed because of the splR in the sleeve. The presence of the split in
the sleeve also creates a shàrp discontinuity on the side of the hole which itself
can become a source of stress concentration. To overcome this problem, it is
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generally recommended that a suitable reaming process be applied to the hole
following cold expansion. This additional procedure further adds to the
production costs of such holes.
Another disadvantage of this method is that in using a tapered mandrel, a
5 residual stress distribution which is non-symmetrical about the mid-plane of the
material is created thus inducing a bending moment which, particularly in the
case ot thin sheets (generally less than 2.3 mm), causes local buckling of the
material. Furthermore, the lack of out-of-plane constraints on the material during
the expansion process often leaves a raised region at the periphery of the hole.Another form of treatment commonly used on holes in aircraft manufacture
is ring pad or stress coining. In the case of ring pad coining, a thin groove isformed which is spaced from and around an existing aperture in the structural
member. U.S. 3,110,086 to Austin Phillips granted 12 November, 1963
discloses a ring pad coining method. The groove is formed by stamping, thereby
15 cold-working the material and creating a residual compressive stress around the
exterior of the hole. Although this method can enhance the fatigue life of
fast~ner holes, several disadvantages are immediately apparent when using this
process. For instance, as the grooved depression is spaced some distance from
the hole, a relatively large amount of deformation is required to achieve a
20 desirable level of residual stress. Further, for many structural members, thepresence of an annular depressed ring around the hole is aesthetically, if not
structurally, undesirable.
Stress coining is similar to ring pad coining, however an entire region
around the hole is stamped or compressed. U.S. 3,434,327 to E. R. Speakman
25 granted March 25, 1969 discloses a stress coining process. Stress coining
relieves to a greater extent the localised stress around the hole; however, withthis solution other problems arise. Generally what happens is that the
impressions formed around the holes can become a stress raiser themsel~es,
especially when the holes are fitted with rivets, bolts or the like. That is, the
30 impressbns reduce the bearing surface and thus raise the bearing stress for
fastener holes. Furthermore, local indentations around the hole can create
problems br members beihg fastened over them, particularly tor thin composite
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plates; wherein the indentations can cause severe local buckling and
delaminations of the plates.
Also when the stress/ring pad coining method is used to treat holes great
care must be taken so that there is not excessive indentation of the area around5 the hole, otherwise this may result in closure of the hole or in the extreme
punching out of the hole which leaves an unusable oversized hole. Excessive
indentation can also lead to shear band cracking in the indented area.
Stress coining as with other prior art methods improves the fatigue
resistance of the structural members but at the expense of other desirable and
10 necessarycharacteristics.
The object of the present invention is to increase the fatigue life of the
structural members containing holes and cut out portions but without some of
the disadvantages of the prior art.
The present invention provides a method of improving the fatigue
15 resistance of holes and cut out portions formed in material sheets for use instructure subjected to fatigue loading wherein at least one area on a sheet
where a hole is to be formed or a portion is to be cut out is compressed, most
preferably forming an indentation, on at least one side and, preferably both
sides, of the sheet prior to the formation of the hole or the portion being cut out.
20 Thus the compressive stress is created in the area surrounding the hole or cut
out portion prior to the formation of the hole or the portion being cut out.
Advantageously, the mathod of the invention provides significantly
improved fatigue life of holes in components. Preliminary tests illustrate at least
a ten-fold increase in fatigue life of open holes. Furthermore, the method of the
25 invention can bs effectively used in r~latively thin shee~s since the undesirable
effects cf the prior art methods such as sheet buckling or peripheral
defo!mations are significantly reduced. Similarly, by the same underlying
physical principle, the fatigue life of cut out portions in which the stress critical
locations have b~en treated by the invention will improve.
Generally the method of the invention would be useful in any application
where the fatigue resistance of the structure and containing a hole or holes is to
be improved.
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The method of the invention has general application in the manufacture of
aircraft and aerospace equipment, where the fatigue resistance of the highly
stressed structures and components thereof including the joining holes and
stress critical locations, is of primary importance. Additionally, the method
5 greatly enhances the fatigue resistance of a structure containing a hole or holes
without additional weight which is most desirable in the aerospace and aircraft
industries. It is also envisaged that the process will be useful in other
applications such as in the manufacture of ships and other high performance
(and highly stressed) vehicles and also in the manufacture of highly stressed
10 process vessels and containers which have to withstand high pressures.
Additionally, bearing in mind the method of the present invention can be
used to enhance the fatigue resistance in thin sheets containing a hole or holes,
the method can be used in any high performance and lightweight applications,
for example, sports equipment such as tennis and squash racquet frames and
15 motorsportstructures.
The method of the invention can be applied to any material which exhibits
an elasto-plastic stress-strain relationship, that is most metals and alloys forexample, those of aluminium and steel.
Preferably, the area on the sheet where the hole is to be formed is
20 compressed from both sides by mandrels, thus most preferably forming
indentations on both sides of the sheet. The mandrels rnay be of any cross-
sectional shape, i.e. square, hexagonal, round etc. and as such, the resulting
hole formed may equally be of any shape. The mandrels, apart from being flat or
chamfered, may also be shaped in order to provide a countersink for the various
25 types of fasteners.
ARer the area has been compressed and most preferably an indentatio
or indentations have resulted from the comprassion, the hols can be ~ormed by
any conventional method including drilling. However, the inventors have
discovered that instead of drilling, one of the mandrels can be removed and the
30 other can then be forced through to shear away the material and form the hole.
Thus providing a simpler, faster method of producing a hole with improved
fatigue life, by decreasing the steps required to form the hole.
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In th~ case of cut out portions, once the stress critical locations have been
compressed and most preferably indentations have been f~rmed by the
compression, the portion is then cut out as normal.
The invention can be further understood with reference to the tollowing
5 illustrations ot a preterred embodiment ot the method of the invention.
Figure 1: A sheet which requires a hole such that it can be tastened to
another structure or sheet.
Figure 2: The same sheet as in Figure 1 with optional clamps applied
to the sheet.
Figure 3: The mandrels in action around the sheet.
Figure 4: Optional step wherein one of the mandrels torms the hole.
Figure 5: The completed hole.
Figure 6: A pretreated (dimpled) sheet with mandrels being aligned
for pre-hole compression.
Figure 7: A pretreated (pilot hole) sheet with mandrels being aligned
tor pre-hole compression.
Figure 8: An oversized hole compared to the indentations I mandr~l
size.
Figure 9: An undersized hole compared to the indentations / mandrel
20 siza.
Figure 10: Indented area to be cut out.
Figure 11: Cut out area.
Figure 12: A graph of the first set of comparative results between
drilling, prior art cold working and the present invention.
25Figure 13: A graph of another set of comparative results between
drilling, prior art cold working and the pres~nt invention.
Figure 14: An illustration of the specimen used in the comparative
examples of Figure 13.
Figure 15: A graph showing fatigue results similar to Figure 13; with
30 the additional comparison of no-hole.
Figure t6: A graph comparing one-sided indentation compared to two-
; sWed indentation prior to hole formation.~
,
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Figur~17: Illustrates a front view and side view of a specimen wherein
three layers of material with holes made by drilling, the prior art cold working or
the invention, are riveted together.
Figure 18: A graph of a set of comparative results wherein the holes are
5 produced by drilling, prior art cold working or the present invention then riveted
together.
With reference to Figures 1 to 5, sheet 10 which requires a hole 20 to be
forrned therein is clamped by clamps 11 in the vicinity of where the hole is to be
made. The holes 13 provided in clamps 11, outline the portion of the sheet 10,
10 where the hole 20 is to be formed. In Figure 3, mandrels 12 which are guided by
clamps 11 commence their compression and indentation on sheet tO. The top
mandrel acting downwardly and the bottom mandrel acting upwardly in the
direction ot the arrows. It has been discovered that the degree of fatigue life
improvement generally increases with an increase in indentation depth. Whilst
15 this could also be true for the prior art methods, excessive indentation for the
coining techniques, as previousty stated, can lead to hole closure or even
complete punch-out.
The mandrels 12 may then be removed and the hole 20 is formed by
conventional methods such as drilling and the like. Alternatively, as is illustrated
20 in Figure 4, only one of the mandrels 12 is removed and the other remaining
mandrel stamps out the reduced section 14 to form the hole 20.
Although the use of the restraining means or clamps 11 is not an essential
part of the process, the use thereof furlher enhances the fatigue life by virtue of
restricting any out-of-plane deformations thereby creating a greater region of
25 residual stress than otherwise possible. In addition, this restraining procedure
results in the structure containing the hole or holes being relatively free from the
distortions which are an undesirable by-product of the prior art cold expansion
methods.
Alternatively, where only pre-hole indentation is required on one side, two
30 sheets (not shown) can be processed simultaneously, wherein as is illustrated in
Figures t-5, inst~ad of item 10 being one sheet, item 10 is two separate sheets
(not shown). Mandrels 12 can then compress and most preferably form
_ .
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indentations ~n the two sheets simultaneously on a single side. Similarly, the
mandrels 12 would be able to shear away the material (from both sheets) to form
the holes in each sheet.
Figure 6 illustrates a sheet 10 which requires a hole 20 to be formed.
In order to ensure that the indentations are formed in the correct hole
area, the sheet is preferably provided with location means (15,16). In Figure 6
the location means 15 is in the form of a dimple. Figure 6 illustrates pre-hole
indentation from only one side occurs prior to forming the hole. The dimple 15
cooperates with protrusion 17 on the mandrel 12. The provision of the location
10 means 15 and cooperating protrusion 17 on the mandrel 12 assists in ensuring
that the indentation is correctly positioned and that the hole area has been
pretreated as required. Figure 7 illustrates another form of location means 16
which is in the form of a pilot hole. Figure 7 illustrates pre-hole indentation from
both sides of the sheet 10. It must be noted that the pilot hole is just a location
15 means to cooperate with and to assist in the centring of the mandrels and is not
the actual fastening hole. Thus, the pilot hole 16 (as do the dimples 15) assists
in ensuring that the indentations are in the correct hole area and in the case of
double-sided indentations, the indentations formed on both sides are correctly
aligned.
Whilst it is envisaged that the drill or punch size in order to form the hole
would generally be the sama size as the indentations formed, improved fatigue
resistant results are also obseNed wherein the indentation is smailer or larger
than the final hol~ size. Reference is now made to Figure 8 wherein the final
hole 20 is larger than the indentations (shown in dotted lines). In this case the
25 hole will be formed by drilling or by using a larger punch / mandrel than theindenting mandr~ls. Figure 9 illustrates the situation wher3in the final hole 20 is
smaller than the indentations 18. Similarly, the hole is formed by drilling or by
using a smaller punch / mandrel than the indenting mandrels. Additionally,
whilst Figures 6 to 9 do not illustrate this feature, it is generally preferred that the
30 sheet 10 be retained in position, by clamps or the like during the indentation and
punching / drilling steps as was shown in Figures 2 to 4.
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Figure6 10 and 11 illustrate the application of the present invention to
areas which are to be cut out. Figure 10 illustrates sheet 110 wherein area 120
is to be cut out. Prior to the cutting out process, stress-critical locations of the
area to be cut out are indented 121 in a similar manner as described and
5 discussed in Figures 1 to 9 above. Once the indentation process is complete,
the area is cut out using conventional techniques resulting in the cut out area
120 as illustrated in Figure 11.
Tests have been conducted based upon the process of the present
invention.
The results of the tests are shown in Figures 12 and 13. Th~ results
shown in Figure 12 were preliminary tests comparing the three processes, that
is, drilled, cold-worked and the process of the present invention. All of the
samples were the same, being dog-boned shaped specimens made from
Aluminium 2024. The dog-bone specimen used in these tests, (the results of
15 which are illustrated in Figure 12) were substantially square in shape, wherein
the specimen resembled a capital UI". The hole was formed in the narrow portion
of the dog-bone. The cross-section in the relevant hole area of the samples was
41.5 mm x 1.6 mm. Each of the holes formed were 4 mm in diameter. The prior
art ~drilled" method involved drilling the hole in the sampls to 4 mm. The 4 mm
20 holes formed in the prior art Ucold expanded" specimens were made using the
Fatigue Technology Inc. (FTI) cold expansion process.
Tha specimens made by the present invention were compressed and
indented by two mandrels (4 mm in diameter), each with a 25 kN load applied
thereto, positioned on either side of the samples and guided by clamps which
25 wers holding the samples. The hole was punched using one of the mandrels. It
should be noted that the indentation depths (and loading) vary with the type ot
material and hole size.
Tests were performed on a 50 kN capacity servo-hydraulic testing
machine with the following ioading conditions:
Frequency of loading s 20 Hz
Ratio of maximum / minimum load = 10.
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Each ef the samples were cyclically stressed until failure of the specimen
occurred. The results of the preliminary tri~s are illustrated in Figure 12.
Stress shown on the graph is obtained by:
STRESS = MAX LOAD - MIN LOAD
CROSS-SECTIONAL AREA
It should be noted that for the drilled" and "cold expanded" specimens the
specimen failed from the hole.
The specimens produced by the method of the present invention began
failing at the specimsn edges (at the top and bottom portions of the dog-bone)
10 and not the hole. Thus, the true lives of the holes produced by the method of the
present imention is assumed to be higher than shown. On reviewing Flgure 12,
it can be seen that specimens produced by the present invention could withstand
over double the number of cycles for a particular stress than the "cold expanded~
hoies and over ten times the numbsr of cycles that a drilled hole could withstand.
Further comparative tests have been conducted comparing the Udrilled~
process, Ucold expanded~ process and the process of the present invention using
a different shaped sample to the previous preliminary tests. Figure 14 illustrates
the sample used in the further tests. The sample made of Aluminium 2024 had
dimensions 175 x 60 x 1.6 mm wherein an arc of radius 85 mm was removed
20 from each of the long sides of the sample, resulting in a smooth dog-bone
shapèd specimen. The narrowest portion of the dog-bona was 45 mm. The 4
mm hole was formed and centred at the narrowest portion. The "drilled~ and
Ucold expanded~ specimens were made using the same processes as in the
preliminary tests. The specimens made by the present invention were
25 compressed and indented by two mandrels (4 mm) each with a 16 kN load
applied thereto, positioned on either side of the samples and guided by clamps
which were holding the samples.
These tes1s were performed on the same 50 kN capacRy selvo-hydraulic
testing machine with the same loading conditions as with the previous
30 preliminary tests.
~ ~ Once again, eæh of the samples were cyclically stressed until failure of
; the specimen occurr~d. lhe results of the tests are illustrated in Flgure 13.
r~
.
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As with the preliminary tests the Udrilled'' and "cold-expanded" specimens
failed from the hole.
Similarly, the specimens produc~d by the present invention failed at the
specimen edges. However, where in the preliminary tests the specimens failed
5 near the fixed end portions, the specimens of the subsequent tests failed closer
to the neck or narrowest portion of sample.
It can be observed from Figure 13, that the specimens prepared by the
present invention outlived the specimens prepared by the other two methods
under the loading conditions considered.
For stress levels of 140 MPa to 200 MPa, the samplcs of the present
invention outlived the FTI specimens by approximately from 21/2 to 11/2 times
respectively. The improvement in fatigue life is even greater below 140 MPa.
Figure 15 illustrates the same results as in Figure 13; however, with the
additional control data of fatigue loading metal samples with no holes, the
15 specimens were the same as the samples with holes formed therein, but withoutholes formed therein. It can be noted with reference to Figure 15, that the
specimens prepared with holes of the present invention perform equally as well,
if not in some cases better, than the "no-hole" specimens. A likely explanation
for this result is that the method of the invention provides compressive residual
20 stresses at the resulting hole perimeter which are so effective in preventingfatigue such that the critical or most likely failure site of the specimen is shifted
from the hole edge to the outer edge of the specimen. Thus, for a given applied
set section stress, the dynamic stress at the outer edge of a specimen without ahole is slightly higher than that of a specimen with a hole produced by the
25 meîhod of the present invention. Similarly, there is a real potential for a
component containing a hole or holes produced by the method of the present
invention to out perform an otherwise identical component which does not
contain a hole at all.
Essentially the process of the present invention introduces sufficient and
30 effective compressive residual stresses such that the critical or most likely failure
site is shifted from the hole edge to the outer specimen edge.
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Figur~ 16 illustrates tests conducted using the present invention
comprising one-sided indentation with two-sided indentation prior to formation of
the hole. It is observed that one-sided indentation can provide as good a resultas indenting from both sides without requiring a significant increase in load. In
5 panicular, it can be seen from Figure 16 specimens produced with a one-sided
indentation of 18 kN, which represents only a 12.5% increase in indentation
force, showed a comparable fatigue resistance to that obtained by the two-sided
indentation process.
Figure 18 shows the results of tests comparing samples which had holes
10 produced in each of three metal sheets by drilling, cold-working and the process
of the present invention, and then joining the three sheets in a manner shown inFigure 17, which illustrates Sample 30, with rivets in sheets 31.
More specUically, the specimens of the tests were constructed from three
sheets of 60 mm wide x 1.6 mm thick, aluminium 2024 sheets. Two 4 mm
15 diameter aircraft grade countersunk rivets spaced at 25 mm were used to fasten
the sheets 32 together as shown in Figure t 7.
The three types of holes were made as follows:
(i) Drilled: Holes were drilled such that a snug push fit with the
rivets was obtained.
(ii) FTI: Holes were made using the Fatigue Technology Inc
cold expansion process as discussed previously. The
maximum allowable cold-working by this method was
obtained - 5% expansion of initial hole.
(iii) The invention: - compression and indentation was made from both
sides with an indenting force of 18 kN, prior to the formation
of the hole.
The same loading conditions as used above in previous tests were
applied to the riveted joint specimens. The results of the tests are illustrated in
Figure t8. Stress shown on Figure 18, is the average fastener stress which is as30 follows:
Average fastener stress = LOAD
4 x cross~sectional area
ot a single rivet
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By con paring the results ot Figure 18, with those of Figure 13, it can be
seen that the results of Figure 18 show a greater degree of scatter compared to
the open hole results. This is not entirely unexpected because of the method of
manufacture of the specimens which involves the additional process ot manually
5 fixing rivets, which itself is subject to variations.
However, it is clear from reviewing the results shown in Figure 18, that the
specimens treated with the process of the present invention, out perform
specimens treated by the prior art cold expanded method and the drilled method
only.
By way of example, for shear stress of 140 MPa. the expected fatigue lives
for the drilled holes, cold-expanded holes and the holes manufactured using the
present method of the invention were approximately 200,000, 400,000 and
2,000,000 respectively. In this particular example, the cold-expanded process
and the process of the present invention showed a two-fold and ten-fold,
15 respectively, improvement over the drilled only hole.
The present invention also provides an apparatus for improving the
fatigue resistance of holes and cut out portions in material sheets comprising ameans for compressing and indenting at least one side of the sheet, preferably
both sides. The apparatus may also comprise restraining means or clamps to
20 retain the sheet in position. Mandrels are preferably used to compress and
indent the sheet. Furthermore, one or both of the mandrels may be capable of
stamping out the hole after indentation.
It is generally considered that the present invention is a preventative
manufacturing method which can be used when manufacturing structures which
25 are prone to fatigue failures such as aircraft and other high performance and highly stressed components.
¦ I~W8UB8TITUTE SHE~T
