U.S. patent application number 15/884431 was filed with the patent office on 2018-05-31 for chamfering of laminate layers.
The applicant listed for this patent is Vestas Wind Systems A/S. Invention is credited to Yohann Bellanger, Christopher Thomson.
Application Number | 20180147683 15/884431 |
Document ID | / |
Family ID | 42799388 |
Filed Date | 2018-05-31 |
United States Patent
Application |
20180147683 |
Kind Code |
A1 |
Bellanger; Yohann ; et
al. |
May 31, 2018 |
CHAMFERING OF LAMINATE LAYERS
Abstract
A method of machining a fibrous sheet for a composite structure
is described. The sheet comprises a resin matrix having a glass
transition temperature, wherein the method comprises cooling the
sheet substantially to maintain the temperature of the matrix below
its glass transition temperature during machining.
Inventors: |
Bellanger; Yohann; (Cowes,
GB) ; Thomson; Christopher; (Portsmouth, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Vestas Wind Systems A/S |
Aarhus N. |
|
DK |
|
|
Family ID: |
42799388 |
Appl. No.: |
15/884431 |
Filed: |
January 31, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13641636 |
Mar 29, 2013 |
9914191 |
|
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PCT/DK2011/050287 |
Jul 19, 2011 |
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15884431 |
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61413078 |
Nov 12, 2010 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B29C 2793/0027 20130101;
F04D 29/38 20130101; B23Q 11/10 20130101; B24B 9/20 20130101; B29C
70/545 20130101; B29C 2035/165 20130101; Y10T 29/49995 20150115;
Y10T 29/54 20150115; B24B 41/068 20130101; B23Q 11/1053 20130101;
B24B 55/02 20130101; B29C 70/54 20130101 |
International
Class: |
B23Q 11/10 20060101
B23Q011/10; F04D 29/38 20060101 F04D029/38; B24B 55/02 20060101
B24B055/02; B24B 9/20 20060101 B24B009/20; B29C 70/54 20060101
B29C070/54; B24B 41/06 20120101 B24B041/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 30, 2010 |
GB |
1012868.4 |
Claims
1. An apparatus for machining a fibrous sheet for a composite
structure, the apparatus comprising: a support for the sheet; a
machining tool movable relative to the support; and a cooling
system for cooling the sheet.
2. The apparatus of claim 1, wherein the cooling system comprises
one or more cooled surfaces against which the sheet is supported
before or during machining.
3. The apparatus of claim 2, comprising opposed cooled surfaces
between which the sheet may be sandwiched.
4. The apparatus of claim 1, wherein the cooling system comprises a
supply for supplying coolant to the sheet before or during
machining.
5. The apparatus of claim 4, wherein the supply is arranged to move
relative to the support in tandem with the machining tool.
6. The apparatus of claim 1, wherein the cooling system comprises
an enclosure for holding the sheet within a cooled environment.
Description
[0001] This invention relates to techniques for chamfering layers
or plies used in composite structures, such as wind turbine
blades.
[0002] Composite structures typically comprise one or more plies,
each ply being a fibre-reinforced sheet that may comprise a
thermoplastic or thermosetting resin matrix. The fibres may be
pre-impregnated with the matrix as a `prepreg` or the matrix may be
impregnated into a fibre sheet during fabrication of a composite
structure, for example during lay-up or injection-moulding
procedures. Alternatively, the fibre-reinforced sheet may be
pre-impregnated on just one side by a resin foil, i.e., a
`semi-preg`.
[0003] Plies are commonly laid atop one another in a layered or
laminated arrangement. Single-ply composite structures are also
possible, with single-thickness plies abutting in edge-to-edge
relationship or overlapping at their edges. The plies are commonly
supported by a foam core to define a skin on or around the
core.
[0004] In some circumstances, it is desirable to chamfer an edge of
a ply. For example, plies may abut edge-to-edge in a composite
structure and it is desirable to maximize the surface area of the
interface between the abutting plies. This is because the shear
strength at the interface is a small fraction--possibly as little
as 5%--of the tensile strength of the plies themselves. The
alternative of overlapping abutting plies leads to stress
concentration and disadvantageously kinks the load path extending
from one ply to another. Also, where plies define the external
surface of a composite structure, an overlap between the plies
makes a smooth finish difficult to achieve.
[0005] It is also well known to taper a composite structure by
reducing the number of plies from one location to another across
the structure. Such tapering is common in aerofoil members such as
wind turbine blades, which taper in both the spanwise direction
from blade root to blade tip and in the chordwise direction from
leading edge to trailing edge. To achieve this, some plies may be
terminated or `dropped` inward of an extremity of the structure,
leaving other continuous plies to extend further toward that
extremity.
[0006] Plies are preferably dropped in a staggered or interleaved
manner to make the transition as gradual as possible. However, each
dropped ply introduces a region of weakness in view of
discontinuity between the neighboring plies, with the possibility
of resin concentrations or gas pockets in any gaps between plies,
especially at the edge of dropped plies. Here, edge chamfering is
helpful to minimize gaps, to straighten the load path and to
maximize the surface area of the interface between plies. This
allows thicker plies to be used, which facilitates the lay-up
process because fewer layers are then required in the laminate to
achieve a required overall thickness.
[0007] Plies for use in composite structures are difficult to
chamfer efficiently, accurately and repeatably, particularly with
the shallow taper angle that is desirable to maximize the surface
area of the edge interface. The plies are flexible and compressible
and so tend to move unpredictably under the forces applied by the
chamfering process. Also, the plies may degrade with heat generated
by the chamfering process. This is a particular problem with
prepregs, if the matrix cures or otherwise transforms with heat.
For example, heat generated during chamfering may cause the
thermoplastic matrix to soften or melt and clog the chamfering
tool. If the matrix softens or melts, it is also possible for the
chamfering tool to drag the ply unpredictably, possibly distorting
it and so undermining the accuracy of cutting.
[0008] Some examples of ply-tapering tools are disclosed in EP
1786617. These include finger cutters akin to hair trimmers, but
finger cutters are not suitable for cutting prepregs in which the
fibres are embedded in a matrix because the matrix prevents the
fingers from penetrating between the fibres. EP 1786617 also
discloses milling cutters with inclined faces, turning about an
axis orthogonal to a plane containing the edge being tapered. When
configured as shown in EP 1786617, milling cutters impart heat to
the ply that may degrade the ply and melt its matrix if the ply is
a thermoplastic prepreg; this is also a problem suffered by
abrading techniques proposed elsewhere in the art, using a belt
sander or the like. Also, when configured as shown in EP 1786617,
milling cutters impart a side force to the ply, parallel to the
tapered edge, that tends to distort the ply and so undermines the
accuracy of cutting. This is also a problem suffered by
knife-cutting techniques proposed elsewhere in the art.
[0009] It is against this background that the present invention has
been made.
SUMMARY OF THE INVENTION
[0010] In accordance with the present invention, there is provided
a method of machining a fibrous sheet for a composite structure,
the sheet comprising a resin matrix having a glass transition
temperature, wherein the method comprises: providing a fibrous
sheet at a first temperature; supporting the sheet for machining;
and cooling at least part of the sheet to a second temperature
below the first temperature, substantially to maintain the
temperature of the matrix below its glass transition temperature
during machining.
[0011] The resin becomes hard and brittle when cooled, which makes
it easier to machine. Generally, the resin becomes harder and more
brittle with decreasing temperature. Therefore, it is preferable to
cool the material to the lowest temperature possible within
realistic practical and economic constraints.
[0012] The glass transition temperature of the uncured resin may
also be referred to in the art as the `cold T.sub.g` or the
`uncured T.sub.g`, and is an intrinsic property of the resin that
will vary from material to material. Put simply, the cold/uncured
T.sub.g is the glass transition temperature of a matrix that has
reacted at ambient temperature, and hence exhibits a relatively low
degree of cross-linking. Material suppliers such as Gurit.TM. can
provide details of the cold/uncured T.sub.g of the materials that
they supply. However, as a matrix ages, some additional cross
linking will occur, causing the cold T.sub.g to increase slightly
with time. The T.sub.g of the uncured resin in typical prepreg or
semi-preg materials used in the construction of modern wind turbine
blades is generally below 0.degree. C., for example around
-2.degree. C. In comparison, when a matrix is cured at an elevated
temperature, it will exhibit a relatively high degree of
cross-linking, resulting in the cured matrix having a much higher
T.sub.g, typically well in excess of 100.degree. C.
[0013] The inventive concept encompasses a method of making a
composite structure, comprising: tapering an edge of a fibrous
reinforcement in accordance with the above machining method; and
incorporating the sheet into a composite structure with the tapered
edge lying against or beside at least one other fibrous
reinforcement sheet.
[0014] The present invention also provides an apparatus for
machining a fibrous sheet for a composite structure, the apparatus
comprising: a support for the sheet; a machining tool movable
relative to the support; and a cooling system for cooling the
sheet.
[0015] The inventive concept also encompasses a composite structure
such as a wind turbine blade produced by the above methods or
apparatus.
[0016] Optional features of the present invention are set out in
the sub claims appended hereto.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] In order that the invention may be more readily understood,
reference will now be made, by way of example, to the accompanying
drawings in which:
[0018] FIG. 1 is a schematic perspective view of an apparatus for
machining a prepreg or semi-preg ply, in which refrigerant is
applied to the ply prior to machining;
[0019] FIG. 2 is a schematic perspective view of the apparatus of
FIG. 1, in which the refrigerant is applied to the machining site
during the machining process;
[0020] FIG. 3 is a schematic side view showing a machining tool
being moveable towards and away from an edge of a ply along an
arc-shaped path; and
[0021] FIG. 4 shows the apparatus of FIG. 1 located within a
climate-controlled environment.
DETAILED DESCRIPTION
[0022] Referring to FIG. 1, a prepreg ply 10 is clamped between
refrigerated steel blocks 12a, 12b, and a grinding wheel 14 is
arranged to translate across a free edge 16 of the ply 10 to remove
material from that edge to create a chamfer. Refrigerant 18 is
applied locally to the free edge 16 of the ply 10 during the
chamfering operation. The apparatus and chamfering technique are
described in further detail below.
[0023] The prepreg ply 10 comprises a sheet of glass fibre fabric,
which has been impregnated with a thermoset matrix, which in this
example is pre-catalysed epoxy resin. The glass fibre fabric
consists of two layers and is commonly referred to as `triax`. The
first layer includes a set of unidirectional (ud) fibres, whilst
the second layer is a layer of `biax`, which has a first set of
unidirectional fibres oriented at an angle of +45.degree. relative
to the fibres in the first layer, and a second set of
unidirectional fibres oriented at an angle of -45.degree. relative
to the fibres in the first layer.
[0024] The steel blocks 12a, 12b are oblongs and include internal
refrigeration channels 20. A refrigerant is pumped through the
channels 20 to cool the blocks 12a, 12b to a temperature of
-50.degree. C., and then continuously pumped through the channels
20 to maintain the temperature of the blocks 12a,12b at -50.degree.
C. Alternatively, the blocks 12a, 12b may be placed in a
refrigerator at -50.degree. C. for several hours prior to the
chamfering operation. In this way, the refrigeration channels 20
may not be required.
[0025] The steel blocks 12a, 12b are placed one on top of the
other, with the lower steel block 12b being located on an
insulating foam block 22 to reduce heat transfer from a work
surface 24 to the cold blocks 12a, 12b. An end portion of the
prepreg ply 10 is sandwiched between the steel blocks 12a, 12b and
the blocks are clamped together by a clamp (not shown) to hold the
ply 10 firmly in place. The upper block 12a is set back from the
lower block 12b by approximately 40 mm to define an elongate ledge
26. The free edge 16 of the ply 10 extends from between the steel
blocks 12a, 12b onto this ledge 26.
[0026] The grinding wheel 14 is arranged to traverse along the
ledge 26 in a direction parallel to the exposed free edge 16 of the
ply 10 as indicated by the arrow 28 in FIG. 1. The grinding wheel
14 has an abrasive cylindrical outer surface 30, which rotates
about an axis parallel to the free edge 16 of the ply 10, i.e.,
parallel to its direction of translation 28 across the ply 10. In
use, the grinding wheel 14 is angled slightly with respect to the
surface 32 of the ply 10 and traversed across the free edge 16 to
create a chamfer of a desired gradient. A shallow chamfer gradient
in the range of 1:20 to 1:10 i.e., approximately 2.8.degree. to
6.degree. is particularly desirable.
[0027] Prior to chamfering commencing, the free edge 16 of the ply
10 is sprayed with tetrafluoroethane refrigerant (R134a) from a
spray can 34. It will of course be appreciated that other suitable
refrigerants may be used for this purpose, for example liquid
nitrogen or liquid carbon dioxide. Spraying the free edge 16 of the
ply 10 with refrigerant cools the ply 10 to well below the glass
transition temperature (T.sub.g) of the uncured epoxy resin in the
prepreg. Typically the T.sub.g of the uncured epoxy is around
-2.degree. C. Maintaining the temperature of the resin below its
uncured T.sub.g during chamfering ensures that the resin remains
hard during the chamfering process. This prevents the resin from
becoming tacky and contaminating or clogging the abrasive surface
30 of the grinding wheel 14, which would otherwise occur if
chamfering was conducted at room temperature. The cold steel blocks
12a, 12b ensure that any heat generated during the chamfering
operation is channeled away from the ply 10.
[0028] Experimental tests have shown that a single application of
the R134a refrigerant to the free edge 16 of the ply 10 prior to
chamfering is sufficient to keep the temperature of the ply 10
below the T.sub.g of the uncured resin. However, if necessary, the
refrigerant may be applied repeatedly or continuously during
chamfering to keep the temperature of the ply 10 below the T.sub.g
of the uncured resin. Applying the refrigerant continuously has the
advantage that a flow of refrigerant will carry heat away from the
worksite. In the example shown in FIG. 2, the refrigerant 18 is
applied during machining and is applied locally at the machining
site 35. A nozzle 36 supplying the refrigerant 18 may be arranged
to move in tandem with the grinding wheel 14 as represented by the
arrows 38 in FIG. 2. Applying the refrigerant 18 locally at the
machining site 35 is advantageous because it concentrates the
refrigerant 18 at the point where heat is generated.
[0029] In order to assist heat dissipation from the free edge 16 of
the ply 10, rather than being translated across the ply 10 in a
single motion, the grinding wheel 14 may be pressed against the
free edge 16 of the ply 10 in a series of pressing operations
across the width of the ply 10. This is represented schematically
in FIG. 3, which shows the grinding wheel 14 being moveable towards
and away from the free edge 16 of the ply 10, i.e., in and out of
contact with the free edge 16, along an arc-shaped path 40.
Refrigerant is continuously applied to the free edge 16 so that
cooling continues between presses, i.e., whilst the grinding wheel
14 is moved out of contact with the free edge 16 of the ply 10.
[0030] Whilst not shown in the above figures, the humidity of the
air surrounding the apparatus is controlled to prevent condensation
from forming on the cold ply 10 or elsewhere on the apparatus
itself.
[0031] Referring to FIG. 4, rather than applying refrigerant
directly to the free edge 16 of the prepreg ply 10, the entire
apparatus is located in a climate-controlled environment 42 that is
sufficiently cold to maintain the epoxy below its uncured glass
transition temperature during the chamfering process. Of course, it
is also possible to apply refrigerant directly to the chamfering
site 35 if necessary when the apparatus is located in a
climate-controlled environment 42 such as this. In this example,
refrigerant channels have been removed from the blocks 12a, 12b,
however it will be appreciated that such channels may be used in
combination with a climate-controlled environment 42.
[0032] It will be appreciated that many modifications may be made
to the techniques described above without departing from the scope
of the present invention as defined by the accompanying claims. For
example, it will be appreciated that the prepreg ply described by
way of example above may be substituted for a semi-preg ply or
other fibrous ply comprising a resinous matrix material. Also,
whilst triax is described by way of example, it will be appreciated
that the invention is not limited to the use of triax. Indeed, the
fibres in the ply may have any other orientation, for example the
fibres may all be unidirectional (ud). In addition, whilst a
grinding wheel has been described above, it will be appreciated
that the invention may be used in connection with any other
machining tool or technique.
* * * * *