FIELD OF THE INVENTION
The present invention relates to a composite laminate structure,
and a method for indicating impact damage in such a structure. In particular, although
not exclusively, the structure may form part of an aircraft component.
BACKGROUND OF THE INVENTION
Many aircraft components are made from composite laminate
materials. For example, a stringer in an aircraft wing, stiffeners and spar structures,
may all be in the form of composite laminate structures. While the structures may
be strong when loaded in the way in which they have been designed to be loaded,
they may be vulnerable to impact damage, which may result in delamination occurring.
Delamination may result in the structure weakening and eventually failing. Particularly
vulnerable are the edges of composite laminate structures (at which the layers of
the composite laminate structure terminate) where an impact occurs, particularly
if the impact is end-on (i.e. when the impact is in a direction perpendicular to
the edge and parallel to the plane of the laminates that make up the structure).
If an impact has a significant component of force in the
end-on direction the impact may be capable of damaging the bonding between the laminate's
layers comprising the composite laminate structure and causing, or adding to the
effect of, delamination. Such impacts may be as a result of workmen dropping tools
whilst working on the structure, during maintenance for example when the end surfaces
of the composite laminate structures are exposed in a way they are usually not.
A further way in which the composite laminate structures may be damaged is as a
result of wear over time, such wear for example resulting from workmen sitting,
or standing, on the structure or contacting the structure directly or by means of
tools, or other equipment. It is for example common for a workman to rest tools
or toolboxes on the end surface of such structures while working on the aircraft.
The edge of a composite laminate structure is typically exposed, at least during
maintenance, to direct wear and direct impacts as described above. Similar impact
threat exists during the manufacturing and assembly of the composite parts.
When designing an aircraft component, factors that affect
the strength and other mechanical properties of composite laminate structures of
the aircraft, such as those factors described above, are taken into account. Thus,
factors such as those described above typically mean that the composite laminate
structures need to be provided with extra strength, resulting in an increase in
size and weight.
One known approach to strengthening the edge of a laminar
composite is to apply an edge protector - for example as described in
Another solution is to form the composite laminate with a roll-form edge - that
is, an edge formed by a folded layer. However a problem with these conventional
methods is that they may not provide sufficient visual evidence of an impact. Such
visual evidence may be a requirement of the structure.
SUMMARY OF THE INVENTION
A first aspect of the invention provides a structure comprising
a composite laminate having an edge; and an impact indicator which is carried by
the edge and comprises a resin which fractures upon impact.
A second aspect of the invention provides a method of indicating
impact damage in such a structure, the method comprising fracturing the impact indicator.
The fracture provides permanent visible evidence of impact
damage, for instance by cracking or by one or more pieces breaking off from the
impact indicator. As well as providing such visible evidence of impact damage, the
impact indicator may also provide an element of impact protection by absorbing part
of the impact energy.
Typically the impact indicator comprises a resin which
is more brittle and less strong than the material forming the composite laminate.
For instance the material forming the composite laminate may be reinforced, and
the resin forming the impact indicator may be un-reinforced.
Typically the composite laminate comprises a thermosetting
material such as an epoxy resin. The material forming the impact indicator may be
formed from the same thermosetting resin, or from a resin which cures at a similar
or lower temperature.
The impact indicator may be limited in length, or may comprises
a strip which runs along at least the majority of the length of the edge.
In certain embodiments of the invention, a plurality of
layers of the composite laminate terminate at the edge. Alternatively the edge may
be a roll-form edge formed by a folded layer.
The impact indicator may be adhered to the edge by co-curing,
co-bonding, or secondary bonding. In the case where the impact indicator is adhered
to the edge by co-curing, the impact indicator may be formed integrally with the
composite laminate (for instance by forming the composite laminate with a resin-rich
In one example the impact indicator is applied to the edge
by extruding resin from a nozzle, and then co-bonding the extruded material to the
edge by curing it.
In another example the impact indicator is applied by providing
a bead of thermosetting resin in a female mould; inserting the edge of the composite
laminate into the female mould; and heating the bead and the composite laminate
so that the composite laminate cures and the bead is co-cured to the edge.
In another example the composite laminate comprises a reinforcement
phase and a matrix resin phase, and the impact indicator is formed by inserting
the edge of the composite laminate into a female mould; providing a gap between
the edge and the female mould; and heating the composite laminate so that the matrix
resin phase flows into the gap and forms the impact indicator.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described with
reference to the accompanying drawings, in which:
DETAILED DESCRIPTION OF EMBODIMENT(S)
- Figure 1 is a perspective view of a
method of applying an impact indicator to a stringer;
- Figure 2 is an end view of the stringer
carrying the impact indicator;
- Figures 3-5 show three steps in the
formation of the stringer;
- Figure 6 is an end view of an alternative
stringer carrying an impact indicator;
- Figure 7 is an end view of a further
alternative stringer carrying an impact indicator;
- Figure 8 is an end view of a roll-form
stringer carrying an impact indicator;
- Figure 9 is an end view of a co-curing
manufacturing process; and
- Figure 10 is an end view of a manufacturing
process in which the impact indicator is formed integrally with the stringer.
A stringer 1 shown in Figure
1 comprises a blade 2 and a pair of flanges 3,4. The stringer is formed
by the process illustrated in Figures 3-5.
In a first step, a planar charge 5a is placed on a male forming tool 6. The charge
5 comprises a laminate structure formed from a stack of sheets, each sheet comprising
a plurality of unidirectional carbon fibres impregnated by a thermosetting epoxy
resin. These sheets are conventionally known as "prepregs". Individual prepreg sheets
are shown schematically in Figure 3 but
not in the other figures for purposes of clarity.
The charge 5a is then deformed over the mould tool as shown
in Figure 4 to form a U-shaped part 5b.
The U-shaped part 5b is then cut into two L-shaped parts 5c,5d as shown in
Figure 5; and the parts 5c,5d are placed
back-to-back as shown in Figure 2. Once
the L-shaped parts 5c,5d have been placed back-to-back, they are co-cured to harden
the stringer and join the parts together.
The direction of the length of the stringer 1 is defined
as 0 degrees, and the direction of the height of the stringer is defined as 90 degrees.
The lay-up of the stringer is represented by a notation representing the percentage
of the fibres in the stringer that are oriented in the directions 0 degree/±
45 degrees/90 degrees. A typical stringer lay-up is 60/30/10. Therefore, 60 percent
of the fibres are oriented in the 0 degree direction, 30 percent in the ± 45
degree directions and 10 percent in the 90 degree direction.
The stringer includes an exposed top edge 8 which is machined
and sealed (after curing) by the process shown in Figure
A shaped machine cutter 10 moves along the edge 8, removing
material to form a groove 11 shown in Figure 2.
A cleaning device 12 removes the material generated by the machining process. A
bead 13 is applied to the groove by extruding liquid epoxy resin from a nozzle 14.
A finishing tool 15 has a slot 16 with a desired shape, and the tool 15 is moved
along the edge behind the nozzle 14 to remove excess resin from the bead 13.
Note that these processes are performed inline so that
the bead 13 is applied at the same time that the edge 8 is machined.
The resin is then cured to co-bond the bead to the edge
8, forming an impact indicator 9 shown in Figure
2 seated in the groove 11.
Note that the machining step may be omitted so that the
top edge of the stringer is flat as shown at 8a in Figure
6. In an alternative embodiment shown in Figure
7, the plies of prepreg may slide with respect to each other during the
deformation step shown in Figure 4, so
that the L-shaped parts have angled edges 8b at which the sheets of prepreg terminate.
Thus the groove is formed in this case without requiring a separate machining step.
In the cases of Figures
6 and 7 the stringer and impact indicators 9a,9b may be co-cured instead
of being cured at different times.
A roll-form stringer is shown in Figure
8. This is formed from a single stack of prepreg plies which is folded
to form a roll-form edge 8c. An impact indicator 9c is attached to the roll-form
edge 8c by extruding a resin bead and co-bonding it to the roll-form edge 8c.
In an alternative manufacturing process shown in
Figure 9, the impact indicator is applied
to the edge of the stringer by providing a bead 20 of uncured epoxy resin in a female
mould 21; inserting the stringer blade 2 into the female mould; and heating the
mould assembly so that the stringer and bead 20 become bonded to each other by co-curing.
In a further alternative manufacturing process shown in
Figure 10 the impact indicator is formed
integrally with the stringer by inserting the stringer blade into the female mould
21; providing a gap 22 between the edge 8 and the female mould; and heating the
stringer so that the epoxy resin matrix in the blade flows into the gap 22 to form
the impact indicator.
The impact indicators described above are formed from a
material which fractures upon impact, resulting in one or more cracks and/or one
or more pieces breaking off. EA9394 epoxy resin has been found to provide the necessary
fracture properties, but other materials may be envisaged. For instance the resin
may cure at room temperature so that the bead does not have to be heated in order
to cure it.
Note that the impact indicators are formed from a resin
which is more brittle and less strong than the material forming the composite laminate.
For example un-reinforced EA9394 resin is more brittle and less strong than the
carbon-fibre reinforced prepregs which form the composite laminate. Also, the impact
indicators each have a curved convex outer surface, and a thickness which varies
across the width of the edge. This has a number of benefits:
- it makes them more prone to fracture than a flat strip;
- it makes it more likely that one or more pieces will break off after fracture;
- it makes any post-fracture cracks or voids more easily visible from the side.
Although the invention has been described above with reference
to one or more preferred embodiments, it will be appreciated that various changes
or modifications may be made without departing from the scope of the invention as
defined in the appended claims.