CA2100548A1

CA2100548A1 - Composite structure for floor panels

Info

Publication number: CA2100548A1
Application number: CA002100548A
Authority: CA
Inventors: Guy Weinand
Original assignee: Euro Composites SA
Current assignee: Euro Composites SA
Priority date: 1992-07-15
Filing date: 1993-07-14
Publication date: 1994-01-16
Also published as: ATE162461T1; ES2112930T3; EP0579000A1; LU88149A1; DE59308016D1; EP0579000B1

Abstract

Abstract of the Disclosure A composite flat structure for lightweight floor panels suitable particularly for use in aircraft comprises a rigid cellular core to each of two opposed surfaces of which inner and outer layers of fiber composite materials are attached. Each of the outer layers comprises a layer of E-glass cloth and a layer of unidirectional carbon fibers both embedded in a phenolic resin. The inner layers comprise a layer of unidirectional R-glass fibers which are embedded in an epoxy resin and which are arranged to lie transversely with respect to the unidirectional fibers in said carbon-fibers layer. The phenolic and the epoxy resins have substantially compatible gelling temperatures, curing times and curing temperatures so that they can be cured together. Preferably the core comprises an aramid honeycomb structure.

Description

?, 1~ 8 A COMPOSITE STRUCTU~E FOR FLOOR PANEI,S

The present inventiorl relates to a lightweight composite flat structure such as can be used, for example, for floor panels in aircraft construction.

Li~htweight flat structures composed of glass, carbon and aramid in various heat-cured artificial-resin matrices, attached to a honeycomb core plate, are employed increasingly in the aeronautics and aerospace, automobile, marine and mechanical engineering sectors. Despite their low specific gravity, these lightweight structures have excellent mechanical, electrical and thermal characteristics.

Known, commercially available flat structures, however, present a substantial disadvantage especially when carbon fibers are used, in that incompatibility between the fibers and the matrix can produce defective interfaces. The problem arises because the reinforcing fibers are very brittle in comparison to the matrix, with the result that the adhesion with e.g. the honeycomb is broken and eventually microcracks can develop. Furthermore, carbon fibers can initiate galvanic corrosion, which in the course of time weakens the connection between fibers and matrix.

Glass fibers and in particular woven E-glass fibers fatigue relatively rapidly.

Aramid fibers are also of only limited applicability because of their poor compression strength. These fibers ., , ~ .
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cannot prevent pronouncecl flexure under load, which is disadvantageous for panels used for aircraft floors.
However, the honeycomb core for these structures is often chosen with insufficient care, especially when non-metallic cores are to be used. Where well balanced mechanical properties are to be achieved without changing the density and/or the thickness of the honeycomb core, i-t is important for the ratio oE fibers to resin in the honeycomb to be optimal. When the fiber content is high (75% or more), the compression strength is normally low whereas the characteristics with respect to shear forces are favorable.
A high resin content (60% or more), however, makes the honeycomb fragile and impairs both its dynamic long-term behavior and its impact resistance.

The disadvalltages associated with these two extremes often result in lightweight flat structures being overdimensioned because they do not reach their theoretical strength or durability. The increase in size increases their cost and weight, both of which are important considerations for structures to be used in aircraft.

Furthermore, many lightweight flat structures have the disadvantage that they, like organic materials in general, are flammable and easily ignited.

The authorities responsible for aircraft construction have rightly reguired these negative characteristics to ~e diminished, especially for applications in the interior of aircraft. In addition, in case of fire as little smoke as possible should be produced, at least for a specific time span, so that the rescue of endangered persons is not impeded; moreover, the smoke that is produced should be no more poisonous than carbon monoxide. These requirements have been established internationally, e.g. in the ATS
1000.001, which prescribes the fire-inhibiting qualities of 2~ 0~3 ~

materials to be used in aircraft interiors, as well as the optical density and toxicity of the smoke they produce.

In attempts to prevent the polymer matrix from continuing to ~urn once it has ignited, additives or chemical ~odifiers such as bromine and antimony (synergic action) have been included. ~owever, there is another class of synthetic resins, the phenolic resins, with better flame-inhibiting properties than those of the epoxy resins commonly used in the manufacture of lightweight flat structures. In the case of phenolic resins, it is simple to make them self-extinguishing: furthermore, when exposed to flame they generate smoke at a low density, because they tend to char rather than decompose with intense smoke emission. Unfortunately, the cured phenolic resins are fragile and have a low fatigue limit. They also have little resistance to peeling at the interface between covering layer and honeycomb core. Therefore phenolic resins are unsuitable for load-bearing structures in heavy use, such as floor panels.

In modern passenger transport systems efforts are continually being made to enhance comfort and increase speed while reducing energy consumption and making travel safer for the passengers.

The object of the present invention is to provide a composite structure for e.g. aircraft floor panels, which is lightweight and which has greater fatigue strength, higher resistance to impact and shear forces, and simultaneously less flexure under load without substantially increasing either its weight per unit surface area or its thickness, over conventional structures.

210~

According to the present: invention there is provided a composite flat structure for floor panels comprising a rigid cellular core to each of two opposed .surfaces of which inner and outer layers of fiber composite materials are attached, and wherein each of said outer layers comprises a layer of E-glass cloth and a layer of unidirectional carbon fibers both embedded in a phenolic resin, each of said inner layers comprises a layer of unidirectional R-glass fibers which are embedded in an epoxy resin and which are arranged to lie transversely with respect to the unidirectional fibers in said carbon-fibers layer, and said phenolic resin and said epoxy resin have substantially compatible gelling temperatures, curing times and curing temperatures so that they ~an be cured together.

As mentioned above, the two resins, phenolic and epoxy, must have approximately the same gelling temperatures, curing times and curing temperatures. By matching the gelling times, the phenolic resin is prevented from diffusing into the epoxy resin and conversely. An independent but nevertheless simultaneous curing of the two resin systems is thereby ensured. Phenolic resins with a gelling temperature in the range of 100C to 120C, a curing time in the range of 60 to 120 minutes and a curing temperature in the range of 120C to 155C can be used together with epoxy resins having a gelling temperature in the range of 80C to lOO~C, a curing time in the range of 60 to 120 minutes and a curing temperature in the range of 120C to 155C.

Preferably, the core comprises an aramid honeycomb structure formed of aramid ribbons running substantially in an L direction perpendicularly to a W direction whilst .

~10~48 s defining honeycomb cells with longitudinal axes parallel to a T direction, which is also perpendicular to the W
direction.

The load that can be supported by the aramid honeycornb structure used as the core differs depending on direction owing to the nature of the honeycomb. The shear strength is greatest in the direction in which the aramid ribbon used in its construction runs within the honeycomb, i,e.
typically in a transverse direction L. The shear strength lO perpendicular to the aramid ribbon, i.e. in a W direction, is about 1.5 to 2 times less than in the L direction.

In one preferred embodiment, the R-glass fibers are applied so that they lie parallel to the L direction of the honeycomb, so as to produce a lightweight flat structure with optimal shear strength. In the outer layer the carbon fibers lie parallel to the W direction of the honeycomb.

Because the standard size of the honeycomb in the L
direction is only between 1,120 mm and 1,220 mm, whereas it is 2,440 mm in the W direction, it can be advantageous in 20 some applications to increase the shear strength of the honeycomb in the ~ direction.
` .
Thus, in an alternative preferred embodiment the R-glass fibers lie parallel to the W direction and the carbon fibers lie parallel to the L direction.
;
;~ In yet another embodiment, the core comprises an aramid honeycomb structure with a cell width between 3.2 and 6.4 mm inclusive and a specific density between 50 and 144 g/cm3 inclusive, and wherein the ratio of said carbon ; and glass fibers combined to the phenolic and epoxy resins 30 combined is between 83:17 and 37:63 inclusive.
.

: .. .

g Because of their special construction, the structures according to the invention meet internationally applicable safety requirements in case of fire, for example ATS
1000.001.

The individual structural elements of the composite structure have the following functions, which they fulfil advantageously.

The E~glass cloth on the surface of each side has four main functions: (1) during lay-up of the lightweight flat structures it facilitates manipulation of the fiber composite materials; (2) the finished panel can be more simply machined; (3) a delamination of fibers during sawing is prevented; and (4) galvanic corrosion during the lifetime of the structure when used as a floor panel in aircraft is prevented.

The underlying carbon fibers impregnated with phenolic resin serve to minimize flexure of the structure when under high load, without increasing its weight, and to cover the layer of epoxy resin. The structure thereby meets internationally applicable safety requirements in case of fire, for example ATS 1000.001.

The R-glass fibers, together with the correctly chosen bonding agent, enable the epoxy resin to bond chemically to the fibers during curing. In this way a quasi-perfect interface is produced between the fibers and the matrix.

The shear stress to which the floor panel is exposed is mainly applied in the flight direction of the aircraft.
Therefore the R-glass fibers can be made to run parallel to the flight direction by laying the panels of the composite structure in the L direction. However, aircraft builders often demand that the W direction of the panels coincide with the flight direction. This facili~ates construction of the floor because the dimensions of the panels are such that when so laid they are cut and glued less often so there is less waste.

Regardless of whether a panel is oriented in the L
direction or the W direction, the R-glass fibers must always run parallel to the flight direction. Because the R~
glass fi~ers are less fragile than the carbon fibers that lie perpendicular to the flight direction, and because the R-glass fibers are embedded in epoxy resin, which is less fragile than phenolic resin, this connection between the R-glass fibers/epoxy resin layer and the covering layer as well as the honeycomb core can be optimally loaded without the production of microcracks while the floor panels are in use.

This fact, combined with a precise curing reaction of the epoxy resin layer and the phenolic resin layer, ensures that no delamination occurs, either between the various layers or between the core plate and the layers. As a consequence, panels manufactured in this way have higher fatigue strength, higher impact resistance, higher shear strength and simultaneously less flexure under maximal load, with no substantial increase in the areal weight or the thickness of the lightweight flat structure.

Fig. 1 is a perspective view of a honeycomb structure with hexagonal cells; and Fig. 2 is a cross-section of a composite structure in accordance with the invention and incorporating a honeycom~
structure as shown in Fig. 1.

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As shown in Fig. 1, in a honeycom~ structure 1 wit~.
hexagonal cells, a T direction (thickness) defined by the direction of the longitudinal axes of the cells is indicated by 5, an L direction defined by the longitudinal direction of the ribbons comprising the honeycomb ~y 7, and a W direction perpendicular to both the T and L directions by 9. The honeycomb cell width is indicated by 3. In Fig.
2, the cross-section is taken in the T direction of the incorporated honeycomb structure to illustrate the construction of the composite structure.

In the invention, a core 16 comprising the honeycom~
structure 1 is covered on both sides first by a carbon fiber layer 12 and then with an E-glass cloth 10, both of which are embedded in phenolic resin. Between the carbon fiber layer 12 and the core 16 is a layer of epoxy-resin-impregnated R-glass fibers 14.

This structure was manufactured as follows, with reference to Table 1, and was subjected to tests, the results of which are summarized in Table 2.

An aramid honeycom~ of type ECA 3.2-96, such as is manufactured and marketed by Euro-Composites S.A., was used as the core 16. The honeycomb comprises hexagonal cells with an average width of 3.2 mm and has a nominal density of 96 kg/m3.

To each side of the core 16 were applied two layers of fiber composite materials consisting of fiber-reinforced artificial resins. In the present case two different types of fi~er composite material were used:
a. The outer fiber composite material 10 comprises E-glass cloth (+ 25 g/m2) and unidirectional car~on fibers 12 : - .
:
;

9 2 ;l ~ 8 (-~ 190 g/m2), lying in -the L direction in this example, both of which are embedded in phenolic resin.
b. The inner fiber composite material 14 comprises unidirectionally oriented R-glass fibers (+ 260 g/m2) embedded in epoxy resin, the fibers lying in the W
direction in this example.

The R-glass fibers used here have a modulus of elasticity equal to 86,000 MPa, a maximal tensile strength of 4,400 MPa and a relative density of 2.55 g/cm3. The carbon fibers have a tensile strength of 3,792 MPa, a modulus of elasticity of 234,000 MPa, and a relative density of 1.78 g/cm3. The gelling temperatures, curing temperatures and curing times of the resin systems are given in Table 1.

Table 1 Gellinq Curing Temperature Temperature Time C _ D C ~ ( min) Phenolic resin 120 135 _ 90 Epoxy resin _90 _ 135 __ 90 .
The panel so produced was tested extensively and compared with a standard floor panel. The results of the two test series are given in Table 2.

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Table 2 STANDARD PANEL ACCORDING
__ PANEL TO THE INVENTION
Panel thickness (mm) 9 35-9.65 9.35-9.65 Areal weight (kq/m~) 2.950-3.15 _ _ 2.450-2.650 Drum peel force, W direction (N/76mm~ 350-390 330-360 _ . .. _ __ . __ 4-point bending force, L direction (N) _ 1,450-1,65 Q 2,000-2,300 Deflection at 446 N, L direction (mm~ 7-8 4-5 _ 3-point shear force, W direction (N)_1,900-2,100 _ 2,500-2,800 ATS 1000.001 did not_pass passed Fatigue limit (load cycles) 850 _ _ 2,450 ,:
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Table 2 documents the improved characteristics of a floor panel manufactured in accordance with the invention ~ over a standard panel.

-~The panel in accordance with the invention has a : deflection under load of only 50% to 70% of that of the standard panel, even though its areal weight is lower by 500 g/m2 for the same thickness.

Despite the significantly lower resin content in the panel in accordance with the invention, and despite the fact that the peeling was parallel to the fiber direction of the inner fiber composite layer, i~e. in the W
direction, the reduction in peeling force between the core :

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plate and the covering is negligible.

Also, ~ecause of a ~etter choice o~ fibers, i.e.
unidirectional carbon fibers as opposed to woven E-glass, the 4-point bending force result was improved by 30% to 40%.

The 3-point shear force result in the W direction was improved by 30-35%, because of the better ratio of fiber to resin in the honeycomb of the core plate.

Furthermore, the ATS 1000.001 regulations prescribing the self-extinguishing properties, smoke dens;ty and toxicity of the smoke produced are met owing to the particular construction of the panel wherein a phenol matrix is outside and an epoxy matrix is inside. The standard panel, which was not so constructed, does not satisfy ATS 1000.001.

In addition, because of the symmetric construction of the lightweight flat structure in accordance with the ~ invention, fewer problems with bowing of the panels are ; encountered during manufacture.

; 20 To simulate the life expectancy of a floor panel inthe laboratory, a procedure was developed on the basis of the following assumptions.

Assume that an aircraft makes 851 flights per year, carrying an average of 210 passengers, and has two aisles, i.e. 105 passengers per aisle. The number of loadings per year will then be:

- Passengers boarding and leaving the aircraft:
105 x 2 x 851 = 178,710 loadings :

2 ~

- Visits to toilets plus perambulation:
1.5 per passenger x 105 passengerq x 851 flights =
134,032 loadings - Use by flight crew, 4 persons, 16 times each:

4 persons x 16 x 851 flights = 54,464 loadings - Use by service trolleys:
4 rollers x 8 x 851 flights = 27,232 loadings The total number of loadings for the aisle region is then 394,438 per year, or 462 loadings per flight, which corresponds to one load cycle.

Table 3 shows the distribution by weight of the loadings in a load cycle for an aircxaft operating in Europe and North America. These weights include a dynamic component of 15% for running, ~umping, and dancing in flight.

Table 3 .~ .
~ 60 kq 2.5% 11 loadinqs :~ 70 kq 5.0% 23 loadinqs ~ 80 ka 15 0% 70 loadinqs , ... .. _ _ 100 kq 25.0% 116 loadinqs_ 110 kq 35.0% 162 loadinqs 120 kq 10.0~ 46 loadinqs 130 kq 5.0% 23 loadinqs 145 kq 2.5% 11 loadinqs : Total 100% 462 loadinqs .~

Thus, for each floor panel an individually reproducible Wohler stress-number curve can be constructed, so that a life span c,an be calculated for each floor panel :
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witll reference to the associated load cycle.

The permanent deformat:ion of the panels may not, however, exceed 1 mm.

It can be seen from Table 2 that the standard panels have a fatigue limit of only 850 load cycles; that is, their life span is about 1 year o~ flight operations.

In contrast, a panel in accordance with the invention, despite its lower weight, tolerates 2,450 load cycles; that is, under flight conditions its life span is about 2.9 years.
The weight saved by using the lightweight panel tested in this experiment, for an aircraft with a ~loor area o~
228 m2, is between a maximum of (3.150 kg/m2 - 2.450 kg/m2) x 228 m2 = 159.6 kg and a minimum of (2.950 kg/m2 - 2.650 kg/m2) x 228 m2 = 68.4 kg.
. .
The life span of a floor panel in accordance with the invention can be dramatically increased by doing without the weight savings in comparison with the standard panel.
The mechanical properties of the panel in accordance with the invention can be improved simply by using a honeycomb core with higher weight per unit volume.

.

Claims

1. A composite flat structure for floor panels comprising a rigid cellular core to each of two opposed surfaces of which inner and outer layers of fiber composite materials are attached, and wherein each of said outer layers comprises a layer of E-glass cloth and a layer of unidirectional carbon fibers both embedded in a phenolic resin, each of said inner layers comprises a layer of unidirectional R-glass fibers which are embedded in an epoxy resin and which are arranged to lie transversely with respect to the unidirectional fibers in said carbon-fibers layer, and said phenolic resin and said epoxy resin have substantially compatible gelling temperatures, curing times and curing temperatures so that they can be cured together.

2. A structure as claimed in Claim 1, wherein said cellular core comprises an aramid honeycomb structure formed of aramid ribbons running in an L direction perpendicularly to a W direction whilst defining honeycomb cells with longitudinal axes parallel to a T direction, which is also perpendicular to the W direction..

3. A structure as claimed in Claim 2, wherein said R-glass fibers lie parallel to the W direction of the core and said carbon fibers lie parallel to the L direction of the core.

4. A structure as claimed in Claim 2, wherein said R-glass fibers lie parallel to the L direction of the core and said carbon fibers lie parallel to the W direction of the core.

5. A structure as claimed in Claim 1, wherein the core comprises an aramid honeycomb structure with a cell width between 3.2 and 6.4 mm inclusive and a specific density between 50 and 144 g/cm3 inclusive, and wherein the ratio of said carbon and glass fibers combined to the phenolic and epoxy resins combined is between 83:17 and 37:63 inclusive.

6. A structure as claimed in Claim 1, wherein said phenolic resin has a gelling temperature in the range of 100°C to 120°C, a curing temperature in the range of 120°C
to 155°C, and a curing time in the range of 60 min to 120 min.

7. A structure as claimed in Claim 1, wherein said epoxy resin has a gelling temperature in the range of 80°C to 100°C, a curing temperature in the range of 120°C to 155°C,and a curing time in the range of 60 min to 120 min.

8. A structure as claimed in Claim 1, wherein said R-glass fibers have a maximal tensile strength of 4,400 MPa, a modulus of elasticity of 86,000 MPa and a relative density of 2.55 g/cm3.

9. A structure as claimed in Claim 1, wherein said carbon fibers have a maximal tensile strength of 3,792 MPa, a modulus of elasticity of 234,000 MPa and a relative density of 1.78 g/cm3.

10. A structure as claimed in Claim 1, comprising - a thickness between 9.35 and 9.65 mm inclusive;
- an areal weight between 2.450 and 2.650 kg/m2 inclusive;
- a drum peel force in the W direction between 330 and 360 N/76 mm;
- a 4-point bending force for the L direction between 2,000 and 2,300 N inclusive;

- a deflection for the L direction at 446 N between 4 and 5 mm inclusive;
- a 3-point shear force for the W direction between 2,500 and 2,800 N; and - a fatigue limit of at least 2,400 load cycles.