Method for producing a profile that has a re-entrant angle in a composite material from a stack of layers of fibres

Abstract

A method for producing, in a composite material, a component that has a re-entrant angle, includes stacking layers of fibers which have been pre-impregnated with a resin so as to obtain a preform and placing the preform in a tooling fixture in order to subject it to a polymerization cycle including a temperature cycle, with a temperature-increase phase and at least one soak at which the temperature is maintained, and a pressure cycle on the outside of the tooling fixture with a pressure-increase phase and a pressure-hold phase, with the layers of pre-impregnated fibers being able to slip against one another when the temperature of the temperature cycle is equal to or higher than a threshold temperature Tf dependent on the resin. The temperature-increase phase includes a soak at a temperature higher than or equal to Tf, the soak beginning before the end of the pressure-increase phase.

Claims

1. A method for producing, in a composite material, a component that has a re-entrant angle, the said method comprising: stacking, in a mold of a tooling fixture, layers of fibres which have been pre-impregnated with a resin so as to obtain a preform having said re-entrant angle, the stack of layers of pre-impregnated fibres comprising a defect of the bridging type at the re-entrant angle; subjecting the preform disposed in the tooling fixture to a polymerization cycle comprising a temperature cycle including a temperature-increase phase and a first soak at which temperature is maintained, a second soak at which temperature is maintained and a third soak at which temperature is maintained, and a pressure cycle which includes subjecting the preform to a pressure at a location outside of the tooling fixture, the pressure cycle including a pressure-increase phase and a pressure-hold phase; determining a threshold temperature Tf of the resin wherein, during the temperature-increase phase, when the temperature is equal to or higher than said threshold temperature Tf, the layers of fibres are able to slip against one another without wrinkling; subjecting the preform to a vacuum-pulling cycle in which a vacuum is pulled on the inside of the tooling fixture, the vacuum-pulling cycle beginning before the start of the pressure cycle and at the same time as the start of the temperature cycle so that the fibres are subjected to tensile loadings before the layers of pre-impregnated fibres are compressed, the defect of the bridging type allowing the fibres in the preform to be stretched out; and causing, during the polymerization cycle, the layers of pre-impregnated fibres to slip against one another when the temperature of the temperature cycle is equal to or higher than the threshold temperature Tf which is dependent on the resin, characterized in that at least one of said first soak, said second soak and said third soak is at a temperature higher than or equal to the threshold temperature Tf, and at least one of said first soak, said second soak and said third soak beginning after the start of the pressure-increase phase, characterized in that the temperature cycle includes subjecting the preform to a temperature decrease phase after said first soak, said second soak and said third soak to bring the temperature to a lower temperature than the temperature of the start of polymerization and ending the pressure-hold phase after start of said temperature decrease phase, and characterized in that the method comprises using a forming tool interposed between the preform and a bag at the re-entrant angle.

2. The method according to claim 1, characterized in that said first soak begins before the end of the pressure-increase phase and said second soak begins after the start of the pressure-increase phase.

3. The method according to claim 2, characterized in that the duration of the first soak prior to the start of the pressure cycle is long enough to obtain a uniform temperature throughout the preform that is higher than or equal to the threshold temperature Tf.

4. The method according to claim 3, characterized in that the duration of the first soak prior to the start of the pressure cycle is greater than or equal to 30 min.

5. The method according to claim 1, characterized in that both the second soak and the third soak are performed during the pressure-hold phase.

6. The method according to claim 5, characterized in that both the second soak and the third soak are performed during the pressure-hold phase.

7. The method according to claim 1, characterized in that said first soak is at a temperature higher than or equal to the threshold temperature Tf.

8. The method according to claim 1, characterized in that said second soak is at a temperature higher than or equal to the threshold temperature Tf.

9. The method according to claim 1, characterized in that said third soak is at a temperature higher than or equal to the threshold temperature Tf.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Other features and advantages will become evident from the following description of the invention, which description is given solely by way of example with reference to the attached drawings in which

(2) FIG. 1 is a perspective view of a portion of an aircraft fuselage frame,

(3) FIG. 2 is a perspective view of a stack of layers of fibres on a rectilinear deformable mandrel with cutaway so as to illustrate the orientation of the fibres in the stacked layers,

(4) FIG. 3 is a cross section through a tooling fixture in which a preform produced by stacking layers of fibres has been positioned,

(5) FIG. 4 is a schematic depiction of temperature, pressure and vacuum-pulling cycles during a polymerization cycle according to the prior art,

(6) FIG. 5A is a section through a preform illustrating a defect of the bridging type and the loads generated by the exterior pressure during a temperature-increase phase in a polymerization cycle according to the prior art,

(7) FIG. 5B is a section through the preform illustrated in FIG. 5A, illustrating the formation of wrinkles during a polymerization cycle according to the prior art,

(8) FIG. 6 is a schematic depiction of temperature, pressure and vacuum-pulling cycles during a polymerization cycle according to the invention,

(9) FIG. 7A is a section through a preform illustrating a defect of the bridging type during a temperature-increase phase during a polymerization phase according to the invention, and

(10) FIG. 7B is a section through the preform illustrated in FIG. 7A illustrating the correction of the bridging defect during the polymerization phase.

DETAILED DESCRIPTION

(11) FIG. 7A depicts, at 70 a preform of pre-impregnated fibres which is obtained by stacking layers 72 of fibres.

(12) The term layer can mean a ply of fibres or a strip comprising several plies. In the remainder of the description, the longest dimension of the layers corresponds to the longitudinal direction.

(13) The stack of layers of fibres 72 is performed on a mould 74 which may also be used during the polymerization phase.

(14) This preform 70 comprises at least one re-entrant angle 76, the surface of the preform 70 which is not in contact with the mould 74 having a shape that is concave in a plane of section perpendicular to the longitudinal direction.

(15) Given the tack of the layers of pre-impregnated fibres, a defect 78 of the bridging type almost always occurs when stacking the layers 72. This defect 78 of the bridging type comprises a space 80 between successive layers 72a and 72b at the re-entrant angle 76. Thus the layers 72a and 72b are no longer in contact with one another between a first point P1 situated on one side of the vertex of the re-entrant angle and a point P2 situated on the other side of the re-entrant angle. Between these points P1 and P2, the arc length of the layer 72a is greater than the length of the arc of the layer 72b.

(16) During the polymerization cycle, the preform 70 is placed in a tooling fixture comprising a mould 74 and a covering. This tooling fixture may be identical to that of the prior art. Thus, to make the depiction simpler, only the forming tool 82 and the bag 84 of the covering have been depicted in FIGS. 5A and 5B. As in the prior art, when the pressure on the outside of the tooling fixture increases, the bag 84 applies a compressive load which tends to press the forming tool 82 firmly against the preform 70.

(17) For preference, the tooling fixture comprises a forming tool 82 positioned between the preform and the bag at the re-entrant angle to ensure that the re-entrant angle is formed correctly. Thus, in general, if the bag 84 is not associated with a forming tool 82, the tooling fixture is unable to achieve the geometry of the re-entrant angle.

(18) As illustrated in FIG. 6, the polymerization cycle comprises a temperature cycle 85 and a pressure cycle.

(19) For preference, the pressure cycle comprises a pressure cycle 86 on the outside of the tooling fixture and a vacuum-pulling cycle 88 on the preform 70 on the inside of the tooling fixture in the zone delimited by the mould 74 and the bag 84.

(20) The pressure cycle 86 on the outside of the tooling fixture begins at the instant T1 and comprises a pressure-increase phase 90, a pressure-hold phase 92 and a return phase, in which the pressure returns to atmospheric pressure, 94, The increase in pressure is very rapid such that the instant T1′ that corresponds to the start of the pressure-hold phase 92 is very close to T1.

(21) The temperature cycle 85 comprises a temperature-increase phase, one or more temperature soaks 96 and a cooling phase 98.

(22) The end of the pressure hold phase 92 begins when the resin has solidified and occurs during the cooling phase 98. Thus, the end of the pressure-hold phase 92 comes after the start of the cooling phase.

(23) By way of example, during the pressure-hold phase 92, the pressure on the outside of the tooling fixture is of the order of 9 bar.

(24) According to the illustrated example, the temperature cycle 85 comprises two soaks 96 and 96′, one at 135° C. and the other at 180° C. As an indication of an order of magnitude, the two soaks have a duration of the order of 120 to 180 min.

(25) Of course, the invention is not restricted to this number of soaks, or to these soak durations, or to these pressure and temperature values. All these features will be adjusted by a person skilled in the art notably according to the rheology of the resin.

(26) During the temperature increase, when the temperature is equal to or higher than a threshold temperature Tf dependent on the resin, the layers of pre-impregnated fibres can slide against one another without wrinkling.

(27) This threshold temperature Tf is determined by testing. It varies according to the resin, to the fibres, to the types of weaving in the layer, etc.

(28) As an indication of an order of magnitude, the threshold temperature Tf varies between 50 and 65° C.

(29) According to the invention, the preform is held at a soak temperature higher than or equal to the threshold temperature Tf before the end of the pressure-increase phase 90. Advantageously, the soak temperature is lower than the temperature at which polymerization begins, which corresponds approximately to the temperature of the first temperature soak 96 of the temperature cycle. For preference, the preform is held at the soak temperature before the start of the pressure cycle.

(30) Thus, according to the invention, the temperature-increase phase comprises a soak 100 at a temperature higher than or equal to the threshold temperature Tf but preferably lower than the temperature of the start of polymerization, the said soak 100 beginning before the end of the pressure-increase phase 90 and preferably before the start of the pressure cycle 86.

(31) The soak 100 is at a temperature of the order of 80° C.

(32) The duration of the soak 100 prior to the start of the pressure cycle needs to be long enough to obtain a uniform temperature throughout the preform that is higher than or equal to the threshold temperature Tf. By way of example, the duration of the soak 100 prior to the start of the pressure cycle 86 is greater than or equal to 30 min.

(33) Advantageously, the vacuum-pulling cycle 88 begins before the start of the pressure cycle 86 and preferably at the same time as the temperature cycle 85. This feature allows the fibres to be subjected to tensile loadings before the layers are compressed in as much as the ends of the fibres are sucked down at the periphery of the preform, and also allows any air included within the layers of the preform to be extracted.

(34) FIG. 7A depicts the preform 70 during the soak 100 prior to the start of the pressure cycle 86. In this case, the bag 84 and the forming tool 82 are applying only a very light load to the preform 70 because of the vacuum that has been pulled. These loads are markedly lower than the compression loads 104 applied by the bag 84 and the forming tool 82 during the pressure cycle 86, therefore these loads have not been depicted.

(35) During this soak 100, the conditions necessary for the layers 72 to be able to slip over one another as illustrated by the arrows 104 are created. The pulling of the vacuum allows tensile loads 106 to be applied to the fibres of the layers.

(36) When the pressure cycle 86 is begun, the bag 84 and the forming tool 82 apply compression loads 102 to the preform. Because the resin is sufficiently fluid, the layers can slip relative to one another, which prevents the wrinkling that is likely to occur in the prior art. At the end of polymerization, the layers are all pressed firmly together without defect, namely without any spaces and without any wrinkles at the re-entrant angles.

(37) According to the invention, the bridging defects no longer give rise to a new defect during the polymerization cycle but rather become an advantage in as much as the fibres experience light tension as a result of the slippage that occurs between the layers. By virtue of the defects of the bridging type, the fibres are also all stretched out in the preform and any potential local excesses in the lengths of the fibres are eliminated.