WO2007027093A1

WO2007027093A1 - Method for producing a reinforced molding

Info

Publication number: WO2007027093A1
Application number: PCT/NL2006/050213
Authority: WO
Inventors: Geerardus Hubertus Joannes Jozeph Roebroeks
Original assignee: Alcoa Inc.
Priority date: 2005-09-01
Filing date: 2006-09-01
Publication date: 2007-03-08
Also published as: NL1029854C2

Abstract

The invention relates to a method for producing a reinforced molding, with the molding being accommodated in a mold, such that the expansion thereof when subjected to a change in temperature is at least partially impeded, with at least one reinforcing part being affixed to the molding at a first temperature, such that the expansion thereof when subjected to a change in temperature is not impeded at least substantially, and with the assembly of molding and at least one reinforcing part being heated to a second higher temperature, at which at least one reinforcing part and the molding are connected to each other using an appropriate means of connection. The invention also relates to a device for implementing the method and a plate-shaped semi- finished product that is obtainable by the method.

Description

Method for producing a reinforced molding

The invention relates to a method for producing a reinforced molding. The invention also relates to a device for producing the reinforced molding.

Moldings made of metal for instance are frequently used in the transportation industry, for example in cars, trains and aircraft. In sectors in which weight savings play a predominant role, for example in the aviation and space industry, such moldings are generally produced out of relatively light metals, such as aluminum or titanium alloys. Fiber-reinforced plastics are also increasingly used due to their high specific mechanical properties (properties divided by density). Moldings for the aviation and space industry, such as wings, fuselage and tail panels, shell panels, stiffeners, etcetera, must be resistant to any damage that may occur, in view of the high risks involved in flying. Such resistance to damage is also referred to in the art as "damage tolerance". Damage tolerance is also understood to mean resistance to the further growth of cracks that have appeared in the molding. Crack growth will generally be driven by the alternating load exerted on the molding in practice (also referred to as fatigue load). The cracks can reach such a length that the molding or a part thereof will fail. Such failure under fatigue is obviously not desired. For this reason prior art moldings are at least locally reinforced with reinforcing parts.

A known method for producing such a reinforced molding, for example (part of) an aircraft wing made of aluminum, comprises affixing at least one reinforcing part, for example a strip made of a relatively strong material that is at least less sensitive to fatigue, to the molding and fastening it using an appropriate adhesive. If fatigue cracks develop in the molding when subjected to the alternating load and they continue to grow under the reinforcing parts, said reinforcing parts will generally remain intact in practice and span the cracks in the molding, thus ensuring at least a deceleration in average crack growth when subjected to alternating load.

Although the formation of cracks under alternating load can at least partially be decelerated using the known method, there is a need for an improved method. It has furthermore proven difficult with the known method to obtain moldings that demonstrate effective molding stability. The object of this invention is to provide an improved method for producing a reinforced molding, with the molding having good mechanical properties and in particular demonstrating improved damage tolerance.

The method according to the invention is thereto characterized as referred to in claim 1. More particularly, the method according to the invention comprises accommodating the molding in a mold, such that the expansion thereof when subjected to a change in temperature is at least partially impeded, affixing at least one reinforcing part to the molding at a first temperature, such that the expansion thereof when subjected to a change in temperature can take place at least substantially without hindrance, heating an assembly of the molding and at least one reinforcing part to a second higher temperature, connecting at least one reinforcing part and the molding to each other at the second temperature using appropriate connecting means, and finally removing the reinforced molding formed in this way from the mold. According to the present invention, a compressive stress is formed by at least partially impeding the molding from expanding when the temperature changes from the first to the second temperature, said compressive stress at least partially remaining, even after connecting the molding and reinforcing part to each other at the second temperature and cooling the connected assembly. This compressive stress probably helps to ensure that a molding produced using the method according to the invention in practice demonstrates a lower crack growth velocity under alternating load than the molding obtained using the known method.

Crack formation and growth in a specific position of the molding are largely determined by the stress field in situ. A crack can thus for example demonstrate the tendency to reproduce in a direction that is substantially perpendicular to the direction in which the largest positive principal stress is located. Knowledge of the stress field in the molding under normal operating conditions can therefore be important in order to determine the positions in the molding that have the greatest risk of cracks forming and further growing. The method according to the invention is furthermore advantageous in that it makes it possible to favorably affect the stress field in the molding if required only at specific positions (locally), by affixing reinforcing parts at these positions using the method according to the invention. A molding for example made of aluminum can expand without hindrance when subjected to a change in temperature that constitutes an increase in temperature for most materials. Although less simple to achieve, there are also other ways of allowing a molding to expand, such as via the intake of moisture. According to the invention, the molding is subjected to a change in temperature, such that the expansion of the molding is at least partially impeded. A possible means of impeding the expansion at least partially is to apply a mold that is provided with at least two retaining plates in the direction in which expansion is to be impeded, between which the molding can be accommodated so that it is almost fitting or tightly fitting. By producing the mold out of a material that expands less than the material of the molding, the retaining plates will not move apart from each other as far as the molding wants to expand when subjected to an increase in temperature, thus forming a compressive stress in the molding. In practice, it is possible to impede the expansion of the molding in any direction - along its length, depth and/or height. In a preferred embodiment of the method according to the invention, however, the molding is relatively flat and expansion thereof is impeded in at least one direction of impedance in the plane of the molding. By applying the method according to the invention on a relatively flat molding - the length and depth of which are therefore significantly greater than the height of it - the required compressive stress is achieved in large sections of the molding, in turn further increasing resistance to cracking.

The geometric form of the at least one reinforcing part can be selected from a wide range. For instance, it is possible to apply three-, two- and/or one-dimensionally formed reinforcing parts, if required with relatively small dimensions with respect to the molding, so that local reinforcement can take place. In a preferred embodiment of the method according to the invention, at least one relatively elongated reinforcing part can be applied, the longitudinal axis of which substantially travels in the intended direction of impedance. Such a reinforcing part is relatively simple to affix.

The reinforcing parts applied in the method according to the invention can be attached to the molding in any appropriate fashion. Appropriate connecting means comprise for example bolt and/or mortise connections, and/or other mechanical connections. These may be affixed at specific points such as along the side edges of the reinforcing part, or be uniformly spread over the surface of the reinforcing part. The connection is preferably achieved by applying a layer of adhesive between molding and reinforcing part, if required discontinuously but preferably over almost the entire interface between reinforcing part and molding. By achieving a connection in this way, a molding is obtained, the reinforced section of which at least is under a mainly uniform compressive stress in an unloaded state. It turned out that such a molding can have considerably higher damage tolerance under specific fatigue loads. For instance, the damage tolerance can be improved by a factor of 2 to 4 compared with a molding that has been produced according to the known method.

Although according to the invention it is possible to apply any appropriate adhesive to connect molding and reinforcing part, the adhesive layer preferably comprises a thermosetting adhesive that can cure at the second temperature. Such adhesives can ensure an almost permanent connection that is furthermore relatively insensitive to time-dependent effects such as creep and stress relaxation. As a result, when the molding is in use, it will demonstrate the intended improved level of damage tolerance for a large part of its life. Appropriate techniques for applying the adhesive layer on the reinforcing part and/or molding comprise lamination, spraying, dipping, coating with a roller, coil coating, immersion, etcetera. It is also possible to apply the adhesive layer in advance onto (sections of) the reinforcing part. If desired, reinforcing part and/or molding are pretreated at the indicated points to protect for example against corrosion, as well as to improve adherence. Examples of such pretreatments include degreasing, chromating, anodizing, roughening, applying a conversion coating and/or primer.

When reference is made in this application to a first and second temperature, it is understood to mean an average temperature. It should be noted that when reference is made for instance to the affixing of the reinforcing part at a first temperature, this means that the assembly of molding, reinforcing part and connecting means has on average assumed the first temperature, subject to local variations. The same applies with respect to the step in the method in which the connection between molding and reinforcing part is achieved and the assembly is at the second temperature. Here too it is possible for there to be a different temperature locally, albeit one that does not differ too greatly from the average second temperature. The level of the first temperature can be selected according to the circumstances, but it will preferably lie in the region of room temperature. The level of the second temperature can easily be selected by a person skilled in the art and is for example dependent on the type of connecting means applied. An appropriate method for heating the structure to the first and/or second temperature is to immerse the entire structure in a heatable pressure vessel or autoclave and to heat it using a heated gas, generally air. Once the structure is heated to the second temperature, the components of the structure can be fastened to each other, if required under pressure. It is also possible to heat the structure up using radiant heat, for example infrared (IR), and to interconnect the structure subject to pressure between pressure plates that are also heated if required.

In a preferred embodiment of the method according to the invention, the first and second temperatures are selected such that the difference between the second and first temperature is at least 80°C. This difference will preferably be at least 110°C, and most preferably at least 140°C. Apparently this forms a higher average residual compressive stress in the unloaded molding, which surprisingly leads to an above-average increase in damage tolerance under alternating load.

According to the invention, there are no particular restrictions to the material out of which the mold, reinforcing parts and molding are produced. For instance, a metal can be used for the mold, reinforcing part and molding, such as aluminum or (lightweight) alloys thereof, stainless steel, cold-deformed steel, electroplated steel, magnesium, lithium, titanium, copper and/or alloys thereof. In view of weight savings, it is advantageous in the aviation and space industry to construct the molding and/or reinforcing parts out of a lightweight metal, such as aluminum or titanium alloys. Fiber- reinforced plastics can also be used due to their high specific mechanical properties

(properties divided by density). In view of operating life, hardness and other properties required for a mold, it is advantageous if the mold material comprises an iron alloy or steel.

In a preferred embodiment of the method according to the invention, the materials for mold and reinforcing part are selected such that the thermal expansion coefficient of the mold material is lower than the thermal expansion coefficient of the material of the reinforcing part. In this embodiment, the mold can for example be provided with means of impedance in the form of retainer plates forming an integral part of the mold or attached thereto. By accommodating the molding at least partially between the retainer plates of the mold and then increasing the temperature, a compressive stress will be created in the molding, because the mold material will demonstrate less expansion than the reinforcing part and the molding can only expand in the direction of impedance as far as the mold material permits. It is furthermore advantageous to attach the retainer plates - and means of impedance in general - to the mold in such a way that they can be detached. This can for example be achieved by inserting holes in the mold and means of impedance, for example in the form of a plate. The impedance plates can then be affixed to the mold in the required position by means of dowel pins that are fitted through the holes. There are various options available to the person skilled in the art. For instance the means of impedance can be secured in the required position on the mold by applying a vacuum pressure.

In a further preferred embodiment of the method according to the invention, the thermal expansion coefficient of the material of the at least one reinforcing part differs from the thermal expansion coefficient of the material of the molding. By this is achieved that the level of the compressive stress formed in the molding can easily be set. It is hereby particularly advantageous to characterize the method in that the thermal expansion coefficient of the material of the at least one reinforcing part is lower than the thermal expansion coefficient of the material of the molding. In the known method for affixing reinforcing parts on a molding, it is considerably disadvantageous for the thermal expansion coefficient of the material of the reinforcing part to be lower than the thermal expansion coefficient of the molding material. Indeed, by connecting both parts to each other at room temperature according to the prior art, a tensile stress will be formed in the molding at a temperature below room temperature, which is undesirable - temperatures of use in the aviation and space industry are generally below room temperature. If both parts are connected at a temperature higher than room temperature, this would also apply to all operating temperatures up to this higher temperature. The present preferred embodiment of the method according to the invention does not have these disadvantages.

It is furthermore advantageous to characterize the method according to the invention in that the ratio of the modulus of rigidity of the material of the at least one reinforcing part to the modulus of rigidity of the material of the molding is at least 85%, and more preferably at least 100%, and most preferably at least 130%. The modulus of rigidity (also referred to as the modulus of elasticity) of the materials is simple to set by a person skilled in the art by making an appropriate choice of material.

It is advantageous to characterize the method according to the invention in that the material of the reinforcing part comprises a fiber-reinforced plastic. Such materials are light and strong. Reinforcing fibers that are appropriate for use include for example glass fibers, carbon fibers, metal fibers, drawn thermoplastic polymer fibers, such as aramid fibers, PBO fibers (Zylon®), M5® fibers, and ultrahigh molecular weight polyethylene or polypropylene fibers, as well as natural fibers such as flax, wood and hemp fibers, and/or combinations of the above fibers. It is also possible to use commingled and/or intermingled rovings. Such rovings comprise a reinforcing fiber and a thermoplastic polymer in fiber form. Examples of appropriate matrix materials for the reinforcing fibers are thermoplastic polymers such as polyamides, polyimides, polyethersulphones, polyetheretherketone, polyurethane, polyethylene, polypropylene, polyphenylene sulphides (PPS), polyamide-imides, acrylonitrile butadiene styrene (ABS), styrene/maleic anhydride (SMA), polycarbonate, polyphenylene oxide blend (PPO), thermoplastic polyesters such as polyethylene terephthalate, polybutylene terephthalate, as well as blends and copolymers of one or more of the above polymers, and thermosetting polymers such as epoxies, unsaturated polyester resins, melamine/formaldehyde resins, phenol/formaldehyde resins, polyurethane, etcetera.

In a preferred embodiment of the method, the fiber-reinforced plastic substantially comprises continuous fibers that extend in two almost orthogonal directions (isotropic woven fabric). In another preferred embodiment, the fiber-reinforced plastic substantially comprises continuous fibers that mainly extend in one direction (UD tissue). It is advantageous to use the fiber-reinforced plastic in the form of a pre- impregnated semi- finished product. Such a "prepreg" can easily be processed and is also advantageous in that the required fiber direction can be set.

To achieve the required high level of rigidity and strength in the direction of impedance, it is advantageous to use a fiber-reinforced plastic in the method that substantially comprises continuous fibers mainly extending in one direction. The fibers are then preferentially oriented at least partially in the direction of impedance. The continuous fibers preferably extend mainly in a direction in which an angle of a maximum of 15 degrees is made with the direction of impedance. More preferably, this angle is at most 10 degrees. Most preferably, the continuous fibers extend substantially parallel to the direction of impedance.

Another particularly appropriate material for the reinforcing part comprises a laminate of at least two metal layers and an intermediate fiber-reinforced plastic layer. Such a material is known to the person skilled in the art under the trade name Arall® (with polyaramid fibers) or Glare® (with glass fibers). This material is preferably used in a prestressed form, whereby the fibers of the intermediary fiber-reinforced plastic layer are on average subjected to a tensile stress and the metal layers to a compressive stress. According to the invention, it is also possible to combine different reinforcing parts in the method, if required with each reinforcing part being produced out of a different material. In a particularly appropriate method according to the invention, a reinforcing part in the form of an aluminum profile is for example connected to an aluminum molding by means of a number of reinforcing parts in the form of strips of Glare® material. Fibrous metal laminates containing other fibers, such as Zylon®, M5® and/or carbon can also be used according to the invention. It is also possible to use reinforcing elements, such as stiffeners and strips, made of fibrous metal laminate, to reinforce moldings made of fibrous metal laminate.

Further features of the invention will emerge from the following examples and accompanying figures, in which:

Figure Ia schematically shows a top view of the mold and starting materials, as applied in an embodiment of the method according to the invention;

Figure Ib schematically shows a side view of the mold and starting materials for the embodiment referred to in Figure 1 ;

Figure 2 schematically shows a product in perspective that is achieved using a different embodiment of the method according to the invention; Figure 3a schematically shows a cross-section of a product that is achieved using the method according to the invention;

Figure 3b schematically shows another cross-section of a product that is achieved using the method according to the invention. Example 1.

With reference to Figures Ia and Ib, a reinforced molding 1 is produced by providing a mold 2 in which the molding 1 can be accommodated or otherwise supported. The mold 1 is provided with means of impedance 3 for at least partially impeding the expansion of the molding 1 when subjected to a change in temperature. In the embodiment shown, the means of impedance 3 are formed by steel plates 3 with support edge 3a attached to the mold 1 via bolts 4. An aluminum molding that is 1000 mm long, 300 mm wide and 2 mm deep is accommodated between steel plates 3, such that each side edge Ia lies as closely as possible (tightly fitting) against support edge 3a. Two reinforcing parts 5 in the form of two aluminum strips that are 2 mm deep and 75 mm wide were then affixed to the molding 1 at room temperature (the first temperature) by means of an intermediate adhesive layer 6, as shown in Figures Ia and Ib. Before applying the adhesive, the aluminum molding was sanded using standard sanding paper. The adhesive layer comprised an epoxy adhesive of type AF 163-2 K, available from 3M. Reinforcing parts 5 were then affixed to the molding 1 such that the expansion thereof can at least take place when subjected to a change in temperature substantially without hindrance. It can be seen in Figure Ia that this is simple to achieve by making reinforcing parts 5 slightly shorter than the distance between the two plates 3. The assembly of mold 2, molding 1 and reinforcing parts 5 was then placed in an autoclave. Subject to a pressure of 6 bar, the assembly was heated to and maintained at a second temperature of 120⁰C for approximately 1 hour to cure the adhesive layer 6 between reinforcing parts 5 and molding 1. The assembly was then cooled until almost room temperature and then removed from the mold.

The average compressive stress created in the molding 1 was determined by measuring the curvature radius of the reinforced molding, and amounted to approximately 6.7 MPa.

Example 2.

In the same way as described above, two reinforcing parts 5 in the form of an aluminum stiff ener 5b and two Glare® glass fiber laminates 5 a were affixed to an aluminum plate 1, as shown in Figure 3b. The same adhesive layer 6 as in example 1 was applied between reinforcing parts 5a and 5b, and molding 1. During the autoclave process, it is ensured that the thermal expansion of reinforcing parts 5 a and 5b can take place with almost no hindrance. Any hindrance that may occur is caused by the curing adhesive layer 6. The average compressive stress formed in the molding 1 amounted to approximately 9.4 MPa.

Example 3 (not shown in the figures).

In the same way as described above, two reinforcing parts in the form of a Glare® glass fiber laminate 5 a were affixed to an aluminum plate 1. The same adhesive layer 6 as in example 1 was applied between reinforcing parts 5a and molding 1. The average compressive stress formed in the molding 1 amounted to approximately 10 MPa.

In all the cases described above, the average compressive stresses formed in the molding 1 by means of the method according to the invention lead to a reduction of at least approx. 50% in crack growth velocity measured under alternating load. With respect to appropriate test methods for determining the crack growth velocity, reference is made among others to the standard publication known to the person skilled in the art: Military Handbook 5.

The moldings obtained using the method according to the invention can be used in industrial applications as lightweight structural elements, for example in civil engineering structures, buildings, vehicles and ships, and are preferably used in applications in which the operating temperatures are generally lower than room temperature, for example in the aviation and space industry.

Claims

1. Method for producing a reinforced molding, comprising a) accommodation of the molding in a mold such that the expansion thereof when subjected to a change in temperature is at least partially impeded; b) affixing of at least one reinforcing part onto the molding at a first temperature, such that the expansion thereof when subjected to a change in temperature is at least substantially not impeded; c) heating of the assembly of molding and at least one reinforcing part to an increased second temperature; d) connection of the at least one reinforcing part and molding to each other at the second temperature using appropriate connecting means; e) removal of the reinforced molding formed in this way from the mold.

2. Method according to claim 1, characterized in that the molding is relatively flat and expansion thereof is impeded in at least one direction of impedance in the plane of the molding.

3. Method according to claim 1 or 2, characterized in that the at least one reinforcing part is relatively elongated and the longitudinal axis thereof substantially travels in the at least one direction of impedance.

4. Method according to any one of the preceding claims, characterized in that the connecting means form an adhesive layer between the molding and at least one reinforcing part.

5. Method according to claim 4, characterized in that the adhesive layer comprises a thermosetting adhesive that can cure at the second temperature.

6. Method according to any one of the preceding claims, characterized in that the thermal expansion coefficient of the mold material is lower than the thermal expansion coefficient of the material of the at least one reinforcing part.

7. Method according to any one of the preceding claims, characterized in that the thermal expansion coefficient of the material of the at least one reinforcing part differs from the thermal expansion coefficient of the material of the molding.

8. Method according to claim 7, characterized in that the thermal expansion coefficient of the material of the at least one reinforcing part is lower than the thermal expansion coefficient of the material of the molding.

9. Method according to any one of the preceding claims, characterized in that the difference between the second and first temperatures is at least 80⁰C.

10. Method according to claim 9, characterized in that the difference between the second and first temperatures is at least 140⁰C.

11. Method according to any one of the preceding claims, characterized in that the ratio of the modulus of rigidity of the material of the at least one reinforcing part to the modulus of rigidity of the material of the molding is at least 85%.

12. Method according to claim 11, characterized in that the ratio of the modulus of rigidity of the material of the at least one reinforcing part to the modulus of rigidity of the material of the molding is at least 130%.

13. Method according to any one of the preceding claims, characterized in that the material of the reinforcing part comprises a fiber-reinforced plastic.

14. Method according to claim 13, characterized in that the fiber-reinforced plastic substantially comprises continuous fibers that extend mainly in one direction.

15. Method according to claim 14, characterized in that the continuous fibers extend mainly in a direction in which an angle of a maximum of 15 degrees is made with the direction of impedance.

16. Method according to claim 15, characterized in that the continuous fibers mainly extend parallel to the direction of impedance.

17. Method according to any one of the preceding claims, characterized in that the material of the reinforcing part comprises a laminate of at least two metal layers and an intermediate fiber-reinforced plastic layer.

18. Method according to claim 17, characterized in that the laminate is prestressed, whereby the fibers of the intermediate fiber-reinforced plastic layer on average are subjected to a tensile stress and the metal layers to a compressive stress.

19. Device for producing a reinforced molding, said device comprising at least one mold that can accommodate the molding, and means of impedance for at least partially impeding the expansion of the molding when subjected to a change in temperature, whereby the device also comprises heating means to heat the assembly of molding and at least one reinforcing part to a second temperature.

20. Device according to claim 19, characterized in that the means of impedance are formed by a retaining plate forming an integral part of the mold or attached thereto, between which at least a section of the molding can be accommodated.

21. Device according to claim 19 or 20, characterized in that the means of impedance are attached to the mold in such a way that they can be detached.

22. Plate-shaped semi- finished product of at least one metal plate and reinforcing parts affixed thereto in accordance with the method according to any one of claims 1- 18.

23. Plate-shaped semi- finished product of at least one metal plate and reinforcing parts affixed thereto, obtainable following the method according to any one of claims 1- 18, with an average compressive stress of at least 5 MPa in at least one metal plate in the direction of impedance at the first temperature.

24. Plate-shaped semi-finished product according to claim 23, with an average compressive stress of at least 5 MPa in at least one metal plate in the direction of impedance at the operating temperature.