NL1029854C2 - Method for manufacturing a reinforced molded part. - Google Patents

Description

Method for manufacturing a reinforced molded part

The invention relates to a method for manufacturing a reinforced molded part. The invention also relates to a device for manufacturing the reinforced molded part.

Molded parts from, for example, metal are frequently used in the transport industry, such as, for example, in cars, trains and airplanes. In sectors where weight saving plays a predominant role, such as, for example, in aviation and space travel, such molded parts are generally manufactured from relatively light metals, such as, for example, from aluminum or titanium alloys. Fiber-reinforced plastics are also increasingly being used because of their high specific mechanical properties (properties divided by density). Shaped parts for the aerospace industry, such as for example wings, hull and tail panels, skin panels, stiffeners, and the like, must be able to withstand any damage caused due to the high risks associated with flying. Such resistance to damage is also referred to in the professional world by the term "damage tolerance". Damage resistance is understood to mean, among other things, that cracks that have once formed in the molded part are subject to resistance as they grow further. Growth of cracks will usually be driven by the alternating load exerted on the molded part in practice (also referred to as fatigue load). The cracks can hereby be of such a length that the molded part or a part thereof collapses. Such a collapse under fatigue is of course undesirable. For that reason, in the state of the art, molded parts are at least locally reinforced with reinforcing parts.

A known method for manufacturing such a reinforced molded part, for example a (part of an) aircraft wing made of aluminum, consists of applying at least one reinforcing part, for example a strip of a relatively strong and at least reduced susceptible sensitive material, to the molded part and attach to it with a suitable glue. If fatigue cracks develop in the molded part under the influence of the alternating load, and if they grow underneath the reinforcing parts, they will in practice usually remain intact, bridging the 10298 54_2 cracks in the molded part, and as a result can at least cause a delay of the average crack growth under the influence of the alternating load.

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Although cracking under alternating load can be at least partially delayed with the known method, there is a need for an improved method. Furthermore, it has proved difficult to obtain molded parts with the known method which exhibit good form stability.

The present invention has for its object to provide an improved method for manufacturing a reinforced molded article, wherein the molded article exhibits good mechanical properties, and in particular an improved damage resistance.

To that end, the method according to the invention has the features as stated in claim 1. More specifically, the method according to the invention comprises the incorporation of the molded part in a mold, and such that expansion thereof is at least partially prevented under the influence of a change in temperature, applying a first temperature of at least one reinforcement part to the molded part, such that expansion thereof can take place under the influence of a temperature change at least substantially unimpeded, bringing the assembly of molded part and at least one reinforcement part to an elevated second temperature, the second temperature mutually connecting the at least one reinforcement part and the molded part by means of suitable connecting means, and finally removing the thus formed reinforced molded part from the mold. According to the present invention, by at least partially preventing the expansion of the molded part upon the transition from the first to the second temperature, a compressive stress is developed herein, which also after the connecting of molded part and reinforcing part at the second temperature and cooling of the connected part assembly at least partially remains intact. This compressive stress probably also contributes to the fact that a molded part produced with the method according to the invention shows in practice a reduced crack growth rate under alternating load than the molded part obtained with the known method.

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Cracking and growth in a certain position of the molded part are largely determined by the stress field at the location. For example, a crack may show a tendency to propagate in a direction that is substantially perpendicular to the direction in which the largest positive main voltage is located. Knowledge of the stress field in the molded part under normal conditions of use can therefore be important to be able to determine the positions in the molded part with the greatest risk for the occurrence and further growth of cracks. The method according to the invention has the additional advantage that it makes it possible to influence the voltage field in the molded part favorably only at certain positions (locally), by providing reinforcement parts at these positions according to the invented method.

Expansion of a molded part from, for example, aluminum can take place without hindrance under the influence of a temperature change, which will be a temperature increase for most materials. Although less easy to realize, other ways of allowing a molded part to expand, such as for example by moisture absorption, can also be used. According to the invention, the molded part is subjected to a temperature change and such that the expansion of the molded part is at least partially prevented. A possible way to at least partially prevent the expansion is to use a mold which, in the direction in which expansion is to be prevented, is provided with at least two retaining plates, between which the molded part can be received virtually fittingly or fittingly. By manufacturing the mold from a material that expands less than it! material of the molded part, with an increase in temperature the retaining plates 25 will be removed less far apart than the molded part would expand, so that this molded part is brought under a compressive stress. In practice, it is possible to prevent expansion of the molded part in any direction - length, width and / or height. In a preferred embodiment of the method according to the invention, however, the molded part is relatively flat and expansion thereof is prevented in at least one preventing direction in the plane of the molded part. By applying the method according to the invention to a relatively flat molded part - the length and width of which are therefore considerably greater than the height - the desired compressive stress is achieved in large parts of the molded part, as a result of which the tear resistance further increases.

1029854_ 4

The geometric shape of the at least one reinforcement member can be selected within wide limits. It is thus possible to use three, two, and / or one-dimensionally shaped reinforcement parts, if desired with dimensions that are relatively small relative to the mold part, in which case local reinforcement can take place. In a preferred embodiment of the method according to the invention, at least one relatively elongated reinforcement part is used, the longitudinal axis of which extends substantially in the intended prevention direction. Such a reinforcement part is relatively easy to fit.

The reinforcement parts used in the method according to the invention can be attached to the molded part in any suitable manner. Suitable connecting means include, for example, bolt and / or pin hole connections, and / or other mechanical connections. These can be arranged at specific locations such as, for example, along the side edges of the reinforcement part, or be homogeneously distributed over the surface of the reinforcement part. The connection is preferably established by applying an adhesive layer between molded part and reinforcing part, optionally interrupted but preferably over substantially the entire interface between reinforcing part and molded part. Such a connecting method results in a molded part of which at least the reinforced part is under a substantially homogeneous compressive stress in the unloaded state. It has been found that such a shaped part can have a greatly improved damage resistance under certain fatigue loads. For example, the damage resistance can improve by a factor of 2 to 4 compared to a molded part that was manufactured according to the known method.

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Although it is possible according to the invention to use any glue suitable for joining molded part and reinforcing part, the glue layer preferably comprises a thermosetting glue, which can be cured at the second temperature. Such adhesives can provide a substantially permanent connection, which moreover is relatively insensitive to time-dependent effects, such as, for example, creep and stress relaxation. As a result, the molded part will exhibit the intended improved damage resistance during a large part of its service life. Suitable techniques for applying the adhesive layer to the reinforcement part and / or the molded part include lamination, spraying, dipping, coating with a roller, strip lacquers, dipping, and so on. It is also possible to apply the glue layer on (parts of) the reinforcement part in advance. If desired, reinforcement part and / or molded part are pre-treated at the designated places for protection, for example against corrosion, as well as to improve the adhesive capacity. Examples of such pretreatments include degreasing, chromatography, anodizing, roughening, applying a conversion coating and / or a primer, and so on.

When in this application there is a first and a second temperature, this means an average temperature. It will be clear that when, for example, the provision of the reinforcement part at a first temperature is referred to, this means that the assembly of mold part, reinforcement part and connecting means has assumed the first temperature on average, whereby local variations may occur. The same applies to the step in the method in which the connection between molded part and reinforcing part is established and the assembly is at the second temperature. Here too it is possible that a different temperature prevails locally, which, although not much different from the average second temperature. The height of the first temperature can be selected according to the circumstances, but will preferably be in the vicinity of room temperature. The height of the second temperature can easily be selected by the person skilled in the art and depends, for example, on the type of connecting means used. A suitable method for bringing the stack to the first and / or second temperature is to receive the entire stack in a heatable pressure vessel or autoclave and to heat it with a heated gas, usually air. Once the stack has been brought to the second temperature, the components of the stack can be bonded to each other under pressure, if desired. It is also possible to heat the stack by means of radiant heat, for example infrared (IR), and to interconnect under pressure between, if desired also heated, pressing plates.

In a preferred embodiment of the method according to the invention, the first and second temperature are chosen such that the difference between the second and the first temperature comprises at least 80 ° C. More preferably, this difference is at least 110 ° C, most preferably at least 140 ° C. It has been found that this results in a higher average residual compressive stress in the unloaded molded part, which surprisingly leads to a disproportionate increase in the damage resistance under alternating load.

The material from which the mold, the reinforcement parts and the molded part are made is not particularly limited according to the invention. For example, a metal can be used for the mold, reinforcement part and molded part, such as, for example, aluminum or (lightweight) alloys thereof, stainless steel, cold-formed steel, galvanized steel, magnesium, lithium, titanium, copper and / or alloys thereof. Because of weight saving, it is advantageous for use in aviation and space travel to design the molded part and / or the reinforcing parts in a lightweight metal, such as, for example, in aluminum or titanium alloys. Fiber-reinforced plastics can also be used because of their high specific mechanical properties (properties divided by density). Because of the useful life, hardness and other properties desired 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 reinforcement part are chosen such that the thermal expansion coefficient of the mold material is lower than the thermal expansion coefficient of the material of the reinforcement part. In this embodiment the mold can for instance be provided with preventive means in the form of retaining plates attached to the mold or forming an integral part thereof. By taking the molded part at least partially between the retaining plates of the mold and subsequently raising the temperature, a compressive stress will arise in the molded part because the mold material will exhibit less expansion than the reinforcing part and the molded part can only expand as far in the direction of prevention as the mold material allows this. It is furthermore advantageous to detachably attach the retaining plates - and preventive means in general - to the mold. This can for instance be done by providing the mold and the preventing means, for example in the form of a plate, with holes. The preventing plates can then be mounted on the mold at the desired position by means of dowel pins which are inserted through the holes. There are several options available to the professional. The preventing means can for instance be held on the mold at the desired position by means of a vacuum pressure.

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In a further preferred embodiment of the method according to the invention, the thermal expansion coefficient of the material of the at least one reinforcement part is different from the thermal expansion coefficient of the material of the molded part. This achieves that the height of the pressure stress generated in the molded part can be adjusted more easily. It is particularly advantageous here to characterize the method in that the coefficient of thermal expansion of the material of the at least one reinforcement member is smaller than the coefficient of thermal expansion of the material of the molded part. In the known method for applying reinforcement parts to a molded part, it has major disadvantages when the thermal expansion coefficient of the material of the reinforcement part is smaller than the thermal expansion coefficient of the molded material. After all, by connecting the two parts according to the prior art at room temperature, a tensile stress will be generated in the molded part at a temperature below room temperature - below which the usual operating temperatures can be classified in aviation - which is undesirable. If both parts are connected at an elevated temperature relative to room temperature, this even applies to all operating temperatures up to this elevated temperature. The presently preferred embodiment of the method according to the invention does not have this disadvantage.

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It is furthermore advantageous to characterize the method according to the invention in that the ratio of the stiffness modulus of the material of the at least one reinforcement part to the stiffness modulus of the material of the molded part is at least 85%, more preferably at least 100%, and most preferably at least 130%. The stiffness modulus (also referred to as elasticity modulus) of the materials can easily be adjusted by a person skilled in the art by a suitable choice of the material.

It is advantageous to characterize the method according to the invention in that the material of the reinforcement part comprises a fiber-reinforced plastic. Such materials are light and strong. Suitable reinforcement fibers to be used are, for example, glass fibers, carbon fibers, metal fibers, stretched thermoplastic synthetic fibers, such as, for example, aramid fibers, PBO fibers (Zylon®), M5® fibers, and ultra-high molecular weight polyethylene or polypropylene fibers, as well as 1 0 2 98 5 4_ - - <4 8 natural fibers such as, for example, flax, wood and hemp fibers, and / or combinations of the aforementioned fibers. It is also possible to use so-called commingled and / or intermingled rovings. Such rovings include a reinforcing fiber and a thermoplastic fiber in fiber form. Examples of suitable matrix materials for the reinforcing fibers are thermoplastic plastics such as polyamides, polyimides, polyether sulfones, polyether ether ketone, polyurethanes, polyethylene, polypropylene, polyphenylene sulfides (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 aforementioned polymers, and thermosetting plastics such as epoxies, unsaturated polyester resins, melamine-formaldehyde resins, phenol-formaldehyde resins, polyurethanes, and the like.

In a preferred embodiment of the method, the fiber-reinforced plastic comprises substantially continuous fibers that extend in two substantially orthogonal directions (so-called isotropic fabric). In another preferred embodiment, the fiber-reinforced plastic comprises substantially continuous fibers that extend substantially in one direction (so-called UD fabric). It is advantageous to use the fiber-reinforced plastic in the form of a pre-impregnated semi-finished product. Such a "prepreg" is easy to process and has the additional advantage that the desired fiber direction is adjustable.

In order to obtain the desired high stiffness and strength in the preventing direction, it is advantageous if a fiber-reinforced plastic is used in the method that comprises substantially continuous fibers that extend substantially in one direction. The fibers are then preferably oriented at least partially in the preventing direction. Preferably, the continuous fibers extend substantially in a direction that makes an angle of at most 15 degrees with the preventing direction. More preferably, this is at most 10 degrees. Most preferably, the continuous fibers extend substantially parallel to the preventing direction.

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Another particularly suitable material for the reinforcement member comprises a laminate of at least two metal layers and an intermediate fiber-reinforced plastic layer.

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Such a material is known to the person skilled in the art under the trade name Arall® (with polyaramide fibers) or Glare® (with glass fibers). This material is preferably used in pre-stressed form, wherein the fibers of the intermediate fiber-reinforced plastic layer are, on average, under a tensile stress, and the metal layers under a compressive stress. According to the invention, it is also possible to combine different reinforcement parts in the method, wherein, if desired, each reinforcement part is made of a different material. In a particularly suitable method according to the invention, for example, a reinforcement member in the form of an aluminum profile is connected to an aluminum molded member by means of a number of reinforcement members in the form of strips of Glare® material. Fiber-metal laminates which contain other fibers, such as, for example, Zylon®, M5® and / or carbon, can also be used according to the invention. It is also possible to use reinforcement elements, such as stiffeners and strips, from fiber metal laminate to reinforce moldings from fiber metal laminate.

15

The invention is explained below with reference to the following examples and accompanying figures, in which:

FIG. la shows a schematic top view of the mold and starting materials, as used in an embodiment of the method according to the invention; FIG. 1b shows a schematic side view of the mold and starting materials for the embodiment of Figure 1;

FIG. 2 schematically represents an article obtained in perspective with a different embodiment of the method according to the invention;

FIG. 3a is a schematic representation of a cross-section of an article obtainable by the method according to the invention;

FIG. 3b is a schematic representation of another cross-section of a product obtainable by the method according to the invention;

Example 1

With reference to figures 1a and 1b, a reinforced molded part 1 is manufactured by providing a mold 2 in which the molded part 1 can be received or otherwise supported. The mold 1 is provided with preventing means 3 for at least partially preventing the expansion of the molded part 1 under the influence of a temperature change. In the embodiment shown, the preventing means 3 are formed by steel plates 3 with supporting edge 3a attached to the mold 1 via bolts 4. An aluminum molded part with a length of 1000 mm, a width of 300 mm and a thickness of 2 mm is received between steel plates 3, and in such a way that each side edge la rests as closely as possible (clamp-fitting) against supporting edge 3a. Subsequently, at room temperature (the first temperature), two reinforcement parts 5 in the form of two aluminum strips 2 mm thick and 75 mm wide were applied to the mold part 1 by means of an intermediate layer of glue 6, as indicated in figures 1a and 1b. Before applying the glue, the aluminum molded part was sanded with standard sandpaper. The adhesive layer comprised an epoxy adhesive of the type AF 163-2 K, available from the company 3M. Reinforcement parts 5 were arranged on the mold part 1 in such a way that expansion thereof can take place at least substantially unimpeded under the influence of a temperature change. Figure 1a shows that this can easily be achieved by making reinforcement parts 5 slightly shorter than the distance between the two plates 3. The assembly of mold 2, mold part 1 and reinforcement parts 5 was then placed in an autoclave. The assembly was brought to a second temperature of 120 ° C under a pressure of 6 bar and held there for about 1 hour to cure the adhesive layer 6 between reinforcement parts 5 and mold part 1. The assembly was then cooled to almost room temperature and removed from the mold.

The average compressive stress developed in the molding 1 was determined from a measurement of the radius of curvature of the reinforced molding and was approximately 6.7 MPa.

Example 2

In the same manner as above, two reinforcement members 5 in the form of an aluminum stiffener 5b and two Glare® glass fiber laminates 5a were applied to an aluminum plate 1, as shown in Figure 3b. The same adhesive layer 6 was applied between reinforcement parts 5a and 5b and mold part 1 as in Example 1. During the autoclaving process, it is ensured that thermal expansion of the reinforcement parts 5a and 5b can take place virtually unobstructed. The only obstruction that may possibly occur is caused by the hardening glue layer 6. The average compressive stress developed in the molded part 1 was approximately 9.4 MPa.

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Example 3 (not shown in the figures).

In the same manner as described above, two reinforcement parts in the form of a Glare® glass fiber laminate 5a were applied to an aluminum plate 1. The same adhesive layer 6 was applied between reinforcement parts 5a and mold part 1 as in Example 1. The average compressive stress developed in the mold part 1 was approximately 10 MPa.

In all cases described above, the average compressive stresses developed with the aid of the method according to the invention in the molded part 1 result in a decrease in the crack growth rate of at least about 50% measured under alternating load. With regard to suitable test methods for determining the crack growth rate, reference is made, inter alia, to the standard work Military Handbook 5 known to those skilled in the art.

15! The molded parts obtained with the method according to the invention can be used as a lightweight structural element in industrial applications, such as for example in civil constructions, buildings, vehicles, ships, and are preferably used in applications in which the operating temperatures are generally lower than room temperature, such as in aviation and space travel.

102Q8 5 $ ___

Claims (25)

Method for manufacturing a reinforced molded part, comprising a) incorporating the molded part in a mold, such that expansion thereof is at least partially prevented under the influence of a change in temperature; b) arranging at least one reinforcement part on the molded part at a first temperature, such that expansion thereof is at least substantially not hindered under the influence of a change in temperature; c) bringing the assembly of molded part and at least one reinforcing part to an elevated second temperature; d) mutually connecting the at least one reinforcement part and the mold part at the second temperature by means of suitable connecting means; e) removing the reinforced molding thus formed from the mold. | 2. Method as claimed in claim 1, characterized in that the molded part is relatively flat and expansion thereof is prevented in at least one preventive direction in the plane of the molded part. Method according to claim 1 or 2, characterized in that the at least one reinforcement part is relatively elongated and its longitudinal axis extends substantially in the at least one preventive direction. 25 A method according to any one of the preceding claims, characterized in that the connecting means form a layer of glue between molded part and at least one reinforcing part. Method according to claim 4, characterized in that the glue layer comprises a thermosetting glue, which can be cured at the second temperature. 10298 54_ * Method according to one of the preceding claims, characterized in that the coefficient of thermal expansion of the mold material is lower than the coefficient of thermal expansion of the material of the at least one reinforcement part. A method according to any one of the preceding claims, characterized in that the thermal expansion coefficient of the material of the at least one reinforcement part differs from the thermal expansion coefficient of the material of the molded part. Method according to claim 7, characterized in that the coefficient of thermal expansion of the material of the at least one reinforcement member is smaller than the coefficient of thermal expansion of the material of the molded part. 9. A method according to any one of the preceding claims, characterized in that the difference between the second and the first temperature comprises at least 80 ° C. Method according to claim 9, characterized in that the difference between the second and the first temperature comprises at least 140 ° C. A method according to any one of the preceding claims, characterized in that the ratio of the stiffness modulus of the material of the at least one reinforcement part to the stiffness modulus of the material of the molded part is at least 85%. Method according to claim 11, characterized in that the ratio of the stiffness modulus of the material of the at least one reinforcement part to the stiffness modulus of the material of the molded part is at least 130%. 13. Method as claimed in any of the foregoing claims, characterized in that the material of the reinforcement part comprises a fiber-reinforced plastic. A method according to claim 13, characterized in that the fiber-reinforced plastic comprises substantially continuous fibers that extend substantially in one direction. 1 0 2 98 5 4_% A method according to claim 14, characterized in that the continuous fibers extend substantially in a direction that makes an angle of at most 15 degrees with the preventing direction. 5 A method according to claim 15, characterized in that the continuous fibers extend substantially parallel to the preventing direction. 17. Method as claimed in any of the foregoing claims, characterized in that the material of the reinforcement part comprises a laminate of at least two metal layers and an intermediate fiber-reinforced plastic layer. 18. A method according to claim 17, characterized in that the laminate is prestressed, wherein the fibers of the intermediate fiber-reinforced plastic layer 15 are, on average, under a tensile stress, and the metal layers under a compressive stress. 19. Device for manufacturing a reinforced molded part, which device comprises at least one mold for being able to receive the molded part, and preventing means for at least partially preventing the expansion of the molded part under the influence of a change in temperature, wherein the device also comprises heating means comprises for bringing the assembly of molded part and at least one reinforcing part to a second temperature. 20. Device as claimed in claim 19, characterized in that the preventing means 25 are formed by a retaining plate attached to the mold or forming part thereof, between which at least a part of the molded part can be received. 21. Device as claimed in claim 19 or 20, characterized in that the preventing means are releasably attached to the mold. 22. Plate-shaped semi-finished product of at least one metal plate and reinforcement parts provided thereon according to the method according to one of claims 1-18. 102 98 54 *% A plate-shaped semi-finished product of at least one metal plate and reinforcing parts mounted thereon, obtainable by a method according to any one of claims 1-18, with an average compressive stress in at least one metal plate and in the preventing direction of at least 5 MPa at the first temperature. 5 A plate-shaped semi-finished product according to claim 23, with an average compressive stress in at least one metal plate and in the preventing direction of at least 5 MPa at the use temperature. 102 Qfi 5 a_