Abstract:
This invention relates to a process for manufacturing a piece of composite material made with a polymer resin and fiber reinforcement from at least two subcomponents comprising the following steps: a) providing the first subcomponent partially cured in a curing cycle in an autoclave at a maximum temperature T 1 , comprised between the resin gelling temperature GT and the resin curing temperature CT, applied for a predetermined time PT 1  such that the exothermal component is released from the first subcomponent in a degree exceeding 50%; b) providing the second subcomponent in a fresh or cured state; c) assembling the two subcomponents and then joining them to one another in a curing cycle in an autoclave at a maximum temperature T 2  comprised between 90% and 100% of the resin curing temperature CT, applied for a predetermined time PT 2.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a process for the manufacture of parts made of composite materials with two curing cycles, and more specifically parts intended for aeronautical structures. 
       BACKGROUND OF THE INVENTION 
       [0002]    Processes are known for the manufacture of many parts made of composite materials made with polymer resins and fiber reinforcements intended for aeronautical structures in which at least one of the subcomponents of the part is subjected to a first curing cycle in an autoclave and the assembly of the part, with all its subcomponents duly assembly, is subjected to a second curing cycle in an autoclave. 
         [0003]    At the same time, the drawback considered due to the exothermicity of the chemical reaction taking place during the polymerization of the resin during curing in the autoclave is well known in the aeronautical industry: the heat produced by the reaction is added to the heat of the autoclave and can cause an unwanted overheating of the piece subjected to the curing cycle. Techniques such as the slow or step-wise increase of the autoclave temperature or optimization techniques such as those described in U.S. Pat. No. 5,345,397, in which the optimal autoclave temperature is periodically recalculated according to the temperature of the piece, having to that end instruments for measuring the controlled variables, are known for controlling this drawback. 
         [0004]    However, effective techniques for reducing the effect of the exothermic reaction in the manufacture of pieces with different subcomponents, and particularly techniques reducing the manufacturing cost, are not known. 
         [0005]    This invention is aimed at solving these drawbacks. 
       SUMMARY OF THE INVENTION 
       [0006]    This invention proposes a process for manufacturing a piece of composite material made with polymer resin and fiber reinforcement from at least two subcomponents comprising the following steps:
       Providing the first subcomponent partially cured in a curing cycle in an autoclave at maximum temperature T 1 , comprised between the resin gelling temperature GT and the resin curing temperature CT, applied for a predetermined time PT 1  such that the exothermal components of the first subcomponent is released in a degree exceeding 50%.   Providing the second subcomponent.   Assembling the two subcomponents and then joining them together in a curing cycle in an autoclave at a maximum temperature T 2  comprised between 90% and 100% of the resin curing temperature CT, applied for a predetermined time PT 2 .       
 
         [0010]    An advantage of the process object of this invention is that it reduces the time of the first curing cycle with the subsequent savings in manufacturing costs. 
         [0011]    Another advantage of the process object of this invention is that part of the exothermicity of the resin is released in the first curing cycle, a smaller exothermal component remaining which may be released in the second curing cycle, without excessively increasing the temperature during the process. 
         [0012]    Other features and advantages of this invention will be understood from the following detailed description of an illustrative embodiment of its object in relation to the attached drawings. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows two subcomponents of a piece manufactured according to the process object of this invention. 
           [0014]      FIG. 2  schematically shows the subcomponents of the piece assembled and prepared for being subjected to the second curing cycle. 
           [0015]      FIG. 3  shows a diagram of a conventional curing cycle. 
           [0016]      FIG. 4  shows a diagram of the first curing cycle following the process of this invention. 
           [0017]      FIG. 5  shows a diagram of the second curing cycle following the process of this invention. 
           [0018]      FIG. 6  shows the diagrams of a first curing cycle following both the process object of this invention and a conventional process in reference to an embodiment of the invention. 
           [0019]      FIG. 7  shows the total enthalpy and residual enthalpy after the first curing cycle. 
           [0020]      FIG. 8  shows the resin gelling in the first curing cycle. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    The conventional process for manufacturing a piece of composite material such as a horizontal tailplane (HTP) skin panel of the Airbus  380  formed by a skin  11  stiffened by T stringers  13  comprises the following steps: 
         [0022]    x) Providing the skin  11  in a cured state. 
         [0023]    y) Providing the stringers  13  uncured. 
         [0024]    z) Co-bonding the skin+stringers assembly. 
         [0025]    Step x) in turn comprises the following basic steps:
       Laminating the composite material in prepreg form on a tool with the shape of the skin  11 .   Placing a vacuum bag on the laminate.   Subjecting the assembly to a curing cycle in the autoclave at a pressure of 135 psi and at a temperature of 185° C. for 2 hours.       
 
         [0029]    Step y) in turn comprises the following basic steps:
       Laminating the composite material in prepreg form on a suitable tool.   Forming the T stringers  13  in a hot forming cycle in a suitable tool.       
 
         [0032]    Step z) in turn comprises the following basic steps:
       Assembling the stringers  13  with the skin  11 . This step may be carried out in different manners and with a different type of tool. For example, one way of doing so consists of first arranging the stringers  13  in a curing tool with the head facing up, secondly placing the skin  11  on them and thirdly turning the tool over so as to place the assembly with the skin  11  in the lower position. The assembly may include the use of adhesives  15  between the components.   Placing a vacuum bag on the assembly.   Subjecting the assembly to a curing cycle in an autoclave at a pressure of 135 psi and at a temperature of 185° C. for 2 hours.       
 
         [0036]    In the conventional process, in the curing cycle of both step x) and step z), the temperature that is reached is 185° C. for 2 hours and the autoclave pressure is 135 psi. The skin is therefore subjected to two curing cycles. 
         [0037]    All heat-setting resin systems develop an exothermal reaction during their polymerization with a temperature increase. The greater the mass per surface unit of resin that is heated, and therefore the greater the thickness of the material, the greater this exothermal reaction. After a certain thickness (different for each type of resin), the exothermal reaction of the resin begins to have visible effects on the resin curing cycle. When the stabilization temperature is reached and the supply of heat to the autoclave is shut off, the exothermal reaction causes a temperature increase I and therefore the resin continues heating up (see  FIG. 3 ). This overheating is detrimental for the material being manufactured given that after a certain value, the temperature increase causes an increase in the brittleness of the material. This overheating could furthermore cause a fire or uncontrolled reaction. 
         [0038]    One possible way of controlling this effect of exothermicity is to reduce the heating rate. However, reducing the heating rate leads to very long and therefore expensive manufacturing processes. 
         [0039]    According to this invention, the process for manufacturing the same piece previously mentioned, i.e. a horizontal tailplane (HTP) skin panel of the Airbus  380  formed by a skin  11  stiffened by T stringers  13  comprises the following steps: 
         [0040]    a) Providing the skin  11  in a partially cured state. 
         [0041]    b) Providing the stringers  13 . 
         [0042]    c) Co-bonding the skin+stringers assembly. 
         [0043]    As the person skilled in the art will understand, steps are used in these steps which are similar to those of the conventional process that has been omitted for the sake of simplification. The basic differences of the process according to this invention and the conventional processes are the following: 
         [0044]    In step a) the curing cycle for the skin  11  is carried out up to a certain temperature T 1  such that, at normal heating rates, on one hand a partial degree of curing is reached which allows the skin  11  to be stiff enough so that its geometry is not modified when the stringers  13  are assembled on it, and on the other hand, part of the exothermal component of the resin is released, without reaching the resin curing temperature CT, such that the exothermal component of the skin  11  during the curing cycle of step c) is so small that no visible overheating of the skin occurs. 
         [0045]    Therefore the temperature of the piece during the first curing cycle must be on one hand greater than the resin gelling temperature GT and on the other less than the resin curing temperature CT so that a sufficient part of the exothermal component is released at a normal heating rate, for example 0.8° C./min, so that the exothermal components of the skin  11  during the curing cycle of step c) is so small that no visible overheating of the piece occurs. This step is depicted in  FIG. 4 , where it can be seen that a curing cycle is carried out at a maximum temperature of 140° C., in which the piece reaches a maximum temperature of T 1 =140° C.+I 1  which, on one hand, does not reach 180° C., which is the curing temperature for the resin used, and which on the other hand is enough to gel the resin. 
         [0046]    It is considered that the exothermal component should be released from the skin  11  in the first cycle in a degree exceeding 50%, preferably 65%. 
         [0047]    The stringers  13 , which can be in a fresh or precured state, are assembled in the skin  11  in step c). If they are fresh, tools consisting of metal angle irons  25  and plates  27  are used for that purpose. As previously mentioned, the skin  11  must be stiff enough so that said metal angle irons  25  and plates  27  doe not leave marks on it. The vacuum bag  29  is placed and then a curing cycle is carried out until the degree of curing the skin  11  and stringers  13  exceeds 90%. 
         [0048]    As shown in  FIG. 5 , the curing cycle can be carried out at a maximum temperature close to the curing temperature for the resin used, i.e. 180° C., because the exothermal component of the resin of the skin  11 , manifested in segment  31  of the curing cycle, is so small that the overheating of the piece due to the effect of exothermicity is insignificant. 
       EXAMPLE 
       [0049]    Described below are the results obtained in a specific embodiment of the invention using as a first subcomponent a panel with 300×300 mm surface area and 50 mm thick with an epoxy resin impregnated carbon fiber material designated Toray 3911-T800, and as a second subcomponent a panel with 50×50 mm surface area and 3 mm thick and a parallel example following the conventional process and using the same subcomponents. 
         [0050]    It is understood that these subcomponents acceptably simulate the skin and a stringer of the coating of a tailplane or wing of an airplane. 
         [0051]    In the first step of the process, the first subcomponent is partially cured following the curing cycle depicted in the lower part of  FIG. 6 , where the temperature is seen to increase at a rate of 0.8° C./min until reaching 140° C., this temperature being maintained for 3 hours. 
         [0052]    According to the measurements obtained by thermocouples located both in the center and at the ends of the panel, the exothermal reaction causes a temperature increase I 2  up to 164° C., a temperature which is less than 180° C., which is the curing temperature for the material. The curing degree of the first subcomponent was also analyzed, obtaining a value of 75%. 
         [0053]    Applying a curing cycle according to the convention process at a maximum temperature of 180° C., depicted in the upper part of  FIG. 6 , the exothermal reaction causes a temperature increase I 3  up to 210° C. and the resulting curing degree is 96%. 
         [0054]    In the third step the curing cycle is carried out conventionally in both cases: the temperature is increased at a rate of 0.8° C./min until reaching 180° C., this temperature being maintained for 2 hours. 
         [0055]    There was not overheating due to exothermicity in any of them. 
         [0056]    The curing degree of the final piece was about 95% using both the conventional process and the process according to the invention. 
         [0057]    The selection of the temperature, time and heating rate parameters of the first curing cycle was based on the differential scanning calorimetry (DSC) and rheometry carried out on preimpregnated material.  FIG. 7  shows the residual enthalpy  35  of the sample of preimpregnated material in comparison to the total reference enthalpy  37 , showing the considerable reduction of the residual enthalpy of the resin (and therefore preventing the potential problems of exothermicity), and  FIG. 8  shows the gelling process  39  for the resin subjected to cycle  41  of 140° C./3 hours with a heating rate of 0.8° C./min in which it can be seen that resin gelling  43  takes place 40 minutes after the isothermal step begins at 140° C. 
         [0058]    The process object of this invention is not only applicable to the manufacture of the HTP skin panel of an aircraft from a precured skin and a plurality of stringers in a fresh state, but to any process for the manufacture of pieces of composite materials from two subcomponents in which a partial curing of one of the subcomponents is carried out in the first step. The second subcomponent can be provided in a fresh or precured state such that co-bonding is carried out in the second step if the second subcomponent is provided in a fresh state or a secondary gluing if the second subcomponent is provided in a precured state. 
         [0059]    The application of the process of this invention is particularly considered for the manufacture of pieces with the following subcomponents:
       Precured skin and fresh stringers.   Precured skin and precured stringers.   Fresh skin and precured stringers.       
 
         [0063]    The stringers can have a T, Ω or U profile, or any other suitable profile. 
         [0064]    It is considered that the proposed solution for the problem of exothermicity is shown to be particularly advantageous in the manufacture of pieces in which the first subcomponent has areas with a thickness exceeding 25 mm. 
         [0065]    In the preferred embodiment described above any modifications comprised within the scope defined by the following claims can be introduced.