Abstract:
A honeycomb core for dimensionally stable components (e.g., a reflector) in sandwich construction with a plurality of cells with a triangular cell cross section made from carbon fiber reinforced plastic. The honeycomb core is assembled from flat laminate strips by plugging, wherein the laminate strips have slots running transversely to the extension direction respectively on their longitudinal side. The laminate strips are connected to one another in such a way that the slots of three laminate strips engage in a positive-locking manner in one another at a respective intersection point to form the corners of the cells of the honeycomb core.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the German patent application No. 10 2015 010 436.5 filed on Aug. 14, 2015, the entire disclosures of which are incorporated herein by way of reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a honeycomb core for dimensionally stable components in a sandwich construction with a plurality of cells with a triangular cell cross section made from carbon fiber reinforced plastic. 
         [0003]    A honeycomb core of this kind is required, for example, for base plates of optical instruments, optical benches and optical mirrors in sandwich construction, as described in EP 0 649 036 A1. Such optical mirrors are required, e.g., for astronomical telescopes, for the transmission of laser beams, or for the concentration of solar radiation. Fundamental requirements for these applications are extreme dimensional stability under all environmental conditions occurring and low weight. 
         [0004]    Conventional lightweight construction core materials for a sandwich construction, such as, e.g., foamed materials, honeycomb cores of aluminum, honeycomb cores of aramid fibers or Nomex honeycombs, have a thermal expansion coefficient that is too high for applications calling for dimensional stability. In many cases, this leads to an intolerable deformation in the event of a temperature change. Due to the pronounced L- and W-directions, honeycomb cores of such materials have direction-dependent, anisotropic mechanical properties that likewise lead to undesirable direction-dependent component behavior. 
         [0005]    The problem of the thermal expansion coefficient can be solved by honeycomb cores made from carbon fiber reinforced plastic (CFRP). However, commercially available honeycombs of carbon fiber reinforced plastic are restricted in respect of possible applications and very expensive. 
       SUMMARY OF THE INVENTION 
       [0006]    An object of the invention is to specify a honeycomb core for dimensionally stable components in sandwich construction, which can be manufactured cost-effectively and permits a high flexibility with respect to possible applications. The honeycomb core is to have a high dimensional stability as well as an adjustable thermal expansion coefficient. 
         [0007]    Another object of the invention is to specify a reflector in a sandwich construction, in particular for an optical mirror with a lightweight construction, as well as a sandwich panel with a lightweight construction, which reflector or which sandwich panel has high dimensional stability with low weight. 
         [0008]    Another object of the invention is to provide a method for the cost-effective manufacture of a honeycomb core for dimensionally stable components in a sandwich construction. 
         [0009]    A honeycomb core with a plurality of cells with a triangular cell cross section made from carbon fiber reinforced plastic is proposed, wherein the honeycomb core is assembled from flat laminate strips by insertion. The laminate strips each have on their longitudinal side, slots running transversely to the extension direction, wherein the laminate strips are connected to one another in such a way that the slots of three laminate strips engage in one another in a positive-locking manner at a respective intersection point to form the corners of the cells of the honeycomb core. 
         [0010]    According to the invention, a honeycomb core with quasi-isotropic triangular cells in a cheap plug-in design is provided, which can be fabricated cost-effectively without elaborate devices. In a honeycomb core of this kind, the thermal expansion coefficient can be set to 0 or another desired value. It is likewise possible to determine the size of the cells of the honeycomb core without great effort. 
         [0011]    The honeycomb core is assembled from a number of laminate strips of a first type, a number of laminate strips of a second type and a number of laminate strips of a third type. The laminate strips of the first type comprise first and second slots distributed at regular intervals, wherein the first slots, starting out from a first longitudinal side of the respective laminate strip, extend by one third transversely to the extension direction. The second slots extend, starting out from a second longitudinal side, by one third transversely to the extension direction of the respective laminate strip. The first and the second slots run in a line. The line thus runs transversely to the extension direction of the respective laminate strip or at a 90° angle to the extension direction. 
         [0012]    The laminate strips of the second type comprise slots distributed at regular intervals, wherein the slots, starting out from a first longitudinal side of the respective laminate strip of the second type, extend by two thirds transversely to the extension direction. Here, too, transversely to the extension direction means a progression at a 90° angle to the extension direction of the relevant laminate strip of the second type. 
         [0013]    The laminate strips of a third type comprise slots distributed at regular intervals, wherein the slots, starting out from a second longitudinal side, extend by two thirds transversely to the extension direction. Here, too, transversely to the extension direction means a progression at a 90° angle to the extension direction of the relevant laminate strip of the third type. 
         [0014]    In this description, first longitudinal sides of the laminate strips of the different types are those longitudinal sides that form, e.g., the front of the honeycomb core. Correspondingly, the second longitudinal sides of the laminate strips of any type are those longitudinal sides that then form the back of the honeycomb core. 
         [0015]    By plugging the laminate strips of the three different types together, a honeycomb core that is stable in itself can be provided. Due to the respective properties of the respective laminate strips, this honeycomb core then has the desired dimensional stability and the set thermal expansion coefficient. 
         [0016]    Beyond the laminate strips, no further elements are necessary to provide a stable structure. The required dimensional stability results solely from the positive-locking connection of the different laminate strips in the area of their intersection points. 
         [0017]    It is expedient if the spacings of the slots running in the transverse direction of a laminate strip of a first type, the spacings of the slots running in the transverse direction of a laminate strip of the second type and the spacings of the slots running in the transverse direction of a laminate strip of the third type are dimensioned in such a way that these correspond to the length of the lateral edges of the triangular cell cross section. The spacings of the slots running in the transverse direction of a laminate strip of a certain type can correspond to the spacings of the slots running in the transverse direction of one or of both other laminate strips of the other types. The spacings of the slots running in the transverse direction can thus be identically dimensioned on the laminate strips of the first, the second and the third type. This results in a honeycomb core that comprises cells of equilateral triangles. 
         [0018]    The honeycomb core is also characterized by the fact that the laminate strips of an identical type are arranged parallel to one another in the honeycomb core. This results in a regular structure of the cells of the honeycomb core, due to which a high dimensional stability as well as isotropic properties of the honeycomb core can be achieved. 
         [0019]    If the cells comprise equilateral triangles and the laminate strips are formed quasi-isotropic in their own right, the cells have quasi-isotropic properties. The honeycomb core has isotropic properties as a result of this. Furthermore, a high dimensional stability is yielded by equilateral triangles. 
         [0020]    It is expedient if the laminate strips comprise cured carbon fiber reinforced plastic. 
         [0021]    To provide quasi-isotropic triangular cells as well as a honeycomb core with isotropic properties, it is expedient if the laminate strips of all types have an identical thickness. The weight of the honeycomb core on the one hand and its stability on the other are set by the thickness of the laminate strips. On the other hand, the isotropy of the honeycomb core can be provided in a simple manner. Different suitable dimensions can be selected according to the application. The dimensions can be determined in a suitable manner by calculation or experiments. 
         [0022]    It can also be expedient for the slots of three laminate strips (of the three different types) engaging in one another to be fixed at an intersection point by an adhesive or a synthetic resin. To this end, the adhesive or the synthetic resin can be introduced into the slot of one of the laminate strips and/or several laminate strips before the laminate strips of the different types are plugged together. 
         [0023]    The honeycomb core is distinguished, according to a further configuration, in that the lateral edges of the laminate strips of at least a main side of the honeycomb core lie in one plane following the completed assembly. A honeycomb core with two parallel main sides can be provided by this. Alternatively, a honeycomb core can be provided in which the lateral edges of the laminate strips of one main side lie in one plane, while the lateral edges of the other main side have the shape of a convex surface, for example. 
         [0024]    An insert, in particular a hollow insert, into which a load can be introduced can be arranged in at least some of the cells of the honeycomb core. Such inserts can be arranged in adjacent cells of the honeycomb core, for example. A particular insert can be arranged in all cells of the honeycomb core. A particular insert can be arranged only in some cells, so that, e.g., a cell with an insert is surrounded by cells without an insert. Which cells are provided with an insert depends substantially on the respective application and the loads occurring that are to be processed. 
         [0025]    The insert expediently has a triangular cross section matched to the cell, wherein a respective insert wall abuts an assigned cell wall. It is ensured by this that the insert is arranged in a particular cell without play and cannot at any point execute an undesirable movement relative to the cell. Moreover, an optimized distribution of force can be facilitated between the insert and the cell walls. 
         [0026]    One or more of the following elements can be used as an insert: a cured CFRP shaped piece; a wet-formed, laminated composite insert; a shaped metal piece, a shaped piece of rigid foam; a shaped piece of plastic. 
         [0027]    The invention also creates a sandwich panel in lightweight construction, in particular for dimensionally stable applications. A sandwich panel of this kind comprises a honeycomb core formed as in this description. In a sandwich panel of this kind, the honeycomb core can be combined with a front and rear covering layer. Such a sandwich panel can also be formed as an optical bench, on which one or more optical elements are arranged on one covering layer. 
         [0028]    The invention also creates a reflector in sandwich construction, in particular for an optical mirror in lightweight construction. Such a reflector comprises a honeycomb core formed as in this description. On such a reflector, the honeycomb core can be combined with a front and rear covering layer, wherein one of the two covering layers can be provided with a mirror backing layer. Such a mirror backing layer can be optically polished and/or coated with a reflection layer. The configuration of such a reflector can take place in principle in any manner known to the person skilled in the art. 
         [0029]    The invention also creates a method for the manufacture of a honeycomb core described as above and below. The method comprises the following steps: 
         [0030]    a) provision of a number of laminate strips of the first type, provision of a number of laminate strips of the second type and provision of a number of laminate strips of the third type; 
         [0031]    b) arrangement of the number of laminate strips of the second type on a level base, so that their slots point away from the base; 
         [0032]    c) plugging of the number of laminate strips of the first type from above into the laminate strips of the second type, so that the slots of the laminate strips of the first type, which face the laminate strips of the second type, engage in the slots of the laminate strips of the second type; 
         [0033]    d) plugging of the number of laminate strips of the third type from above into the number of laminate strips of the first type, so that the slots of the laminate strips of the third type engage in the slots of the laminate strips of the first type, which face the laminate strips of the third type. 
         [0034]    According to the spacings of the slots of the laminate strips of the respective types and the wall thickness of the respective laminate strips, the honeycomb core can be adapted flexibly to design requirements of a target application. 
         [0035]    In one configuration of the method, it is provided to arrange the number of laminate strips of the third type parallel on the base. 
         [0036]    In another configuration, the number of laminate strips of the third type are arranged in a direction perpendicular to their respective extension direction with identical spacing on the base. 
         [0037]    Another expedient configuration of the method provides that the number of laminate strips of the second type is aligned in such a way that assigned slots of adjacent laminate strips of the second type form a line. Along this line a laminate strip of the first type is then connected by insertion in a positive-locking manner to the number of laminate strips of the second type arranged in parallel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    The invention is explained in greater detail below with reference to a practical example in the drawing. 
           [0039]      FIG. 1  shows a top view of a honeycomb core according to the invention; 
           [0040]      FIG. 2  shows an enlarged representation of a triangular cell formed in the honeycomb core from  FIG. 1 ; 
           [0041]      FIG. 3  shows a representation in perspective of a honeycomb core according to the invention; 
           [0042]      FIG. 4  shows a section of a laminate strip of a first type in a side view; 
           [0043]      FIG. 5  shows a section of a laminate strip of a second type in a side view; 
           [0044]      FIG. 6  shows a section of a laminate strip of a third type in a side view; 
           [0045]      FIG. 7  shows a section of a laminate strip of the second type, wherein openings are shown by way of example in two sections between adjacent slots; 
           [0046]      FIG. 8  shows an enlarged section of a honeycomb core according to the invention in a top view, wherein inserts for load introduction are arranged in some cells of the honeycomb core; and 
           [0047]      FIG. 9  shows a representation in perspective of an insert for load introduction. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0048]      FIG. 1  shows a top view of a honeycomb core  1  according to the invention. The honeycomb core  1 , which is round in the top view, comprises a plurality of cells  2  with a triangular cell cross section. As can readily be recognized, the cells are formed of equilateral triangles, so that their cell walls assume an angle of 60° in relation to one another. 
         [0049]    The honeycomb core  1  has three recesses  8  distributed over the outer circumference, which are offset respectively by 120° to one another and in which no cells are provided. The recesses  8  are used to take up holding elements, which are not shown, in order to be able to attach the honeycomb core  1  via one or more brackets. Since neither the recesses  8  nor the brackets are of significance for the present invention, a more detailed description has been dispensed with. 
         [0050]    The triangular cell cross section of the cells  2  is evident from the enlarged representation in  FIG. 2 . The cell  2  shown in the center has three corners  3 ,  4 ,  5 , which are also described as intersection points  6  of the respective cell walls in the following description. As described, the cell walls  7  of these cells  2  assume an angle of 60° in relation to one another. 
         [0051]    The thickness of the cell walls  7  is preferably of identical dimensions. Quasi-isotropic properties of a cell  2  and thus of the honeycomb core  1  as a whole can be provided by this. The wall thickness of the cell walls  7  is between 0.4 and 0.8 mm, for example. In principle the wall thickness can also be chosen differently according to the requirements of a target application. The length of the cell walls  7  is likewise selected according to the target application. The longer the walls selected, the fewer the cells a honeycomb core has for a given diameter. It can be easily constructed thereby. If greater stability is the aim, a correspondingly shorter wall length of the cell walls  7  is selected. The person skilled in the art can determine a precise measurement by calculation and/or experiments for a given target application. 
         [0052]    The honeycomb core  1  as shown in  FIG. 1  is assembled from a plurality of slotted, cured, flat and quasi-isotropic CFRP laminate strips. The cells in the form of equilateral triangles then result from the assembly, as shown in  FIG. 2 . 
         [0053]    So that a quasi-isotropic honeycomb core  1  can be produced with the triangular cells  2 , three different types of laminate strips are provided, which are each shown in a section in  FIGS. 4 to 6 . 
         [0054]      FIG. 4  shows a section of a laminate strip  10  of a first type, which extends in the direction of the arrow  9  from left to right in the drawing plane. A first longitudinal side of the laminate strip  10  is identified by  11 , a second longitudinal side by the reference sign  12 . The first and the second longitudinal side run, e.g., parallel to one another. The laminate strip  10  has first slots  13  and second slots  14  distributed at regular intervals. The first slots  13  extend, starting out from the first longitudinal side  11 , by a third transversely (i.e., at a 90° angle) to the direction of extension  9 . In a corresponding manner, the second slots  14  extend, starting out from the second longitudinal side  12 , likewise by a third transversely to the direction of extension  9 . As is clearly visible, the first and second slots run in a straight line transversely to the direction of extension. The web  15  remaining between a slot  13  and a slot  14  (which lie in a straight line) accordingly likewise has a width of a third of the overall width  18  between the first and the second longitudinal side  11 ,  12 . The spacing  16  formed between two adjacent slots  13  and  14  corresponds to the width of the cell wall  7  as shown in  FIG. 2 . 
         [0055]      FIG. 5  shows a laminate strip  20  of a second type, wherein only a section of the laminate strip  20  extending in the direction of the arrow  9  is shown. The laminate strip  20  comprises slots  23  distributed at regular intervals, which extend, starting out from a first longitudinal side  21 , by two thirds transversely to the extension direction  9 . A web  25  remaining between the end of the slot  23  and the second longitudinal side  22  thus takes up a third of the width  28  of the laminate strip  20 . A distance  26  extending in the extension direction  9  between two adjacent slots  23  is identified by the reference sign  26 . In cells  2  that have the shape of an equilateral triangle, the spacing  26  corresponds to the spacings  16  and  17  of the first laminate strip  10 . 
         [0056]      FIG. 6  shows a section of a laminate strip  30  of a third type, which extends with its long side again in the direction of the extension direction  9 . The laminate strip  30  has a first longitudinal side  31  and a second longitudinal side  32 . The laminate strip  30  comprises slots  34  distributed at regular intervals. The slots  34  extend, starting out from the second longitudinal side  32 , by two thirds transversely to the extension direction. This means in turn that the web  35  (which forms an extension of the slot  34 ) remaining between the end of the slot  34  and the first longitudinal side  31  has a third of the width  38  of the laminate strip  30 . Two adjacent slots  34  have a spacing  36  relative to one another. In a cell that has the shape of an equilateral triangle in cross section, the spacing  36  corresponds to the spacings  16 ,  17  of the laminate strip  10  and the spacing  26  of the laminate strip  20 . 
         [0057]    The widths  18 ,  28 ,  38  of the laminate strips  10 ,  20 ,  30  are preferably identical. It is likewise preferable if the material thickness (i.e., the thickness of the laminate strips  10 ,  20 ,  30  resulting in a direction perpendicular to the sheet level) is the same. 
         [0058]    The honeycomb core shown in  FIG. 1  is produced by plugging the laminate strips  10 ,  20 ,  30  together. The honeycomb core  1  is produced in this case in such a way that first the laminate strips  20  of the second type are arranged on a flat base, so that their slots  23  point away from the base. Here the number of laminate strips  20  of the second type is arranged parallel on the base. The arrangement of the laminate strips  20  likewise takes place in such a way that these are arranged in a direction perpendicular to their respective extension direction with equal spacing on the base (equidistance). The laminate strips  20  are oriented in this case in such a way that their respective slots  23  of adjacent laminate strips  20  form a line. 
         [0059]    The laminate strips  10  of the first type are then plugged from above into the laminate strips  20 , so that the slots  14  of the laminate strips  10  of the first type, which face the laminate strips  20 , engage in their slots  23 . Then the laminate strips  30  of the third type are inserted from above into the laminate strips  10 , so that the slots  34  of the laminate strips  30  engage in the slots  13 , which face the laminate strips of the second type. 
         [0060]    No device is necessary to assemble the honeycomb core  1  in the manner described above. 
         [0061]    To fix the inserted laminate strips  10 ,  20 ,  30 , the slots  13 ,  14 ,  23 ,  34  of the laminate strips  10 ,  20 ,  30  of one or more types can be provided with adhesive or synthetic resin. This can be done prior to assembly. The application of the adhesive or synthetic resin can also take place following assembly. 
         [0062]    The width  18 ,  28 ,  38 , the length (in extension direction  9 ) and the number of laminate strips  10 ,  20 ,  30  determine the height, length and width of the honeycomb core  1 . This is shown in an exemplary manner in a representation in perspective in  FIG. 3 , wherein the directions of the laminate strips  10 ,  20 ,  30  are illustrated by an arrow system shown outside the honeycomb core  1 . As is clearly visible, the laminate strips  10  of the first type extend from bottom left to top right. The laminate strips  20  of the second type extend from top left to bottom right. The laminate strips  30  of the third type substantially extend from left to right. It is unnecessary to note that the extension directions described here refer only to the representation shown in  FIG. 3 . 
         [0063]    The laminate strips  10  and/or  20  and/or  30  can have uninterrupted cell walls, i.e., cell walls in which the sections  40  between two slots are not provided with any opening. The sections  40  can also be provided with openings  41 , wherein the openings can be formed as a ventilation opening  43  or lightening hole  42 . The latter are used to reduce the weight of the honeycomb core  1  without substantially influencing the stability. This is shown by way of example in  FIG. 7 , for example, which shows a section of the laminate strip  20 . The openings  42  can also be seen in the representation in perspective in  FIG. 3 , wherein here the openings are provided in all types of laminate strips  10 ,  20 ,  30  in a plurality of cells. 
         [0064]    The honeycomb core  1  can be provided for local reinforcement of the cells  2  with reinforcing inserts (in short: inserts)  50  if this should be required for reasons of load introduction. Such cells, which are provided with reinforcing inserts  50 , can be provided in the area of the recesses  8  of the honeycomb core  1 , for example.  FIG. 8  shows a section from the honeycomb core  1  in a top view, wherein several adjacently arranged cells  2  are provided with reinforcing inserts  50 . The reinforcing inserts  50  have a triangular cross section matched to the respective cell  2 , wherein a particular insert wall  51  of a reinforcing insert abuts an assigned cell wall  7  of the cell  2 . 
         [0065]    A cured CFRP shaped piece, a wet-formed, laminated composite insert, a shaped metal piece, a shaped piece of rigid foam or a shaped piece of plastic can be used as a reinforcing insert, for example. A reinforcing insert  50  of this kind is shown in a representation in perspective in  FIG. 9 . The respective cell walls  51  are provided with an opening  52  only by way of example. 
         [0066]    The honeycomb core  1  thus produced can be fabricated cost-effectively from any carbon fiber with any resin system. 
         [0067]    Due to a freely selectable cell width (i.e., the distance between two slots of a laminate strip) and its wall thickness, the honeycomb core can be adapted flexibly to the design requirements of a target application. 
         [0068]    While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority. 
       REFERENCE SIGN LIST 
       [0069]      1  Honeycomb core 
         [0070]      2  Cell 
         [0071]      3  First corner of the cell  2   
         [0072]      4  Second corner of the cell  2   
         [0073]      5  Third corner of the cell  2   
         [0074]      6  Intersection point 
         [0075]      7  Cell wall 
         [0076]      8  Recess 
         [0077]      9  Extension direction 
         [0078]      10  Laminate strip of the first type 
         [0079]      11  First longitudinal side 
         [0080]      12  Second longitudinal side 
         [0081]      13  First slot 
         [0082]      14  Second slot 
         [0083]      15  Web remaining between first and second slot  13 ,  14  lying in a line 
         [0084]      16  Spacing between two adjacent first slots  13   
         [0085]      18  Width of the laminate strip  10   
         [0086]      17  Spacing between two adjacent second slots  14   
         [0087]      20  Laminate strip of the second type 
         [0088]      21  First longitudinal side 
         [0089]      22  Second longitudinal side 
         [0090]      23  Slot 
         [0091]      25  Web remaining between slot  23  and second longitudinal side  22   
         [0092]      26  Spacing between two adjacent slots  23   
         [0093]      28  Width of the laminate strip  20   
         [0094]      30  Laminate strip of the third type 
         [0095]      31  First longitudinal side 
         [0096]      32  Second longitudinal side 
         [0097]      34  Slot 
         [0098]      35  Web remaining between slot  34  and first longitudinal side  31   
         [0099]      36  Spacing between two adjacent slots  34   
         [0100]      38  Width of the laminate strip  30   
         [0101]      40  Section between two slots of a laminate strip 
         [0102]      41  Opening 
         [0103]      43  Ventilation opening 
         [0104]      42  Lightening hole 
         [0105]      50  Insert 
         [0106]      51  Insert wall 
         [0107]      52  Opening