Wagon body element for a wagon body of a rail vehicle

Description

[0001] The invention relates to a car body element for the car body of a rail vehicle, with a support structure and at least one covering element, as well as a car body and a method for producing a car body element.

[0002] The car body of a rail vehicle forms the load-bearing structure for attaching all of the vehicle's components and also forms the outer shell of the passenger compartment. Car bodies are usually made of steel or aluminum. During the manufacture of car bodies from steel, load-bearing structures are created, e.g., by riveting or welding. The load-bearing structures consist of longitudinal beams, cross beams, and pillars. The load-bearing structures usually consist of semi-finished products, such as U-, Z-, L-, or T-profiles. To form the shell of the car body, skin elements are applied to the load-bearing structure, or the load-bearing structure is covered with skin elements. The skin elements also have a stiffening function or reinforce the load-bearing structure. The skin elements are usually made of sheet metal that is welded to the load-bearing structure.Such car bodies are described in DE 195 28 610 C1, EP 2 555 957 A1, and DE 10 2008 048 083 A1. Furthermore, the subsequently published EP 3 798 079 A1 describes a car body element for the car body of a rail vehicle, comprising a support structure and at least one shell element, wherein the support structure is applied additively to the shell element.

[0003] In order to provide the car body with windows, cutouts can be made in the shell of the car body, for example for the installation of windows or doors. Because the shell of a car body usually also has a load-bearing function, increased forces and stresses can occur in the area of ​​these cutouts during operation of the rail vehicle. For example, when the car accelerates or brakes or enters a curve, bending and torsional moments are introduced into the car body via the coupling and the bogies of the rail vehicle, which are then transmitted via the shell to the cutouts. These loads lead to tensile or compressive stresses in the car body wall, which are concentrated primarily at the corners of the cutouts and can compress or stretch them.In order to counteract increased loads in the car body wall, the load-bearing structure is reinforced in the area of ​​increased loads, for example by increasing the sheet thickness or by adding additional beams or ribs.

[0004] DE 10 2008 048 083 A1 proposes manufacturing the car body shell from sheet metal parts of varying thicknesses. This allows for thinner sheet metal to be used in areas subject to lower loads, saving weight and material.

[0005] EP 2 555 957 A1 describes a car body wall with a shell made of sheet metal with different properties. The sheets are butted against each other and are joined by continuous laser welds to form a smooth exterior. For example, in areas of the exterior sheeting where increased stresses occur, sheets with thicker walls are used.

[0006] Disadvantages of the outer walls of known car bodies are the relatively high manufacturing costs resulting from the production of suitable sheet metal parts and the necessary welding work, as well as the continued high weight of the car bodies.

[0007] The invention is therefore based on the object of providing a car body which has a high strength, a low weight and is easy to manufacture.

[0008] This object is achieved according to the invention with a car body element according to claim 1, a car body according to claim 13 and a method for producing a car body element according to claim 14.

[0009] According to the invention, the support structure is applied additively to the casing element in a first section, i.e., printed using a 3D printing process. The support structure in the first section is preferably created entirely by the 3D printing process. The terms "additive manufacturing" and "3D printing" are to be understood as synonyms in the following.

[0010] The 3D printing process used preferably involves deposition welding, in particular additive laser powder deposition welding (LPA) or laser welding wire deposition welding. By additively applying the support structure to the shell element in the first section, virtually any desired geometry can be realized in the support structure.

[0011] In a second section, the support structure is formed by at least one prefabricated support element. The support element is preferably fully prefabricated before being arranged on the enveloping element. The support element can, in particular, be prefabricated from sheet metal. The sheet metal for the support element is preferably cut to size using a laser and folded into the intended shape. The support element used in addition to 3D printing makes it possible, in particular, to connect areas of the enveloping element that cannot be connected to the support structure using 3D printing, particularly for manufacturing reasons, to the force flow of the support structure.

[0012] According to a preferred embodiment of the invention, a support element formed from a sheet metal is formed from sheet metal, in particular bent and / or welded. The enveloping element preferably has a receptacle for the support element, into which the support element is inserted during manufacture of the car body element. According to the invention, the support element forms a connection between a first section of the support structure and the enveloping element, in particular between a peripheral section of the enveloping element and the first section of the support structure and / or between a frame profile and the first section of the support structure and / or between a longitudinal profile integrated into the enveloping element and the first section of the support structure.

[0013] According to a preferred variant of the car body element according to the invention, the first section, created using the 3D printing process, is integrally connected, in particular welded, to the second section. In a further variant according to the invention, the second section is alternatively or additionally integrally connected to the shell element. The integral connection is preferably formed by a welded joint.

[0014] Preferably, the support structure is formed by a second section at at least one connection interface. In particular, the connection interface is formed by at least one sheet metal element, in particular a laser-cut and / or folded sheet metal element. The connection interface serves to connect the car body element to adjacent areas of a car body.

[0015] In a further preferred embodiment of the invention, the second section forms a connection area for connecting the support structure to the enveloping element or a frame profile. This allows connections of the support structure to complex geometries of the enveloping element or the frame profile that are not achievable using the 3D printing process. The support element can be placed on the enveloping element and / or inserted into indentations and / or depressions and / or undercuts of the enveloping element.

[0016] The second section can be designed as a hollow profile. Cutouts, e.g., laser-cut cutouts, can be provided to optimize the weight of the second section, provide drainage, and / or apply corrosion inhibitors and thermal / sound insulation to the cavity. By cutting and bending, a truss structure can be formed, allowing for high strength while maintaining low weight.

[0017] The enveloping element can preferably be designed in a flat or plate-like manner. The first section of the support structure is printed onto the enveloping element using a 3D printing process. The enveloping element thus forms the basis for the construction of the support structure. In particular, the support structure can thus be integrally bonded to the enveloping element in a single step.

[0018] For example, the casing element can be formed from a metal alloy, in particular a steel alloy. Accordingly, the support structure can be formed from a metal alloy.

[0019] The support structure preferably has at least one support structure profile. The support structure profile can be provided with a variable profile cross-section. The profile cross-section can be adapted to the locally occurring loads in order to achieve the best possible ratio between material consumption and strength.

[0020] In a preferred variant of the invention, the support structure or support structure profile comprises at least one chord and at least one web arranged between the chord and the enveloping element. The web forms a force-transmitting connection between the chord and the enveloping element. While the chord preferably bears a large portion of the loads acting on the car body or conducts a large portion of the force flow resulting from loads on the car body, a portion of the loads acting on the chord is transferred via the web to the enveloping element, which thus performs a supporting function.

[0021] In an advantageous embodiment of the invention, the at least one belt and the at least one web are applied to the casing element by means of additive manufacturing. Thus, in one work step, the web can first be applied, i.e., "printed," to the casing element, and immediately thereafter, the belt can be fabricated onto the web.

[0022] According to one embodiment of the invention, the support structure has openings adjacent to the sheathing element. In a support structure comprising a belt and a web, the openings are preferably provided in the web.

[0023] The at least one web of the support structure preferably has a width and / or thickness of no more than 2 mm, preferably no more than 1.5 mm, most preferably no more than 1 mm. The width and / or thickness and height of the web can vary along the longitudinal extent of the web. The web can have cavities and / or cutouts, wherein the width and / or thickness is reduced adjacent to the cavities and / or cutouts.

[0024] The at least one belt of the support structure preferably has a width and / or thickness that is a multiple of the width and / or thickness of the web. In particular, the belt can have a width and / or thickness of approximately 2 to 3 mm.

[0025] Ideally, the at least one belt and the at least one web are formed from a common, one-piece body or manufactured as a single piece. The belt and web can preferably be made of a homogeneous material, allowing virtually any desired geometry that optimally supports the primary functions of the belt, namely force transmission, and the primary functions of the web, namely the arrangement of the envelope module on the belt. The belt and web can be additively manufactured in a single operation. This avoids structural changes between the belt and web, which could lead to a reduction in strength.

[0026] To achieve optimal force distribution in the web and the shell element, or optimal force transmission between the web and the shell element, the flange and the web can form cross-sections such as a house-shaped profile (two parallel side walls with a tapered roof arranged on the side walls), a HY-shaped profile, or an I-shaped profile. In another preferred variant, the flange forms an open and / or closed hollow profile. The cross-sections can be variably designed to accommodate higher bending moments with the same mass.

[0027] According to an expedient development of the invention, the hollow profile formed by the web and / or flange has openings adjacent to the enveloping element. The openings can form through-holes or cavities between the web and the enveloping element. Alternatively, the openings can be designed as an interruption of the web.

[0028] The openings reduce heat input and thus distortion of the enveloping element during the application of the web and flange. Furthermore, the openings provide an easy drainage option, allowing water caused by condensation, for example, to flow away and reducing the likelihood of corrosion. The openings also allow for the interior coating of the hollow structure with corrosion protection agents and / or the implementation of thermal or acoustic insulation. The distance between two openings is 30 mm, for example, comparable to a welded crotch seam.

[0029] In a further advantageous embodiment of the invention, the flange and the web extend essentially along a load path occurring during operation of the rail vehicle. During operation of the rail vehicle, the car body of a rail vehicle is subjected to loads that are concentrated in the wall of the car body along load paths. These load paths can be determined, for example, using computer-aided calculations or simulations, in particular using the finite element method. If the path of the support structure is then adapted to the path of the determined load paths, increased rigidity of the car body can be achieved while simultaneously reducing weight.

[0030] Ideally, the support structure covers a large number of possible load paths acting on the car body. The support structure can comprise a multitude of webs and chords that essentially run along the determined load paths and are jointly applied to the shell element using additive manufacturing. The webs and chords of a car body element can merge into one another. In order to connect the car body element to an adjacent car body element or other load-bearing structures of a car body in a force-transmitting manner, the chords can extend to the edge of a car body element. At the edge of the car body element, the chords can form fastening and / or force-transmitting interfaces.The fastening and / or force transmission interfaces can, for example, be formed by contact surfaces which, when the car body is assembled, rest on contact surfaces of adjacent elements of the car body or are welded to them.

[0031] The support structure may preferably have an inhomogeneous material distribution across its length or width to accommodate occurring loads. The material and / or mass of the support structure may be concentrated at locations where increased loads occur during operation.

[0032] At least one web and / or chord of the support structure can have an arcuate shape. Additionally, the at least one web and / or chord can have at least one rectilinear section.

[0033] According to a preferred variant of the invention, the support structure covers only a fraction of the inner surface of the enveloping module. Preferably, the support structure covers less than 30% of the inner surface of the enveloping module, or less than 20% of the inner surface, most preferably less than 10% of the inner surface of the enveloping module. This allows for more space for cutouts and / or installations.

[0034] For optimal force transmission and rigidity of the car body element, the height and / or width and / or thickness of the flange and / or web can vary along the longitudinal extent of the support structure. This allows the web's path to be optimized or adapted to the load paths, enabling the most direct force transmission possible through the web. Ideally, at least one flange follows a load path along which the force flow is concentrated during operation of the rail vehicle.

[0035] According to a further variant of the invention, the car body element has a cutout. The cutout can be provided for the installation of a window or a door. The cutout can be substantially rectangular. The support structure or at least one web ideally borders the corners of the rectangular cutout. In this way, the loads that are concentrated at the corners of the cutout during operation are diverted through the web.

[0036] The cutout is preferably bordered at the edges by a frame profile. The frame profile can form a door or window frame. Forces acting on the cutout via the car body shell can be absorbed and dissipated by the frame profile. Ideally, the support structure, in particular the at least one chord, borders the frame profile. The frame profile is therefore preferably bordered at least partially by the support profile and in particular by the at least one chord in a force-transmitting manner, and / or the chord is connected to the frame profile in a force-transmitting manner.

[0037] In a further advantageous embodiment of the invention, the car body element according to the invention forms a car body module with standardized interfaces. The interfaces of the car body module can be formed by one or more chords and / or webs and / or enveloping elements for the transmission of forces. To construct a car body, the car body elements are preferably welded together at their interfaces.

[0038] A car body according to the invention comprises at least one car body element with the features described above. Corresponding car body elements can form side walls, end walls, roofs, and floors.

[0039] According to a method according to the invention for producing a car body for a rail vehicle, the following steps are provided:

[0040] First, a shell element is provided. Then, at least one web is applied to the shell element. The web is preferably applied using additive manufacturing. Ideally, the web is bonded to the shell element. For this purpose, a material that is the same as or equivalent to the material of the shell element can be used for the web.

[0041] The web can be manufactured with openings by starting with the web on spaced-apart surfaces of the envelope element. A flange bridging the openings can then be fabricated on the web.

[0042] Following the fabrication of the web, at least one belt is manufactured. The belt is applied to the webs, spaced apart from the sheathing element.

[0043] In order to achieve optimal load distribution on the shell element, a large number of belts and webs can be applied to the shell element simultaneously by additive manufacturing.

[0044] The additive manufacturing of the web and / or the belt is preferably carried out by laser powder deposition welding or laser welding wire deposition welding.

[0045] For optimal load distribution on the enveloping element, in a further variant of the method according to the invention, the load paths can first be calculated, preferably within a predefined installation space, on a predefined enveloping element. Ideally, computer-aided topology optimization can be performed, with the course of the flanges and webs subsequently being designed according to the determined optimal topology. Computer-aided topology optimization is first performed. Subsequently, the flanges and webs are preferably designed for production and / or manufactured with a course that essentially corresponds to the optimal shape determined by the topology optimization.

[0046] Because additive manufacturing of the flanges and webs also allows for variable cross-sections and curved profiles, the geometry of the flanges and webs can be designed to vary according to the determined load paths. Accordingly, according to a further preferred process step, the shape of the cross-section, the height, the width, and / or the length of the flanges and webs are selected based on the results of a previously calculated topology and / or material thickness optimization.

[0047] In a method for topology optimization according to the invention, the basic shape of the support structure is designed with computer support. The topology (basic shape) is preferably determined with computer support using the finite element method (FEM). The installation space for the support structure is defined, on the one hand, by the envelope element on the outside and, on the other hand, by the maximum available installation space for the support structure or the passenger interior on the inside. In particular, the installation space defined for the definition of the support structure is limited by the envelope element. The geometry of the envelope element is preferably defined before the start of the topology optimization, with the installation space of the support structure beginning on one side at the envelope element.

[0048] The chords and webs extending toward one end of the car body element preferably border the end of the enveloping element. The ends of the chords and webs can form force-transmitting connection interfaces for the installation of the car body element into a car body. The connection interfaces are thus located on the load paths. By connecting two car body elements at the connection interfaces, e.g., by welding, a continuous force flow can be achieved via adjacent support structure modules. The support structure modules of two adjacent car body elements can ideally dissipate the loads acting on the car body elements essentially completely.

[0049] The connection interfaces can, for example, be formed by welding surfaces to which adjacent support structure modules can be welded together and / or load-bearing profiles can be connected.

[0050] The belt and / or the web are preferably widened in the area of ​​the connection interface to enable high force transmission at low tension.

[0051] According to a further variant of the method, the car body element is designed with a cutout. The cutout can be intended for the installation of a window or a door. The cutout is preferably surrounded by a frame profile. The installation space defined for the definition of the support structure is then ideally delimited, at least in sections, by the frame profile. The installation space can surround the frame profile. For this purpose, the geometry of the frame profile is preferably defined before the start of the topology optimization. The installation space of the support structure preferably begins at the frame profile. Accordingly, the support structure can begin at the frame profile.

[0052] Thanks to the topology-optimized design and the corresponding additive manufacturing of the support structure, the component costs of a car body element according to the invention can be reduced by approximately 40%. Furthermore, a weight saving of approximately 15% is possible relative to the additively manufactured structure.

[0053] The accompanying drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention, but are not limited thereto. In the drawings: Fig. 1 an embodiment of a car body element according to the invention for a car body of a rail vehicle; Fig. 2 a first enlarged section of the embodiment of the Fig. 1 ; Fig. 3 a second enlarged section of the embodiment of the Fig. 1 ; Fig. 4 a third enlarged section of the embodiment of the Fig. 1 .

[0054] In the following, with reference to the Figuren 1 bis 4 a preferred embodiment of a car body element according to the invention is described.

[0055] Fig. 1 shows a car body element 1 according to the invention. The car body element 1 has a shell element 2 provided with a support structure 3. Two cutouts 4 are provided in the car body element 1, which serve for the installation of windows.

[0056] The enveloping element 2 is essentially made of sheet-like or plate-shaped metal sheets. The enveloping element 2 forms the outer surface of a car body or rail vehicle and essentially ensures the spatial separation between the interior and the surrounding area. To ensure optimal utilization of the available clearance profile on a railway line, i.e., to maximize passenger compartment volume, the enveloping element 2 is curved toward the outside. The support structure 3 is thus mounted on a concavely curved surface of the enveloping element 2.

[0057] The support structure 3 is largely applied to the inner surface of the enveloping element 2 via additive manufacturing in first sections 14 and is integrally bonded to it. In areas where a structure based solely on additive manufacturing does not offer the desired properties or capabilities due to greater loads and / or complex geometries of the enveloping element 2, the support structure 3 is prefabricated in second sections 15 and subsequently welded to the first sections 14 and the enveloping element 2.

[0058] The second sections 15 comprise support elements 16, 17, 18, which are not manufactured using a 3D printing process, but are formed from sheet metal—preferably laser-cut and bent sheets. The sheets are cut out, preferably by laser, then bent, preferably by bent, and finally welded to the adjacent first section 14 and the casing element 2.

[0059] At its upper end, the shell element 2 comprises a longitudinal profile 5 extending in the longitudinal direction L, which reinforces the car body element 1 and forms a transition to the car body roof (not shown here). The longitudinal profile 5 can form a longitudinal member of a car body. The support structure 3 extends to the longitudinal profile 5.

[0060] The cutouts 4 are surrounded by frame profiles 6. The frame profiles 6 support the cutouts 4 at their edges. The frame profiles 6 form window frames for accommodating window seals and panes. The frame profiles 6 are in turn supported by the support structure 3. For this purpose, the support structure 3 borders the frame profiles 6. The frame profiles 6 are preferably made of semi-finished products and welded to the casing element 2 in order to achieve the most precisely manufacturable interface for the windows to be installed.

[0061] For the connection to a frame profile 6, a second section 15 formed from a laser-cut, folded sheet metal is provided.

[0062] The webs 7 and / or flanges 8 can have an arcuate course, which essentially corresponds to the arcuate course of a load path occurring during operation. In the embodiment of the Fig. 1 Curved belt sections 10 are provided, enabling optimized force transmission. Furthermore, straight belt sections 11 are present. The majority of the straight belt sections 11 run essentially diagonally to the longitudinal direction L, thus forming a stiffened shear field. The straight belt sections 11 thus serve, in particular, to increase the torsional rigidity of the car body.

[0063] Fig. 2 is an enlarged section of the lower end of the car body element of the Fig. 1 . The support structure 3 comprises webs 7 and straps 8. The webs 7 are applied directly to the covering element 2.

[0064] In the floor area, the chords 8 and webs 7 border the end of the enveloping element 2, where force-transmitting connection interfaces 13 are provided for connection to a car body floor. The connection interfaces 13 are formed by support elements 18. The support elements 18 are designed as hollow profiles. Recesses 19 serve to reduce the overall weight of the support structure 3, provide drainage, and, if necessary, apply corrosion protection agents and heat / sound insulation in the cavity. Struts 20 are provided between the recesses 19, forming a truss-like structure and thus ensuring weight-optimized stiffening of the support element 18.

[0065] The first section 14 of the support structure 3 branches into two branches, forming a fork 21. Welded onto the fork is a support element 22 designed as a support plate, which supports the two branches of the fork 21 against each other and simultaneously serves as a fastening interface for rails for passenger seat fastening.

[0066] Fig. 3 shows an enlarged section of the car body element 1 at a section 4. The frame profile 6 for accommodating a window is integrally connected to the support structure 3. For this purpose, a first section 14 of the support structure 3, which is printed onto the shell element 2 using a 3D printing process, is integrally connected via a second section 15 of the support structure 3. The second section 15 is formed by a support element 17, which is formed from a sheet metal.

[0067] The support element 17 extends in a bridge-like manner from the first section 14 of the support structure 3 to the frame profile 6. The support element 17 runs essentially parallel to the casing element 2.

[0068] Fig. 4 is a further enlarged section of the car body element 1 in the roof area. The longitudinal profile 5 curves inward over adjacent areas of the cladding element 2. If the support structure 3, produced using a 3D printing process, in particular by deposition welding, is applied to the cladding element 2 in a direction that is essentially perpendicular to the large-surface areas of the cladding element 2, the longitudinal profile therefore forms an obstacle for the welding device.

[0069] The use of a separately manufactured support element 16 allows it to be inserted, for example, transversely to the 3D printing direction into the shell element 2 and welded to other sections of the support structure 3. Undercuts or cavities, such as the cavity formed by the longitudinal profile 5, can thus be connected to the support structure 3.

[0070] The support element 16 is like that in Fig. 2 The support element 18 shown is provided with recesses 19, which are bridged by struts 20. The resulting truss structure also enables a weight-saving, high-strength construction, combined with the possibility of an internal corrosion protection coating and the implementation of thermal / sound insulation. List of reference symbols

[0071] 1Car body element 2Shell element 3Support structure 4Cutout 5Longitudinal profile 6Frame profile 7Web 8Belt 9Opening 10Curved belt section 11Straight belt section 12Connection interface (side) 13Connection interface (floor) 14First section 15Second section 16, 17, 18Support element 19Recess 20Strut 21Fork 22Support element

Claims

1. Car body element (1) for the car body of a rail vehicle, having a support structure (3), a frame profile (6) and at least one covering element (2), wherein the support structure (3) is printed onto the covering element (2) in a first section (14) using a 3D printing process and is formed in a second section (15) by at least one prefabricated support element (16, 17, 18, 22), wherein the at least one prefabricated support element (16, 17, 18, 22) forms a connection between the first section (14) of the support structure (3) and the covering element (2) and / or between the frame profile (6) and the first section (14) of the support structure (3) and / or between a longitudinal profile integrated into the covering element (3) and the first section (14) of the support structure (3).

2. Car body element (1) according to claim 1, wherein the first section (14) is a peripheral section of the covering element (2) and / or wherein the first section (14) is welded to the second section (15) and / or the second section (15) is welded to the covering element (2).

3. Car body element (1), according to one of the preceding claims, wherein the support structure (3) is formed at at least one connection interface (12, 13) by at least one sheet metal, in particular a laser-cut and / or folded sheet metal.

4. Car body element (1), according to one of the preceding claims, wherein the second section (15) forms a connection region for connecting the support structure to the covering element (2) and or the frame profile (6).

5. Car body element (1) according to one of the preceding claims, wherein the support structure (3) has openings (9) adjacent to the covering element (2).

6. Car body element (1) according to one of the preceding claims, wherein the support structure (3), in particular the first section (14) of the support structure (3), extends substantially along a load path occurring during operation of the rail vehicle.

7. Car body element (1) according to one of the preceding claims, wherein the support structure (3) covers the majority of possible load paths acting on the car body.

8. Car body element (1) according to one of the preceding claims, wherein the support structure (3) comprises at least one belt (8) and at least one web (7) arranged between the belt (8) and the casing element (2).

9. Car body element (1) according to claim 8, wherein the height and / or width of the belt (8) and / or web (7) vary along the longitudinal extent of the support structure (3).

10. Car body element (1) according to one of claims 8 or 9, wherein the at least one belt (8) follows a load path along which the force flow is concentrated during operation of the rail vehicle.

11. Car body element (1) according to claim 10, wherein the belt (8) together with the web (7) forms a house-shaped, HY-shaped or I-shaped profile.

12. Car body element (1) according to one of the preceding claims, wherein the car body element (1) has a cut-out (4) and the cut-out (4) is bounded at the edge by the frame profile (6), wherein the frame profile (6) adjoins the support structure (3) in a force-transmitting manner at least in sections and / or is connected to the support structure (3) in a force-transmitting manner.

13. Car body for a rail vehicle, characterised by a plurality of car body elements (1) according to one of claims 1 to 12.

14. A method of manufacturing a car body element according to any one of claims 1 to 12, comprising the steps of: a. manufacturing a shell element (2) and a frame profile (6), b. manufacturing a support element (16, 17, 18, 22), in particular from a sheet metal, c. additive manufacturing of a support structure (3) on the covering element (2), d. welding the support element (16, 17, 18, 22) to the support structure (3) and / or the covering element (2), such that the support element (16, 17, 18, 22) forms a connection between the first section (14) of the support structure (3) and the covering element (2) and / or between the frame profile (6) and a first section (14) of the support structure (3) and / or between a longitudinal profile integrated into the covering element (3) and the first section (14) of the support structure (3).

15. Method according to claim 14, characterised by the additional steps: a. computer-aided topology optimisation, where the installation space is defined by the covering element, b. subsequently manufacturing a support structure (3) with a shape that essentially corresponds to the optimum shape determined by the topology optimisation.