RU2431189C2 - Method of optimisation of structural design of composite panel reinforced with stiffeners - Google Patents

Abstract

FIELD: machine building. ^ SUBSTANCE: method consists in following stages: making imitation model of panel; determination of variables of design of panel reinforced with stiffeners; determination of restricting condition which design solution of panel reinforced with stiffeners should meet; creation of imitation expert module of family of kinds of damages for panel reinforced with stiffeners; iteration change of variables at designing imitation model (9) reinforced with stiffeners to optimise target variable. ^ EFFECT: raised specific strength of panel due to consideration of changes at distribution of loads and to confirmation of compliance to restricting conditions and requirements of safety factor related to preliminary specified kinds of damages with usage of imitation expert modules. ^ 5 cl, 6 dwg, 4 tbl

Description

FIELD OF THE INVENTION

The present invention relates to a method for optimizing the design process of a panel of a composite material reinforced by stiffeners, such as casing of an aircraft structural box.

BACKGROUND OF THE INVENTION

Work on the use of composite materials in the design of aircraft structures began after the Second World War. The first materials, mainly fiberglass pressed into polyester resins, were used in the design of superstructure elements such as fairings to protect antennas and fuselages. Continuous development in the development of new materials has led to an increase in the use of these materials in more numerous components of the aircraft.

Twenty-five years of accelerated development in the study of both the properties of materials and their behavior have passed from the time of certification of the first main structural element of a passenger aircraft, which was completely designed using composite materials, successfully completed in the 80s, to the present, when knowledge is acquired in the design of most of the structure of the aircraft made of composite material.

The composition of the composite materials most used in the aviation industry includes fibers or bundles of fibers pressed into a matrix of thermosetting or thermoplastic resin, in the form of pre-impregnated material or "prepreg".

In the list of advantages of composite materials, there are three most basic:

- their high specific strength in comparison with metals. This is the ratio of strength / weight;

- their excellent fatigue behavior;

- the possibilities of structural optimization hidden in the anisotropy of the material, and the possibility of combining fibers with different orientations, providing the ability to design elements with different mechanical properties, focused on different needs in terms of applied loads.

Methods of designing aircraft structures using the advantages inherent in these optimization opportunities are not known from the prior art, and the present invention is directed to solving this problem.

SUMMARY OF THE INVENTION

The present invention provides a method for computer-aided design of a panel structure made of composite material reinforced by stiffeners, which consists in optimizing the target variable, comprising the following steps:

a) obtaining a simulation model (SM) of the panel, reinforced with a stiffener, including all relevant information for the analysis of the design of this panel, from a common finite element model (GFEM);

b) determining at least one variable panel design variable reinforced by stiffeners;

c) determining at least one restrictive condition that the design solution of the panel reinforced with stiffeners must satisfy;

d) the creation of at least one simulation expert module of a family of types of panel damage reinforced by stiffeners;

e) an iterative change in the design variables of the simulation model (SM) of the panel, reinforced by stiffeners, in order to optimize the target variable, taking into account at each iteration the changes in the load distribution caused by the previous iteration, and confirmation of satisfaction of at least one specified restrictive condition, and safety margin requirements associated with predefined types of damage using the specified at least one simulation expert module.

The use of a simulation expert module of families of types of damage provides the opportunity to fully optimize the design for all possible damage criteria and their relationships and, therefore, achieve the result of the process, is very close to the final design decision of the design.

A relevant feature of the present invention is that said relationships can and do lead to non-linear changes and potential sudden changes in safety margin, and thus to an increase in the complexity of the optimization process.

Other features and advantages according to the present invention will be disclosed in the following detailed description of an illustrative embodiment, its purpose with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Figa is a perspective view of the casing of the caisson, which can be designed according to the method, which is the object of the present invention.

Fig.1b is a view of Fig.2a in section along the axis A-A '.

Figure 2 - structural element of the casing of the caisson of the aircraft structure and its idealized representation in the finite element model.

Figure 3 - structural element of the casing of the caisson aircraft structure with a local coordinate system.

FIG. 4 is a flowchart of a method for optimizing a design process for a casing of an aircraft structure according to the present invention.

Figure 5 is a simplified example of said skin.

6 is a skin of a caisson of an aircraft structure, divided into optimization zones.

DETAILED DESCRIPTION OF THE INVENTION

The approach to the problem of design optimization is mainly in finding the minimum of the objective function. In this case, a suitable target function is the minimum weight of the structure, due to a number of conditions that must be fulfilled simultaneously, such as positive safety factors, special design criteria, production and stability constraints to prevent aeroelastic phenomena, etc.

As is known, the optimization problem of the type under consideration can be formulated by the expression min f (x _i ), in which f is the objective function (in this case, in the preferred embodiment, it is the minimum weight), x _i are the task variables, and g _j (x _i ) - restrictions.

In the case when the material of the structural element, which is the object of optimization, is isotropic, the number of variables in the problem is much smaller than in the case of using composite materials. Among some reasons for the occurrence of this fact, one can name the dependence of stiffness on the orientation and order of fiber laying, the discreteness of the thickness values, the discontinuity of the variables, etc.

The noted extremely high degree of complexity implies the need to develop a method aimed at reducing the complexity of the task to reasonable parameters that ensure that the goal can be reached in a safe, as well as fast and efficient manner.

The following is a description of an example embodiment of the present invention to obtain an optimized design solution for reinforced casing of an aircraft structural box.

The main function of the skin of the casing 9 of an aircraft structure, similar to that shown in Fig. 1, relating to a lifting force element, such as a wing or an airplane stabilizer, should form a continuous surface that receives and distributes aerodynamic pressure caused by the lifting of this element. These aerodynamic forces act in the direction normal to the skin.

On the other hand, in the presence of these external loads, the wing or stabilizer behaves like a cantilever beam, which determines the internal distribution of shear forces, bending and torques along the span. In this case, axial and bending loads are perceived by both the panels and the skin stringers, and shear forces and torques acting on the wing are perceived by the skin panels and stringer walls. In other words, claddings are combined:

- axial loads, arising mainly due to the bending of the caisson, creating tension in one casing and compression in the opposite casing;

- shear loads resulting from torque and shear forces;

- lateral loads that may occur for the following several reasons:

- wedge-shaped caisson;

- Poisson effect;

- local loads resulting from the fastening of metal parts of other elements, such as rudders, flaps, etc.

The configuration design of the skin 9, which is most used to perceive the mentioned combination of loads, consists of a panel 11 with a very low curvature due to the aerodynamic profile supported by ribs and stringers with a certain degree of restriction of the angle of rotation and reinforced in the longitudinal direction by the stringers 13.

Panel 11 is composed of carbon fiber layers previously impregnated with resin. The layers are very thin (have a thickness of less than 0.25 mm) and are formed of carbon filaments, all of which are oriented in the same direction (tape) or in two directions (fabric) and are pressed into an uncured resin. Each layer is oriented in a given direction, usually 4 laying directions are set: 0 °, 90 °, + 45 ° and -45 °, however, this rule is not unchanged. On the other hand, the number of layers in each direction is determined by the requirements for mechanical properties put forward by the designer. The panel acquires its characteristic features after the curing process of the resin under conditions of high temperature and pressure.

On the other hand, the manufacture of stringer 13 is performed in the same way. The connection between the two elements can be achieved by joint curing of both elements or the use of an intermediate adhesive. In either of the two cases, for analytical purposes and ensuring the high quality of the obtained compound, it can be assumed that both elements work as a whole.

A method for optimizing the design process of the indicated section according to the present invention, the detailed description of which is given below, contains (see FIG. 4) three large phases:

- phase 21 of the preparation;

- phase 51 of the simulation;

- phase 91 presentation of the results.

Phase 21 of the preparation, in turn, contains two stages:

- step 25 of obtaining a simulation model (SM) skin 9 from

general finite element model (GFEM) 23;

- step 25 of determining the variables and constraints.

The starting point of preparation phase 21 is the determination of the caisson GFEM 23 and the external loads applied to it. In other words, the initial given value will be the structure of the problem, understood as the geometric arrangement of the various elements forming the caisson:

- location of stringers;

- the location of the ribs that allow you to determine the length of the panels;

- the step of the stringers, which obviously can be constant or variable;

- orientation of the stringers, i.e. parallelism of stringers to one another or to some stringer.

The data presented are not an object of optimization, i.e. they remain constant throughout the process. If it is necessary to assess the effect of a change in any of these data on the weight of the structure, the specified modification of the GFEM 23 itself and the restart of the process are required. Obviously, the approach to the indicated data as task variables is ideal, however, the indicated assumption transforms the task into something uncontrollable. Despite this, the method according to the present invention allows to achieve an extremely high speed of execution of the complete process and, thus, provides the ability to perform numerous studies at a very low cost of calculations and even at the same time facilitate the comparison of results and selection of the optimal integrated solution.

Despite the fact that only skin 9 is the object of optimization, GFEM 23 may include all elements of the caisson. In other words, no conversion of the original model used will be required to obtain internal loads.

Sheathing 9 may be continuous or have holes. The only basic requirement is a separate idealization of stiffeners and paneling, as well as their correct delimitation as a result of idealization of stringers and ribs.

Inclusions in GFEM 23 cladding 9 properties of elements, thicknesses, materials, areas, etc. not required. However, the main thing is to determine the orientation of the material at an angle of 0 °, i.e. axis relative to which the orientation of the laying of the layers of the composite material is determined.

On the other hand, the GFEM 23 of the skin 9 should also include the determination of the finite elements forming the area of each structural element, as well as their designation in accordance with established standards.

The second stage of the preparation phase 21 is to obtain the SM 25 skin 9 based on the concept of the structural element.

The structural element 17 is defined as the smallest section of the skin 9, operating as a unit, which, as shown in figure 2, is formed by a section of the stringer S _m limited by the ribs R _n and R _{n + 1} and its adjacent panels 15, 15 '(passing from the stringer S _m to the adjacent stringer S _{m + 1} or S _m-1 ), which can be idealized by one or more finite elements (three elements for each panel 15, 15 'are shown in FIG. 2).

For each structural element 17, the SM 25 contains all relevant information for the structural analysis process, discussed below:

- geometry:

- width and length of panels 15, 15 ';

- the angle φ of the orientation of the laying of the material relative to the local axes (X, Y, Z) of the structural element 17 (see figure 3);

- the shape of the straight section of the stringer S _m ;

- dimensions of the stringer S _m .

Table 1 shows some of the initial geometric data of the casing according to the example presented in figure 5, consisting of three panels and two T-shaped stringers.

Table 1 Initial sizes Stringer base width (mm) 70 Stringer base thickness (mm) four Stringer shelf height (mm) 65 Stringer Shelf Thickness (mm) 3 Panel Thickness (mm) 8

Mechanical properties:

- laying of panels 15, 15 ';

- laying stringers S _m along its entire straight section;

- properties of the material used during installation.

The mechanical properties of the layer.

Table 2 shows some initial data when laying the paneling according to the example presented in figure 5.

table 2 Initial sizes The percentage of layers with an orientation angle of 0 ° 55 The percentage of layers with an orientation angle of 4 ° 35 The percentage of layers with an orientation angle of 90 ° 10

- Loads in panels 15, 15 'and stringer S _m .

- Safety factors for all types of damage in the structural element 17.

For example, for casing according to the example presented in figure 5, safety factors exceeding 1.0 are set for the following types of damage:

- local deflection of the stringer;

- local deflection of the casing;

- deflection of the casing between the rivets;

- subsequent warping in case of local loss of stability of the stringer and sheathing;

- general deflection of the structure;

- tolerance for damage in the casing and in the base and shelf of the stringer;

- potential riveted joints in the casing and in the base and shelf of the stringer to ensure maintainability of the component.

The third phase of preparation phase 21 is the definition of design variables and constraints 27.

The definition of a design variable means the designation of a predefined feature in the "structural element 17", which is subjected to analysis of its sensitivity to the result of the analysis of the structure.

For example, if the panel thickness is defined as a design variable, the optimization motor calculates the change in safety margin for each relevant damage criterion before a single change in the value of the variable. This makes it possible to direct the process to design options with a minimum weight relative to the set variable.

The number of design variables that can be defined in a design element essentially depends on its geometric complexity and can be very large. Therefore, the method allows the user to select design variables to be optimized. To correctly make this choice, consider variables that have a greater impact on the objective function, such as:

- thickness of panels;

- thicknesses of various stringer segments;

- sizes of stringers;

- percentages of the possible orientations of the panel layers;

- percentages of the possible orientations of the layers of various stringer segments.

The described method is not limited by the number of design variables given, and this restriction is determined more likely by the capabilities of computing equipment and mathematical optimization algorithms.

For casing according to the example presented in figure 5, the following are considered as design variables:

- the width of the base of the stringer;

- thickness of the base of the stringer;

- the height of the shelf stringer;

- the thickness of the shelf stringer;

- panel thickness;

- percentage of layers with an orientation angle of 0 °;

- percentage of layers with an orientation angle of 90 °.

The limitations of these variables are defined as the range of variability or the value of a parameter, which may or may not be a design variable. For example, the thickness of a panel can be defined as a design variable, and at the same time, its minimum and / or maximum value can be defined as a limitation.

However, the minimum value for the stringer area is the ratio of the area of the panel, which does not have to be a design variable and can be defined as a limitation.

Another type of constraint that must be defined is the maximum ratio of the values of the design variables corresponding to neighboring elements, subject to design rules that guarantee the possibility of manufacturing a component.

Thus, if a parameter is defined as a design variable, this means that it is an object of analysis for sensitivity to safety factors, whereas if this parameter is defined as a limitation, it only confirms its compliance by means of an optimal solution.

The following are the manufacturing limitations used in the case of the casing shown in FIG. 5:

- effective panel width. The ratio of the stringer's step and the width of its base;

- the ratio of the longitudinal stiffness of the panel and its shear stiffness, E _x / G _xy ;

- the diameter of the rivet used in the maintainability criterion;

- minimum panel thickness;

- thickness and length of the base of the stringer;

- thickness and length of the stringer shelf;

- the relationship between the thickness of the shelf of the stringer and the thickness of its base;

- the relationship between the thickness of the shelf of the stringer and the height of its shelf;

- the relationship between the thickness of the base of the stringer and the thickness of the panel;

- percentage of panel layers in a direction with an orientation angle of 0 °;

- the percentage of panel layers in a direction with an orientation angle of 90 °.

To assess the effect of changes due to the optimization method described below on the distribution of internal loads, data transfer between the SM 25 and the GFEM 23 should be automatic. It should be borne in mind that in this method the change in the value of the optimization variables is focused on the minimum weight options, which can cause relative changes in stiffness between structural elements forming the casing, and, consequently, changes in the trajectories of the loads on it.

After describing the preparation phase 21 of the method for optimizing the design process of the skin 9 according to the present invention, the following is a description of the simulation phase 51, which contains the use of the SM 25 optimization engine 53 in the iteration cycle 52, which uses simulation modules 55 of the families of types of damage, regarding stability 57, permissible damage 59, maintainability 61, or other factors 63, changing the values of the design variable selected in order to reduce the weight of the skin 9 in each iteration cycle 52.

Possible typical options for the destruction of the skin 9 subjected to the said loads are:

- loss of stability;

- gap taking into account the presence of permissible manufacturing damage and defects;

- connections between various elements: ribs, spars, fittings, etc.

On the other hand, the maintainability of the component should be guaranteed taking into account its service life. This ratio is taken into account by a preliminary determination of repair work such as riveted patches.

During the sensitivity analysis to be performed by the optimization engine 53, each of these types of damage should be considered. This is ensured by the creation of simulation expert modules 55, which make it possible to effectively implement the basis of analysis, and this efficiency should be interpreted from three points of view: the security of simulation, its reliability and the reduction in the cost of consumed computing resources.

The concept of simulation expert module 55 is limited by the following parameters:

- It does not use simple and general formulas of the specialized literature for determining the net destructive loads of simplified elements, but rather uses complex methods for determining the indicated allowable considered loads:

• Any combination of load patterns in more than two shear directions.

• The influence of the stringer base on local and general buckling.

• Different orientations when laying both panels and stiffeners, consisting of various materials.

However, the exclusively expert module is characterized by its suitability for taking into account the relationship between the different types of damage under consideration, so that it allows you to determine not only the main, or pure, types of damage, but also the possible effect of one of them on the other in accordance with the level of load applied to each of them. The following is an example explaining this relationship:

• Local deflection occurs in a panel located between two stringers at a load level P, which causes a greater total deflection.

• Under these conditions, when the inertia of the stringer is sufficient to prevent general deflection, in the panel reinforced by stiffeners, subsequent warping develops, causing the load to be redistributed on the stringer.

• Finally, the assembly is destroyed either due to damage, for example, the support of the stringer, or due to the separation of the stringer from the panel under a load of P + ΔP.

• Prior to the previous paragraph, the application of the first paragraph related to the use of complex processes is possible. However, the expert module provides an additional step as a result of correlating the previous sequence with the effect that the secondary load distribution resulting from the loss of stability can have other general damage criteria such as permissible damage or maintainability. In other words, the safety factors of these two criteria take into account the loads caused by the redistribution of initially unapplied loads in the event of such a redistribution.

In a preferred embodiment of the present invention, each type of damage has a corresponding simulation expert module 55. This provides greater flexibility of the method for the following reasons:

- Simulated expert modules 55 may or may not be related to optimization and, at the same time, provide the opportunity to study the effect of taking into account each damage criterion on the weight of the optimal design.

- Corrections or improvements to the simulation expert module 55 can be carried out completely regardless of the characteristics of the method for optimizing the design process of a particular component.

- New simulation expert modules 55 can be completely seamlessly included in a way to optimize the component design process.

In a preferred embodiment, the simulation phase 51 comprises the following steps:

a) Running the model with the initial data to ensure the correct distribution of internal loads between the various elements 17 of the structure of the skin 9 in accordance with their relative stiffness.

b) Subsequent receipt by motor 53 of optimization of safety factors corresponding to various simulation expert modules 55 for each structural member 17.

c) Motor 53 performing an optimization of the sensitivity analysis for each type of damage after obtaining the corresponding safety factors.

d) Change in design variables that are considered the most promising in terms of reducing the weight of the skin 9, taking into account the results obtained in the previous analysis and the remaining certain limitations.

e) Obtaining a new distribution of internal loads in the various elements 17 of the casing 9, taking into account the potential changes in the coefficients of relative stiffness causing these changes by changing the model and restarting it.

f) Repeating the whole process from step b) as amended, providing the optimization engine 53 with the possibility of a progressive reduction in the initial weight at successive iterations, converging to one solution, characterized in that no subsequent solution when introducing any change and fulfilling the initial constraints will provide further weight loss.

g) Performing the final stage, which consists in fully determining the sequence of laying the casing from the optimal percent of layers for each of the orientations considered.

The last phase 91 of the method for optimizing the design process of the skin 9 according to the present invention is the phase of presenting the results of the optimal solution 93.

These results, which can be presented graphically or numerically, usually contain the following information:

- The optimal value obtained for each of the optimization variables.

- Safety factors, corresponding to each type of damage contained in the optimization method, and their corresponding loads.

- Critical safety factors.

- Cases of critical loads.

- Types of damage associated with critical safety factors.

- The initial and final result of the objective function, as well as

their development for all sequential iterations.

For casing according to the example shown in Fig. 5, Table 3 shows the data on the change in the weight of the casing in the simulation phase 52, in which the optimal solution was achieved at six iterations.

Table 3

Table 4 shows the data on changes in the used optimization variables.

Table 4 Initial sizes Optimized sizes Stringer base width (mm) 70 76 Stringer base thickness (mm) four 2,4 Stringer shelf height (mm) 65 51 Stringer Shelf Thickness (mm) 3 1.7 Panel Thickness (mm) 8 3 The percentage of layers with an orientation angle of 0 ° 55 40 The percentage of layers with an orientation angle of 90 ° 10 twenty

Based on the data presented, it can be concluded that the optimization method allows to increase the width of the base of the stringer, reduce its thickness, reduce both the height and thickness of the shelf of the stiffener, and thus reduce its inertia, as well as reduce the thickness of the panel and angles orientation layers. These changes lead to a 40% weight loss.

The optimization process of all structural elements that can be defined in the entire skin can be extremely expensive and complicated, and, in addition, taking into account the restrictions imposed by the production process, its results can be optimal, but impossible solutions. And finally, in a preferred embodiment of the present invention, SM 25 is not implemented at the level of structural element 17, but rather at the level of the zone, understood as a group of structural elements 17 having one or more common structural parameters, as well as limitations and variable optimizations, understood as same.

Figure 6 presents a partition of the skin 9 into zones 19.

The present invention can be applied to the design of aircraft structural elements, such as horizontal and vertical stabilizers, cantilevers and the middle parts of the wing, including high lift surfaces such as rudders, flaps, slats, which contain elements formed by stiffeners in their architecture with small curvature.

In addition to use in the design of cladding, the method according to the present invention can be used for:

- identifying factors that have the greatest impact on design, and their impact on the final objective function of the type:

- critical types of damage,

- case of critical loads.

- Studies of viability type:

- the impact of changes in the mechanical properties of materials,

- the influence of the orientation of the rosette-like structure that defines the angles of the layers,

- the influence of the overall flexibility of the component on its aerodynamic behavior.

The deformation of a component changes its shape and, therefore, the aerodynamic loads to which it is subjected. Therefore, total rigidity could be included as a design constraint.

In the preferred embodiment described above, any changes may be made without departing from the scope of the invention as defined in the claims that follow.

Claims (5)

1. A method for computer-aided design of a panel structure (9) from a composite material reinforced with stiffening elements, which consists in optimizing the target variable, comprising the following steps: obtaining a simulation model (25) of the panel (9), which includes all relevant information for analyzing the design of this panel , from the general finite element model (23); determining at least one variable panel design variable (9) reinforced by stiffeners; determination of at least one restrictive condition that the design solution of the panel (9) reinforced by stiffeners must satisfy; the creation of at least one simulation expert module (55) of a family of types of damage to a panel (9) reinforced by stiffeners, and at least one simulation expert module (55) of a family of types of damage to a panel (9) reinforced by stiffeners the relationships that exist between the different types of damage in the family of types of damage to the panel; iterative change of the design variables of the simulation model (25) of the panel (9), reinforced by stiffeners, in order to optimize the target variable, taking into account in each iteration the changes in the load distribution due to the previous iteration, and confirmation of satisfaction of at least one specified restrictive condition, and also the safety margin requirements associated with predefined types of damage using the specified at least one simulation expert module (55). 2. The method according to claim 1, characterized in that the target variable is the weight of the panel (9) reinforced by stiffeners. 3. The method according to claim 3, characterized in that it provides for the creation of several simulation expert modules (55) from families of types of panel damage reinforced by stiffeners, and the fact that the relationships existing between types of damage in various simulation expert modules (55), are taken into account when confirming compliance with safety factors associated with predefined types of damage using the specified expert modules. 4. The method according to claim 4, characterized in that the family of types of damage in these simulation expert modules (55) include one or more of the following: loss of stability, permissible damage and maintainability. 5. The method according to claim 1, characterized in that said panel (9), reinforced with stiffeners, is a casing of an airplane box.

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