Targeting Composite Wing Performance Optimising the Composite Lay-Up Design

Transcription

1 Targeting Composite Wing Performance Optimising the Composite Lay-Up Design Sam Patten Optimisation Specialist, Altair Engineering Ltd Imperial House, Holly Walk, Royal Leamington Spa, CV32 4JG Abstract This paper demonstrates the application of innovative optimisation functionality within Altair HyperWorks for the design of optimised composite structures. In the field of composite design there are an almost unlimited number of ways in which a laminate could be constructed by modifying the laminate ply shapes, position, orientation, material and lay-up sequence. With such a range of possibilities determining the optimum design has previously been extremely time-consuming and traditional optimisation methods cannot cope with problems of this scale and complexity. The development of Altair s composite optimisation technology has made it not only possible to optimise complex composite structures but also complete the optimisation in a short time frame. This paper shows how Altair OptiStruct, part of the HyperWorks suite, is used to provide a complete solution when designing with laminated composites, taking the design through concept stages to producing the final ply lay-up sequence. The technology is applied to the design of a laminated wing cover to produce a mass optimised design which meets the requested structural targets. Keywords: Laminate Boundaries, OptiStruct, Free Element Sizing 1. Introduction The aerospace industry provides many challenges which require the use of leading edge technologies to keep up with increasing performance demands. This paper shows how Altair s optimisation technology can be applied to the design of composite wing covers to provide a detailed composite design and to determine: i. Patch shape & position ii. Number of plies for each shape, position & orientation iii. Ply stacking sequence In determining the optimum composite design, the following structural & manufacturability targets were applied to the structure: i. Stiffness targets ii. Laminate lay-up rules The laminate lay-up rules to be applied are: i. The final design must have / layers on the outside ii. The lay-up must be symmetric iii. The & layers must be balanced Altair Engineering

2 iv. The lay-up must not have more than four plies of the same orientation adjacent to each other The OptiStruct optimisation process involved using: Free Element Sizing (FES) optimisation was used to determine the ply shapes & positions (concept design phase) Size optimisation was used to determine the required number of plies for each ply patch & orientation (system level design phase) Stacking sequence optimisation was used to determine the best lay-up sequence subject to meeting the composite stacking rules (detailed design phase) 2. Free Element Sizing 2.1 FES Modelling method An existing finite element model of the aircraft wing was provided, containing a number of in flight, take-off and landing loading conditions. Structural targets were also provided for each loading condition: Static displacements at the wing tip Rotational stiffness targets The objectives of the free element size optimisation are to: i. Produce concept ply shapes for upper and lower covers of the aircraft wing ii. Save significant model set-up & run time compared to other optimisation methods iii. Visualise which ply orientations are doing the most work iv. Minimise the mass of the composite covers v. Meet the required stiffness targets 2.2 Free Element Sizing Method & Super-Ply Concepts Free element sizing allows the thicknesses of individual shell elements to be varied independently. In the case of composite structures, the thicknesses of each ply within each element are varied independently. The method is based upon similar principles to topology optimisation, as opposed to having actual independent design variables for each element thickness. This has the following advantages: i. Ease of model setup; one option to vary all elements independently within the model ii. Single design variable per component; independent thickness changes handled internally iii. Fast solution time; converges in few iterations Altair Engineering 29 Targeting Composite Wing Performance- 16-2

3 T 9 PCOM P T - Figure 1: Free Element Sizing The concept of a super-ply is to group plies of the same type together such that the number of plies in the model is significantly reduced. The thickness of each super-ply can then be varied as a method of simulating addition or removal of plies in the laminate. OptiStruct allows an element formulation to be used which smears the available stiffness from the plies uniformly throughout the element thickness. This is similar to dividing each ply into a number of infinitely thin plies and mixing them evenly, allowing the composite to be modelled as a super-ply but simulated as if it were a uniformly shuffled stack, ignoring the effects of ply position with the lay-up. Ply Level Stack Super-Ply Stack Smear Formulation (layers uniformly mixed) Figure 2: Super-Ply Methodology Since the element thicknesses are varied independently, the solution has the benefit of highlighting the optimum location for laminate patches as well as their thickness. Optimised (concept) ply shapes are automatically generated as a part of the optimisation solution. 3. Concept Design Phase 3.1 Modelling method The elements in the upper and lower cover were arranged into a new component group and assigned a super-ply property. The laminate is made up of four super-ply layers (,,-, 9 ) each of which has an orthotropic material definition. The orthotropic material zero direction was aligned with the wing length and has a longitudinal stiffness that is significantly higher than the transverse stiffness (E1 >> E2). The super-ply uses the SMEAR option to simulate an evenly shuffled laminate stack. This property was used by the free element size optimisation to determine the optimum thicknesses of each element in each of the four plies. This gives the optimisation a large amount of flexibility in producing an optimum design as it has the potential to vary thickness vales, equivalent to the number of elements in Free Size component multiplied by Altair Engineering 29 Targeting Composite Wing Performance- 16-3

4 the number of independent composite layers. The composite stack is simulated as being symmetric, using the SYSMEAR formulation. 9 Figure 3: Optimisation Model The composite covers included in the optimisation are shown below in Figure 4. Figure 4: Wing Covers for Optimisation The design requires that the number and positioning of the º and the º layers must be identical. This is to make sure the laminate remains symmetrical throughout all design phases and to minimise the likelihood of introducing manufacturing stresses, such as torsion. An optimisation constraint was applied to link the +º and º layers, ensuring that they produce the same ply shapes. In order to meet all the design targets in the final stages of the optimisation, a number of additional composite constraints were applied to the Free Element Size study: i. Maximum & minimum total laminate thickness to prevent very thick areas occurring & to redistribute the material accordingly ii. Minimum thicknesses for the & layers to ensure material is retained for outer cover layers iii. Maximum & minimum ply percentages to ensure enough different ply orientations are available to meet lay-up sequence rules iv. Enforce the optimiser filter out very small (unrealistic) ply shapes Altair Engineering 29 Targeting Composite Wing Performance- 16-4

5 The free element sizing optimisation study is set up as follows: Design variables: Thickness of each ply within each element in the º, º & 9º layers in the upper and lower composite wing covers º layer thickness is linked to the º layer thickness Objective: Minimise the mass of the laminate covers Design constraints: Achieve the required bending and torsional stiffness for all of the loading conditions, defined as the wing tip displacement targets Laminate thickness constraints Ply percentage constraints Minimum ply thickness constraints Minimum patch size constraints 3.2 OptiStruct FES Optimisation Results A FES optimisation was completed to calculate the thicknesses for each of the four ply orientations, giving the minimum mass whilst achieving the stiffness targets. The +º and - º degree layers were linked such that their resulting thicknesses are identical. The optimised thicknesses for each of the ply layers are shown below: Figure 5: Ply Thickness Results from Free Element Sizing Optimisation (º left; º/º middle; 9º right) The optimization took approximately fifteen minutes on a Windows desktop and converged in 38 iterations, requiring only 334Mb of memory. The set-up time for the optimisation study is also minimal; a few minutes. The results show that the º ply requires the highest number of layers. This is because the º ply is doing the most work being orientated in the direction of the load path. The º & - º layers are identical, meeting the manufacturing requirement, and show that a number of º /º layers are required in the centre of the covers. The 9º ply, being orientated out of plane to the loading, is not being worked and consequently requires very few layers. Altair Engineering 29 Targeting Composite Wing Performance- 16-5

6 Figure 6: Element Thickness Results from Free Element Sizing Optimisation The thickness is maintained at leading & trailing edge of the wing centralised region, providing torsional stiffness as well as bending stiffness. The thickness is reduced as much as possible in other regions to minimise the mass. At the end of the Free Element Sizing optimisation, OptiStruct automatically generates ply shapes based upon the optimisation results. For each ply orientation, the optimised ply thickness is split into a number of layers of different shapes, the default being 4 shapes per orientation. Figure 7: Automatically Generated Ply Shapes The ply shapes that were generated by OptiStruct were edited using HyperMesh. This allows the shape of the plies to be based around the optimisation results but also be made realistic & manufacturable. Any infeasible ply shapes can be removed and shapes which are too complicated can be simplified. This process was completed for each of the layers that were generated automatically, producing a total of 62 ply shapes. Altair Engineering 29 Targeting Composite Wing Performance- 16-6

7 Figure 8: Ply Shapes Modified Using HyperMesh For each ply shape, four plies were created with material orientations,, and 9 producing a total of 248 available ply types. These plies will be used in the system level optimisation when determining how many plies are needed. 4. System Design Phase 4.1 Size optimisation In order to determine how many plies are required for each ply shape and orientation, size optimisation can be used. This will tune the thicknesses of the different plies in discrete levels as a simulation of adding & removing plies in the laminate lay-up. Consequently when using this method, the ply shapes are fixed and the thicknesses of all elements within a ply are varied being together as a group. T 9 PCOM P T - Figure 9: Ply Thickness Optimisation Whilst carrying out composite sizing optimisation it is possible to impose any combination of the following laminate constraints, if required: Maximum & minimum total laminate thickness Altair Engineering 29 Targeting Composite Wing Performance- 16-7

8 Maximum & minimum ply thickness Maximum & minimum ply percentages Non designable plies within a stack Linking thicknesses of different ply orientations 4.2 System level optimisation The size optimisation method was applied to the wing covers to determine the required number of plies to meet the bending and torsional stiffness targets. A thickness design variable was assigned to each ply within the top & bottom covers resulting in a total of 248 design variables. The thickness design variables were discrete, only allowing changes in increments of a single ply thickness. The design variables for the & orientated plies were linked such that they would produce a design with the same number of and orientated plies. The objective for the optimisation was to minimise the mass. In addition to this a number of ply constraints were added: Maximum & minimum total laminate thickness, to prevent large changes in thickness Minimum ply thickness for / to ensure that plies are retained for the outer layers of the covers Maximum & minimum ply percentages to ensure enough plies are retained to meet stacking rules in the final optimisation The setup was performed using HyperMesh & submitted to OptiStruct for optimisation. During the study, each ply thickness is tuned to meet structural requirements and any unneeded plies are removed by reducing their thickness to zero. The optimisation took 11 iterations and 331 Mb of memory on a Windows desktop. The run time for the solution was 11 minutes. The thicknesses of the optimised covers are shown below. Material is retained in similar areas to the regions determined by FES; however the plies that are used have all been designed to be manufacturable. Figure 1: Total Thickness After Size Optimisation Of the 62 ply shapes available in the study, 59 have been retained. The other 3 have been automatically removed by having their thicknesses tuned to zero. Altair Engineering 29 Targeting Composite Wing Performance- 16-8

9 Figure 11: Removed Ply Shape (left); Optimised Ply Thickness (right) After completing the optimisation, OptiStruct automatically creates physical plies for the detailed design stage of the optimisation. This is achieved by dividing the optimised thickness by the discrete step size (thickness of a single ply) to determine the number of physical plies to create. The sizing optimisation automatically generated a new model which contained: 174 plies with orientation 57 plies with orientation 57 plies with orientation 53 plies with 9 orientation A total 341 ply layers Figure 12: Optimised Ply Converted into Multiple Plies The model now has optimum shaped plies and also the number of each ply has been tuned. To complete the process, it is necessary to also meet the ply lay up rules. This is addressed in the next phase of the design. 5. Detailed Design Phase 5.1 Stack sequence optimisation When orthotropic materials are used, the part properties can be adjusted by changing the order of the plies within the laminate. This technique is known to produce changes to the structural performance such as: Bending and torsional stiffness Buckling factors Stresses, strains and reserve factors Altair Engineering 29 Targeting Composite Wing Performance- 16-9

10 To take advantage of this, OptiStruct can perform a laminate stack sequence optimisation to find the best lay-up sequence whilst meeting a combination of ply lay-up rules Figure 13: Plies Automatically Reordered to Improve Structural Performance 5.2 Stack sequence optimisation set-up The objective of the optimisation is to reorder the plies to maximise performance characteristics. This will be achieved by minimising the weighted compliance of the model across all the load cases. The setup is carried out by adding two stack sequence design variables (DSHUFFLE) to the model. The following ply stacking rules were applied for damage tolerance and resistance to de-lamination: Cover; / Maximum number of successive plies; 4 The stacking sequence rules that were applied are shown in the images below. 9 9 COVER:, Figure 14: Ply Sequence Enforced on Outside of Laminate The COVER option is applied to enforce the outer plies to use a predefined sequence, such that ply shuffling will only occur on the internal plies. Altair Engineering 29 Targeting Composite Wing Performance- 16-1

11 Too many adjacent plies of same orientation Figure 15: Ply Sequence Reordered Automatically to Distribute Plies The maximum successive plies constraint prevents too many plies of the same configuration from being layered directly together. The plies must be divided by a different type to improve the laminate integrity. Additional stacking sequence constraints available in OptiStruct are: Pairing; ensure two ply types always occur together within a stack Reversed pairing; as pairing but with the order flipped at each occurrence Core; a predefined stack sequence at the laminate core The stacking targets can be used in any combination and can also have weighting factors applied to assist in determining the best sequence. 5.3 Detailed design results The stack sequence optimisation converged in 7 analysis iterations, taking 68 minutes on a Windows desktop. The memory used was 5412mb. The optimisation history plot (below) shows that the total weighted compliance has been reduced by reordering the stacking sequence, giving an improvement in structural performance. Altair Engineering 29 Targeting Composite Wing Performance

12 Figure 16: Optimisation History; Compliance Reduced by Modifying Stacking Sequence OptiStruct automatically screens the responses for improved efficiency. It is only necessary to monitor the responses which drive the design. These are written out by default in the retained responses table. The displacement results for the main retained responses are shown below. This shows that the final design has stiffness values slightly higher than the optimisation targets (approx 5%). Load case Normalised displacement Normalised target Table 1: Final Design Displacement Vs. Targets; Displacement Contours The stack sequence history throughout the optimisation is written automatically as an HTML report. This shows the global stack sequence for each of the two covers vs. design iteration. The plies are coloured orientation, giving an overview of the laminate ply book. The top cover stack sequence shows some general trends: plies moved towards the outside of the covers 9 & plies moved towards centre plies distributed to break up large ply groups Altair Engineering 29 Targeting Composite Wing Performance

13 Optimised Ply Sequence Iteration Figure 17: Global Stack Sequence (Top Cover) The optimisation very quickly determines where each ply type is best utilized & then fine tunes the design to meet the stacking rules; by iteration three, the changes in the stack sequence appear to be small. As the plies can have different shapes it is not possible to tell from the global stacking sequence if the lay up targets have all been met. A more detailed investigation is needed to see how the lay-up is constructed locally across different regions of wing. This was achieved using OptiStruct to convert the plies into property regions with the output option OUTPUT,PCOMP,YES. Elements which have the same layup are grouped together in the traditional PCOMPG format, allowing a more detailed investigation of the layup in each region. Figure 18: Local Property Regions Altair Engineering 29 Targeting Composite Wing Performance

14 By reviewing the lay-up within each property region, it is possible to show that the lay-up meets the stack sequencing rules across all localised zones. For a more detailed review of the layup, a utility was created to convert the shell model into solids for visualisation purposes. The utility allows thickness scaling and also applies the element & laminate shell reference plane offsets. Figure 19: Bottom Wing Cover Solid Visualisation of Plies (Thickness x5) The solid visualisation makes it possible to see: How the plies transition / overlap across different zones Thickness drop off Manufacturing sequence (ply book) Also when using a solid visualisation in conjunction with the cross sectioning tools in HyperView, it is possible to perform a detailed investigation into the layup throughout the length & width of the covers. Another benefit would be for generating an inner mould surface for the laminates. 5.4 Discussion of Results The optimisation studies show that the laminate cover lay-up has a direct effect upon the global wing bending and torsional stiffness. The studies have shown that the optimum location for material is at the centralised region of the leading & trailing edges of the wing. This provides the required torsional as well as bending stiffness. The thickness should be reduced as much as possible in other regions to minimise the mass. The majority of plies should be orientated at º to the length of the wing as this is the direction of the main load path. The other ply orientations (º,º and 9º) are required to meet the laminate stacking rules and will take any additional (e.g. transverse) loading. The final design has a stiffness value approximately 5% higher than the minimum allowable. This suggests that it may be possible to further reduce the mass of the design by removing Altair Engineering 29 Targeting Composite Wing Performance

15 a few plies & pushing the stiffness to the design target. This could be achieved by rerunning the sizing & shuffling phases on the optimised model. The study was successful in finding a minimum mass design, meeting the stiffness targets as well as: Determining the optimum laminate patch shapes & locations Determining required number of plies Producing a ply book which meets the stacking rules To take the design optimisation further, additional design constraints could be included into the sizing & shuffling phases, such as: Stress / strain targets Failure index & reserve factor targets 6. Conclusions The study has shown that OptiStruct can be used to produce a very detailed composite design. The optimised design of the laminate wing covers meets the requested structural targets, uses the minimum mass and meets ply lay-up rules. The composite tools available within Altair HyperWorks can be used to take the design from blank sheet to complete ply book in a very short time frame. The efficiency of the final design is dependent upon the user s interpretation of the ply shapes after free size optimisation. It may be possible to produce a lower mass design by having additional plies with more detailed ply shapes could but may impact the manufacturing feasibility, cost & complexity. A number of areas have been identified in which further research could be performed: Include ply strain targets in sizing and shuffling optimisation Size effects; thickness and lay-up dependent laminate strain targets Iterate the sizing and shuffling optimisation phases to further reduce mass Couple composite optimisation with stringer shape changes Use a wider set of load cases Include ply drop off targets to prevent large changes in thickness across adjacent elements 7. References [1] Optimization Driven design of shell structures under stiffness, strength and stability requirements P Cervellera, M Zhou, U Schramm 6th World Congresses of Structural and Multidisciplinary Optimization Rio de Janerio 3 May - 3 June 25 Brazil [2] Targeting Composite Wing Performance Optimum Location of Laminate Boundaries Marc Funnel, Altair Engineering CAE Technology Conference 27 [3] Altair RADIOSS/OptiStruct Version 1. Reference Guide, Altair Engineering Inc, 29 Altair Engineering 29 Targeting Composite Wing Performance