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Airbus R&T PhD Day - 2009

Improving stress-loads interface andevaluation proceduresQUB: Shaun McGuinness, Cecil Armstrong, Adrian Murphy

Airbus: James Barron, Mark Hockenhull, Eddie Barber



Page 3DiPaRT Workshop, Nov 2010

Overview

Numerical simulations develop millions of external loading conditions.Each flight/ground load case is translated into shear, moment, and torquevalues for each of the wing and fuselage stations

Global Finite Element model (GFEM)

analysed for ~1000 down-selected loadcases

Load case down-selection

Internal loads extracted

Internal GFEM loads combined withadditional loads and local reserve

factors calculated




• Current approach

Limited communication between stress and loads

departments

Load case downselection based on ‘interesting quantities’

(IQs)

Load case alteration and optimisation difficultStructural specific load case downselection

– only performed for specific components

– can be resource intensive

– based on engineering judgment

Background to Aerospace Structural design

Page 6




Characteristic load methodology

Page 7




• Objective

Relate high level loads parameters (IQ’s) to downstream

structural behaviour

– Improved stress-loads communication

– Provides basis for loadcase downslection and alteration

• Procedure

90% or more of GFEM analysis is linear static

– Characterise aircraft loading

– Characterise airframe structural behaviour

Overview

Page 8




• Loads characterisation

Can external aircraft loads be characterised by a relatively smallnumber of ‘characteristic loads’?

• A320 study performed (M. Hockenhull, EDGL)

Set of 274 load cases (SMT format) from an A320 aircraftMixed set, containing flight and ground cases

Separated into 2 subsets

– 100 Master cases

– 174 Slave cases

Singular value decomposition (SVD) performed on master set and

the first 20 singular values used to construct ‘characteristic cases’

Slave set recreated from the ‘characteristic cases’

Loads characterisation

Page 9


http://slidepdf.com/reader/full/2dipartqub 7/31Page 10DiPaRT Workshop, Nov 2010

Characteristic loads: A320 study

Page 10

• Characteristic loads produced from

SVD analysis

First is similar to a typical lift

distribution

Second is similar to a typical

ground handling case

Others tend to be local

perturbations e.g. analogous to

aileron deflection etc



Reconstruction of down-selected load case A320 study

•Slave case 10

•Flight case

•Randomly selected from outer envelope



Load characterisation

• SVD reconstructs flight and ground loads on the convex hull

accurately, even for 20 characteristic loads• Cases inside the convex hulk not approximated as well, however

there will be an LC with a much larger magnitude local loading

(compare to 2D down-selection envelope)

• More characteristic loads can be used for increased accuracy



Structural characterisation

Page 14



• Structural characterisation

Can structural behaviour be captured and related to

‘characteristic loads’?

• Performance envelope

Perform GFEM analyses for ‘characteristic cases’

Construct design criteria (e.g. max strain) in ‘characteristic

load’ space

Relate characteristic structural behaviour to external load

cases

Overview

Page 15



Performance envelope

Page 16

FE model analyzed for‘characteristic cases’

All design criteria based onstress, strain, deflection, force

etc

Analysis of characteristic loadsand their super-position allowsstress, strain, deflection etc tobe related to the ‘characteristic

loads’

Hence design criteria are nowrelated to ‘characteristic loads’in the form of a ‘performance

envelope’

Stress

Loads



Performance envelope (2 load example)

P1

P2

Quadratic constraint

von Mises stress forelements m-n

Global failureenvelope made up

of only a fewelements

Linear constraint

e.g. buckling



Load case downselection

P1

P2

Load casescause failure andare selected for

analysis

Load cases arenot severe andare not selected

Critical LCs forgiven locationsmay also be of

interest



Structural insight/redesign

P1

P2

Certain areas/criteriaover designed

Failure locationand type

identified forgiven LC

Relative positionof structure to LCsallows for guided

redesign



Loads insight/redesign

P1

P2

Identify LCclusters and thestructure they

define

Identify when itsbetter to alter anLC rather thanlots of structure

Identify how bestto alter loads

Relates loadsand their impact

on physicalstructure

P f l h ll f d



Loads envelope vs. Performance envelope

Page 21

Performance envelope vs convex hull of down-selected load cases

P1

P2

Under designedstructure

Over designedstructure

Efficient structure

Convex hull ofdown-selected load

cases

Performance envelopeof non-redundant

constraints

See also: Malte Werwer, SVD for Fast Reserve Factor Estimation - TTEK2 - Ref. V55PR1010551 -Issue 1



• Finding the performance envelope

Linear criteria analysed quickly using existing software (CDD)Quadratic criteria analysed using

– Linear faceting techniques

– Software developed in collaboration with ETH Zürich

Higher order criteria – Linear faceting techniques

– Perform reserve factor calculations for a scattering of points and facet(relatively quick)

• CriteriaLinear

– Composite laminate strain, Stringer buckling, Inter-rivet buckling, Fastenerfailures etc

Quadratic – Von Mises, Principal strain, Damage tolerance (stringer impact), Local

buckling interaction etc

Higher order – Global buckling

Performance envelope

Page 22



Box beam example

20m semi-span

Colourindicates skin

thickness

21 reference stations for loadapplication (per semi-span)



• 20 ‘characteristic loads’ from the A320 study were mapped

onto the structure• Performance envelope developed in 20D ‘characteristic

load’ space for

Local buckling of upper skin elements

Max stress/strain

von Mises criterion

• 125 constraints form the inner performance envelope

• These can then be quickly checked to determine if a load

case causes failure. Load case 4 most critical.

Box beam example

Page 24



Box beam example: load case 4

Page 25

•Element 338 S12

• Actual RF = 0.926

•Predicted RF = 0.927

•Error = 0.1%



• What?

Two complimentary developments – Derivation of small number of representative ‘characteristic loadcases’ using Singular Value Decomposition of many existing loadcases

– Formulation of a ‘performance envelope’ which identifies• A loads envelope within which the aircraft is safe

• All possible ways in which the structure can fail (the non-redundantconstraints)

• The load cases which cause failure in particular areas of the structure

Characteristic loads can accurately recreate the global load cases

A PE/failure envelope can be constructed.

• Why?

Improved down-selectionStructural redesign and optimisation

Loads redesign and optimisation

Improves loads-stress interface

Conclusions

Page 26



Simulation modelling tools

•Mesh generation (auto structured multiblock for CFD)

• Cellular modelling

Any given model can be defined as a 3D, 2D, 1D or 0Dcell plus its boundaries.

Interfaces defined by adjacencies between cells

• Virtual topologyEach solid, face or edge can be divided into smaller

parts

Each edge, face or solid body can be composed of theunion of separate adjacent parts

• Equivalencing

3D region -> virtual topology -> 3D elements

2D faces -> virtual topology -> element faces etc




• Short term

Non-redundant constraints as Strescan IQs – Track max / min of all non-redundant RFs during manoeuvre

history by 20x20 matrix multiplication at each time step

Re-evaluation of existing design under new loading e.g. 1st flight ->certification

Other component specific applications – Fuselage

– Wing spars

– Ribs

Nodal loads integration

Applications – Rapid structural analysis

– Rapid loads analysis

– Nodal loads development

Future work

Page 28




• Long term

Re-casting current RF calculations as linear, quadratic orlow-order convex criteria in load space

Multi-scale analysis

– Loads model / global FEM / local FEM

– Transmission of loading, analysis results and non-redundantconstraints up and down the levels of maturity and complexity

Support from structures for new approach and non-traditional

methods

Future work

Page 29




Cellular Modelling

• Multi-disciplinary analysis

requires partitioning solid model

into separate ‘cells’For application of boundary conditions

For meshing (thin sheets, long slenderregions, ‘chunky’ parts)

For manufacturing (fabrication, casting,

..)

• Fluid volumes need to be

extracted and solid / fluidinterfaces definedShould be designing space rather than

solid parts or gas paths




• Original model

Virtual topology

• Virtual top surface is superset oforiginal surfaces

in Abaqus CAE




Cellular Modelling – bolted connection

Fluid B

Fluid A

Casing A1 Casing A2

Casing B1 Casing B2

Bolt1

Nut1

Washer1




Cellular Modelling – interfaces

Cellular Modelling




Cellular ModellingWorkflow Automation

• Interfaces defined by adjacencies in cellular model

• 1D bolt model

3D sliding of bolt in hole implies which freedoms couple1D bolt nodes to contact surfaces

• Thin sheet mesh to unstructured tet mesh couplingimplied by interfaces between thin sheet region and

flange

(c)

(b)

(b) (c)

Cellular Modelling



Cellular ModellingWorkflow Automation

• Mesh transitions defined

• MPCs applied automatically

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