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Birdstrike: approaches to the analysis of impacts with penetrationmore

by Luigi Castelletti

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Birdstrike: approach

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es to the analysis ofimpacts with penetration

Marco Anghileri, Luigi-M L Castelletti & Valerio Mazza Dipartimento di Ingegneria Aerospaziale,

Politecnico di Milano, ItalyAbstractBirdstrike is one of the most dangerous threats for flight safety and, there-fore,

aircraft before being put into operational service have to be certifiedfor a proven level of bird impact resistance. Since the

experimental tests areexpensive and difficult to perform, numerical techniques are usually usedto develop high-

efficiency bird-proof structures. In this work, bird strikeand (subsequent) penetration inside the airframe are

investigated. Differentmodels of bird are considered: advantages and disadvantages of which havebeen discussed with

regard to the impact of a bird against the intake of aturbofan engine.1 IntroductionSince the early beginning of

aviation history, birdstrike has been one of themost dangerous threats for the aircraft [1] that causes every

year damagesfor more than eighty millions US-dollars - though most of times it has nottragic consequences.A

birdstrike is characterised by loads with high intensity and short du-ration [2]. The materials undergo high strain rates,

large deformations andinelastic strains. In addition, a deep interaction exists between the

impactloads and the response of the structure. For those reasons, only with thedevelopment of explicit codes based on

Finite Element Method (FEM), atthe end of the eighties, it became possible to numerically analyse the

eventwith a degree of accuracy. Nowadays, computer modelling is commonly usedto develop high efficiency

(i.e. high-resistance low-weight) bird-proof struc-tures.WIT Transactions on Engineering Sciences, Vol 49Impact Loading of Lightweight Structures, M. Alves & N. Jones (Editors)c

2005 WIT Press, www.witpress.com, ISSN 1743-3533 

64 Marco Anghileri, Luigi-M L Castelletti & Valerio MazzaWith specific regard to the structure of an

intake, this is designed to carryaerodynamic and thermal loads. Therefore, it is not surprising that it mightcollapse

when struck by a bird. This event is extremely difficult to modelalso because, since the intake is in large part

manufactured using compos-ite materials, the failure comes abruptly and with mechanisms difficult topredict

a priori.The failure of the intake is tolerated, but the fly-home capability has tobe guaranteed. In particular, for

the certification, it has to be demonstrated(with an experimental test) that the bird, after strike and

penetration, doesnot damage the equipments necessary for the correct functioning of theengine that are usually

placed behind the intake (Fig. 1): the intake musteither deflect the bird or absorb the most part of the impact energy.In this

work, using LSTC/LS-Dyna [3], a numerical model to analyse theimpact and the subsequent penetration of a bird inside

the airframe of theintake of a turbofan engine is presented. Initially, different approaches tothe bird modelling are

presented and discussed. Hence, a birdstrike onto aturbofan-engine intake is considered.Figure 1: Scheme of a

turbofan engine.2 Bird modellingThe numerical model of the bird -geometry

(shape and dimensions) andmaterial(constitutive law and equation of state) - is central in a

birdstrikeanalysis when using explicit FE codes. On that depend the impact loads.Customarily, the bird is modelled like a

cylindrical water bullet [4]. Since abird is primarily water and the impact velocities are rather high, this modelis

commonly accepted - though it is not free from criticisms [5].WIT Transactions on Engineering Sciences, Vol 49Impact Loading of Lightweight Structures, M. Alves & N. Jones (Editors)c

2005 WIT Press, www.witpress.com, ISSN 1743-3533

 

Birdstrike: analysis of impa

cts with penetration 652.1 Problem overviewSince late eighties, explicit FE codes have been used to develop high

effi-ciency bird-proof structures [4]. Nevertheless, the analysis of birdstrike ontocompliant structures by means of these

codes, which adopt a Lagrangianapproach, is particularly troublesome. In fact, after the early instants of theimpact, the

distortions in the mesh of the bird cause a loss in accuracy,a reduction in the time-step and, eventually, an error

termination of thesimulation.In that, techniques based onEulerianorArbitrary Lagrangian Eulerian

(ALE) approach seem to have an advantage and, therefore, some explicitFE codes have implemented also an Eulerian or an ALE

solver. Drawbacksof these solvers (which eventually limit their use) are: thediffusivity

, thelack of sharp boundaries, and the large computational resources necessaryto run the analysis.A 

different approach to the problem that recalls theDiscrete Element  Method (DEM) consists of modelling the bird with a

set of nodes endowedwith a mass. Indeed, when applied to the analysis of birdstrike, thenodalmasses

(NM) model provided a relatively good numerical-experimental cor-relation [5,6].Recently, solvers based on

SmoothedParticleHydrodynamics(SPH) Methodhave been developed and implemented in the framework of an explicit

FEcode to analyse events characterised by large deformations. In fact, being(genuinely)meshless

, the SPH Method does not suffer the problems of meshdistortion.Another known meshless method is the

Element Free Galerkin(EFG): de-veloped for crack growing analysis, it is not particularly suitable for

bird-strike analysis [5].2.2 Experimental dataThe data collected during experimental

tests are fundamental in the devel-opment of a numerical model.In 1996, an intense test programme has been carried

out to characterisethe numerical model of the bird [7]. Using an air-gun, 4lb-chickens werelaunched

with an initial velocity of 265ktstoward a massive (rigid) targetinclined of 30deg

. The target was instrumented in order to acquire the profilein time of the impact force. Birdstrike tests are rather difficult to

perform.Nevertheless, the tests were repeatable (as shown by the close agreementamong the data obtained) and

the data acquired reliable (when comparedwith the ones of the tests previously carried out [2]).WIT Transactions on Engineering Sciences, Vol 49Impact Loading of

Lightweight Structures, M. Alves & N. Jones (Editors)c2005 WIT Press, www.witpress.com, ISSN 1743-3533

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