NASA Technical Memorandum 87231

Computational Engine Structural Analysis

(NASA-T_]-87231) CG_!PUTIL_.iCNAL _NGINE

,_RU_.TU_AL ANAL_SiS (NASA) 2i.% p[C A02/M_ AOi CSCL 20t<

G 3/39

N86-I 9663

Christos C. Chamis and Robert H. Johns

Lewis Research Center

Cleveland, Ohio

Prepared for the

31st International Gas Turbine Conference and Exhibit

sponsored by the American Society of Mechanical Engineers

Dusseldorf, West Germany, June 8-12, 1986

https://ntrs.nasa.gov/search.jsp?R=19860010192 2018-06-12T21:36:15+00:00Z

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TABLE OF CONTENTSPage

SUMMARY ...................... L ........... l

INTRODUCTION - LEWIS'S RESEARCH ACTIVITIES ON

COMPUTATIONAL ENGINE STRUCTURAL ANALYSIS ................ l

AEROELASTICITY OF BLADES ROTORS ..................... 2

HIGH VELOCITY IMPACT OF FAN BLADES .................... 2

BLADE LOSS TRANSIENTS .......................... 2

ROTOR/STATOR/SQUEEZE-FILM-BEARING INTERACTION .............. 3

BLADE-FRAGMENT/ROTOR-BURST CONTAINMENT .................. 4

ADVANCED TURBOPROP STRUCTURAL BEHAVIOR .................. 4

SUMMARY OF RESULTS ............................ 5

REFERENCES ................................ 5

COMPUTATIONALENGINESTRUCTURALANALYSIS

Chrlstos C. Chamlsand Robert H. JohnsNational Aeronautics and SpaceAdministration

Lewis Research CenterCleveland, Ohio 44135, U.S.A.

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SUMMARY

A significant research activity at the NASA Lewis Research Center is the

computational simulation of complex multldlsclplinary engine structural prob-

lems. This simulation is performed using computational engine structural

analysis (CESA) which consists of integrated multldlsclpllnary computer codes

in conjunction with computer post-processlng for "problem-speclflc" applica-

tion. A variety of the computational simulations of specific cases aredescribed in some detail in this paper. These case studies include (1) aero-

elastic behavior of bladed rotors, (2) high velocity impact of fan blades, (3)

blade-loss transient response, (4) rotor/stator/squeeze-film/bearlng interac-tion, (5) blade-fragment/rotor-burst containment, and (6) structural behavior

of advanced swept turboprops. These representative case studies were selected

to demonstrate the breadth of the problems analyzed and the role of the com-

puter including post-processlng and graphical display of voluminous outputdata.

INTRODUCIION

The need for lightweight, durable, fuel-efflcient, cost-competltlve air-craft requires engine structures which are made from advanced materials

including composites, and which resist higher temperatures, maintain tighterclearances and reduce maintenance costs. These requirements tend to increase

the complexity of aircraft engines which are already intricate structural

systems consisting of dynamically interacting components. Prediction of the

integrated response of these systems requires complex analytical models (finite

element, for example) as well as complex static, periodic, transient, and

impact analyses. A major engine structures program (fig. l) is currently beingconducted by NASA Lewis, with emphasis on computational engine structural

analysis (CESA). Several independent computer codes for specific aspects of

engine structures modeling and/or structural analysis are being developed as

part of this program. These codes include: (1) interactive finite elements

for bearlng/shaft interaction, (2) engine structural component modeling, (3)

automatic finite element generators, (4) nonlinear analysis with various levels

of sophistication, (5) nonlinear constitutive relationships, (6) automatic

thermal load transfer, (7) blade-loss transient analysis, (8)aeroelastIc

analysis of bladed disks, (9) multimode impact analysis, and (lO) structuraltailoring of engine blades.

Several of these computer codes were briefly described in a previous paper

(ref. l). The objective of this paper is to illustrate typical computational

results obtained from these codes for: (1) aeroelastlc behavior, (2) high-

velocity impact, (3) blade-loss transients, (4) rotor/stator/squeeze film/-

bearing interaction, (5) blade fragment containment, and (6) advanced turbo-

prop structural behavior. The role of the computer in the solution of these

complex multldlscipllnary problems, the post-processlng, and the graphic

display of voluminous results obtained therefrom are emphasized.

AEROELASTICITY

The multldlscipllnary analysis required for predicting aeroelastlcbehavior including flutter is performed by a speclal-purpose code (ref. 2).

This code couples appropriate aerodynamics with the structural response ofbladed, shrouded disks. Flutter is an important aeroelastlc design consider-

ation for fan blades. When it occurs it subjects a blade to hlgh-amplltude

cyclic loading which can rapidly degrade its structural integrity and lead toblade failure.

Blade failure induces rotor imbalance which, if of sufficient magnitude

can cause rotor stage failure and/or bearing failures, both of which can leadto catastrophic engine failure, if only very infrequently.

The computer code predicts the flutter speed and the blade motion near or

at this speed. The code uses cyclic symmetry substructurlng concepts and the

wldely-used NASTRAN structural analysis computer code. A computer plot of a

typical sector of a bladed shrouded disk analyzed is shown in figure 2. Com-

puted graphical results of the motion of this sector near flutter speed are

shown exaggerated in figure 3. The character of the motion and location of

high bending (fatigue) stresses are readily observable in this figure.

HIGH VELOCITY IMPACT OF FAN BLADES

High-veloclty impact which can result in foreign object damage (FOD) is an

important design consideration in aircraft turbine engine fan blades. Some

research activities at NASA Lewis focused on developing technology for impact-

resistant fan blades using advanced design concepts and several computer codes

(ref. l).

A computer-produced three-dlmenslonal finite element model for one of theseadvanced blade concepts is shown in figure 4. Stress contour plots obtained

using thls flnlte element model are shown in figure 5 for several blade depth

locations or "surfaces." Impact location and blade root stresses obtained

using modal synthesis for transient analysis are summarized in figure 6.

The amount of information associated wlth hlgh-veloclty impact analysis is

very voluminous. Computer post-processlng and computer graphics including

computer-produced movies is the practical way to handle it. Furthermore,

computer color graphics enhance the interpretation of the output relative to

displacement magnitudes, stress distributions and magnitudes, and transient

motion of the impacted blade.

BLADE-LOSS TRANSIENTS

Partial or full blade loss due to impact, for example, causes stage

_mbalance which can induce rotor transient whirling. This type of imbalance

and the resulting transient and steady-state motion must be predictable sothat appropriate tolerances and operating restrictions can be provided. Pre-diction of blade loss transient response requires an integrated structuraldynamic analysis where the bladed rotor, shaft, and bearings are all modeledin the analysis. A computer code (Turbine Engine Transient ResponseAnalysis[TETRA]ref. l) was developed for such an integrated analysis.

TETRAuses the componentelement analysis method with progressively reducedmodel complexity as shownin figure 7. TETRA output is also voluminous. Com-

puter graphics is the practical way to display this type of output. Typically

TETRA produces dlsplacement/tlme history plots as shown in figure B, or cor-

responding frequency-domain plots as shown in figure 9. Both of these plots

are valuable and necessary in interpreting the blade loss transient responseand in assessing the adequacy of the design.

ROTOR/STATOR/SQUEEZE-FILM/BEARING INTERACTION

Squeeze-film damper bearings are designed for specified blade-loss transi-

ents and rotor imbalance and vibration. Rotor/squeeze-film/bearlng/ stator

interactions are inherently nonlinear because of: (1) the inherent behavior

characteristics of squeeze films, particularly during transient situations,and (2) potential structural interactions wherein either large deformation

kinematics or material nonlinearity (plastlcity) are excited.

Because of such behavior, the overall rotor-bearlng-stator computational

simulation must be able to incorporate the various sources of nonlinearity.Additionally, to enable efficient solutions in situations wherein the behavior

of the model components is linear, the overall simulation scheme must incorpo-

rate substructurlng capabilities. Furthermore, since transient problems need

to be considered, such features must be accommodated by the various integration

algorithms used to solve the governing model equations.

A general purpose squeeze-film-damper/Interactlve-force finite element

model has been developed as part of the NASA Lewis program (refs. 3 to 4).This finite element model was implanted in a general purpose finite element

computer code. Three different numerical integration methods were also incor-

porated to solve the governing structural dynamics equations of the interactive

finite element. The general purpose code is used subsequently to predict the

structural dynamic response of the rotor/stator coupled structure subjected toimbalance and impulse type excitations. The structural dynamic responses pre-

dicted include: (1) bearlng/rotor trajectories, (2) stator trajectory, (3)

rotor orbit, and (4) force, velocity and acceleration histories at a given

location in the coupled structure. Three-dimenslonal post-processors are used

to graphically display the predicted results in isometric views. Typical com-

puter post-processlng results are shown in figure lO for a shaft/slngle bearing

interaction, and in figure II for a shaft/multlbearlng interaction. These

graphical results clearly indicate squeeze film pressures, orbit motion, orbit

stability, clearances, and respective margins.

BLADE-FRAGMENT/ROTOR-BURSTCONTAINMENT

Blade-fragment containment and rotor-burst containment are critical designrequirements for turbine engines for commercial aircraft. The computationalsimulation of blade-fragment and/or rotor-burst containment require complexanalyses including fragment tracking, nonlinear material behavior, and predic-tion of finite strains, large rotations and large displacements. These complexanalyses were developed as part of the NASALewis program and incorporatedinto an integrated computer code using nonlinear finite element theory (ref. 5).

The containment computational simulation using thls code starts with the

initial configuration of the containment ring, rotor speed, fragment size, and

fragment location. The simulation proceeds to determine the fragment kinetic

energy, momentum, direction, and impact point on the containment ring. The

transient structural response of the rlng from this contact point Is tracked

as a function of time. The transient structural response generally includes

displacements, strains and stresses.

Because of the importance of fragment containment, extensive experimental

studies have been conducted on specific rotors and containment rings. Thecomputational simulation results for one of these cases are included herein.

The containment ring geometry and properties and the rotor characteristics are

shown In figure 12. A computer plot of the shape of the deformed ring at about

its maximum deformation is shown in figure 13. Computer plots for the stresses

at the corresponding time are shown in figure 14 for the outer surface and in

figure 15 for the inner surface.

ADVANCED PROPELLER STRUCTURAL BEHAVIOR

The use of thin, highly swept, twisted propeller blades is important to

the development of fuel-efflclent and quiet advanced aircraft. Hlgh sweep

angles produce significant reductions in noise and are therefore a desirable

design feature. However, blades of thls type (thin, highly swept, twisted)

exhibit complex structural response under a centrifugal force field, which

requires special analysis techniques for accurate characterlzatlon. These

techniques are required to establish the structural dynamic response of the

turboprop blades at operating rotor speeds including the avoidance of possible

leading edge buckling under centrifugal loads. Computational simulation

studies (refs. 6 to B) were performed at NASA Lewis to determine analytically

the structural behavior and possible leading edge buckling of advanced, highly

swept turboprop airfoils In centrifugal force fields. Computational simulation

studies were also performed in order to identify any advantages of using "high

performance" composites in order to change the regions of instability. The

theoretical studies were performed using an In-house program (COBSTRAN, ref. l)

designed for composite blade analysis. Schematics of the rotor, the propeller

and the finite element model used in the analyses are shown in figure 16.

Computer output results for rotor speed effects on the vibration mode

shape and potential for leading edge buckling are shown in figure 17. Computer-

plotted mode shapes of the propeller blade superimposed on a Campbell Diagram

are shown in figure 18. Display of the amount of computer output information

summarized In figure 18 would be impractical without computer post-processlng

and graphical display. The availability of all this information in one figure

makes it convenient to assess the adequacy of the design on a quantitativebasis.

SUMMARY OF RESULTS

The complex behavior of integrated aircraft engine structures requires the

solution of interacting multidlsclplinary problems. Some recent research

activities at NASA Lewis focus on computational engine structural analysis

(CESA). CESA is essentially the computational simulation of complex engine

structural behavior using multldlsclpllnary integrated computer programs in

Conjunction with extensive computer post-processlng and graphical display of

computer output. Representative case studies were selected and are describedin some detail to illustrate the role of the computer on the computational

simulation of complex multldlsclpllnary engine structural problems. These

case studies include: (1) aeroelastic behavior of bladed rotors, (2) high

velocity impact of fan blades, (3) blade-loss transient response, (4) rotor/

stator/squeeze-fllm/bearing interaction, (5) blade-fragment/rotor-burst con-

tainment, and (6) structural behavior of advanced swept turboprops. These

representative case studies further demonstrate the indispensable utility of

computer post-processing and graphical display of voluminous analysis results

which make it possible to readily assess the adequacy of a design on a quanti-tative basis.

REFERENCES

I. Chamls, C.C., "Integrated Analysis of Engine Structures,: NASA TM-B2713,1981.

2. Smith, G.C.C. and Elchurl, V., "Aeroleastic and Dynamic Finite Element

Analyses of a Blader Shrouded Disk," Textron Bell Aerospace Co., Buffalo,

NY, 1980. (NASA CR-159728)

3. Adams, M.L., Padovan, J., and Fertis, D.G., "Finite Element for Rotor/

Stator Interactive Forces in General Engine Dynamic Simulation,"

EDA-2OI-3A, Akron Univ., Akron, OH, 1980. (NASA CR-165214)

4. Padovan, J., et al., "Engine Dynamic Analysis, with General Nonlinear

Finite Element Codes," Akron Univ., Akron, OH, 1982. (NASA CR-167944)

5. Rodal, J.J.A. and Witmer, E.A., "Finlte-Straln Large-Deflection Elastic-

Vlscoplastlc Finlte-Element Transient Response Analysis of Structures,"

ASRL-TR-154-15, Massachusetts Institute of Technology, Cambrldge, MA,

1979. NASA CR-159874)

6. Aiello, R.A. and Chamls, C.C., "Large Displacement and Stability Analysis

of Nonlinear Propeller Structures," Tenth NASTRAN User's Colloquium, NASA

CP-2249, 1982, pp. I12-132.

7. Chamls, C.C. and Aiello, R.A., Tensile Buckling of Advanced Turboprops,"

Journal of Aircraft, Vol. 20, No. II, Nov. 1983, pp. 907-912.

8. Hirschbeln, M.S., et al., "Structural and Aeroelastlc Analysis of theSR-TL Propfan," NASA TM-86877, 1985.

FOREIGNOBJECTDAMAGE STRUCTURAL DESIGN CONCEPTS

,_ ,, ,, COMPOSITEFOLDEDBU.NE,• IMPACT ANALYSIS _,_ lg/ FRAME

• LOCAL DAMAGEANALYSIS I_[-_... __" ,_lll_

• LARGE BLADE DEFLECTION " " .-(-----_ _ _ " _II

OPTIMIZATION AND _ __-q__ I

_ _ " HIGH TEMPERATURESTRUCTURES

£.AUTO AT,C_-_: F -_" _ TURBINE BLADEMETHODS

WITH COOLING COMPLEX• OPTIMIZATION ENGINESYSTEMS BLADE

CRITERIA MODEL TO DISK• DESIGN ATTACHMENT

TAILORING• MANY DEGREESOF FREEDOM• AUTOMATIC MESH GENERATION• INTEGRATIONOF STATIC STRESS,

THERMAL, AND VIBRATIONANALYSES

PASSAGES LOADING

• COMPLEX GEOMETRIES• CREEPAND PLASTICITY• COMPLEXTHERMAL AND

MECHANICAL LOADING• NONLINEAR EFFECTS

Figure I. - Engine structural components amenable to computational simulation.

/

BLADE- 64 PLATEELS

SHROUD - 4 PLATEELS

Z

DISK - 12 SOLID ELS

NASA LEWIS ROTOR12FINITE ELEMENTMODELFOR AEROELASTICANDMODAL ANALYSIS

Figure 2. - Finite element model foraeroelastic analysis of bladedshrouded disks.

UNDEFORMED MODE I785 Hz

MODE 21830 Hz

Figure 3. - Aeroelastic finite element analysis of bladed shrouded disks(bell code).

• 3 ELEMENTS THICK

• 306 ELEMENTS

• .504NODES

LEADING

TRAILING EDGE

CS-81-1470

Figure 4. - Superhybrid composite blade finite element model.

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Figure 12. - Geometric, test, and modeling data for the 4130 steel containment ringsubjected to tri-hub T58 rotor burst in NAPTCtest 201.

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Figure 16. - Turbopropstagepropellerandfinite elementmodel.

SUMMARY OF TENSILEBUCKLING ROTORSPEEDS

TURBOPROP ROTOR SPEED,rpm

UNSWEPT-COMPOSITE

600 - SWEPT-TITANIUM

600 - SWEPT-COMPOSITE

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Figure 17. - Structural behavior of advanced turboprops.

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Figure 18. - Composite turboprop vibration analysis.

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.

NASA TM-87231

5. Report Date4. Title and Subtitle

Computational Engine Structural Analysis

7. Author(s)

Chrlstos C. Chamls and Robert H. Johns

9. Performing Organization Name and Address

National Aeronautics and Space AdministrationLewis Research Center

Cleveland, Ohio 44135

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington, D.C. 20546

6. Performing Organization Code

505-63-II

8. Performing Organization Report No.

E-2898

10. Work Unit No.

Ill. Contract or Grant No.

13. Type of Report and Period Covered

Technical Memorandum

14. Sponsoring Agency Code

15. Supplementa_ Notes

Prepared for the 31st International Gas Turbine Conference and Exhibit,sponsored by the American Society of Mechanical Engineers, Dusseldorf, WestGermany, June 8-12, 1986. Invited paper.

_16. Abstract

A significant research activity at the NASA Lewis Research Center is the.compu-tational simulation of complex multidlsclpllnary engine structural problems.This simulation is performed using computational engine structural analysis(CESA) which consists of integrated multldlsclpllnary computer codes In conjunc-tion with computer post-processing for "problem-specific" application. A vari-ety of the computational simulations of specific cases are described in somedetail in this paper. These case studies include (l) aeroelastlc behavior ofbladed rotors, (2) high velocity impact of fan blades, (3) blade-loss translentresponse, (4) rotor/stator/squeeze-fllm/bearlng interaction, (5) blade-fragment/rotor-burst containment, and (6) structural behavior of advanced swept turbo-props. These representative case studies were selected to demonstrate thebreadth of the problems analyzed and the role of the computer including post-processing and graphical display of voluminous output data.

17. Key Words (Suggested by Author(s))

Engine structures; Structural response;Multidisplinaryanalysis;Cow,outercodes;

Aeroelasticity;High velocityimpact;Impact

containment;Squeeze-filmbearings;Advanced

turboprops

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