On the Balance and Integration of Simulation and Test in ... · MANUFACTURE & MAINTAINENANCE . These later phases will have their own uncertainties to be managed: First, in MANUFACTURE,

On the Balance and Integration of Simulation and Test in Engineering

Structural Dynamics

by David Ewins

Imperial College London & University of Bristol

NAFEMSvQ – June 2017

1. A Historical Introduction

2. A Strategy for Today’s Challenges in Structural Dynamics

3. Challenges from Practical Case Studies

4. The Future

Further details can be found in: “Exciting vibrations: the role of testing in an era of supercomputers and uncertainties” by D J Ewins (2016) Meccanica (2016) 51:3241-3258 DOI 10.1007/s11012-016-0576-y




4. The Future

NAFEMS – June 2017

Cranfield 7-9 July 1993

A TOPIC FAMILIAR TO NAFEMS 2 DECADES AGO

Windemere 3-5 July 1996

A BRIEF HISTORY OF THE NAFEMS DYNAMICS & TESTING WORKING GROUP (DTWG)

1978 Founding of informal Modal Testing Users Group at Imperial College

1990 Founding of Dynamic Testing Agency (DTA)

1993 1st Joint NAFEMS/DTA Conference on Modelling, Testing, Correlation

1996 2nd Joint NAFEMS/DTA Conference on Modelling, Testing, Correlation

1999 3rd Joint NAFEMS/DTA Conference on Modelling, Testing, Correlation

1999 DTA completes Mission; seeks ‘home’

2004 Dormant DTA resuscitated; contact with NAFEMS re-established

2005 1st NAFEMS Seminar ‘Simulation vs Test’

2006 Formation of DTWG proposed; Joint participation in 2nd ‘S vs T’

2007 DTWG 1st Workshop on Integration of Test and Analysis

2017 DTWG 10 years old!




4. The Future

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THE SUBJECT MATTER

STRUCTURAL DYNAMICS OF CRITICAL STRUCTUERS Structural dynamics plays a critical role in the performance of a

family of structures (and vehicles and machines) for which structural integrity is the primary concern. These are referred to as critical structures and predominate in aerospace, defence and high-performance machinery industries.

THE INTERACTIONS BETWEEN SIMULATION AND TESTS The case which will be argued here is that in these critical

applications, dynamic testing activities are just as essential as analysis or simulation – and perhaps even more essential – notwithstanding the huge advances made in numerical and computational capabilities.

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THE STRUCTURES

Picture courtesy of Federation of American Scientists.

Picture courtesy of Rolls-Royce plc

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THE SIGNIFICANCE OF STRUCTURAL DYNAMICS

In general, vibration in structures leads to one of three types of problem which must be anticipated at the design stage and eliminated or controlled in the final product to an acceptable (prescribed) degree to assure its performance

A Structural Failure – short term, or long term B Malfunction - of accessories, on-board equipment C Disturbance – discomfort, noise Category A, and to some extent Category B, issues are

concerned with Structural Integrity where the cost of failure to meet design targets can be very severe.

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A NEW PERSPECTIVE: STRUCTURAL PERFORMANCE

All these effects can have a major influence on the overall performance and commercial viability of the products involved.

Not only can they impair the functional performance of the machine or vehicle (e.g. by loss of efficiency) but they also can limit its life.

Thus it is appropriate to consider performance in two parts:

1. Functional Performance (sfc, thrust, power..) and 2. Structural Performance (life, reliability, wear ….)

NDE SHM

MATERIALS STRUCTURAL DYNAMICS

STRUCTURAL INTEGRITY

STRUCTURAL PERFORMANCE

FATIGUE

DEGRADATION

WEAR

FOD

TARGET: LIFE PREDICTION & VERIFICATION

DYNAMIC PROPERTIES

SECONDARY DAMAGE

THE ROOT CAUSE OF MOST LIFE REDUCTION AND UNRELIABILITY

© D J EWINS 2015

THE PRIMARY TASKS: DESIGN & DEMONSTRATION

So, the LIFE PREDICTION AND VERIFICATION target is, in effect, for the structural dynamicist: DESIGN and DEMONSTRATION

1. To enable the DESIGN of structures to have a reliably- predictable, cost-effective structural performance; 2. To DEMONSTRATE attainment of these performance properties to customers and authorities.

The first of these requires a mathematical model and so is ANALYSIS driven, while the second requires test rig and so is is TEST driven

MATH MODEL

TEST RIG

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SO, WHAT’S THE PROBLEM? USE SIMULATION TO DESIGN AND TEST TO DEMONSTRATE

But bear in mind, as quoted by Dr McNeal of MSC (NASTRAN) at the NAFEMS/DTA conference on integration of Analysis and Test in 1993:

“No-one believes computer predictions except the person who has done the analysis, while ….

Everyone believes the results from a test, except the person who has made the measurements”

© 2015 Rolls-Royce plc

EXAMPLES OF DESIGN AND DEMONSTRATION

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Basically, the problem is that we don’t get things right first time. Sometimes it takes several iterations. A major underlying reason for this is the uncertainties that apply to our analysis and test activities, because of the simplifications, assumptions, approximations made. It is important to recognise the 2 different types of Uncertainty: ALEATORIC UNCERTAINTY which relates to inaccuracy, or not being sure of the precise values for parameters of interest, and EPISTEMIC UNCERTAINTY which relates to inadequacy, or not being sure whether sufficient parameters have been included These two types require quite different approaches to resolve.

THE PROBLEM: UNCERTAINTIES

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Essentially, the best way to proceed is to use both Analysis and Test in each phase of the overall activity. • Use Tests to ensure that the models being developed

for the Design Analysis are good enough, and • Use Analysis to devise and interpret the most

appropriate Tests to Demonstrate the full performance.

In both cases, the inter-related Analysis and Test processes serve to reduce the uncertainties to an acceptable level.

SOLUTION TO MANAGING UNCERTAINTIES: CAREFUL INTEGRATION OF TEST AND ANALYSIS

VALIDATION

VERIFICATION

KEY THEME 1

KEY THEME 2




4. The Future

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FOCUS ON TESTING FOR VALIDATION OF THE MODELS REQUIRED FOR DESIGN

Model Validation: The primary procedure which integrates Analysis & Test in Structural Dynamics in order to reduce uncertainties to an acceptable level so that the model is good enough

_ Test Strategy

Test Planning

Modal Test

Reference Data

Correlation Updating

PRELIMINARY MODEL

VALIDATED MODEL

Verification

Upgrading

Model Validation using Test Data

© D J EWINS 2015

The 1st Challenge : Joints

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Courtesy: University of Kassel Courtesy: University of Kassel Courtesy: University of Kassel

MODELLING APPROACH FOR BOLTED FLANGE JOINTS

Area representedby the frictioncontact element

The referencepoint,A, used fordeterminingcontact stresses

a)

b)

N ( )x

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-6 -4 -2 0 2 4 6 8 x 10 -6

RELATIVE DISPLACEMENT (mm)

-8 -25

-20

-15

-10

-5

0

5

10

15

20

25

FRICTION FORCE

(N)

A set of hysteresis loops, measured at different applied normal loads.

50 N

40 N

30 N

20 N

10 N

Normal Load

...AND THE ROLE OF TESTING?

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THE CHALLENGE OF MODELLING JOINTS In many complex structures, the joints are a major source of uncertainty. Experimental evidence suggests that often a low-order model is capable of providing adequate representation of the structural dynamics, BUT…. ….such a model is extremely difficult to predict…. …..AND it is generally nonlinear…… An international activity is under way to advance joints modelling technology: ASME Research Committee: The Mechanics of Jointed Structures.

Further details can be found at: http://joints.rice.edu

The 2nd Challenge : Nonlinearity

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EFFECT OF NONLINEAR JOINT DYNAMICS ON ENGINE STRUCTURES

1N 10N 20N

30N 40N

50N 70N

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IMC-CCOC interface

INCORPORATING NONLINEAR JOINT BEHAVIOUR INTO FE MODELS

rigid connections with hinge

shell elements

bolt centre line

combination of linear and non-linear

springs and dampers

Z

R

F

F

offset beam elements

rotation about tangential axis

d

Modelling Approach for Bolted Flange Joints Courtesy: University of Kassel

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NONLINEARITIES PRESENT AN EPISTEMIC UNCERTAINTY THAT CALLS FOR MODEL UPGRADING

Dealing with nonlinearities in Model Validation introduces both types of Uncertainty. In particular, the need for elements to have additional coefficients to describe nonlinear characteristics requires model upgrading before updating is possible The first task – MODEL UPGRADING - addresses the question of the model being COMPLETE ENOUGH for its intended use. The second task – MODEL UPDATING - addresses the question as to whether it is ACCURATE ENOUGH. Further details can be found in: “Modal testing for model validation of structures with discrete nonlinearities” by D J Ewins, B Weekes & A Delli Carri (2015) Phil.Trans.R.Soc.A373:20140410. http://dx.doi.org/10.1098/rsta.2014.0410

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© D J Ewins 2015

MODEL WING-PYLON RIG

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Pylon Driving Point

© D J Ewins 2015

UPGRADED AND UPDATED MODEL – RESPONSE OF FIRST MODE

Further details can be found in: “ Extending modal testing for model validation of engineering structures with sparse nonlinearities: A first case study” by A Delli Carri, B Weekes & D J Ewins Mechanical Systems and Signal Processing (2016): http://dx.doi.org/10.1016/j.ymssp.2016.04.012

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FOCUS ON TESTING FOR PRODUCT VERIFICATION

Verification Testing is …….. the experimental demonstration – to Customers and the Authorities - that a product meets its design specification under test conditions that closely match those in service. Verification tests are PASS/FAIL tests

The 3rd Challenge:

Representative Extreme Event Tests

8 (34)

© 2014 Rolls-Royce plc

35 © 2014 Rolls-Royce plc

Further details can be found in: “Non-linear dynamics of a simplified model of an overhung rotor subjected to intermittent annular rubs” by A Zilli, R J Williams & D J Ewins (2015) ASME J Eng Gas Turbines Power 137(6):065001

APPENDIX 6 SLIDES NOT PRESENTED IN LECTURE

The 4th Challenge: Representative Endurance Tests

ENDURANCE QUALIFICATION TESTING

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QUALIFICATION TESTS TO DEMONSTRATE SURVIVABILITY IN SERVICE ENVIRONMENT

Picture courtesy of Federation of American Scientists.

© British Crown Owned Copyright 2015/AWE

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TYPE 1: TRADITIONAL TESTS: REPLICATE ATTACHMENT VIBRATION LEVELS

TWO APPROACHES TO TESTING - 1


Control Locations

Response Locations (Uncontrolled)

In-axis Overtest

In-axis Undertest Cross-axis Overtest


TYPE 1: TRADITIONAL TESTS

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TYPE 2: NEW ‘SMART’ TESTS: REPLICATE ATTACHMENT VIBRATION LEVELS AND SIMULATE STRUCTURAL INTERACTIONS


TWO APPROACHES TO TESTING - 2

Further details can be found in: “Next-Generation Random Vibration Tests” by P M Daborn, C Roberts, D J Ewins & P R Ind 2014 Proceedings IMAC XXXII, Conference Proc SEM DOI 10.1007/978-3-319-04774-4_37


TYPE 2: NEW ‘SMART’ TESTS

Response Locations (Uncontrolled)




4. The Future

FURTHER DEVELOPMENTS

We have focussed here on the first and second of these 4 phases – DESIGN & DEMONSTRATION but there are 2 more phases to be managed later in the product’s life – MANUFACTURE & MAINTAINENANCE

These later phases will have their own uncertainties to be managed: First, in MANUFACTURE, there will be significant new aleatoric uncertainties in the form of the scatter which results from manufacturing tolerances. Second, there will epistemic uncertainties introduced in service by predictable wear or damage to individual components; these will need to be identified during MAINTENANCE and used for diagnosis and prognosis of health monitoring.

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Structural Dynamics can be set in a positive context using the concept of STRUCTURAL PERFORMANCE with the expectation that safe working life can be reliably predicted in design In modern Structural Dynamics, TESTING activities are equally important as ANALYSIS – if not more. They are often the same order of cost. Both approaches are subject to a range of uncertainties – both aleatoric and epistemic – and the use of the other ‘branch’ to manage these uncertainties through systematic VALIDATION AND VERIFICATION procedures is the best way forward. Achieving the correct BALANCE AND INTEGRATION of Test and Analysis is vital for the economic viability of these structures

SUMMING UP: STRATEGIC CONCLUSIONS

My thanks go to … a generation of students, research students, associates and colleagues around the world whose contributions and collaborations have provided most of the material presented in this talk… …and to the industrial sponsors who have provided both the application needs and the funding for most of these activities.

ACKNOWLEDGEMENTS

Documents

On the Balance and Integration of Simulation and Test in ... · MANUFACTURE & MAINTAINENANCE . These later phases will have their own uncertainties to be managed: First, in MANUFACTURE,