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Further Developments in

Aerospace Structural Dynamics with particular reference to experimental technologies

by

Professor David Ewins University of Bristol and Imperial College London

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Structure of Talk

1. Background

2. Technology Needs

3. Strategic review

4. Current priorities

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Structure of Talk

1. Background

2. Technology Needs

3. Strategic review

4. Current priorities

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Aerospace Structural Dynamics Based on 50 years working in specific areas of

Structural Dynamics with particular relevance to

Aerospace application areas: Specifically,

• Aero engines

• Helicopters and

• Defence

While it is not true to say that all Lightweight Structures are

Aerospace, it is true to say that all Aerospace Structures

are Lightweight.

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BLADES

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7

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Structure of Talk

1. Background

2. Technology Needs

3. Strategic review

4. Current priorities

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Technology Needs in Structural Dynamics

Helpful to put Structural Dynamics into perspective and

context. Do this by discussing Functional Performance (of

almost any product) and its Structural Performance.

MATERIALS STRUCTURAL

DYNAMICS

NDE

STRUCTURAL

INTEGRITY

Functional Performance is about cost per mile/SFC/.

Structural Performance is about Life

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TARGET:

LIFE PREDICTION

& VERIFICATION

Frequency

Vibration

Amplitude

Frequency

Vibration

Amplitude

Position

Frequency

Vibration

Amplitude

Position

VIBRATION SENSITIVITY TO EXCITATION CONDITIONS F

V

F

V

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Appropriate to define the mission of the Structural

Dynamics community in general and aerospace in

particular….

To provide the technology which

(i) Ensures that machines and structures can be designed

to be free of unwanted dynamic characteristics

(malfunction, wear, fatigue, instability,… excessive

noise, vibration disturbance etc…. )

(ii) Enables the prediction of operating life of critical

structures, and

(iii) Facilitates monitoring to ensure maintenance of

performance and reliability throughout working life

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CAPABILITIES required by the Structural Dynamics

community in order to fulfil this role ….

Ability to predict the dynamic characteristics of a structure or

machine under any prescribed operating conditions

Ability to specify design changes such that a structure’s dynamic

characteristics satisfy design criteria of acceptability

Ability to measure and to interpret data to reveal underlying

dynamic behaviour of actual structures and to monitor their

maintenance of the specified criteria

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Specific TOOLS required to provide these capabilities …

THEORETICAL MODELLING (M) (of the structural elements, of

damping phenomena, and of excitation forces,…)

NUMERICAL ANALYSIS (A) (to permit the efficient prediction of

specific levels of dynamic response under arbitrary loading

conditions..)

EXPERIMENTAL MEASUREMENTS (X) (selection and measurement

of appropriate parameters under controlled or operating

conditions, and extraction of useful information from these data)

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THEORETICAL MODELLING

EXPERIMENTAL MEASUREMENTS

& TESTS

NUMERICAL ANALYSIS

THE STRUCTURAL DYNAMICIST’S TOOLKIT

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and the TECHNOLOGIES that can be performed using

these tools....

SIMULATION (prediction of behaviour of structures)

VALIDATION (ensuring that an product is fit for purpose)

IDENTIFICATION (interpretation of observed data to

reveal underlying physics of structural behaviour)

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THEORETICAL MODELLING

EXPERIMENTAL MEASUREMENTS

& TESTS

NUMERICAL ANALYSIS

‘SIMULATION’

‘VALIDATION’

‘IDENTIFICATION’

THE STRUCTURAL DYNAMICIST’S TOOLKIT

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Experimental Procedures

• Measurements: quantification of physical parameters

• Experiments: use of measurements to observe (and then to

understand and explain) physical phenomena

• Tests: use of measurements to prove or ‘test’ a theory (i.e.

validation)

• Trials: use of measurements to demonstrate the overall

performance of a machine or structure (eg. Certification, Verification)

• Monitoring/Diagnostics: repeated measurement of selected

parameters to detect changes in structural condition or differences

between nominally identical structures

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Some important Concepts and Terminology

At this point, it is very important that we establish some

issues:

Need to understand the difference between:

VALIDATION and VERIFICATION

…and between the UNCERTAINTIES which result in

INACCURACY and INADEQUACY

..and want to learn

1. how TESTS can improve the VALIDATION theoretical

models for design, and

2. how THEORETICAL MODELS can inform the design

and conduct of VERIFICATION TESTS at pass-off

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Structure of Talk

1. Background

2. Technology Needs

3. Strategic review

4. Current priorities

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BLADES

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Testing for model validation (CURRENT)

_ Test

Strategy

Test

Planning

Modal

Test

Reference Data

Verification Correlation Updating

PRELIMINARY

MODEL

VALIDATED

MODEL

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Strategic Planning

Make a list of the major challenges/opportunities that

need to be taken on in order to achieve the top level

objective, and classify them according to the

Theoretical, Numerical, Experimental subgroups

This will vary from topic to topic, product to product

….. Here we report on specific the application to Aero

engines, but rather similar results will apply to many

other aerospace areas

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Some identified critical issues:

M1 Models of joints and interfaces

M2 Supermodels for complex components

M3 Improved models for complex phenomena and environments

M4 Representative models for structures in operating conditions

X1 Test strategy for validation testing design and planning

X2 Advanced full-field test methods for static and rotating structures

X3 Advanced test methods for non-linear structures

X4 Advanced test Methods to simulate operating service conditions

A1 Highly efficient and reliable numerical analysis of dynamics of

complex nonlinear systems

A2 Accommodation of variability issues (aleatoric uncertainty)

A3 Identification from incomplete measured data (epistemic u/c)

A4 Reliable identification from in-service measured data (e.g. OMA)

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Some identified critical issues:

M1 Models of joints and interfaces

M2 Supermodels for complex components

M3 Improved models for complex phenomena and environments

M4 Representative models for structures in operating conditions

X1 Test strategy for validation testing design and planning

X2 Advanced full-field test methods for static and rotating structures

X3 Advanced test methods for non-linear structures

X4 Advanced test Methods to simulate operating service conditions

A1 Highly efficient and reliable numerical analysis of dynamics of

complex nonlinear systems

A2 Accommodation of variability issues (aleatoric uncertainty)

A3 Identification from incomplete measured data (epistemic u/c)

A4 Reliable identification from in-service measured data (e.g. OMA)

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Structure of Talk

1. Background

2. Technology Needs

3. Strategic review

4. Current priorities

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4. Current priorities

Current priorities have clearly shifted towards

satisfying the needs of the models being produced and

the associated testing to be highly representative of

operating service conditions (and not of the idealised

environment of the test lab).

This means (i) allowing for higher loads and responses,

and more regular incursions into the nonlinear regime,-

in turn, (ii) taking better account of the influence of the

many joints on these complex structures, and (iii)

managing the additional complexity and cost of the

different stages of engagement – experimental,

theoretical and numerical.

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Critical Area 1: Modelling Of Joints and Interfaces

Almost certainly, the single most critical issue in the

current capabilities for structural dynamics is our

inability to predict the dynamic behaviour of the joints

and contact interfaces used in structural assemblies.

The accuracy of our current FE modelling methods for

individual components is of the same order as the

accuracy of our manufacturing capability. But the

accuracy of our models of structural assemblies is

significantly worse than this.

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Aero-engine Casings Test Configuration

Bolt Joints

Bearing Joints

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IMC-CCOC

interface

Effect of Joint Dynamics

on Dynamic Behaviour of 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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Research Committee on Mechanics of Jointed Structures ... - ASME

committees.asme.org/K&C/TCOB/BRTD/MJS/

The Research Committee on Mechanics of Jointed Structures, established in

2010, investigates a broad spectrum of issues associated with the theoretical,

experimental, and computational aspects of mechanics of joints and the mechanics of

jointed structures. Its activities include the generation of new knowledge and

development of guidelines for use by engineers and scientists in measurement,

analysis, prediction, and design of mechanical joints and jointed structures

A International Research Community

MECHANICS OF JOINTED STRUCTURES

Based on an ASME Research Committee

Full Details of Previous Workshops 2006-2009:

http://www3.imperial.ac.uk/medynamics/research/future

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Critical Area 2: Measurement on Nonlinear Structures

Many modern structures exhibit a degree of non-

linearity in their dynamic behaviour. There are many

causes of this but it is increasingly necessary to be able

to undertake reliable vibration tests on structures which

have a non-trivial degree of nonlinearity.

Jointed structures present a classical example, where

an essentially-linear structure has a number of discrete

localised nonlinear elements in it.

How to measure its properties?

Test Data Obtained Using Force-Control Test

1N 10N 20N

30N 40N

50N 70N

© D J EWINS 2012

Variation of Damping with Displacement

Amplitude

0.0E+00

1.0E-01

2.0E-01

3.0E-01

4.0E-01

5.0E-01

6.0E-01

7.0E-01

8.0E-01

9.0E-01

0.00E+00 5.00E-05 1.00E-04 1.50E-04 2.00E-04 2.50E-04 3.00E-04 3.50E-04

Amplitude

ET

A(%

)

70N

50N

40N

30N

20N

10N

1N

CONS-AMPLITUDECONST RESPONSE

© D J EWINS 2012

Nonlinear vibration

characteristics of composites

Mode-2 Mode-3

Validation principle

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Am

pli

tud

e

Frequency

Unmodelled non-linear effects represent a degree of uncertainty

that can render a model invalid

A LINEAR model can predict a response amplitude lower than a

NON-LINEAR model and therefore predicting an incorrect HCF life

PRACTICAL CASE STUDY- BENTLEY NEVADA RIG

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Critical Area 3: Validation of Structural Models with

Nonlinear elements

There is now a need to adapt the existing model validation

methodology which is based on linear behaviour and

extend it to accommodate nonlinear effects.

The switch from linear to nonlinear is a huge step and so

it is prudent to explore possible specific cases before

addressing the most general case.

Here, it is noted that many practical structures possess

only a small number of elements which are not linear.

These are often the result of complicated behaviour in

the joints.

MODAL TESTING OF STRUCTURES WITH NONLINEAR ELEMENTS - 1

We have a structure For which we want a model

Capable of predicting response

Under operating conditions

For design,

modification assessment

and monitoring The model will be validated

using modal test data

but when making the measurements Significant nonlinearities

are often discovered

These are traced to joints

In the structure

Which means the

model needs

enlarging

MODAL TESTING OF STRUCTURES WITH NONLINEAR ELEMENTS - 2

We start with a conventional

Validation procedure but at

Low levels of excitation/response

From which we seek a

validated

Underlying Linear Model

We then undertake tests at

higher and more

representative levels

We need to determine:

Degree of NL; Type of NL;

Location of NL;

Quantification of NL:

Which can be analysed by

Special modal analysis

methods

Detection D

Localisation L

Characterisation C

Quantification Q

NLMT

But this is only part of

the picture: showing how

the modes vary with level

?

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http://www.bristol.ac.uk/news/2012/8770.html

A new, 5-year, 5-institution Research Project

based at the University of Bristol

2012-2017

The structural dynamics landscape

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Concluding Remarks I have discussed many points in this talk, and have introduced

some specific technical developments that might be of general

interest, but would like to emphasise small number of key issues.

These are primarily the Philosophical points which are the various

needs…

To integrate the THEORETICAL MODELLING, NUMERICAL ANALYSIS

and EXPERIMENTAL MEASUREMENT skills that are used

throughout the subject, and to ensure that there is the correct

BALANCE between theory and test.

To recognise that NONLINEARITY is increasingly a factor that must be

accommodated in all our activities related to modern aerospace

structures – in measurement, analysis and modelling

To understand that there are two types of UNCERTAINTY that cause

us to have less than perfect results and to be fully aware of the

limitations to our various activities that arise from both

INACCURATE and INADEQUATE data The latter is CRITICAL

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Concluding Remarks – Post Script

Most of this discussion has been concerned with the role of

experimental structural dynamics in ensuring that the FE models

we construct for designing these super-efficient lightweight

structures are good enough. i.e. The VALIDATION process

The tables can be turned when designing tests for VERIFICATION

purposes – where we need to demonstrate that a given product will

perform according to its design specification, including its

STRUCTURAL PERFORMANCE.

Many such ‘QUALIFICATION’ tests are performed according to

longstanding military-based test criteria. But these tests are

known to be seriously deficient. Recently, we have used the

detailed mathematical models of some test structures to devise

VERIFICIATION tests which are several orders of magnitude

superior to those currently practiced around the world.

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Examples of IMMAT in Practice

(Dummy air-to-air missile)

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Control Locations

In-Axis

Undertest

In-Axis

Overtest

Cross-axis

Overtest

The results from current state-of-the-art for vibration testing Z

-Axis

X

&Y

- A

xes

Cross-Axis

Undertest

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Results - IMMAT vs current state-of-the-art