further developments in aerospace structural dynamics
TRANSCRIPT
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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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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