SE 163 Course Information/Syllabus

Report
SE 265 Lecture 2
January 12, 2005
Topics
1. Brief History of Structural Health Monitoring
2. Operational Evaluation
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Brief History of Vibration-Based Damage Detection
• Heuristic forms of vibration-based damage detection
(acoustic) have probably been around as long as man
has used tools.
• Developments in vibration-based damage detection are
closely coupled with the evolution, miniaturization and
cost reductions in Fast Fourier Transform (FFT)
analyzers and digital computing hardware.
• The development of vibration-based damage detection
has been driven by the rotating machinery, aerospace,
offshore oil platform, and highway bridge applications.
• To date, the most successful applications of vibrationbased damage detection has been for condition
monitoring of rotating machinery.
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Health Monitoring of Rotating Machinery
• Economic benefits have driven the development of
machine condition monitoring
• Two types of monitoring:
– “Protective Monitoring,” e.g. identify data features
that are indicative of impending failure and shut
machines down
• Must establish absolute values on acceptable
levels of feature change.
– “Predictive Monitoring,” e.g. identify tends in data
features that allow for proper and cost effective
maintenance planning.
• Requires knowledge of the feature’s time rate
of change.
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Rotating Machinery Application
FIIS - PROCESS VACUUM BLOWE R
EB1-1
-BND BLOWER NON-DRIVE END 45 DEG
0.30
Max Am p
.38
0.20
0.10
R MS Ac celeration in G-s
0
03-APR-97
18-APR-96
01-APR-96
21-MAR-96
21-MAR-96
21-MAR-96
20-MAR-96
0
1000
2000
3000
4000
5000
Fr equency in Hz
Before Bearing Replacement
Spectral response of
machine vibrations before
(bottom trace) and after
bearing replacement
Engineers at semiconductor
fab measure vibrations on a
vacuum blower motor
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Offshore Structures
• Oil Industry spent
$millions during the
70’s - 80’s to develop
health monitoring for
offshore platforms.
• Studies include
numerical modeling
efforts, scale-model
and full-scale tests.
• Many practical problems were encountered:
– Machine noise, Non-uniform inputs, Hostile environment
for instrumentation, Marine growth, Changes in
foundation with time
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Offshore Structures
• What They Learned:
– Changes in structural
stiffness near the deck
has small effect on
modal properties.
– Marine growth, water
ingress, and water
motion causes
significant shift in
modal properties
– Ambient excitation is more practical than forced or impact
excitation, but limited to low-frequency excitation.
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Highway Bridge Monitoring
• Study SHM techniques to
augment federally mandated
visual inspections.
• Driven by several catastrophic
bridge failures over last 20 yrs.
• Rudimentary Commercial
systems for bridge health
monitoring are being marketed.
• Asian governments are
mandating the companies that
construct civil engineering
infrastructure periodically certify
the structural health of that
infrastructure.
Tsing Ma Bridge, $16
million for 600 sensors
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Example of Recent Catastrophic Bridge Failure
• Seoul, South Korea.
• 8:00AM October 21, 1994
(during rush hour)
• A 3800 ft-long bridge
• 32 people killed and 20
injured
• Constructed in 1979
• Cause of failure:
Structural fatigue
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Overview of Aerospace Applications
Damage to 1988 Aloha Airlines flight motivated the
development of an FAA Aging Aircraft Center at
Sandia National Laboratory
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Rotorcraft Health Monitoring
• Integrated health monitoring
system for rotorcraft. Fault
diagnosis of:
– Drivetrain, Engines, Oil
system, Rotor System
• Difficult to operate rotorcraft
and obtain data when
damaged
• Heath and Usage Monitoring Systems (HUMS) for
transmission and engine applications endorsed by FAA
• Full coverage system between $150K-250K/unit
• One system that monitors 73 structurally significant items
has been shown to provide cost saving of $175/hr flight time
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Space Shuttle Orbiter Structure
• Space Shuttle system was first
vehicle designed to repetitively
be subjected to launch,
spaceflight, and landing
• Needed reliable method for
SHM of components sensitive
to fatigue such as control
surfaces, fuselage panels, and
lifting surfaces
• Modal testing was chosen
because it does not require • Eight situations where
changes in modal properties
removal of thermal protection
correctly identified damage.
system (TPS) tiles.
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X-33 Reusable Launch Vehicle
• During the mid 90’s interest in
creating a completely reusable
launch vehicle has driven the
need for a new global SHM
procedures can facilitate 1
week turn-around.
• Composite fuel tanks are
surfacing as one of the critical
items for long term health
monitoring.
• Two types of sensors: Fiber optic (strain, temperature
hydrogen leak) sensors and acoustic emissions sensors for
crack propagation detection (Temp. range: -252C – 121C)
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International Space Station
• In the late 80’s, space station
SHM evolved into using modal
properties as a tool to detect
damage in the structure.
• Several data sets from trusslike test articles drove advanced
numerical approaches to detect
and locate damage.
• Because finite element
modeling is so prevalent in the
aerospace field, model-based
damage identification
procedures resulted.
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Z-GraDE (Zero-Gravity Damage Evaluation)
• Engineering students
from University of
Kentucky and University
of Houston performed
modal testing of a
planar truss in NASA
zero-g KC-135 aircraft
• Students were able to
identify damage using
modal parameters as
features when truss
element completely
remove.
University of Houston
Undergraduate Student
Testing the Damaged Truss
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Final Comments
• This class will be somewhat different than most of your
courses to date.
– Structural Health Monitoring is emerging technology
– In most cases this technology has not made the transition from
research to practice.
– We will be taking a much more probabilistic, data-driven
approach to structural condition assessment whereas most of
you previous undergraduate classes take a deterministic, firstprincipals, physics-based approach.
• As such, there is a better opportunity to demonstrate
your creative thinking than in most undergraduate
classes, particularly though the group projects.
• Your responsibility: ASK QUESTIONS!!!
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Structural Health Monitoring Process
• The Structural Health Monitoring process includes:
1. Operational evaluation of the structure
2. Data acquisition
3. Feature extraction
4. Statistical model development
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Operational Evaluation
• Operational evaluation begins to answer questions
regarding implementation issues for a structural health
monitoring system.
– Provide economic and/or life-safety justifications for
performing the monitoring.
– Define system-specific damage including types of
damage and expected locations.
– Define the operational and environmental conditions
under which the system functions.
– Define the limitations on data acquisition in the
operational environment.
• Operational evaluation will require input from many
different sources (designers, operators, maintenance
people, financial analysts, regulatory officials)
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Technical Justification for Implementing a SHM System
• Directly coupled with economic/life-safety justifications
for developing and implementing a SHM system is the
technical justification for such system development.
• At a minimum, you must be able to answer the following
questions:
– What are limitations of currently employed
technology?
– What are advantages and limitations of proposed
SHM system?
– How much will it cost to develop and test?
– How long will it take to develop?
– How much will it cost to deploy and maintain?
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Economic and/or Life-Safety Justifications for SHM
• Outside of a research studies, funds will not be devoted to
SHM unless there is a economic or life-safety motive.
– Commercial airframe and jet engine manufactures want
lease their products and assume maintenance
responsibilities. Reducing maintenance cost increases
profits!
– Oil companies invest over a billion dollars for deep
water offshore platforms.
– Cost of down time is exorbitant for high capital
expenditure manufacturing.
– Loss of transportation infrastructure has significant
impact on entire economy.
– Life safety is also an issue for most of these examples.
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Defining System-Specific Damage
• In general, the more specific one can be with regard to
defining the damage to be detected, the better the
chances that the damage can be detected at an early
stage.
• If possible, one should specifically define:
– Type of damage to be detected (e.g. crack, excessive
deformation, corrosion)
– Anticipated location of damage
– Critical level of damage that must be detected (e.g.
crack completely through the member that is 15 mm
in length)
– Time scale for damage evolution
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The Conditions Under Which the System Functions.
• Operational conditions will influence loading that
produces the monitored dynamic responses.
– Traffic loading on bridges
– Machinery and fluid storage on offshore platforms
– Speed of rotating machinery
– Flight maneuvers (altitude, speed) and fuel level for
aircraft
• Environmental conditions can produce changes in
dynamic response that must be distinguished from
changes cause by damage.
– Temperature changes on bridges
– Sea states for offshore platforms
– Air turbulence for aerospace structures
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Limitations on Data Acquisition
• Cost and accessibility are common limiting factors
• For aerospace structures weight restrictions pose
significant limitations
• Spark initiation is a limitation when monitoring structures
containing flammable material
• RF interference poses challenges for wireless telemetry
• Many portions of a structure will not be easily accessible
for instrumentation (bridge deck, below-water-line
portions of oil platforms)
• Hostile Environments (e.g. radiation, temperature,
moisture)
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Summary of Operational Evaluation
• Need to define the justification, goals for, and the
limitations of the SHM system in as quantifiable manner
as possible.
• Operational evaluation is the process of assembling as
much a priori information regarding the SHM system
requirements as possible.
• Such information can come from a wide variety of
sources.
• Quantified operational evaluation will impact the
development of all other portions of the SHM process.
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