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
E B1-1
FI IS - P ROCE S S V ACUUM BLOW E R
-BND BLOW E R NON-DRIV E E ND 4 5 DE G
0. 30
Max Amp
.38
0. 20
0. 10
RMS Acceleration in G-s
0
03 -AP R-97
18 -AP R-96
01 -AP R-96
21 -MAR-96
21 -MAR-96
21 -MAR-96
20 -MAR-96
0
10 00
20 00
30 00
40 00
50 00
Fr e que ncy i n 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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