Engineering Education Initiative

SE 265 Lecture 2January 12, 2005

Topics1. Brief History of Structural Health Monitoring2. 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 vibration-based 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

Before Bearing ReplacementBefore Bearing Replacement

Engineers at semiconductor fab measure vibrations on a vacuum blower motor

Spectral response of machine vibrations before (bottom trace) and after bearing replacement

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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 removal of thermal protection system (TPS) tiles.

• Eight situations where changes in modal properties correctly identified damage.

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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 truss-like 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, first-principals, 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.