SE 265 Lecture 2

1

• SE 265 Lecture 2
• January 12, 2005
• Topics
• Brief History of Structural Health Monitoring
• Operational Evaluation

2
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.

3
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 features time rate of
  change.

4
Rotating Machinery Application
Engineers at semiconductor fab measure vibrations
on a vacuum blower motor
Spectral response of machine vibrations before
(bottom trace) and after bearing replacement
5
Offshore Structures

• Oil Industry spent millions during the 70s -
  80s 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

6
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.

7
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
8
Example of Recent Catastrophic Bridge Failure

• Seoul, South Korea.
• 800AM 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

9
Overview of Aerospace Applications
Damage to 1988 Aloha Airlines flight motivated
the development of an FAA Aging Aircraft Center
at Sandia National Laboratory
10
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

11
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.

12
X-33 Reusable Launch Vehicle

• During the mid 90s 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)

13
International Space Station

• In the late 80s, 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.

14
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
15
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!!!

16
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

17
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)

18
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?

19
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.

20
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

21
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

22
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)

23
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.