Structural Health Monitoring (SHM)

By Kevin Wynter

Outline

• SHM Basics• Invasive Methods• Non-Invasive Methods• Experimental Methods

SHM Basics What is SHM?

• Structural Health Monitoring, is an approach used for damage detection and behavioral characterization of structures, in order to evaluate performance

• Can be large scale (bridge) or small scale (helmet)• Long-term analysis (material creep effects)• Short-term analysis (impulse testing)• Think of SHM systems as complex transducers:

physical properties optical/electrical/acoustic response (stress, strain, resistivity)

SHM BasicsStatistical Pattern Recognition Paradigm [1]

• Operational Evaluation• Data Acquisition• Extraction and Data Compression• Modeling for Feature Discrimination

SHM System • Structure• Sensors• Data acquisition/transfer/management system• Data processing and interpolation software

SHM BasicsExamples

• Huey P. Long Bridge, Louisiana• More than 800 sensors• Axial and bending

• Comanche Helicopter• Active rotor system• Our of plane loading effects

Invasive MethodsStrain Gauges

• Surface mounted electromechanical sensors• Measures the state of strain in an object• Correlates the mechanical deformation of structure to resistance of the coil by

the gauge factor• Wheatstone bridge design• Extensive surface preparation required

Destructive Methods• Enveloping, cutting or otherwise permanently changing physical state• Decommissioning • Combines with some other technique (strain gauge, optical, ultrasound etc.)• Usually performed according to cyclic schedule or specific event• Structure is rendered inoperable – tested to failure

Invasive Methods

Non-Invasive MethodsCarbon Nanotube (CNT)

• Main idea is to integrate thread directly into the fibers of the composite.• CNT thread was chosen due to its highly favorable electrical and mechanical

properties• Fibers are small in size (10-30 µm in diameter) [1] allowing them to be

integrated into the structure without compromising reliability• Can be implemented as one sensory circuit, and provide real time monitoring

of the structural health of a component

Two-thread yarn. [2]

CNT thread being pulled from the forest b) Close up of thread c) Two threads in twist d) CNT ribbon [2]

Non-Invasive MethodsElectronic Speckle Pattern Interferometry (ESPI)[3]

• Optical method using lasers and sophisticated cameras to detect subtle changes in a surface

• Surfaces must be non-uniform• Laser reflects off of surface, and the interference of multiple light fields create a

speckle image representing the surface• Surface will appear the same if the phase difference of the light fields is an

integer multiple of 2π

Non-Invasive MethodsDigital Image Correlation (DIC)[4]

• Optical method similar to ESPI which tracks small changes in an object associated with deformation

• Surface is prepared with optical element, and cameras are used to associate each point on the surface with a reference coordinate

• A stochastic intensity pattern on the surface is used to create 3D and 2D imaging of the property being analyzed by way of a correlation algorithm

Non-Invasive MethodsUltrasonic Scan (UT)[5]

• Acoustic method in which an object is submerged and using pulses, internal characteristics of the object are examined

• Frequency used is typically <1 - 50 MHz• Comprised of a pulser/receiver, transducer and display devices• Very useful for complex structures and detection of small flaws and changes• A direct image of subsurface and its properties can be obtained

Experimental Methods 3D Technology

Acoustic Signatures

Questions?

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Sources[1] Farrar, C. R.; S. W. Doebling and D. A. Nix (2001). "Vibration-Based Structural Damage Identification". Philosophical Transactions of the Royal

Society: Mathematical, Physical & Engineering Sciences 359 (1778): 131–149. doi:10.1098/rsta.2000.0717.[2] Edge Delamination in Angle-Ply Composite Laminates

S. S. Wang. 1984.

[3] Jones R & Wykes C, Holographic and Speckle Interferometry, 1989, Cambridge University Press[4] http://www.dantecdynamics.com/Default.aspx?ID=1030

[5] http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Introduction/description.htm