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 Basics Statistical 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 Basics Examples Huey P. Long Bridge, Louisiana More than 800 sensors Axial and bending Comanche Helicopter Active rotor system Our of plane loading effects

Invasive Methods Strain 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 Methods Carbon 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 Methods Electronic 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 Methods Digital 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 Methods Ultrasonic 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 < 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? ???

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: /rsta doi /rsta [2]Edge Delamination in Angle-Ply Composite Laminates S. S. Wang [3] Jones R & Wykes C, Holographic and Speckle Interferometry, 1989, Cambridge University Press [4] [5]