The majority of MDC’s extensive SHM experience has made use of piezoelectric material. These elements can be used as sensors by measuring voltage differences across parallel electrodes when cyclically strained, or alternatively they can be used as actuators by inducing expansion and contraction with an applied alternating electric field. Materials with piezoelectric properties are particularly attractive for SHM applications due to their high-frequency response and overall wide-bandwidth characteristics. Most research at MDC has indicated piezoceramic elements, specifically PZT (lead zirconate titanate), to be the most suitable for practical SHM efforts since these wafers have balanced actuator and sensor constants, they are accessible, have well vetted properties and reasonable thermal stability. MDC’s assembly service strives to provide customers with robust PZT packages for repeatable testing using proven techniques to eliminate electrical interference, cross-talk, signal attenuation, and non-uniformities caused by typical fabrication and installation practices such as soldering wires or dilled-hole electrodes.
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Structural Health Monitoring
MD7 Digital SHM System
Current in-service monitoring techniques utilize a dense web of analog sensors connected by individual wires routed to centralized data acquisition and processing units. This traditional approach has a significant weight penalty, can be complex to install and is susceptible to EMI. To resolve these issues, MDC has developed a fully digital SHM solution. The MD7 system is composed of 3 core elements: the IntelliConnector™ miniature node for distributed data acquisition, the VectorLocator™ sensor assembly for guided-wave phased-arrays, and the HubTouch™ data accumulator for remote diagnostic processing. Each element of the MD7 system is networked on a 6-wire serial bus that carries the differential communication, synchronization and power signals, typically using flat-flexible-cable (FFC). Benefits of this distributed infrastructure approach include higher fidelity data through digitizing sensor signals at the point of measurement, reduced computational burden through local signal processing and feature reduction, and overall minimal mass through the elimination of cables, connectors and bulky off-the-shelf hardware.
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PZT-based SHM
The majority of MDC’s extensive SHM experience has made use of piezoelectric material. These elements can be used as sensors by measuring voltage differences across parallel electrodes when cyclically strained, or converselythey can be used as actuators by inducing expansion and contraction with an applied alternating electric field. Materials with piezoelectric properties are particularly attractive for SHM applications due to their high-frequency response and overall wide-bandwidth characteristics. Most research at MDC has indicated piezoceramic elements, specifically PZT (lead zirconatetitanate), to be the most suitable for practical SHM efforts since these wafers have balanced actuator and sensor constants, they are accessible, have well vetted properties and reasonable thermal stability. MDC’s assembly service strives to provide customers with robust PZT packages for repeatable testing using proven techniques to eliminate electrical interference, cross-talk, signal attenuation, and non-uniformities caused by typical fabrication and installation practices such as soldering wires or dilled-hole electrodes.
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Concentric Arrays
The original PZT configuration developed by MDC was the concentric pair, where a ring-shaped actuator element circumscribes a disk-shaped sensor (or vice-versa). While simple in concept, this configuration is powerful in practice in that it enables pulse-echo style guided wave methods, where the direct scatter response wave is sensed virtually from the original excitation point. This method is more than 4x as effective in terms of coverage area than common pitch-catch methods, where indirect scatter response waves are sensed from multiple remote locations, which suffers from weak signals that decay as a function of incident reflection angle. Furthermore, using separate actuator and sensor elements decouples noise and crosstalk from signals by eliminating switches and bridge-circuits, and the “direct wave” travelling from the actuator through the sensor can also be used for various compensation algorithms since the elements' positions are fixed with respect to each other.
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Phased Arrays
While phased arrays are more complex from a signal generation and processing point of view than unitized sensor/actuator pairs, they are a powerful approach for both active (guided waves) and passive (acoustic emission) SHM methods. Traditional phased arrays can be fabricated by strategically placing actuator elements such that their outgoing excitations will constructively and destructively interfere with each other creating a beam that can be steered through a range of angles to be monitored. Alternatively, an omnidirectional excitation source can be used in conjunction with deliberately placed sensors such that the phase of the response signals can be used to reconstruct the effective steered beam response from a scatter source (such as is the case with the MD7 developed VectorLocator™). MDC has fabricated a wide variety of phased array configurations for various customers in both configurations, as well as developing software to design the required element spacing and algorithms to interpret and visualize the resulting data.
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Custom Arrays
A core business for MDC has been the assembly of prototype quantities of custom PZT arrays for our clients. MDC’s world-class piezoelectric experts can assist with material selection. Wafers can be purchased flat or curved, they can be diced round, rectangular or in rings, and can range from 0.5 mm to 10 cm in size. These elements can be coated with a variety of electrodes to prevent oxidation or promote solder or adhesive bond strength, including gold, silver or nickel. Wafers can be poled in a variety of orientations including through-thickness, in-plane, in-shear, or with interdigitated electrodes. Flexible circuits are designed to align PZT elements precisely in any configuration of actuators and sensors, including single element, phased array, and concentric pair schemes. A wide selection of connectors can be chosen to mate with assemblies ranging from micro-miniature coaxial to SMA. Arrays can be packaged to survive virtually any harsh environment.
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CNT-based SHM
MDC has partnered with the Technology Laboratory for Advanced Materials and Structures (TELAMS) in the Department of Aeronautics and Astronautics at the Massachusetts Institute of Technology (MIT) to develop the next generation of advanced SHM technologies through the use of embedded carbon nanotubes (CNTs) to enable multi-physics, multi-functional capabilities within composite laminates. Several studies have shown that CNTs possess exceptional mechanical stiffness (as high as ~1 TPa) and strength, as well as excellent electrical conductivity (~1000x copper) and piezoresistivity (resistivity change with mechanical strain). Thus, they can be used to not only to reinforce composite structures to improve impact and delamination resistance, but also to enable novel SHM and NDE techniques. Vertically or horizontally aligned CNT forests can be transferred to composite pre-preg at room temperature through a “nanostitch” process. Radially aligned CNT can be grown in-situ on dry fiber tows or fabric to create “fuzzy-fiber” reinforced polymers (FFRP).
