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 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, or radially aligned CNT can be grown in-situ on dry fiber tows or fabric to create “fuzzy-fiber” reinforced polymers (FFRP).
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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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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).
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Resistive Methods
Due to the fact that FFRP are more than a million times more conductive that traditional CFRP, resistance measurements have proven to be a simple approach to SHM in CNT-engineered composite laminates. Specimens provided by MIT have been instrumented by MDC with non-invasive silver-ink electrode grids, inspired by flat panel liquid crystal display (LCD) technology, with an “active” layer of electrode columns on one surface of the laminate, and a second layer of electrode rows as “passive” ground on the opposite side. Damage alters the CNT-link network around the affected zone in the structure, and consequently its local resistivity. Thus, by selecting particular combinations of rows and/or columns, local in-plane and through-thickness resistivity changes can be monitored and subsequently visualized with nearly-unlimited full-field resolution. Experiments on several composite laminates have validated that even small, beneath the visible surface impact damage is readily captured and quantified using this method.
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Acoustic Emission
MDC and MIT’sTechnology Laboratory for Advanced Materials and Structures (TELAMS) have demonstrated that the multi-functional capabilities of CNT-engineered laminates can be exploited for many other purposes that go beyond traditional resistance-based methods. One of the most intriguing alternative SHM methods is using the CNT network to create virtual acoustic emission (AE) sensors. Due to the fact that CNTs are piezoresistive, their resistance value changes dynamically as a function of their induced strain field. Consequently, as an acoustic and/or stress wave propagates along a CNT-engineered laminate the local resistance value will momentarily change. Therefore by placing a pair of electrodes anywhere on the laminate surface, a virtual in-situ AE sensor can be created. Resistance changes corresponding to impinging waves can be measured by simply placing a fixed current across the electrode and monitoring the resulting voltage at a high enough sampling rate. This method could be used for impact or leak detection, as well as ground vibration test instrumentation.
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Thermal NDE
The use of thermographic imaging in metallic structures provides high probability of detection (POD) for even small flaws. Thermography is more challenging for plastic and reinforced-plastic parts because of the fact that they are natural thermal insulators which hinders the penetration of thermal energy through thick cross-sections. The most successful thermographic imaging of plastic parts has been achieved in monitoring the cool-down of injection-molded parts for quality control purposes, since a great deal of thermal energy is present right after molten plastic leaves its heat source. MDC and MIT TELAMS have demonstrated that the introduction of CNT into laminates can greatly enhance traditional thermography for composite materials. Owing to the fact that aligned embedded CNT networks have a finite but small homogeneous resistance, the application of even low voltage levels can induce rapid self-heating throughout the laminate cross section, allowing for quick and accurate thermographs to be captured.
