NASA Spinoff

Smart Sensors Assess Structural Health

The materials used to make airplanes and space shuttles do not last forever. That is why NASA frequently inspects launch vehicles, fuel tanks, crew habitats, and other components for structural damage. The timely and accurate detection of cracks or other damage can prevent failure, prolong service life, and ensure safety and reliability.

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The Composite Crew Module (CCM) is a full-scale test version of the Orion spacecraft. As seen here, the CCM has fiber optic and traditional strain gauges attached.
To perform quick, nondestructive evaluation and monitoring of aerospace vehicles and structures, NASA pursues the development of structural health monitoring (SHM) systems. SHM aims to build a system with a network of sensors placed in critical areas where structural integrity must be maintained, such as vehicle stage sections, separation interfaces, solid motors, and tanks. The sensors send information to a computer that is programmed to recognize patterns of electrical signals that represent damage, such as strains, breaks, or cracks. These systems can automatically collect and process data.

One approach to SHM is the Stanford Multi-Actuator Receiver Transduction (SMART) Layer, a patented technology from Stanford University. The thin material layer is embedded with a network of piezoelectric sensors and actuators that can be mounted on metal structures or embedded in composite structures. The piezoelectric sensors give off a small amount of electricity when they undergo mechanical pressure. Similar to a medical ultrasound, the sensors generate a wave that propagates through the structure and is picked up by other sensors. The aim is not only to detect structural damage, but to provide early warning before a failure takes place.

To enhance and commercialize the SMART Layer, Acellent Technologies Inc. was founded in 1999 in Sunnyvale, California. Soon after its founding, the company created an SHM system consisting of the SMART Layer, supporting diagnostic hardware, and data processing and analysis software.

Partnership

In 2001, Acellent started working with Marshall Space Flight Center through a Acellent develops advanced active and passive diagnostic systems using built-in networks of actuators and sensors.

Customarily, SHM uses sensors and actuators arrayed at various locations on a structure. In contrast, the SMART Layer contains an entire sensor and actuator array, making it unnecessary to install each sensor and actuator individually. The SMART Layer is pre-networked and pre-positioned, making it very easy for installation. The layer can be mounted on an existing structure or integrated into a composite structure during fabrication. Different types of sensors, such as piezoelectric and fiber optic, can both be embedded in the sheet to form a hybrid network.

Capable of being integrated into new or existing structures, SMART Layer technology is used to automate inspection and maintenance including structural condition monitoring, load and strain monitoring, impact detection, damage detection, impact damage assessment, crack growth monitoring, debonding detection, process monitoring, materials cure monitoring, and quality control.

The main benefits of SMART Layer technology are its flexibility, light weight, ability to adapt to any structure, ease of installation, durability, and reliability under different environments. The size and shape of the technology varies, and Acellent manufactures SMART Layers in numerous sizes, shapes, and complexities—from single sensor flat strip to multisensor 3-D shells. In addition to the sensor network, Acellent provides the diagnostic hardware and data analysis software for different SHM applications from monitoring large composite structures to localized damage detection in composites and metals.

Acellent has
been funded to
monitor civil
infrastructure
such as pipelines,
buildings,
and bridges.

According to Acellent, customers and partners include aerospace and automotive companies; construction, energy, and utility companies; and the defense, space, transportation, and energy industries. Private and government research labs have purchased the technology to gauge its performance in particular environmental conditions.

Acellent has had sales contracts with many major aerospace companies, including Boeing, Lockheed-Martin, Bombardier, and Airbus. Acellent is also customizing the technology for the U.S. Army to monitor fatigue cracks in rotorcraft structures. Recently, BMW used the SMART Layer technology on a concept vehicle to help verify the design and modeling, and to improve manufacturing techniques.

Over the last couple of years, Acellent has been funded to monitor civil infrastructure such as pipelines, buildings, and bridges, including a recent project with the National Institute of Standards and Technology. “We have been asked to develop a system—not just to monitor damage to a structure—but to monitor the overall framework or architecture for monitoring bridges across the Nation,” says Beard.

In addition to winning sales for research and testing, the company has won additional NASA SBIRs to apply the technology to large scale structures like rocket motors. Acellent’s long-term goal, however, is to have the technology become part of the design process. Instead of applying SHM after a new spacecraft is designed, Beard says the company envisions SHM being included in the original design of the structure. “If you design sensors into the structure in the beginning, you can optimize the structural design and reduce the overall weight of the vehicle. That is the direction we would like to see the technology go.”

SMART Layer® is a registered trademark of Acellent Technologies Inc.
SMART Suitcase™ is a trademark of Acellent Technologies Inc.

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