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.
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.
In 2001, Acellent started working with Marshall Space Flight Center through a Small Business Innovation Research (SBIR) award to develop a hybrid SMART Layer for aerospace vehicles and structures. The hybrid layer utilized piezoelectric actuators and fiber optic sensors. As a result of the SBIR, the company expanded the technology’s capability to utilize a combination of sensors for various applications, such as monitoring strain and moisture.
A development known as the hybrid piezoelectric/fiber optic structural diagnostics system was intended to perform quick nondestructive evaluation and longer-term health monitoring of aerospace vehicles and structures, and could potentially monitor material processing, detect structural defects, detect corrosions, characterize load environments, and predict life. Piezoelectric actuators were embedded along with fiber optic detection sensors including Fabry-Perot fiber optic strain gauge sensors and fiber Bragg grating sensors.
Additional SBIRs with Marshall helped Acellent to improve and optimize its technology. The SMART Suitcase was developed and used for testing the SMART Layer and other SHM technologies. A NASA Space Act Agreement through Marshall investigated radio frequency attenuation transmission and detection using the SMART Layer approach. Marshall also provided knowledge advancement through testing opportunities with composite over-wrapped pressure vessels, which were being developed and studied for potential use in several space programs. Under a 2004 SBIR, Acellent tested and evaluated the performance of a SMART Tape, based on the SMART Layer, in harsh cryogenic conditions.
“We used the same base technology in all of the SBIRs, but for different applications requiring new innovations. This increased the reliability of the system and made it more robust,” says Shawn Beard, chief technology officer at Acellent.