A novel structural health monitoring (SHM) concept of embedded nondestructive evaluation with piezoelectric wafer active sensors (PWAS) has been developed. PWAS can be structurally embedded as both individual probes and phased arrays. They can be placed inside closed cavities during fabrication/ overhaul (such as wing structures), and then be left in place for the life of the structure. The embedded nondestructive evaluation (NDE) concept opens new horizons for performing insitu damage detection and structural health monitoring of a multitude of thin-wall structures.

A general concept of a sensor-array aircraft Structural Health Monitoring System. Networks of embedded sensors would be clustered around structural “hot spots.” The networks could be wirelessly connected to a data repository and knowledge base site.
Structural health monitoring research interests in this area include development of embedded active sensors, processing of active sensor data and diagnostics of the health of the structure, and prognostics of the remaining structural life. For the health monitoring of an actual structure, networks of embedded active sensors are envisioned. Such sensory networks would be clustered around structural “hot spots.” The networks could be wirelessly connected to a data repository and knowledge base site.

Current ultrasonic inspection of thin-wall structures (aircraft shells, storage tanks, large pipes, etc.) is a time-consuming operation that requires meticulous through-the-thickness C-scans over large areas. One method to increase the efficiency of thin-wall structures inspection is to utilize guided waves (e.g., Lamb waves) instead of the conventional pressure waves. Guided waves propagate along the mid-surface of thin-wall plates and shallow shells. They can travel at relatively large distances with very little amplitude loss, and offer the advantage of large-area coverage with a minimum of installed sensors. Conventional Lamb wave probes are too heavy to be considered for widespread deployment on an aircraft structure as part of a SHM system. Hence, a different type of sensors than the conventional ultrasonic transducers is required for the SHM systems.

PWAS are inexpensive, non-intrusive, unobtrusive, and minimally invasive devices that can be surface-mounted on existing structures, inserted between the layers of lap joints, or inside composite materials. The main advantage of PWAS over conventional ultrasonic probes lies in their small size, light weight, low profile, and low cost. In spite of their small size, these devices are able to replicate many of the functions of conventional ultrasonic probes.

PWAS are a key component being examined to localize incipient flaws, principally corrosion and cracking, in the complex geometric structure of aircraft. Most effort to date has been directed at increasing the sensitivity of existing inspection tools and developing rapid scanning platforms that allow fast automated scanning of large areas on the aircraft. Since significant improvements have already been made in this area, the next frontier to address in this area is the use of onboard health monitoring. Structural health monitoring that provides owners and maintainers of long-life-expectancy vehicles is being envisioned to localize potential problem areas in the acreage of aircraft skin and underlying structure. This knowledge of evolving “hot spots” will then be used to target inspection techniques that will allow detailed characterization of the structure, which can be used to determine the appropriate maintenance and/or repair needs.

One conceptual vision for a structural health monitoring system is that of an artificial central nervous system. PWAS are seen as leading candidates to serve as one specialized type of “neuron” in this system-level concept.

This work was done by Victor Giurgiutiu of the University of South Carolina, and John H. Barnes and Lt. Dustin Thomas of the Air Force Research Laboratory. AFRL-0203


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This article first appeared in the October, 2011 issue of Defense Tech Briefs Magazine.

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