Knowledge of integrity of in-service structures can greatly enhance their safety and reliability and lower structural maintenance cost. Current practices limit the extent of real-time knowledge that can be obtained from structures during inspection, are labor-intensive and thereby increase life-cycle costs. Utilization of distributed sensors integrated with the structure is a viable and cost-effective means of monitoring the structure and reducing inspection costs. Acellent Technologies is developing a novel system for actively and passively interrogating the health of a structure through an integrated network of sensors and actuators. Acellent's system comprises of SMART Layers™, SMART Suitcase™ and diagnostic software. The patented SMART Layer™ is a thin dielectric film with an embedded network of distributed piezoelectric actuators/sensors that can be surface-mounted on metallic structures or embedded inside composite structures. The SMART Suitcase™ is a portable diagnostic unit designed with multiple sensor/actuator channels to interface with the SMART Layer™, generate diagnostic signals from actuators and record measurements from the embedded sensors. With appropriate diagnostic software, Acellent's system can be used for monitoring structural condition and for detecting damage while the structures are in service. This paper enumerates on the SMART Layer™ and SMART Suitcase™ and their applicability to composite and metal structures.

By implementing a built-in sensor network on a composite structure, crucial information regarding the condition, damage state, and service environment of the structure can be obtained. In this study, methods for integrating piezoelectric sensor networks into a composite structure during different fabrication processes, including the resin transfer molding (RTM) and filament winding processes, are examined. To integrate sensor networks with different contours of structures, the method to fabricate a three-dimensional (3-D) diagnostic layer is developed. It is demonstrated that a large number of sensors supported on a thin flexible dielectric film, called a SMART Layer, offers a simple and efficient way to integrate a large sensor network onto a complex 3-D structure. The sensor network permanently embedded inside the composite structures can be used with either active sensing or passive sensing to monitor the health condition of a structure throughout its lifetime.

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Utilizing the SMART Layer technology as a basis, a real-time active pipeline integrity detection (RAPID) system is developed for built-in in situ assessment of the health of new and existing pipelines. The RAPID system consists of a sensor network permanently mounted on the host pipeline, portable electronic hardware and diagnostic software. Three moduli, including image display, damage sizing, and corrosion depth, are built into the diagnostic software to help in visualization of the approximate location and the extent of corrosion, and to quantify the corrosion sizing and depth. The main advantages of the RAPID system include: (1) ease of use, (2) ability to provide a well-defined resolution, (3) reliability with self-diagnostic and environmental compensation, and (4) quantified corrosion sizing. To verify the detection capability of the RAPID system, a series of tests have been conducted on a 6.7 m long steel pipe with a diameter of 610 mm and a wall thickness of 7.14 mm with ten different types of corrosion flaws. Test results demonstrated that the depth detection limit could be as low as 0.125 mm for general corrosion with an area of 60 mm × 60 mm under laboratory conditions, while a pinhole with 6.35 mm diameter and 3.5 mm depth can be detected with the given sensor density. Some practical issues for field applications of the RAPID system are also discussed.

An active diagnostic system using built-in piezoelectric actuator/sensor networks was developed for monitoring crack growth in a rocket engine pipe. The diagnostic system combines a sensor network, portable diagnostic hardware and data analysis software which allows for real-time in situ monitoring and long term tracking of the structural integrity of pressure vessels. Experimental data shows that the system can detect a surface crack as small as 4 mm and a through-crack as small as 2 mm in the high pressure engine pipe made of Inconel 718. It was found that the actuator-sensor paths that are most sensitive to crack growth are the ones in which the crack is growing away from, rather than towards the path. This discovery will provide important guidelines for the design of a sensor network for crack detection. It was also observed that the bending mode (equivalent to the A 0 mode in plates) is more sensitive than the longitudinal mode (equivalent to the S 0 mode in plates) to crack growth.