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.

A hybrid piezoelectric/fiber optic diagnostic system has been developed for quick non-destructive evaluation and long term health monitoring of aerospace vehicles and structures. The hybrid diagnostic system uses piezoelectric actuators to input a controlled excitation to the structure and fiber optic sensors to capture the corresponding structural response. The system consists of three major parts: a diagnostic layer with a network of piezoelectric elements and fiber gratings to offer a simple and efficient way to integrate a large network of transducers onto a structure; diagnostic hardware consisting of an arbitrary waveform generator and a high speed fiber grating demodulation unit together with a high speed data acquisition card to provide actuation input, data collection, and information processing; and diagnostic software to determine the condition of the structure. This paper presents key development issues related to the manufacturing of the hybrid piezoelectric/fiber optic diagnostic layer and integration of a highly portable diagnostic hardware. Validation and proof testing of this integrated diagnostic system are also presented.

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.

There currently exists a need to develop a cost-effective, in-service structural health monitoring (SHM) system for determining the initial quality of a bonded repair and assessing the long-term durability of the bonded repair on an aircraft structure. In this paper, a real-time active smart patch system (SPS) based on SMART layer technology is introduced for monitoring the integrity of bonded repairs. First, an overview of the SPS is given for typical metal and composite repairs. To illustrate the capability of the SPS, three applications are presented: (1) monitoring of the cure progress of the bonded repair adhesive, (2) detection of the initial artificial disbond between the composite patch and the metal structure, and (3) monitoring of damage in and around a bonded repair during fatigue cycling. The results show that, through the use of a real-time active SPS approach of using sensors placed in, on or around the repair, the initial quality and long-term durability of the repair can be evaluated and monitored.