The manufacture of composite structures with a built-in network of piezoceramics

Abstract

A manufacturing method was developed for integrating a network of distributed piezoceramic actuators/sensors onto laminated carbon/epoxy composite structures. The network of built-in actuators/sensors is used to monitor the health of the host composite structure by acquiring information about the condition of the structure throughout its service life. The manufacturing method applies the printed circuit technique to fabricate a thin flexible layer with a network of piezoceramics. It is used as an extra ply that is either inserted into or bonded onto the surface of a composite laminate to give it actuating and sensing capabilities. This layer that provides the added functionality is referred to as the “SMART Layer” (Stanford Multi-Actuator-Receiver Transduction Layer). Using the developed method, several SMART Layer prototypes have been fabricated and embedded inside carbon fiber composite laminates successfully. Tests were conducted on composite panels integrated with the SMART Layer to validate the design and the manufacturing procedure.

Introduction

Carbon-fiber-reinforced composite structures are widely used in the aerospace industry because of their advantages of having high strength, high stiffness, and low weight. However, composite structures are susceptible to impact damage during service, which can cause internal damages (e.g. delaminations) that are not visible from the surface. Over time, these damages can grow and lead to catastrophic structural failure; hence, composite structures need to be inspected frequently.

Current available methods for composite inspection employ non-destructive evaluation (NDE) techniques, such as X-ray or c-scan, to obtain an image of the internal damage. To perform these NDE procedures, expensive equipment and specialist operators are needed, and the structures have to be taken out of service. This is inconvenient and inefficient for most aerospace applications because it results in vehicle downtime and incurs excessive expense, and for other applications such as satellites, it is simply impossible.

Owing to the drawbacks in the current inspection methods, recent developments in structural health monitoring systems have attracted significant attention. The proposed structural health monitoring system offer potentials in increasing structural reliability, reducing operating cost, and minimizing vehicle downtime. Among the different monitoring techniques, structural health monitoring systems can be roughly divided into two categories based on the type of inputs to the system: passive sensing diagnostics and active sensing diagnostics. A passive sensing system contains only sensors so it takes unknown external inputs to excite the structure and then monitors the structural response. Examples of a passive sensing system include structures with distributed strain gauges [1], fiber optic sensors [2], and accelerometers [3]. An active sensing system contains actuators as well as sensors so it can generate known (controlled) inputs internally to excite the structure and then monitor its response. Although the actuation power of such system is often too small for structural control applications [4], it is adequate for generating ultrasonic waves for non-destructive evaluation purpose. An example of an active sensing system is a structure integrated with a network of distributed piezoceramics (Fig. 1) [5], [6], [7]. This type of system is the focus of study here.

The development of structural health monitoring systems is typically separated into three parts: sensor, software, and integration. Most works [8], [9], [10] done to date have been focused on the development of sensor technologies and application software for interpreting information acquired from structures. However, little work has been done on the integration of sensors with structures, especially for piezoceramic sensors that are needed for the active sensing diagnostic system.