An approach for improving SHM systems with GUW detection and embedded sensors by a planar gradient acoustic impedance matching

Abstract

For technical structures, such as airplanes, the service life must be increased to satisfy the requirements for high efficiency in terms of costs and sustainability. Monitoring the system's current health and providing solutions to maintain it reliably is one way to accomplish this objective. A structural health monitoring system, in which actuators induce a guided ultrasonic wave or wavefield monitored by a sensor or a network of sensors, is a standard for thin‐walled structures, which are made of, for example, fibre‐metal‐laminates. The sensor measurements provide information that can be used to determine the respective health state of the system. However, embedding the actuator and sensors in the system typically results in a distortion of the wavefield, for example, mode‐conversion and reflections. The distortion can lead to over‐ or underestimation of the quantity, location and severity of the damage. Due to these false detections, both an unnecessary high effort for retrofitting the structure and a reduction of the structure's technical reliability may be needed. The objective of the current contribution is to reduce the wave‐field distortion/reflections caused by the sensors to prevent or minimise these false detections. To reduce the distortion and reflection of the wavefield, a functionally graded material with an acoustic impedance matching based on a mechanical model is used. In order to control and adjust the acoustic impedance, which is the leading parameter concerning distortion and reflections of waves, tungsten particles were added to the uncured epoxy resin. The design of an interphase between the sensor or its glass housing and the surrounding structure is achieved by this novel approach. The interphase properties are varying in the radial direction to the sensor and are depending on the tungsten particle content in the epoxy resin. In this work, several models regarding the radial distribution of the tungsten particle content and the excitation frequency are investigated. Additionally, the dependence on geometrical influence is also addressed. The approach results, for specific configurations, in (i) reduced reflections from the sensor (less distortion) and (ii) amplified measuring signals of the sensor. Numerous numerical examples demonstrate the benefit of the presented approach.