Damage detection in bonded repairs using piezoceramics

Abstract

A smart structure is one that senses its internal state andexternal environment, and based on the information gained responds in a mannerthat fulfils its functional requirements. The primary advantage of movingtowards smart structures technology is the potential cost benefit ofcondition-based maintenance strategies and the prospective life extension thatmay be achieved through in situ health monitoring. The monitoring ofoperational health and performance, and diagnosis of any faults as they occur,is a relatively new concept that is being developed globally to provideadvantages of safer , more reliable and affordable structures. Healthmonitoring can be achieved by positioning (embedded or surface mounted) sensorsystems on a structure to measure those physical parameters that areinformative with respect to the state of the structural health. Informationrelating to the severity and location of damage, as well as to the nature ofthe loading is of obvious importance to this endeavour.In the aerospace industry bonded composite patches are increasingly being usedto extend the operational life of ageing aircraft. The application of bondedcomposite patches to repair or reinforce defective metallic structures iswidely acknowledged as an effective and versatile procedure. Such patches havebeen successfully applied to the repair of cracked structures, to thereinforcement of components subject to material loss due to corrosion damageand as a general means of stress reduction through the provision of asupplementary load path. However, certification requirements mandate the needfor a methodology for monitoring the damage state of both the defectiveunderlying structure and of the repair. In this case the concept of smartstructures can be used to detect damage in the repair itself as well asmonitor damage growth in the parent structure.This paper reports on the development of a `perceptive repair' or `smart'system which will provide information on the in-service performance of therepair and the associated structure. In this respect, this paper focuses onthe detection of disbond in the adhesive layer between adhered and themetallic parent structure. One of the criteria of this smart system is that itmust be economical, reliable and, preferably, self-powered. To this end, itwas proposed that piezoceramic of piezoelectric material be utilized becauseof their ease of application. These materials were chosen because they can beused both as a actuator and as a sensor. This paper presents a set ofnumerical investigations performed to highlight the viability of using thismaterial system, and the associated signal analysis that can be employed todetect the presence or the development of disbonds in the adhesive in a bondedrepair situation.