Monitoramento de integridade estrutural de materiais compostos sujeitos a impactos empregando a técnica da impedância eletromecânica

Abstract

Nowadays, composite materials have increasingly replaced traditional materials in applications that require improved performance, safety, economy and durability. Nonetheless, such materials exhibit complex damage mechanisms that cannot be easily detected in early stages. Failures can occur during manufacturing, operation or even in the course of maintenance. In aeronautical structures, a major cause of damage is the collision with foreign objects, such as birds, hailstones, stones of working tools used for maintenance interventions. Hence, it becomes very interesting to detect incipient damage provoked by impact in composite structural components. In this context, the aim of this Dissertation is to evaluate the use of a structural health monitoring technique based on the concept of electromechanical impedance for detection and evaluation of the severity of low-energy impact damage induced in composite plates. The electromechanical impedance method is a non-destructive technique which explores the electromechanical coupling of piezoelectric materials bonded to the surface of the monitored structure. The methodology adopted consists in performing impacts in composite specimens using an ad hoc pendulum-type device conceived in such a way that different values of impact energy can be achieved. For each specimen, the electromechanical impedance functions are measured before and after the impact tests, and such functions are used to compute a damage metric which establishes the difference between the impedance functions in the two conditions. The experimental tests are planned in such a way to encompass a relatively broad range of test conditions, as related to impact energy and dimension of the impacting piece as well as the condition of single or multiple impacts. Also, it was investigated the influence of the frequency band in which the impedance functions are acquired and the influence of the position of the piezoelectric transductors, either on the impacted surface or on the surface opposite to the impacted one. The results of the experiments show that the electromechanical impedance technique is efficient in detecting the presence of damage caused by impacts and can distinguish, in a significant part of the cases, between different damage severity levels induced by different impact conditions. Additionally, statistical metamodels were built aiming at establishing functional relations between the values of the damage metric and impact energy for single and multiple impacts. Such techniques have proved to be useful, especially for the inverse evaluation of the impact conditions from measured values of electromechanical impedance.