Delamination identification for FRP composites with emphasis on frequency-based vibration monitoring-a review

Abstract

Fibre reinforced polymer (FRP) composite laminates are now commonly used in many structural applications, especially in the aerospace industry, where margins of safety are kept low in order to minimise weight. Timely detection and assessment of damage (in particular delaminations) in composite laminates are therefore critical, as they can cause loss of structural integrity affecting the safe operation of the composite structures. The current trend is towards implementation of structural health monitoring (SHM) systems which can monitor the structures in situ without down time. In this paper, first, the current available SHM techniques for delamination detection in FRP composites are briefly reviewed, including acoustic emission, fibre optic sensors, Lamb wave-, impedance-and vibration-based methods. Among different vibration-based methods, frequency monitoring is the simplest to implement, requiring only single point measurement, and is relatively accurate and reliable, thus it becomes the main focus of present paper. A comprehensive review of frequency-based vibration monitoring is conducted in terms of the various aspects of delamination identification in FRPs through frequency shifts, including review of theoretical models for free vibration of delaminated FRP beams, survey of finite element modelling of delaminated composite structures, summary of experimental modal analyses on FRP composites with delaminations, and inverse algorithms for frequency-based delamination assessment. This paper aims to help the readers to get an overview of the available SHM techniques for monitoring the integrity of FRP composites, with a special emphasis on delamination assessment through frequency-based vibration monitoring.