Damage identification and assessment in composite structures

The paper presents multidisciplinary methods and techniques oriented towards damage identification and assessment in composite structures. Two types of materials have been used: carbon and glass fibres reinforced composites. Particularly the paper is dedicated to elastic waves propagation phenomenon, scanning laser vibrometry, electromechanical impedance and terahertz spectroscopy. It also includes a variety of techniques being related to diagnostics (damage size estimation and damage type recognition) and prognostics. Selected numerical aspects of the phenomena related to the mentioned methods are addressed. Moreover it covers the main disciplines which are related to above mentioned techniques as piezoelectric sensors and transducers, and signal processing. The signal processing approach is crucial allowing extracting damage related features from the gathered signals. Investigated damage is in the form of mechanical failures as cracks, delaminations, debonding, voids. Also methods dedicated to thermal degradation, moisture and chemical contamination are shown. Presented methods are also suitable for performance of bonded joints assessment. Problem of external factors on investigated methods is also discussed in this paper. The characteristic of each method is summarized by a critical look. The laser vibrometry is a non-contact technique that allows to measure vibrations of structure excited by shaker or piezoelectric transducer. In guided waves-based technique, the interaction of elastic waves with failures can be seen after appropriate signal processing. The electromechanical impedance method is based on electromechanical coupling of piezoelectric transducer with a host structure. Damage in structures caused frequency shift of certain resonance frequencies visible in resistance characteristic. The terahertz spectroscopy method is based on an electromagnetic radiation in the range: 0.1-3 THz. It allows for reflection and transmission measurements. Reflection mode is more feasible for real structures where access to the structure is only from one side. During current research time signals as well as sets of signals creating B-scans have been analysed. Authors address also the problem of adhesive bonding in the case of CFRP samples. Techniques for detection of weak bonds are presented together with signal processing approaches. The reported investigations concern contaminated bonds caused by both manufacturing (e.g. release agent) and in-service contaminations (e.g. de-icer). Promising combination of selected techniques should lead to an innovative approach to ensure safety operation of structures. Local and global methods have been applied and validated by experimental studies. (C) 2016 The Authors. Published by Elsevier Ltd.