In the past 25 years, the scientific and industrial communities have made big efforts on the fields of Damage Detection and Structural Health Monitoring (SHM). However, no single approach has proven appropriate for all situations. Composite materials, which are receiving an increasing attention in the aeronautical industry, namely Carbon Fibre Reinforced Plastics (CFRP), are very sensitive to impacts of medium and low energy. Typically, Barely Visible Impact Damage (BVID) will occur, constituting an unsafe failure of difficult assessment. To assess (detect, locate and quantify) damage in this kind of material is still a challenge, especially if a huge amount of sensors or expensive equipments at hand are not used. In this work, a methodology that makes use of a reduced amount of conventional sensors is explored, with the aim of locating damage for a low cost on components that are subjected to impacts during service. This represents a considerable benefit, namely for the assessment of damage in aeronautical components, compared to most methods used today. This work can also give a major contribution to the research community since uncommon approaches will be used to model damage in composite materials, namely the modal damping factor as the main feature for damage localization.

The low specific weight of composite materials, along with their excellent mechanical properties, makes them suitable to be widely used in many modern engineering structures.However, composite materials are quite sensitive to impacts: a specific kind of damage, called Barely Visible Impact Damage (BVID), may occur, constituting an unsafe failure of difficult assessment. In the past years several methods have been developed with the aim of assessing this type of damage. In this paper, a vibration-based technique that combines both the natural frequencies and the modal damping factors as damage sensitive features is tested for locating impact damage in carbon fibre reinforced laminates.The method is intended to be used for locating damage in real laminated composite structures that undergo in-service impacts, such as an airplane's fuselage or wings. A minimum of one response coordinate is the strict requirement during each inspection, because it uses the dynamic global parameters of the structure as damage features. This is possible because the method assumes that, at least for BVID, the mode shapes remain practically unchanged.The theory is summarized and the method is tested with experimental examples where damage is introduced at different locations. Additionally, the hypothesis that different damage morphologies on composite materials have different contributions to the damage features is addressed.

When considering structural loss in vibration problems, the Young's modulus is a complex value that may be defined in the Argand plane as the vectorial sum of the real part of the Young's modulus itself and the loss modulus. By performing simple quasi-static forcedisplacement tests in a universal testing machine it is possible to experimentally measure the complex elastic modulus of a material, even if the loss factor is very small. In this work, the authors present the experimental results of the complex Young's modulus measurements on a specimen of a quasi-isotropic carbon fibre reinforced laminate, considering the constant hysteretic damping model. The setup is fully described and results are presented in detail, giving a special attention to the experimental problems so that this work can become a useful contribution for further studies.

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