Keywords: damage detection, structural health monitoring, vibration, delamination, composites.

The development of techniques that evaluate changes in vibration parameters appeared as a way for estimating damage on complex structures, since the dynamic response of a system depends on the stiffness and damping local properties. Such complex structures include those that are built using composite laminate plates, widely used by the aeronautical and automotive industries. Typically, composites are very sensitive to impact that often occurs when laminated plates are used as structural body panels. Impact from foreign objects such as hailstones or mishandled tools may lead to unsafe failures or difficult assessment.

Currently, a number of techniques exist for the identification and location of damage, but because all the techniques have their own specificities, there is not a general algorithm that allows for the resolution of all kinds of problems in all sorts of structures. Also, a considerable investment in the development of instrumentation techniques has been observed in the last years.

To assess (detect, locate and quantify) damage in composite laminate materials is still a challenge, especially if a great number of sensors or expensive equipment is not available. In this article, a technique that solves an inverse problem making use of vibration-based parameters for damage detection, the natural frequencies and damping loss factors, is presented. The methodology uses a reduced amount of conventional sensors with the aim of locating damage on laminated components that are subjected to impacts during service for a low cost. This is possible because the method assumes that the mode shapes remain essentially unchanged. The mode shapes of the structure are required for computing a function, called the plane shape function (PSF).

To obtain the mode shapes and the PSFs with a reduced number of sensors (or budget equipment) and with a fine geometrical description is only possible with the use of finite element (FE) modelling. Thus, some of the computational issues that a FE modeller is concerned about, from the point of view of the user, are also briefly discussed here: from the choice of the element types, approximate value of the optimal number of elements, material behaviour, influence of added mass, etc. Validation and choices are accomplished by comparing the FE models with experimental results obtained using a scanning laser doppler vibrometer (LDV).

The resort to other adequate computational tools is also required. As such, computational tools have been designed and developed with the aim of processing the data in an efficient way. While one of this tools has a narrow field of application (and is still in development), it must be understood that the other, BETAlabR, was conceived to be used in a wider sense, namely in the field of modal identification.

The index was tested with several experimental examples, providing very encouraging results for application of the whole procedure to real structures.

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