Keywords: composite, impact damage, embedded fibre Bragg grating sensor, finite element model, dynamic strains, natural frequencies, identification.

The detection of an impact event and characterization of delamination damage in fibre reinforced polymer structures are a major issue in aerospace, naval and civil applications. Vibration based structural health monitoring methods that allow detection, characterization and, to a certain extent, localization of damage, based on changes of the natural frequencies of a structure, have been used over the past several years. However, these methods are based on accelerometers and optical techniques for the acquisition of experimental data. Embedded fibre Bragg gratings (FBG) allow new approaches in the structural monitoring of fibre reinforced composite structures. The ability of acquiring dynamic strains at high rates of up to 100 kHz makes FBG sensors appropriate devices for monitoring strain signals during an impact event and for experimental modal analysis [1].

In this paper, the strains from embedded fibre Bragg grating sensors are used to measure the change of natural frequencies of a composite plate due to an impact event. The measurements are compared to modal analysis based on velocity response measurements using a laser Doppler vibrometer. The relation of these eigenfrequency changes to the damage area is studied by carrying out a series of impact experiments at different energies. Numerical models, based on detailed three-dimensional images of the damaged plates obtained from high resolution x-ray computed tomography, are used to study the influence of impact damage to the natural frequencies. Based on these findings a homogenized damage model employing two damage factors penalising the transverse shear moduli is developed.

The experimental tests show that for selected mode shapes the natural frequencies systematically change with increasing impact energy. Modes where the damage zone is located on a nodal line are mostly affected. The detailed finite element model, including the delaminated interfaces, confirms that the change of eigenfrequencies can be attributed mainly to delamination damage. The relation between the change of eigenfrequencies and the size of the delamination damage can be thoroughly reproduced. Moreover the applicability of a homogenized damage model for the prediction of the relative eigenfrequency change is demonstrated. The damage factors for the through-the-thickness shear moduli are identified to be approximately 90% for the observed damage pattern. Based on this model, the eigenfrequency changes of a composite plate can be predicted for any given damage size. This homogenized damage model can then serve in an inverse numerical-experimental identification technique of the equivalent size of a damage zone. Based on these developments, the use of embedded FBG sensors can lead to an in-situ damage detection and characterization method.

1

J. Frieden, J. Cugnoni, J. Botsis, Th. Gmür, D. Coric, "High-speed internal strain measurements in composite structures under dynamic load using embedded FBG sensors", Composite Structures, 92(8), 1905-1912, 2010. doi:10.1016/j.compstruct.2010.01.007

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