Publication
The behaviour of brittle matrix fibre reinforced-composites is dependent of the properties of its constituents (fibre, matrix and fibre-matrix interface) and is affected by the mechanical and environmental loads. The fracture of a unidirectional composite material is the result of various micro-mechanisms (matrix and/or fiber fractures, crack fibre interactions, interface failure, delamination, …) that generally occur simultaneously. Among these, one distinguishes the failure by fibre-matrix debonding and the frictional sliding along the composite interfaces. The initiation and debonding growth are influenced by the specimens' geometry, the loading conditions, the nature of the constituent materials, the residual stress and the fibre-matrix interface. Several works have been carried out that deal with theoretical and experimental aspects of micro-mechanisms of fracture. However, a complete understanding is still lacking due to the difficulties to characterise quantitatively the individual contribution of these different micro-mechanisms. In the recent past, optical Fibre Bragg Grating (FBG) have been used as embedded strain sensors. They are particularly adapted to polymer composites where they can provide accurate internal strain measurements at selected locations. Until today, short FBGs have been often used in uniform loading conditions. In such cases, the grating parameters do not vary along the grating length and when uniform variations in strain and/or temperature occur, the FBG spectral response exhibits a simple shift of the Bragg peak without modification of the spectrum shape. This wavelength shift has been used extensively as a global indicator of internal strain in classical sensing applications. This approach however is not realistic when the FBG is found near damage, i.e. crack, delamination, residual strain field, etc. To achieve non-uniform strain measurements using long gauge FBG sensors, a new Optical Low-Coherence Reflectometry (OLCR-based) technique has been developed and tested in order to study crack-fiber interaction and the interface between the glass fibre and epoxy matrix material during composite damage process. When a long FBGs is subjected to non-homogeneous distributed strains, the grating parameters are position-dependent. Due to the extensive grating length, the optical sensor undergoes substantial non-uniform variation along the fibre direction. Inhomogeneity of the grating is a result of the application of a non-uniform strain or temperature field in the interface region and/or the FBG writing process (manufacture). The coupled-mode formalism provides a mathematical tool to describe the interaction of light propagation. This analysis leads to the introduction of a unique complex coupling coefficient to be determined from the FBG complex impulse response. A novel optical low coherence reflectometry acquisition system, designed at EPFL, allows precise measurement of the FBG complex impulse response with high precis
John Martin Kolinski, Chenzhuo Li, Xinyue Wei