Optical Fibre Bragg Grating (FBG) sensors are excellent non-destructive tools for internal strain characterization of composite materials and structures. They can be embedded at selected locations during material preparation to provide accurate in-situ measurements.
In this study, long-gauge-FBGs are introduced in cylindrical specimens of epoxy. This configuration is particularly attractive because it simplifies the study of some relevant phenomena in micromechanics of composites, for instance residual stresses and fracture of the fibre-matrix interface.
Because the matrix epoxy shrinks during the polymerisation process, the optical sensor undergoes substantial non-uniform strain along the fibre. The response of a FBG to a non-uniform strain distribution is investigated using a new Optical Low-Coherence Reflectometry (OLCR) technique developed at EPFL. This method provides a direct reconstruction of the optical period and the corresponding strain distribution along the grating without any a priori assumption about the strain field.
Considering the non-uniform residual strain as a reference state, new Bragg wavelength distributions are obtained for two configurations.
First, a new Bragg wavelength distribution is measured as a function of the depth of circular cracks machined in the radial direction. These measurements lead to the knowledge of (a) the zone of perturbation of the reinforcing fibre on the residual stresses and (b) the effect of the presence of the mechanically induced crack on the residual stress state in the specimen. A finite element modelling of the residual stress field based on an equivalent thermo-elastic approach is also proposed, showing a very good agreement with experimental data.
Second, an interface crack (debonding) between the epoxy and the fibre is introduced by fatigue and monitored using a specifically designed video acquisition system. The induced variations in the FBG response are measured when the fibre is unloaded and then subjected to an axial static load. As preliminary results, a debonding length comparable to the one observed by the video system is found from the Bragg wavelength distribution. Moreover, for the two cases, the measurements clearly indicate that the fibre is in tension in the debonding region while compressive stresses (due to the matrix shrinkage) prevail in the intact part of the specimen.
An optical fiber Bragg grating (FBG) embedded in an epoxy matrix is indubitably subjected to non-negligible residual stresses arising from the cure, especially for a strong fiber-matrix interface. The spectral response of the FBG sensor is clearly influenced by the presence of the residual non-homogeneous strain field along the grating and results in a distortion (chirp) of the reflected spectrum. Direct applications for distributed strain sensing, without tracking the residual field into account, can lead to inaccurate results. In the present work the reflected spectrum of a single FBG sensor embedded in an epoxy specimen at the end of the post-curing process is recorded and characterized using an analytical model which accounts for a distributed residual strain profile along the axial direction of the fiber. In addition an equivalent thermo-elastic problem for the matrix material is considered in finite elemetns simulations of the actual specimen. Both approaches show good agreement for the axial field, with some differences in the radial direction, presumably due to the simplifications introduced by the shear lag simplifications in the adopted analytical model. A level of about 20 MPa of compressive residual stresses is found in the vicinity of the fiber matrix interface.