Mr. Gabriel R. Dunkel Profile

Gabriel R. Dunkel

at EPFL

SPIE Involvement:

Author

Publications (1)

Proceedings Article | 16 May 2005 Paper

Residual stress and debonding analysis using a fiber Bragg grating in a model composite specimen

F. Colpo, G. Dunkel, L. Humbert, J. Botsis

Proceedings Volume 5758, (2005) https://doi.org/10.1117/12.599707

KEYWORDS: Fiber Bragg gratings, Epoxies, Composites, Interfaces, Sensors, Glasses, Data modeling, Chemical elements, Optical fibers, Polymerization

Read Abstract +

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.

View contact details

UPDATE YOUR PROFILE

Is this your profile? Update it now.

Sign into your SPIE.org account

Don’t have a profile and want one?

Create an account on SPIE.org

Keywords/Phrases

Search In:

Publication Years