5.1         Axial strain field distribution measurements

In this section, we describe the experiments conducted on a FBG located at the center of an epoxy sample subjected to non-homogeneous axial stress. First, the FBG behavior under axial stress is presented in section 5.1.1. The experiment is described in section 5.1.2. The OLCR measurements for different loading conditions, the derived spectral response and reconstructed complex coupling coefficients are presented in sections 5.1.3 to 5.1.5. From the complex coupling coefficient, an effective Bragg wavelength distribution l_eff is defined (l_eff(z) = 2n₀L_eff(z) where L_eff(z) is the effective period defined in equation (3-19) and n₀ is the effective refractive index of the fiber), which is used to calculate the axial strain distribution. A comparison with a finite element calculation is finally presented (5.1.6).