5.1         Axial strain field distribution measurements

5.1.2        Experiment description

A preliminary experiment has been conducted to study non-homogeneous axial stress fields and to apply the reconstruction method presented in Chapter 4. A 12 mm FBG has been placed in the middle of an epoxy sample (Fig. 5-3 left).

An axial stress has been applied on the sample using the device presented in Fig. 5-3 (right). Due to the two notches in the epoxy bloc, the resulting stress field in the grating region will be non-homogeneous. The sample was connected to a strain gauge and an extensometer was placed on half the sample height to measure the average displacement. A measurement of the spectral intensity and the complex OLCR response with 20 mm OPLD resolution were performed before and after embedding, and for four loading conditions (384, 299, 207 and 116 N).

The measurement procedure is presented in Fig. 5-4. First, the FBG is completely characterized before the embedding in the epoxy sample. The epoxy sample is then subjected to an axial loading of about 425 N (obtained by turning the force controlling wheel shown in Fig. 5-3 right). As the stress-applying device is not force-controlled, the relaxations in the sample lead to a force decrease. A relaxation time of about 8 hours was observed and then for a force value of 384 N a complete characterization of the FBG is conducted. Three other measurements have been performed for three loading conditions (299, 207 and 116 N). The relaxation time t between these measurements is 1 hour. A complete unloading was necessary after the measurement at 299 N due to a loss of the force and deformation calibration. Nevertheless, the duration of this unloading and re-loading was only one minute and then, parasitic relaxation can be neglected. After this measurement, the sample is removed from the stress-applying device and a relaxation time of 8 hours is observed. The last measurement is then performed for the 0 N force condition.

Fig. 5-3 Left : Sample used in experiment using a FBG and subjected to an axial stress; Right : Stress-applying device

Fig. 5-4 Loading procedure