## 2 Sensor Design
### Interface Adhesion  
The characterization of interfacial interaction between the surrounding matrix and the sensor is of fundamental importance for a reliable sensor use. Therefore, the adhesion between the FBG and the patch carrier was investigated by using the single fiber push-out method. The fibers investigated with this method had different coatings as polyimide, Ormocer(r), and acrylate. Uncoated fibers were investigated, too. The arrangement around the integrated fiber was evaluated by optical investigations of polished micrograph sections (Figure 4). In Figure 4, it can be seen that the thick and relatively soft acrylate coating was significantly deformed during the manufacturing process of the patch. It was assumed that during the curing process, the surrounding matrix shrinks and the soft acrylate is pressed to the sides due to residual stresses developed in the material. Furthermore, the glass fiber cladding does not stick properly to the coating. In case of external forces acting at the patch, deformations take place and the fiber may totally lose its contact to the coating and may freely move or slip inside of it like in a tube. In this case of  
Figure 3: Cure cycling of a patch monitored with embedded FBG sensor. debonding, the specimen and patch deformation, and thus the forces, can no longer be transferred into the sensitive Bragg grating. However, in case of bare fibers embedded in patches, a good adhesion between the fiber and the surrounding matrix could be observed (Figure 5). The bare fiber shows perfect connection to the surrounding matrix. Since the bare glass fiber is relatively stiff compared to a soft acrylate coating, it is therefore able to take the forces built up during curing and shrinking without large deformations. In the case of bare fibers, the shrinkage builds up forces pressing, even more so, the surrounding epoxy resin and the bare fiber together, leading to a good connection, even under external forces. Thus, external strain can directly be transferred into the optical fiber without additional deformations of the soft coating between the fiber and the matrix. Figure 4: Micrograph section of an embedded acrylate-coated optical fiber. In this particular case, the fiber is delaminated from its coating. Figure 5: Embedded bare fiber without coating. The fiber has an excellent adhesion to the matrix. For push-out tests, the polished micrograph sections were prepared as 500 \(\upmu\)m thick slices. A flat cone indenter with a diameter of 100 \(\upmu\)m was used to push the fiber out of this slice specimen. During the test, force and the indenter displacement were recorded. The shear stress acting at the fiber surface was calculated from the recorded force data and contact area. According to the optical observations under the microscope, the acrylate-coated fiber showed the worst adhesion, while the embedded bare fiber showed the best adhesion (Figure 6a,b). As shown in Figure 6, the maximum shear stress represents the estimated interfacial strength. For the embedded bare fiber (Figure 6a), it is approximately 34 MPa. The acrylate-coated fiber (Figure 6b) can easily be pushed out of the coating. Thus, the maximum shear stress is only 0.65 MPa. The frictional stress for bare and completely debonded fibers was evaluated as in between 17 \(\upmu\)m to 19 \(\upmu\)m indenter displacement, and is between 15 MPa and 18 MPa. The frictional stress of the debonded acrylate-coated fiber was evaluated between 30 \(\upmu\)m and 34 \(\upmu\)m and is 0.25 MPa. It can be concluded, then, that the low shear strength between the glass fiber and the coating prevents the transfer of strain into the FBG, even when the thick and soft acrylate coating may prevent the FBG from strong lateral forces and local stress concentrations. The obtained results from the push-out test for two other coating types, polyimide and Ormocer(r), are shown in Figure 7a,b. The interfacial shear strength for Ormocer(r)-coated fibers is around 20 MPa and for polyimide-coated fibers about 13.