## 2 Sensor Design
### Fatigue Behavior  
The fatigue behavior of the patches and the error resulting due to fatigue effects have to be characterized in order to utilize the patches, not only for measurements of statically loaded structural elements, but also for dynamically loaded ones. Important parameters are the maximum dynamic load that the patch is able to withstand and the corresponding strength of the adhesive bond between patch and specimen. A spectral analysis was not in the focus for the fatigue tests. Since practical users are usually only interested in strain information or, at most, the Bragg wavelength change, spectral changes due to perturbation effects have not been considered as long as the measurement unit could identify the Bragg peak. Referencing with resistance strain gauges is not suitable in the case of dynamic loads, because resistance strain gauges are limited in their fatigue life. Furthermore, resistance strain gauges are designed for measurements of classical materials like steel or concrete. For highly tensile materials like cfrp or gfrp, it is necessary to use appropriate referencing techniques. Most common are optical reference techniques that scan the surface of the specimen, like digital image correlation (DIC) or laser extensometer. In this case, a laser extensometer was chosen as the reference method. Four patches were used in tension fatigue tests. Figure 11 shows the set-up of the tensile fatigue test. The patches were applied to cfrp tensile test coupons and strained in 10 steps up to 10 mm/m. Each step consisted of 10,000 load cycles carried out with a frequency of 3 Hz. The tension fatigue test was used to determine the maximum strain that the patch can withstand. This parameter was then afterwards used to set up a dynamic fatigue test. Half of the analyzed patches had embedded FBGs with acrylate coating; the others were embedded as bare fibers. It became evident during the test that the patches with integrated acrylate-coated FBG endured the tensile fatigue test until the specimen broke (Figure 12). In Figure 13, the strain measured with the patch for maximum and minimum load is shown together with the measured strain of the laser extensometer. The specimen broke after 111,890 loading cycles. The broken specimen revealed that the patch started to debond at its edges. Also small cracks in the adhesive were observed. This behavior is considered as a first indication for a potential failure of the patch itself. Figure 11: Set-up of the fatigue test. The patches are applied to the back and front side of a cfrp specimen. A laser extensometer is used to reference the strain values at the small side of the specimen. Figure 12: Broken specimen after tensile fatigue test. The crack initiation zone lies under the patch, but the patch did not peel off from the surface of the specimen. The difference in strain measured with the patch and with the laser extensometer, as observed in Figure 13, can probably be explained by a change in the gauge factor at higher loads. As observed during indentation tests, the embedded acrylate fiber shows delamination between glass fiber and acrylate coating. With increasing loads, the adhesion between glass cladding/core and coating/matrix decreases, and, consequently, the gauge factor declines. The error due to a change in gauge factor is significant in the case of embedded acrylate-coated fibers, and leads to misinterpretation of the measured patch response at higher dynamic loads, and, consequently, to an error in the specimen strain of about 10%. Because of the extremely bad performance, it was decided not to continue the investigations with patches containing embedded acrylate-coated FBG. The attached patches with integrated bare fibers, on the contrary, failed before the specimen broke (Figure 14). This graph shows the minimum and maximum strain during loading cycles of the patches and the laser extensometer. It could clearly be seen that both patches failed shortly one after the other after around 80,000 loading cycles.