## 3 Results and Discussion
### Mechanical Testing
For the Xth cycle of the loading the equation is as follows:  
\[\Delta R_{REL(CYCLE\ X)}=\frac{R_{15N(CYCLE\ X)}-R_{15N(CYCLE\ X)}}{R_{after \ implementation}}.\]  
According to the measured data, it is evident, that all types of carbon fiber tows investigated here show piezoresistive behavior. A positive change in measured electrical resistance was observed for specimens, in which the sensor was integrated on the tension side of the specimen (between layers 1 and 2), see Figures 7 and 8. The dependency between the number of cycles and the measured resistance for the sensors can be neglected in the case of material T300 before impact. Impacted sensors made of material T300 also show quite limited dependency between the number of cycles and the measured resistance. All specimens show a change lower than 0.7% in the peak-to-peak relative values between the first cycle and the 200 th cycle, with the exception of specimen T300-32, which shows a change of 7%. The influence of the number of cycles on the measured signal is much more pronounced for the specimens with integrated sensors made of pitch carbon fiber tow 95 A and 70 A. Compressive loading has a bigger impact on the change in the measured signal for both of these materials. For material 70A, we also have data for cyclic loading after impact loading. Figure 7 shows that the bigger the change is in the measured electrical resistance after impact, the greater is the influence of the number of cycles after impact on the change in the measured electrical resistance. Peak-to-peak values for sensors made of the most brittle material 95 A are not given. The reason is that the specimens with sensors of this type were not subjected to a second campaign of cyclic loading, because damage in the contact area was detected for this type of CFS already after implementation, see Section 3.3. After 200 cycles of flexural 3 PB loading, the specimens were subjected to impact loading according to the configuration depicted in Figure 3. According to our hypothesis and in accordance with a previous experimental investigation published in [27], sensors made of a material with lower ultimate elongation should be more sensitive to impact  
Figure 8: Configuration of the 3 PB test and the measured signal from the integrated CFSs: (**a**) results for specimen with CFS between the 1 st and 2 nd layer during 3 PB (measured data of T300-30), (**b**) results for specimen with CFS between the 4 th and 5 th layer during 3 PB (measured data of T300-37), (**c**) results for specimen with CFS between the 7 th and 8 th layer during 3 PB (measured data of T300-16). loading. The relative change in the measured electrical resistance of the integrated sensor after impact should be greater for sensors made of a more brittle carbon fiber tow. This hypothesis was not confirmed for all types of specimens. For the specimens with sensors on the impacted side, the biggest change in electrical resistance was measured for sensors made of material 95 A, which is the most brittle. For the specimens with sensors on the opposite side to the impact, the biggest change in electrical resistance was measured for material 70 A. This may have been caused by the fact that the sensors made of 95A had damaged electrical contacts already after implementation. Based on the measured data, positioning the CFS sensor on the opposite side to the impact seems to be more appropriate (specimens T300-1