# Monitoring the Joint Area of Composite Membrane Materials
## 3 Results and Discussion
### Embroidery and HF Welding with Conductive Sewing Thread
#### 3.3.3 Experimental and Virtual Testing of Textile Strain Sensors in Membrane Welds  
As performed in the case of sensor yarn, the embroidered sensor was tested for cyclic elongation, and the electrical response signal was measured (see Section 2.2.4, Figure 3b). Figure 14 shows the results of shear testing for a welded, sensor-equipped membrane sample. The sensor behaved very similarly to the sensor yarn Y2 used in its unprocessed form. The functionality of the textile sensor was proven even after being subjected to the high-frequency welding process.  
The accurate model of two welded membranes with embedded sensor thread was subjected to tensile stress in a shear load condition, according to the physical test introduced in Section 2.2.4. The reactional strain of the sensor thread in the model can be employed to validate the electrical signal obtained by the sensor, which was used to interpret the strain field in the membrane joint area. The result of the virtually loaded sensor thread is presented in Figure 15. It is revealed that the level of strain is not uniform along the sensor thread. The sensor is elongated along the membrane tensile direction and compressed in the transverse direction due to transversal material contraction.  
Figure 13: Visualization of failing welds for shear loading (intact (**a**); destroyed (**b**)) and peel loading (intact (**c**); destroyed (**d**)).  
The electrical signal, which was measured during the experiment, appears as the sum over the complete sensor length, whereas the virtual investigation yields local results. An increase in electrical resistance in the areas where virtual results indicate a sensor compression is not expected.  
To validate the use of this sensor system, the virtual results of the sensor strain were compared with those of the membrane strain in the membrane joint area in addition to the results obtained from the electrical signal provided by the sensor thread during the physical experiment (see Figure 16).  
The results presented in Figure 16 show the simulated strains in the weld seam and the corresponding strains calculated from the sensor signal that was obtained during experimental testing. Despite a slight discrepancy in the absolute values of the strain, similar curve progressions are evident. Since it was not possible to determine the local strain of the tensile specimen in the seam area at the time of the investigations, the values for this strain were recalculated from the sensor signals. This calculation was based on the K-factor for the sensor yarn determined from the yarn tension tests. It is assumed that the deviations result from this calculation of the strain, based on the sensor signal. So for example, the sensor sections that are transverse to the tensile direction, which are compressed under loading (see Figure 15), could not be taken into account for the calculation, although affecting the sensor signal. For the precise calibration of the sensor model, further tests including  
Figure 14: Investigation of the functionality of an embroidered strain sensor, placed in the weld seam of a textile membrane sample (diagrams below: Detailed view of the first 75 s of the experiment).  
Figure 15: Visualization of strain in a loaded strain sensor (result of the FEA model).  
measurements of the actual strain in the weld seam are necessary. The matching of the investigation results (see Figure 16) with recorded realistic elongation values of the weld seams will provide further insight into the quality of textile-based measurements as well as the simulation model.