# Monitoring the Joint Area of Composite Membrane Materials
## 3 Results and Discussion
### Embroidery and HF Welding with Conductive Sewing Thread  
Generally, it is not recommended by machine manufacturers to place any type of metallic object in the welding area during HF welding. Nevertheless, several tests were successfully conducted by placing conductive sewing threads between textile membranes and by carefully selecting the welding parameters, yielding favorable results.  
To embroider the sensor thread on the membrane, the embroidery with a lockstitch seam had to be modified. Unlike in the case of standard lockstitch, the crossing points of the needle and bobbin thread should not be positioned in the middle of the embroidery ground, i.e., the membrane material. Instead, the sensor thread should be positioned straight on the membrane surface to be fixed by the second embroidery thread system. This is necessary for three reasons:  
1. Sensor functionality: If the sensor thread would be placed in loops, as it would be the case for the classic lockstitch (see Figure 2, Figure 8 (left)), the resulting thread reserve would impede the detection of a clear correlation between membrane and sensor thread elongation.
2. Weldability: A clear physical separation between the welding electrode and any metallic objects in the welding area is a decisive factor for processability; details will be provided in the following paragraph.
3. Embroidery: The sensor yarn tears very easily when exposed to strong mechanical stress, which includes the passage through the membrane.  
The positioning of interlacing points can be influenced by selecting suitable embroidery parameters. When processed, the tension of the sensor yarn must be significantly higher than the tension of the holding thread to ensure an undulation-free application. As the bobbin thread system is subjected to  
Figure 8: Influence of the pre-tension of thread systems on the position of crossing points in a double lockstitch seam; needle thread (NT) in red, bobbin thread (BT) in blue.  
Figure 7: Investigations on the suitability of yarns (**a**) Y1 and (**b**) Y2 for strain measurements in the range of 0–16% elongation (diagrams below: Detailed view of the first 500 s of the experiment).  
less mechanical load compared to the needle thread system during the embroidery process, the sensor yarn was employed as bobbin thread (see Figure 8).  
When processing the weld, direct contact between electrode and the conductive sewing thread must be avoided. To ensure a stable HF welding procedure, it is essential that the electrode is only exposed to the stitch line made by the nonconductive needle thread (see Figure 9). Any direct contact of the metallic sewing thread with the electrode would result in the ignition of sparks, and subsequently, material damage. No thickening of threads should be present along the stitch line. The probability of spark ignition increases if the surface below the electrode is uneven. So all stitch lines have to be even and flat. The pressure exerted by the electrode during the welding process should be adjusted so that potential contact situations between electrode and conductive threads can be safely excluded as the electrical current is to be kept constant during the welding process. It is, however, possible to increase the pressure on welded seams during cooling. The width of the welded seam was 20 mm.  
Finite Element Analysis of the Strain Conditions at Membrane Welds and Textile Integrated Strain Measurement