# Monitoring the Joint Area of Composite Membrane Materials
## 1 Introduction  
Textile membranes are made of a textile carrier structure and a polymer surface coating. These composite structures offer an enormous potential for the specific demand-oriented adaptation of functional membrane properties, thus tapping into a broad application spectrum. Due to their low weight, favorable mechanical properties, and suitability for highly productive manufacturing (weaving, coating) and processing (welding) methods, textile membranes are predestified for large-surface and weight-critical applications [1]. Hence, they are commonly employed as load-bearing components of textile architecture, e.g., unsupported convertible roofs of halls or stadiums, temporary buildings, large-volume consumable media storage [2], as well as mainsails in sailing [3]. As a result of their large dimensions, these textile structures typically do not consist of a single panel but of several welded sheets. Thus, the coating material is melted locally in the joining zone, and sheets are joined under pressure. The coating material in the joining zone is solely responsible for the transmission of force between the coated woven sheets [4]. It is well known for a long time that, within the first four years, successive degradation of the seam strength occurs, especially in the area of membrane joints [5; 6; 7]. In industrial sectors such as the automotive industry [8], the wind energy sector [9], and civil engineering, cost- and safety-related aspects, including in situ sensor solutions for the structural monitoring of compositematerials, are increasingly important. This results in an emerging demand for the application of textile architecture and textile membrane constructions. The state of the art in the monitoring of textile membranes consists in regular manual inspections of relevant structures. The use of appropriate special equipment is expensive, and time-consuming, while only providing a snapshot of the membrane condition [10]. Attempts to enable continuous membrane monitoring by applying classical sensors, such as strain gauges, to the membrane surface have repeatedly failed due to a lack of fatigue strength. In addition, there are serious doubts regarding to the weldability of such sensors. The same also applies to the concept of printed strain sensors, such as is pursued in [11; 12]. Another disadvantage of all sensor systems applied to the membrane surface is that they are not directly attached to the load-bearing textile core structure of the membrane. Such a direct integration into the carrier structure, however, can be expected to provide more detailed information about the strain state of the membrane elements relevant for its load-bearing capacity. For this reason, systems for strain measurement which can be directly integrated into membranes in the textile manufacturing process are being developed at the Institute of Textile Machinery and High Performance Material Technology (ITM) of the Technische Universitat (TU) Dresden. In the past, the carbon fiber-based detection of mechanical strain conditions has been successfully implemented in composites, so that early failure detection for carbon fiber-based products has been enabled. A correlative and reproducible electrical response to the mechanical strain state in the target area could be measured [13]. In [14; 15; 16], corresponding possibilities for component monitoring of fiber-reinforced plastics (FRP) are presented. However, their use in textile membranes poses higher-quality demands to the sensor yarn. A strain range of up to 15% must be covered, which is considerably larger compared to FRPs. In addition, the joinability of the sensor-equipped membranes based on high-frequency welding must be guaranteed. In the study presented in this paper, development efforts toward the early detection of degradations in textile membranes were carried out.