Composite materials are nowadays used in many technical fields. Blades of wind power plants or aircraft parts, for example, are made out of these materials. In order to detect possible cracks or fractures, however, an immense effort is necessary. Conventional methods of material testing such as ultrasound, X-ray or thermography are very time-consuming, if they are possible to employ whatsoever. Other methods such as the use of strain gauges or glass fiber based systems can take measurements permanently, but they can only indirectly detect a fracture in the material. The invention is a novel, continuous measuring method, which is also suitable in particular for large-area applications such as, for example, wind turbine blades. The sensor consists of a braid of wires whose breaking behavior corresponds to that of the fibers in the composite material. The crossing points are connected via diodes. An electronic evaluation unit can switch between electrical paths through specific diodes so that the defective part can be identified in the breaking event.