Since the second half of the last century, the meaning of lightweight design has raised significantly. The aviation is a well-known application of lightweight design. Another application is the usage for classical mechanical engineering, e. g. for moving parts or rotary machines. To achieve the goals of lightweight design, the classical methods of structural design, using proven safety factors, are not suitable. In consideration of these safety factors it would result in an overdesigned structure. In order to fulfill the requirements of lightweight design and to ensure the required load capacity, special thin-walled construction elements are often used. One example is the wing spar of an aircraft which consists of the upper and lower flange and a sheet, which is called web. For safety-oriented applications, therefore it is important that the construction is able to withstand the applied loads during the whole period of use. At least there are two possibilities to ensure the safety of structure. The first approach is the preventive maintenance, in which the components are replaced before a critical failure occurs. The second option is a permanent monitoring of the structure. In this case it is important to know the state of the structure, as well as the ability to withstand the current load and the prevailing environmental conditions. This is called Structural Health Monitoring. Thus, a SHM-system monitors the structure permanently to be able to detect, locate, qualify and quantify damages. Consequently, the system provides the information if the structure can withstand the required loads. To monitor a structure there are many different methods known. One of them is the application of guided waves. These waves are transmitted into the structure via coupled actuators. Hence these waves travels along the structure. Furthermore there are sensors, which are mounted on the structure to sense these waves. If the wave travels through a damage in the structure, the damage, e. g. a crack, interacts with the wave. Thus certain parameters of this wave changes and depending on the way how it has changed, it should be possible to detect and asses the damage. This thesis deals with the mechanisms of guided waves and subsequently with their applicability for the detection and assessment of damages. A numerical study of selected types of damages is carried out. After a discussion of the simulation results regarding a defined criteria specimens with preassigned failures are prepared for a measurement to validate the simulations.