Varying ohmic loss in the winding of electrical machines, which are operated atvarious operating points, results in temperature changes during operation. Particularly, when the temperature is varying dynamically, the insulation system suffers from repeated thermal-mechanical stress, since the thermal expansion coefficients of the insulating materials and copper conductors are different. For the appropriate design of an insulation system, the effect of thermal-mechanical stress must be known. In the present work, motorettes are subjected to repeated thermal cycles. The expected lifetime is estimated and compared to the life time which is achieved by applying a lifetime-model which only considers thermal aging while ignoring thermal-mechanical stress effects. In addition, the hotspot temperature is simulated, the lifetime at the hotspot is estimated as the worst case. As expected, the results indicate that the thermal-mechanical stress plays a significant role during dynamic thermal aging of the winding insulation system. To better understand the thermal-mechanical stress effect, the resulting thermal-mechanical stress in a single wire is analyzed by the finite element method. A preliminary analysis of the aging mechanism of materials due to cyclic thermal-mechanical stress is performed with the theory of material fatigue.