In-service detection of material failures based on an integrated sensor network is the main goal of structural health monitoring (SHM). In this paper, we report on a millimeter-wave Doppler radar sensor operating at 102GHz that enables accurate and non-contact mechanical vibration sensing. Thanks to the unique properties of mm-wave radiation it is possible to penetrate through many non-conducting materials. In contrast to Laser-Doppler-vibrometry this modality enables non-contact vibration measurements behind barriers such as glass-fiber-reinforced plastics, sandwich structures, foams, ceramics etc. Experimental measurements are shown for damage detection and non-linear motion sensing. The unique properties of mm-wave radiation promise great potential for a multitude of future SHM-applications.