This paper presents the design of a carbon nanotube-based sensor to detect crack propagation in aluminum structures underneath composite patching. Initial tests are utilized to determine the correct procedure and materials to properly fabricate a carbon nanotube (CNT) sensor, which is then placed in between a composite patch and the aluminum structure. CNTs have been utilized as sensors in previous studies but only for sensing crack propagation within the composite itself. This study focuses on crack propagation in the base material and is not concerned with the composite. In this application, the composite is only a patch and can be replaced if damaged. This study utilizes both tension and fatigue testing to determine the usefulness of the CNT sensor. The CNT sensor is shown to be effective in giving an indication of the crack propagation in the aluminum. Correlation is done between the propagation length and the increase in resistance in the CNT sensor for tensile testing as the crack width is large enough to obtain an appreciable resistance change.