In this work, a method for monitoring fatigue crack growth in a metal to composite bonded joint based on the strain field is proposed and applied in the framework of a visualization tool based on Augmented Reality (AR). This tool superimposes some virtual objects, which are the data acquired by the sensors and the crack length, directly on top of the specimen under inspection and in real time. By finite element (FE) analyses, a good correlation between the crack tip position and the strain field in a single lap specimen is found and this feature is exploited to monitor the crack length during fatigue tests and to feed the AR system to virtually visualize the crack on the real specimen. An array of electrical resistance strain gauges is bonded to the surface of one adherend. A Matlab function collects values from the strain gauges mounted on the specimen under investigation analyses them on the basis of the FE analysis and finally feeds the AR system. The validation of this process is done by measuring the crack by optical microscope. This procedure is also tested with the use of Fiber Bragg Gratings (FBG) optical strain gauges.