A method of determining the fatigue status of a metal-containing specimen (19), where the method utilized is benign to the fatigue properties of the tested specimen, in spite of the presence of an electrolyte (15), and the claimed method and the Electrochemical Fatigue Sensor (EFS) (10) device applying the claimed method can measure fatigue damage at different points during the service life of a metallic structure. The tested specimen is placed in an EFS cell containing an electrode (12) and an electrolyte, which does not cause any degradative effects to the tested specimen and is in contact with the tested specimen. The electrolyte is used with EFS device and method of the invention is capable of being incorporated in a gelling material. A voltage is then applied, or galvanically created, between the specimen and the electrode, and the current is measured passing through the electrolyte during a period in which the specimen is subjected to cyclic stresses. By analyzing the elastic and plastic deformation transient current components the fatigue status and remaining fatigue life of a specimen are determined and/or estimated.