Method and apparatus for measuring parameters in ferromagnetic steel structures, e.g. railway rails and drill pipes, to detect mechanical stress, damage and deterioration. On the monitored structure, devices for magnetization or demagnetization e.g. an electromagnet, will be installed. Two or more electrodes for feeding of a pulse shaped electrical voltage are installed. Across another pair of electrodes, a voltage response signal is measured, and compared with another response signal measured under known conditions or compared with calibration data for the structure. The deviation between these voltages is analyzed to quantify the relative or absolute deviations, and the condition of the steel structure is estimated. The device can measure the transient voltage curves with reference to one or more of the magnetization curves and includes algorithms for analyzing the voltage response curve for determination of mechanical stress and/or fatigue and/or cracks and/or metal loss in steel materials.