Sensing device for the non-destructive evaluation of steel structures or components

A portable, easily operable magnetic sensing device for the non-destructive evaluation of steel structures or components which have undergone an extended period of service in order to determine the extent of micro-structural degradation or residual stress within said structures or components which comprises the combination of a probe head adapted to be located proximate the structure or component to be evaluated and signal processing means, characterised in that said probe head comprises a sensing core of nonocrystalline Fe-based material having a first pair of coils wound thereabout such that in normal condition when current flows through said first pair of coils, each half of the sensing core is symmetrically magnetised and said signal processing means comprises a toroidal ferrite core having a second pair of coils wound thereon, one of the terminals of each of said first pair of coils of the sensing core being earthed while the other terminals thereof are each connected respectively to one terminal of each of said second pair of coils on said toroidal core, the other terminals of the second pair of coils being joined together and connected to an excitation source capable of generation of a frequency of 5 kHz, the combination of said first and second pairs of coils constituting a primary, coil, the sensed signal having different harmonics which emanates from said primary coil when the probe head is placed proximate a structure or component being evaluated in order to assess the micro-structural degradation or residual stress thereof being transmitted to the input of a secondary coil mounted on said toroidal core, the output of said secondary coil being connected to a filter means active to suppress only a first harmonic signal and to permit other harmonic signals to pass onward for measurement thereof.