The magnetoelastic properties of steel wires are used for measuring a mechanical load in tension. A microcomputer-controlled measuring instrument was designed. The dependence of magnetic behaviour on a previous state is eliminated by the suggested method of commutated measurement of amplitude permeability from saturation to the demagnetizing state. It was observed that the change of permeability with tension was linear during technical saturation of the specimen: low-carbon steel wire used in building industry. Temperature influenced the permeability curve and was found to be positive. The effect decreased with magnetizing field, but it increased under mechanical loading. The coefficient of relative change of permeability with both tension and temperature was compared. The resolution and the reproducibility of the method is described. The dispersion of magnetic properties of specimens has been shown to be less than 1.5% in the determination of resulting stress. Because of the internal stress of specimen evaluation this method allows one to measure the tension of loaded wire, especially in civil engineering applications.

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The EM method is a valuable tool for Civil Engineering for estimation of the real stress in the prestressing tendons, quality control during construction period, calculating the stress loss due to friction and relaxation, long-time monitoring of stress changes due to concrete creep, temperature changes, traffic load etc. Overview of EM technique with practical applications in the Civil Engineering is presented. The new generation of EM measuring devices is based on more than 15 years experience, including health monitoring systems for the nuclear power plant and a large span cable stayed and suspension bridges. Examples of laboratory and field tests are presented, including uncertainty of stress measurement, long-time stability, resolution and reliability.

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