Modeling and experiments in transient potential drop measurements for non-destructive evaluation
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Electromagnetic non-destructive evaluation methods are commonly used in inspection and health monitoring of steel structures. The electromagnetic properties of ferromagnetic steel are well known for being correlated with microstructure, stresses and plastic deformation, factors that are central to the integrity of structures. Potential drop methods constitute a type of electromagnetic measurement where surface voltages due to injected electrical currents are measured using point contacts. Potential drop sensors rely on electrical contact with the material and are suitable for use as permanently installed sensors. In transient potential drop measurements, pulsed excitation current is used as a source which results in a transient voltage that contains information on electromagnetic properties and their variation with depth due to the electromagnetic skin effect. This thesis considers physical modeling of the response of transient potential drop sensors, an important fundamental step for their use in health monitoring applications. The modeling is focused on the characterization of materials with applications to health monitoring in mind. Experimental work has been done to validate the models and to demonstrate the use of transient measurements for fast and efficient material characterization. A thorough study has been made on the transient response of fourpoint probes on conducting plates. We have evaluated analytically the transient step response of four-point probe measurements on conducting plates, thereby extending previously published results which were limited to a conducting half space. By including material thickness, the results are more applicable for applications in non-destructive evaluation and can be used to efficiently evaluate the impact of thickness and probe geometry on measurements. Furthermore, we have validated the results experimentally and at the same time demonstrated applications of the results to rapid characterization of the conductivity, permeability and thickness of materials. The impact of non-linear magnetization on transient potential drop measurements has been studied. We find that non-linearity and hysteresis in measurements made on structural grade carbon steel can be modeled satisfactorily by a quadratic magnetization model based on the Rayleigh law of magnetization in terms of the initial relative permeability and the Rayleigh coefficient quantifying the non-linear dependence. Finally, a study was made to evaluate the sensitivity of transient potential drop measurements to plastic deformation. By using cylindrical tensile test specimens, simple physics-based models for the potential drop measurements were used to quantify the magnetic permeability of the specimens. In the steel grade investigated we find that the initial relative permeability decreases linearly with the work hardening, while the Rayleigh coefficient decreases linearly with the square root of work hardening. These results have bearing for the use of potential drop sensors to evaluate the impact of large loads exceeding the yield strength of materials.