Long-range, torsional guided waves generated in pipes using magnetostrictive sensors (MsSs) have great potential for applications to the structural health monitoring (SHM) of hard-to-inspect pipes. This paper reports an improved MsS technique (when compared to related techniques currently used for the NDT of pipes) that uses polymeric magnetic tape material that is suitable for use in a variety of industries as an SHM tool for pipes. Improvements include increased efficiency, reduced cost and increased long-term survivability of the sensor system. Transduction efficiency was increased by reducing the sensor eddy current losses and by using a field concentrator strip. For long-term monitoring, a low-cost magnetic oxide based MsS material (video recording tape) having the required magnetic properties was used. The MsS strips were oriented to generate non-dispersive torsional guided ultrasonic waves that propagate long distances with minimal mode conversion. Further, considering both safety and long-term survivability of the sensor, low-power ultrasonic instrumentation was developed and tested. Measurements reported here demonstrate the sensitivity of this sensor to both radial notches (saw cuts) and drilled holes. Results also show that magnetic anisotropy of the strip plays a role in generating torsional waves. It is envisioned that results obtained from the present study will significantly enhance the ability to monitor the long-term structural health of piping systems.

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A simplified experimental technique to measure the acoustic radiation induced static strain during the longitudinal acoustic wave propagation in solids is proposed. Experiments have been carried out to extract the static displacement (dc) component without resorting to electronic filters, as in the case of previously reported experimental measurements. This prevents the influence of the filter time response characteristics on the measurement. The dependence of the static displacement amplitude on the square of the fundamental amplitude of the input wave, its being independent of the burst width of the tone burst, and its abnormal variation at low input amplitudes are reported.

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Nonlinear ultrasonic (NLU) harmonic generation system was used to characterize the fatigue damage in a flat hour-glass, high strength Al–Cu–Zn–Mg alloy, AA7175-T7351 specimens. Experiments were carried out to introduce controlled levels of fatigue damage under constant amplitude loading to determine the NLU response using surface acoustic wave (or Rayleigh mode) at regular intervals of fatigue life. The NLU parameter (A2/A12) plotted as a function of percentage of fatigue life shows two peaks for all the samples tested, independent of the amplitude of fatigue loading. The first peak appeared between 40%–50% of fatigue life and the second peak between 80%–90% of fatigue life. Among the two flat surfaces of the specimen, a higher nonlinearity response was observed on the surface which had the first crack initiation. The appearance of two peaks in the nonlinear response during fatigue damage progression is explained based on the dislocation dynamics and dislocation-crack interaction present in the specimens during the fatigue process.