PhD Thesis Defence Presentations - Christos Tapeinos

Abstract

In this work, magnetoelastic sensors (in ribbon and microwire form) were used to detect cracks in concrete beams, as well as to measure the bending vibration frequencies in concrete and metal beams. The dissertation is divided into three parts.

In the first part, an attempt was made to measure the bending vibration frequencies of an aluminum cantilever beam indirectly by using smaller plastic beams printed with a 3D printer. The metal beam was fixed in place using a press and connected to a mechanical oscillator, which in turn was connected to a frequency generator. The plastic beam was attached with tape to the free end of the metal beam. The sensor was placed both on the outer surface of the plastic beam and inside it at various depths. A detection coil and a multimeter were added to the setup. Thus, when the oscillator operated, it excited (forced) the beam at specific frequencies, and when resonance occurred, the sensor generated an electromotive force (EMF), which was detected by the multimeter. In this way, all the natural frequencies of the metal beam were recorded. It should be noted that the measured frequencies differed from those of a simple metal beam due to the addition of the plastic beam. Finally, the same experiment was repeated several times with the sensor placed externally and at various depths inside the plastic beam, in order to determine whether depth affects detection.

In the second part, an attempt was made to detect cracks in concrete beams with free ends using a ribbon-type sensor by identifying the natural vibration frequencies of the beam. The beams, with dimensions 4×4×16 cm³, were made in special molds from commercial cement according to EN 76 196-1 standards in a specialized laboratory. The sensors were attached to the surface of the beams using special double-sided adhesive tape. For supporting the beam, a metal and a wooden base were used for comparison, and signal detection was achieved using a coil. The coil was connected to the microphone port of a computer, and the spectrum analysis software provided the vibration frequencies of the concrete beams after a Fourier transform. By comparing the frequencies of the beam with and without a crack, the crack was detected from differences in the signal.

In the third part, the behavior of the two sensors—ribbon and wire—was studied comparatively. The hysteresis loops and the response of the sensors to the application of an external alternating magnetic field of 1–300 kHz were measured. Subsequently, again in a comparative manner, the bending vibration frequencies and the crack in concrete beams were detected using a setup similar to that of the second part. Finally, the frequencies of a concrete beam were detected with the sensors embedded inside the beam.