Damage imaging of structural components in the field of Non Destructive Evaluations (NDE) and Structural Health Monitoring (SHM) using ultrasonic waves is usually performed by conventional imaging techniques, such as DelayAnd-Sum (DAS), by back-propagating the recorded waveforms to identify locations and size of defects and damages. This technique results in sidelobes and artifacts that worsen the accuracy of the damage identification. Here we propose a novel imaging approach that derives from the well-known technique of Matched Field Processing (MFP), often used in underwater acoustics and seismology. In MFP, the source or damage is located by a matching procedure between measurements (“data vector”) and expected responses (“replica vectors”) computed for each point of the imaging volume. In this work, we apply this matching approach only to selected features extracted from the recorded waveforms. These features, for example time-of-flights or amplitudes, will be selected for multiple modes of propagation of the ultrasonic waves (longitudinal and shear in bulk waves, multiple guided modes in waveguides). By considering multiple features and multiple wave modes, it is possible to increase the performance of this matching procedure, which can be possibly further improved by also combining different signal frequencies and excitation sources in analogy with biomedical ultrasonic imaging. A correlation metric showing high computational efficiency in the image reconstruction process will be tested as matching coefficient. Applications of this imaging approach to a metallic plate with holes and simulated defects will be shown.
This study aims to investigate the mechanical behavior of FRCM composite-strengthened concrete beams using
embedded FBG sensors. FBG sensors were installed both on the tensioned surface of the concrete beam and on the PBO
mesh woven, that had been applied using cementitious mortar without any epoxy resin. Conventional strain gauges were
used to compare results measured from the FBG sensors. Under three-point bending, a marked difference between strains
measured in the concrete and those gotten on the reinforcement net was observed. A theoretical model is presented to
explain the observed discrepancy.
The paper reports some preliminary results obtained on two polymer-coated fiber Bragg grating based humidity sensors
fabricated using poly(methyl methacrylate) (PMMA) or poly(vinyl acetate) (PVAc) as sensing material. The calibration
of the two sensors has been carried out in the 20 - 70 %RH range at three different temperatures (15, 30 and 45°C) using
a two-pressure humidity generator. Repeatability and time response were assessed by means of multiple reversible RH
step inputs. The PMMA-coated sensor prototype displayed overall much better performance when compared to the
PVAc-coated one, although it manifested a somewhat higher temperature sensitivity.
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