One implementation of the vibro-modulation technique involves monitoring the amplitude modulation of an ultrasonic vibration field transmitted through a cracked specimen undergoing an additional low frequency structural vibration. If the specimen is undamaged and appropriately supported, the two vibration fields do not interact. This phenomenon could be used as the basis for a nondestructive testing technique. In this paper, the sensitivity of the technique is investigated systematically on a set of mild steel beams with cracks of different sizes and shapes. A damage index was measured for each crack. The correlation obtained between the crack size and the strength of the modulation is fairly poor. The technique proved extremely sensitive to the initial state of opening and closing of the crack and to the setup due to the modulating effects of contacts between the specimens and the supports. A simple model is proposed which explains the main features observed and approximately predicts the level of sideband obtained experimentally.

International audienceThis paper presents a new approach for the derivation of fragility curves, named FRAgility through Capacity spectrum ASsessment (FRACAS). FRACAS adapts the capacity spectrum assessment method and uses inelastic response spectra derived from earthquake ground motion accelerograms to construct fragility curves. Following a description of the FRACAS approach, the paper compares the predicted maximum interstory drift (MIDR) response obtained from FRACAS and nonlinear time history analyses (NLTHA) for two case-study buildings subjected to 150 natural accelerograms. FRACAS is seen to represent well the response of both case-study structures when compared to NLTHA. Observations are made as to the sensitivity of the derived fragility curves to assumptions in the capacity spectrum assessment and fragility curve statistical model fitting. The paper also demonstrates the ability of FRACAS to capture inelastic record-to-record variability and to properly translate this into the resulting fragility curves