Thermo-hygric weathering of Carrara marble non-destructively monitored with nonlinear resonant ultrasound spectroscopy
Résumé
Carrara marble is an emblematic marble variety materialising some of the greatest artworks. However, it can develop various alterations as it ages, for instance when exposed to weather conditions. Indeed, previous studies have shown that marble is extremely sensitive to thermal weathering, which can even be enhanced in presence of relative humidity variations. Yet, the effects of prolongated thermal and thermo-hygric cycling have been little investigated so far. In this work, the degradation of Carrara marble samples is followed along such cycles in the mild temperature range (40 – 105 °C) to simulate realistic outdoor exposure conditions. The sample mechanical state during cycling is followed by means of a non-destructive acoustic technique, nonlinear resonant ultrasound spectroscopy (NRUS), through the evolutions of the resonant frequency and of the nonclassical nonlinear parameter. The evolution of both parameters show that Carrara marble is mainly impacted by the very first steps of thermal and thermo-hygric cycling: stiffness decreases and micro-damages increase significantly. Therefore, even a limited outdoor exposure could significantly deteriorate Carrara marble artworks. It also highlights that thermo-hygric fluctuations, which are closer to natural outdoor conditions, have a greater impact on Carrara marble degradation than thermal variations alone. Moreover, micro-damage increases over a large number of thermal and thermo-hygric cycles, whereas the macroscopic stiffness does not vary much after the very first cycles.
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