www.univ-gustave-eiffel.fr

Work Package

Objectives

NCWI tracks early damage in very heterogeneous materials like concrete - see e.g. (Zhang et al, 2012, Hilloulin et al, 2016, 2014) for laboratory studies and (Legland et al, 2017) for an on-site application. The physical idea behind NCWI is to activate, non destructively, the lips of cracks, including closed cracks, with a mechanical pump wave (clapping, sliding) and to detect this activation with coda waves. Very recent works link the new NCWI observables, initially obtained only experimentally, to intrinsic properties of the cracked zones via numerical modelling with the spectral element method (Chen et al., 2017, 2019). Laws connecting dimensions of the cracked area, its actual viscoelastic properties, the sizes of cracks, to NCWI observables were obtained numerically and validated in the laboratory with controlled cracks. Imaging with NCWI is still in its infancy but ongoing laboratory work (Smangin et al., 2020, Chen, 2019) suggests possible solutions for future on-site applications.

The aim of the thesis is to develop and validate a new imaging method, based on NCWI observables, that will locate and characterize closed cracks or micro-cracked areas in heterogeneous environments, that are inaccessible to linear ultrasonic imaging methods. The activation of the crack lips by ambient noise will be addressed. The optimization of the use of embedded sensors (type, number, location) to generate and record the coda wave probe for a given required resolution will also be studied.

Main tasks:
1) Define the characteristics of ambient noise compatible with relevant activation of crack lips through numerical and laboratory experiments
2) Develop an inverse problem with a forward model that uses NCWI observables, in reverberating and non reverberating media. Validate with dedicated numerical and laboratory data using an active pump.
3) Based on 1) and 2), adapt the NCWI imaging technique to passive pump, and design a controlled laboratory experiment that uses simulated ambient noise for the pump for validation.
4) Develop a methodology to optimise experimental set-ups that make use of embedded sensors to improve the resolution.
5) Validate the newly developed semi-passive NCWI imaging method with embedded sensors through a controlled laboratory experiment and on-site case studies recognized as pertinent in 1)

Planned intersectoral, interdisciplinary and international secondments

• Isamgeo, Simone Re, 2 months, M11-M12: Data processing of ambient noise, seismic applications.
• UBRI, Anthony Croxford, 2 months, M18-M19, Building of small-scale polymer samples with embedded source from UBRI and testing.
• ENI, Giuseppe Giunta, 2 months, M26-M27: Performing NCWIi on composites for closed cracks under real ambient noise conditions.