Fibre-optic sensor for fatigue monitoring

Strain and crack width measurements are primary concerns for health monitoring and damage assessment of large concrete structures in civil engineering. Indeed, the effective detection of damage, its localisation and to certain extent its quantification are essential components to establish an optimised maintenance schedule that may lead to a minimum life-cycle cost. For damage detection especially in the case of large concrete structures, distributed fibre optic sensing techniques is a promising alternative to usual visual inspection which is time consuming, expensive and low reliable. Moreover, fibre optic sensors are small dimensions, low sensitivity to electromagnetic perturbation and they are easy of installation. For over a decade, many works on the development and the application of optic fibre sensing techniques for SHM have been carried out and significant progress have been made in the recent years. However, some technological challenges must still overcome. Indeed, the reliability of fibre optic measurements and the mechanical durability of fibre optic sensors for long term monitoring continues to raise interrogations. Besides the issues of spatial resolution which can affect significantly measurement results especially when the sensing length of the fibre optic sensor is shorter than the spatial resolution, the slippage between core fibre (which is the sensing element) and coating materials influences also the accuracy of measurements. Seveval packages and methods of installation of fibre optic sensor will be tested in order to improve the confidence in strain and crack width. In addition for practical purposes, the fibre optic sensor location will be optimised in accordance to the need of structure engineers (in relation to WP3) and the issue of cross-sensitivity between strain and temperature will be addressed. The experimental work will be carried out on representative samples that can reproduce actual wind turbine foundation and bridges. Measurements will be carried out in a large scale climatic chamber to control and henceforth take into account the influence of temperature. The in situ implementation will be performed on reference structures provided by the stakeholders involved in the project.

Host Institution

IFSTTAR, Bouguenais, France

ESR2

Antoine Bassil

Local academic supervisor

Dr. Xavier Chapeleau (IFSTTAR)
xavier.chapeleau(at)ifsttar.fr
Tel: +33 (0)2 40 84 59 63  

Industrial co-supervisor

Dr. Fabian Kirsch (GuD)
kirsch(at)gudconsult.de
Tel: +49 030 - 78 90 89 - 0  

PhD enrolment

University of Nantes (Graduate school: SPIGA)
PhD Director: Dr. Dominique Leduc (GeM-University of Nantes)
PhD co-Director: Dr. Odile Abraham (IFSTTAR)

Start date: 1st November 2016
Duration: 36 months

Synopsis

Main aim is to improve the understanding of strain transfer between a host material in concrete and an embedded fibre optic for more accurate strain and crack width measurements. This include numerical simulations, experimental tests of different types of fibre optic cables embedded in laboratory size concrete models and further optimisation and implementation on a real structure.

Objectives

Cracking is a primary concerns for health monitoring and damage assessment of large concrete structures in civil engineering. However, it is generaly difficult to locate cracks. Recent publications have shown that embedded fibre-optic sensors can be used for cracking detection and localisation. However, some technological issues must still be resolved and in particularly the quantitative interpretation of measured strain at cracks. It is linked to the strain transfer function of the fiber optic.

The project for ESR2 has three objectives:

  • Assess the strain transfer mechanisms from the host or substrate material to the fiber optic by experimental test.
  • Develop of a realistic model of the strain transfer in presence of cracks for proper evaluation of crack width.
  • Implement fiber optic sensors on real concrete structure.

Tasks and methodology

  • Numerical simulations of the strain transfer function by taking into account geometrical parameters and mechanical properties of fibre-optic.
  • Improvement of models for crack width measurements: combination of different mechanisms (debonding, slippage elasto-plastic and plastic phase of the coating deformation…).
  • Lab investigations: quasi-static and fatigue tests on concrete samples of small dimensions with different types of embedded fiber optic.
  • Optimisation of experimental procedure (integration with other types of sensors used in ESR1 and ESR3), development of guidance documents.
  • Implementation on a real structure: wind turbine.

Results

  • Improvement of the confidence in strain and crack width measurements obtained by fiber optic sensor.
  • Demonstration implementation at a real structure.
  • 2 (accepted) peer-reviewed papers.
  • Successfully defended PhD thesis.

Dissemination

  • Reviewed papers.
  • Presentations at national/international conferences.
  • Presentations at workshops and for potential end-users.

Secondments

  • GuD (Berlin, Germany)
    December 2017, January & February 2018
    Foundation design and assessment, implementation of monitoring concepts for foundations of wind turbine
  • NeoStrain (Krakow, Poland)
    November, December 2018 & January 2019
    Industrial implementation and calibration of fibers optic

Research field

Non-Destructive Testing

Keywords

Fiber optic, monitoring, concrete, cracks, strain transfer function

Key publications

  • X. Chapeleau, T. Sedran, L-M. Cottineau, J. Cailliau, F. Taillade, I. Gueguen, J- M. Henault
    Study of ballastless track structure monitoring by distributed optical fiber sensors on a real scale mockup in laboratory
    Engineering Structures 56, pp1751-1757, 2013
  • X. Chapeleau, L-M. Cottineau, T. Sedran, A. Brisson, S. Kolodziejski, C. Masson, J. Cailliau
    Monitoring of Temperature Gradient Effects on the Mechanical Behavior of a Real Ballastless Track Structure
    International Workshop on Structural Health Monitoring, Stanford University, September 2015
  • J. Waeytens, B. Rosic, M.P. Limongelli, P.E. Charbonnel, E. Merliot, D. Siegert, X. Chapeleau, R. Vidal, V. le Corvec, L-M. Cottineau
    Comparison of model updating techniques using strain sensor outputs to detect damages in a 8 meters post-tensioned concrete beam
    International Conference on Recent Advances in Rehabilitation and Sustainability of Structures, june 2015