Optical Waveguide Theory

In-service structural health monitoring (SHM) of engineering structures has assumed a significant role in assessing their safety and integrity. Fibre Bragg grating (FBG) sensors have emerged as a reliable, in situ, non-destructive tool for monitoring, diagnostics and control in civil structures. The versatility of FBG sensors represents a key advantage over other technologies in the structural sensing field. In this article, the recent research and development activities in structural health monitoring using FBG sensors have been critically reviewed, highlighting the areas where further work is needed. A few packaging schemes for FBG strain sensors are also discussed. Finally a few limitations and market barriers associated with the use of these sensors have been addressed.

Fibre Bragg gratings in structural health monitoring—Present status and applications

Renewable energy sources have gained much attention due to the recent energy crisis and the urge to get clean energy. Among the main options being studied, wind energy is a strong contender because of its reliability due to the maturity of the technology, good infrastructure and relative cost competitiveness. In order to harvest wind energy more efficiently, the size of wind turbines has become physically larger, making maintenance and repair works difficult. In order to improve safety considerations, to minimize down time, to lower the frequency of sudden breakdowns and associated huge maintenance and logistic costs and to provide reliable power generation, the wind turbines must be monitored from time to time to ensure that they are in good condition. Among all the monitoring systems, the structural health monitoring (SHM) system is of primary importance because it is the structure that provides the integrity of the system. SHM systems and the related non-destructive test and evaluation methods are discussed in this review. As many of the methods function on local damage, the types of damage that occur commonly in relation to wind turbines, as well as the damage hot spots, are also included in this review.

https://iopscience.iop.org/article/10.1088/0957-0233/19/12/122001/pdf

Structural health monitoring for a wind turbine system: a review of damage detection methods

The paper provides a state of the art review of guided wave based structural health monitoring (SHM). First, the fundamental concepts of guided wave propagation and its implementation for SHM is explained. Following sections present the different modeling schemes adopted, developments in the area of transducers for generation, and sensing of wave, signal processing and imaging technique, statistical and machine learning schemes for feature extraction. Next, a section is presented on the recent advancements in nonlinear guided wave for SHM. This is followed by section on Rayleigh and SH waves. Next is a section on real-life implementation of guided wave for industrial problems. The paper, though briefly talks about the early development for completeness,. is primarily focussed on the recent progress made in the last decade. The paper ends by discussing and highlighting the future directions and open areas of research in guided wave based SHM.

https://iopscience.iop.org/article/10.1088/0964-1726/25/5/053001/pdf

Guided wave based structural health monitoring: A review

This article reviews recently developed methods for constitutive parameter identification based on kinematic full-field measurements, namely the finite element model updating method (FEMU), the constitutive equation gap method (CEGM), the virtual fields method (VFM), the equilibrium gap method (EGM) and the reciprocity gap method (RGM) Their formulation and underlying principles are presented and discussed These identification techniques are then applied to full-field experimental data obtained on four different experiments, namely (i) a tensile test, (ii) the Brazilian test, (iii) a shear-flexural test, and (iv) a biaxial test Test (iv) features a non-uniform damage field, and hence non-uniform equivalent elastic properties, while tests (i), (ii) and (iii) deal with the identification of uniform anisotropic elastic properties Tests (ii), (iii) and (iv) involve non-uniform strain fields in the region of interest

/pdf/overview-of-identification-methods-of-mechanical-parameters-3cd298ssf4.pdf

Overview of Identification Methods of Mechanical Parameters Based on Full-field Measurements

This paper describes a method to effectively image structural features and defects using local estimates of the wavenumber of propagating guided Lamb waves at a fine grid of spatial sampling points. The guided waves are rapidly excited at each grid point using a scanning Q-switched laser system and sensed by a single fixed ultrasonic transducer. Through reciprocity, this produces a full-wave-field time history of a virtual wave being excited from the transducer. Using frequency–wavenumber processing, localized wavelength estimates are obtained by isolating each wave mode, narrowband filtering to one or more high-energy frequency bands, and identifying the center-wavelength of a sliding wavenumber band-pass filter that maximizes the energy at each grid point. The approach was capable of producing detailed images of hidden wall-thinning in an aluminum plate and a steel pipe section and local impact delamination in a complicated composite component.

Structural imaging through local wavenumber estimation of guided waves

Progress in frequency selective surface-based smart electromagnetic structures: A critical review

The identification of mechanical parameters for real structures is still a challenge. With the improvement of optical full-field measurement techniques, it has become easier, but in spite of many publications showing the feasibility of such methods, experimental results are still scarce. In this paper we present a first step towards a global approach of mechanical identification for composite materials. The chosen mechanical test is an open-hole tensile test according to standard recommendations. For the moment, experimental data are provided by a moire interferometry setup. The global principle of the identification developed in this paper is to minimize a discrepancy between experimental and theoretical results, expressed as a cost function, using a Levenberg-Marquardt algorithm. This approach has the advantage of having high adaptability, largely because the optical system, the signal processing as well as the mechanical aspects, can be taken into consideration by the model. In this paper we consider different types of cost functions, which are tested using an identifiability criterion. Although mechanically based cost functions have been studied, a simpler mathematical form is finally more efficient. Two different models were tested. The first is an analytical model based on the Lekhnitskii approach. This approach has the advantage of being a meshless solution; however, the results appeared to be partially false due to boundary effects, leading to a second approach, a classical finite element analysis. The resulting identified values are similar to values from classical mechanical tests (within 6%). which, in practice, validates our approach.

Identification of the four orthotropic plate stiffnesses using a single open-hole tensile test

In this paper, fiber Bragg gratings (FBGs) were applied to measure dynamic strains inside a subscale wing under real-time wind tunnel testing. Two re-coated FBGs were embedded in the wing skin. The FBG sensor system includes a wavelength swept fiber laser with a wavelength indicator and fast signal processing modules. The agreement among the three kinds of sensor inside the subscale wing (FBG, electric strain gauge and PZT sensor) was confirmed in the bench test. The optical fiber strain sensors had an excellent resolution (< 5μe) in the time domain and could detect a frequency response up to 100 Hz. Through the wind tunnel test of the subscale smart wing, the flutter was experimentally detected using FBG sensors and their usefulness as an in-flight health monitoring system was demonstrated.

https://iopscience.iop.org/article/10.1088/0964-1726/12/1/317/pdf

Jung-Ryul Lee

Papers

Structural health monitoring for a wind turbine system: a review of damage detection methods

Structural imaging through local wavenumber estimation of guided waves

Progress in frequency selective surface-based smart electromagnetic structures: A critical review

Identification of the four orthotropic plate stiffnesses using a single open-hole tensile test

In-flight health monitoring of a subscale wing using a fiber Bragg grating sensor system