A strain sensor has been fabricated from a polymer nanocomposite with multiwalled carbon nanotube (MWNT) fillers. The piezoresistivity
of this nanocomposite strain sensor has been investigated based on an improved three-dimensional (3D) statistical resistor network
model incorporating the tunneling effect between the neighboring carbon nanotubes (CNTs), and a fiber reorientation model. The
numerical results agree very well with the experimental measurements. As compared with traditional strain gauges, much higher sensitivity
can be obtained in the nanocomposite sensors when the volume fraction of CNT is close to the percolation threshold. For a small
CNT volume fraction, weak nonlinear piezoresistivity is observed both experimentally and from numerical simulation. The tunneling
effect is considered to be the principal mechanism of the sensor under small strains.

Tunneling effect in a polymer/carbon nanotube nanocompositestrain sensor

As the demand for wind energy continues to grow at exponential rates, reducing operation and maintenance (OM) costs and improving reliability have become top priorities in wind turbine (WT) maintenance strategies. In addition to the development of more highly evolved WT designs intended to improve availability, the application of reliable and cost-effective condition-monitoring (CM) techniques offers an efficient approach to achieve this goal. This paper provides a general review and classification of wind turbine condition monitoring (WTCM) methods and techniques with a focus on trends and future challenges. After highlighting the relevant CM, diagnosis, and maintenance analysis, this work outlines the relationship between these concepts and related theories, and examines new trends and future challenges in the WTCM industry. Interesting insights from this research are used to point out strengths and weaknesses in today’s WTCM industry and define research priorities needed for the industry to meet the challenges in wind industry technological evolution and market growth.

/pdf/wind-turbine-condition-monitoring-state-of-the-art-review-29t23u97yb.pdf

Wind Turbine Condition Monitoring: State-of-the-Art Review, New Trends, and Future Challenges

In-service structural health monitoring of composite aircraft structures plays a key role in the assessment of their performance and integrity. In recent years, Fibre Optic Sensors (FOS) have proved to be a potentially excellent technique for real-time in-situ monitoring of these structures due to their numerous advantages, such as immunity to electromagnetic interference, small size, light weight, durability, and high bandwidth, which allows a great number of sensors to operate in the same system, and the possibility to be integrated within the material. However, more effort is still needed to bring the technology to a fully mature readiness level. In this paper, recent research and applications in structural health monitoring of composite aircraft structures using FOS have been critically reviewed, considering both the multi-point and distributed sensing techniques.

/pdf/fibre-optic-sensors-for-structural-health-monitoring-of-izgdedczqo.pdf

Fibre Optic Sensors for Structural Health Monitoring of Aircraft Composite Structures: Recent Advances and Applications.

This tutorial reviews various noncontact optical sensing techniques that can be used to measure distances to objects, and related parameters such as displacements, surface profiles, velocities and vibrations. The techniques that are discussed and compared include intensity-based sensing, triangulation, time-of-flight sensing, confocal sensing, Doppler sensing, and various kinds of interferometric sensing with both high- and low-coherence sources.

Optical methods for distance and displacement measurements

Aircraft operators are faced with increasing requirements to extend the service life of air platforms beyond their designed life cycles, resulting in heavy maintenance and inspection burdens as well as economic pressure. Structural health monitoring (SHM) based on advanced sensor technology is potentially a cost-effective approach to meet operational requirements, and to reduce maintenance costs. Fiber optic sensor technology is being developed to provide existing and future aircrafts with SHM capability due to its unique superior characteristics. This review paper covers the aerospace SHM requirements and an overview of the fiber optic sensor technologies. In particular, fiber Bragg grating (FBG) sensor technology is evaluated as the most promising tool for load monitoring and damage detection, the two critical SHM aspects of air platforms. At last, recommendations on the implementation and integration of FBG sensors into an SHM system are provided.

Fiber optic sensors for structural health monitoring of air platforms.

Estimation of dynamic structural displacements using fiber Bragg grating strain sensors

The structure shape itself is of great interest for many aerospace applications. For example, the stability of the surface shape of large, high precision or space reflectors is essential for the communication performance. The knowledge of static and dynamic displacements of these structures would provide the possibility to enhance their performance by appropriate countermeasures. During operation, however, the direct measurement of displacements of the whole structure is often difficult. This study investigates whole displacement field estimation using strain measurement and a displacement–strain-transformation approach. The use of fiber Bragg gratings (FBGs) as strain sensors for this application offers the possible implementation of an integrated sensor network including many measurement points within only a few optical fibers. This paper discusses many issues related to the displacement field estimation of a dynamically excited plate using a transformation matrix based on a modal approach. In order to reduce systematic displacement estimation errors due to aliasing, a parametric study was performed and the sensor locations were optimized. Experimental validation was also conducted using a cantilever plate equipped with 16 FBG sensors in an optimized configuration. The estimated displacements showed good agreements with those measured directly from laser displacement sensors.

https://iopscience.iop.org/article/10.1088/0964-1726/18/2/025006/pdf

Displacement field estimation for a two-dimensional structure using fiber Bragg grating sensors

This study investigates shape estimation of rotating structures using displacement–strain relationships and strain data measured with distributed fiber Bragg grating (FBG) sensors. A displacement–strain transformation based on a modal approach is applied to a beam model, and the locations of the strain sensors are optimized by minimizing the condition number of the displacement–strain transformation matrix. In a numerical simulation of a rotating beam, the beam shapes are successfully estimated using the proposed displacement–strain transformation. Experimental validations for transient and steady rotations of the beam are also performed. The estimated shapes of the rotating beam on the basis of FBG signals are compared with the directly captured shapes from photographs taken with a high-speed camera. The results show very close agreement, demonstrating the potential of the proposed shape estimation technique based on displacement–strain transformation using FBG sensors.

http://iopscience.iop.org/article/10.1088/0964-1726/20/3/035011/pdf

Shape estimation with distributed fiber Bragg grating sensors for rotating structures

Nondestructive evaluation of GFRP composite including multi-delamination using THz spectroscopy and imaging

A possible approach to meet the increasing performance requirements of lightweight structures in various engineering
fields is the application of smart structures. One of the functions, which are required, is the observation of the structures'
shape. During operation, however, the monitoring of displacement fields is difficult. This paper discusses the
displacement field estimation of a dynamically excited plate using fiber Bragg grating strain sensors. Using a modal
approach, it is possible to derive a transformation matrix to estimate the displacement field using only a few strain
measurements. To reduce systematic estimation errors due to residual modes, a parameter study was performed and the
sensor location optimized using the condition number of the transformation matrix as an objective function. An
experiment with an optimized sensor configuration including 16 fiber Bragg grating strain sensors was performed to
verify the method and the simulation results.

Lae-Hyong Kang

Papers

Estimation of dynamic structural displacements using fiber Bragg grating strain sensors

Displacement field estimation for a two-dimensional structure using fiber Bragg grating sensors

Shape estimation with distributed fiber Bragg grating sensors for rotating structures

Nondestructive evaluation of GFRP composite including multi-delamination using THz spectroscopy and imaging

Dynamic shape estimation by modal approach using fiber Bragg grating strain sensors