Condition-based maintenance refers to the installation of permanent sensors on a structure/system. By means of early fault detection, severe damage can be avoided, allowing efficient timing of maintenance works and avoiding unnecessary inspections at the same time. These are the goals for structural health monitoring (SHM). The changes caused by incipient damage on raw data collected by sensors are quite small, and are usually contaminated by noise and varying environmental factors, so the algorithms used to extract information from sensor data need to focus on sensitive damage features. The developments of SHM techniques over the last 20 years have been more related to algorithm improvements than to sensor progress, which essentially have been maintained without major conceptual changes (with regards to accelerometers, piezoelectric wafers, and fiber optic sensors). The main different SHM systems (vibration methods, strain-based fiber optics methods, guided waves, acoustic emission, and nanoparticle-doped resins) are reviewed, and the main issues to be solved are identified. Reliability is the key question, and can only be demonstrated through a probability of detection (POD) analysis. Attention has only been paid to this issue over the last ten years, but now it is a growing trend. Simulation of the SHM system is needed in order to reduce the number of experiments.

/pdf/structural-health-monitoring-for-advanced-composite-53zg0e4jun.pdf

Structural Health Monitoring for Advanced Composite Structures: A Review

Conventional damage detection techniques are gradually being replaced by state-of-the-art smart monitoring and decision-making solutions. Near real-time and online damage assessment in structural h...

https://journals.sagepub.com/doi/pdf/10.1177/14759217211036880

Machine learning and structural health monitoring overview with emerging technology and high-dimensional data source highlights:

Sensors for process and structural health monitoring of aerospace composites: A review

In recent years, with the development of materials science and architectural art, ensuring the safety of modern buildings is the top priority while they are developing toward higher, lighter, and more unique trends. Structural health monitoring (SHM) is currently an extremely effective and vital safeguard measure. Because of the fiber-optic sensor's (FOS) inherent distinctive advantages (such as small size, lightweight, immunity to electromagnetic interference (EMI) and corrosion, and embedding capability), a significant number of innovative sensing systems have been exploited in the civil engineering for SHM used in projects (including buildings, bridges, tunnels, etc.). The purpose of this review article is devoted to presenting a summary of the basic principles of various fiber-optic sensors, classification and principles of FOS, typical and functional fiber-optic sensors (FOSs), and the practical application status of the FOS technology in SHM of civil infrastructure.

https://www.mdpi.com/1424-8220/20/16/4517/pdf

Recent Progress of Fiber-Optic Sensors for the Structural Health Monitoring of Civil Infrastructure.

Concrete failure will lead to serious safety concerns in the performance of a building structure. It is one of the biggest challenges for engineers to inspect and maintain the quality of concrete throughout the service years in order to prevent structural deterioration. To date, a lot of research is ongoing to develop different instruments to inspect concrete quality. Detection of moisture ingress is important in the structural monitoring of concrete. This paper presents a novel sensing technique using a smart antenna for the non-destructive evaluation of moisture content and deterioration inspection in concrete blocks. Two different standard concrete samples (United Kingdom and Malaysia) were investigated in this research. An electromagnetic (EM) sensor was designed and embedded inside the concrete to detect the moisture content within the structure. In addition, CST microwave studio was used to validate the theoretical model of the EM sensor against the test data. The results demonstrated that the EM sensor at 2.45 GHz is capable of detecting the moisture content in the concrete with linear regression of R2 = 0.9752. Furthermore, identification of different mix ratios of concrete were successfully demonstrated in this paper. In conclusion, the EM sensor is capable of detecting moisture content non-destructively and could be a potential technique for maintenance and quality control of the building performance.

Embedded Smart Antenna for Non-Destructive Testing and Evaluation (NDT&E) of Moisture Content and Deterioration in Concrete.

Fiber-optic sensors cannot measure damage; to get information about damage from strain measurements, additional strategies are needed, and several alternatives are available in the existing literature. This paper discusses two independent procedures. The first is based on detecting new strains appearing around a damage spot. The structure does not need to be under loads, the technique is very robust, and damage detectability is high, but it requires sensors to be located very close to the damage, so it is a local technique. The second approach offers wider coverage of the structure; it is based on identifying the changes caused by damage on the strain field in the whole structure for similar external loads. Damage location does not need to be known a priori, and detectability is dependent upon the sensor’s network density, the damage size, and the external loads. Examples of application to real structures are given.

Structural Health Monitoring in Composite Structures by Fiber-Optic Sensors

A novel approach to detect and locate buckling from distributed optical sensors is proposed in this paper by means of a second derivative analysis of the strain measurements. This methodology demonstrates that non-linear events are prone to significant changes in the full-field strain shape obtained using a dense distributed optical network, whose measurements were processed earlier using a Savitzy-Golay filter. Moreover, these non-linear events can be identified without needing to know the load step value. A buckling test was conducted in a stiffened composite panel representative of a cockpit fuselage skin stiffened by two omega stiffeners plus two fastened metallic frames. A distributed fibre optic network was bonded to the flat panel side in a crooked configuration. The panel was also instrumented using conventional back-to-back strain gauge rosettes and a high-speed-camera to establish a qualitative comparison with the distributed strain contour map. Through the proposed methodology, the detection and location of the global and local buckling were successfully conducted.

Buckling detection of an omega-stiffened aircraft composite panel using distributed fibre optic sensors

Three composite stiffened panels of 1300 mm x 600 mm manufactured by Resin Liquid Infusion were tested until failure. Two of these panels are representative of a wing skin reinforced with stringers and were tested under compression. The third panel, with two stringers and a manhole, was tested under tension and compression loads. In order to perform the structural tests, each panel is instrumented with strain gauges and advanced methods for strain monitoring, such as photogrammetry and optical fiber sensors. The distributed sensing technique has been chosen because it allows a huge number of sensors along the fiber length with high spatial resolution and strain accuracy. Several optical fibers were bonded onto the surface of the stringer covering the full surface and the stringers in order to analyze the strain fields. The strain all along the fiber length was obtained with the OBR and compared with strain gages. In order to identify the failure mode each distributed measurement line is analyzed to localize failure initiation points. From the obtained results it has been shown that the distributed measurement technique can be used to locate the initiation point before fatal failure of the panel. doi: 10.12783/SHM2015/380

Distributed Fibre Optic Sensors for Stiffened Skin Panels Strain Monitoring and Failure Mode Detection

Delamination is the most characteristic damage found in composite materials. Low velocity impacts may lead to delamination onset, likely to occur during manufacturing processes or in operating service life.Furthermore, areas near the free-edge of the structures,as well as ply drop off and integrated reinforcements have demonstrate a high vulnerability to impacts. This paper focuses on the delamination detection based on an embedded or fixed fiber optic network in a composite laminate. The sensor network uses the Rayleigh scattering through an Optical Backscatter Reflectometer (OBR)that it obtains a continuous strain monitoring on a bare opical fiber. For this purpose, several optical fibers were attached on the structure surface close to the free edge of the specimens in order to identify the first-ply failure and the delamination growth. Experimental results were compared to an ultrasonic C-Scan inspection which present a good agreement with the data of sensor network.

Free-edge delamination location and growth montoring with an embedded distributed fiber optic network

An integral composite aircraft cabin had been instrumented with two different technologies: distributed fiber optic sensors (DFOSs) and Fiber Bragg Gratings (FBGs). This structural test aims at strain field monitoring in any part of structure when cabin pressurization is applied in order to simulate flight conditions. The Distributed fiber optic network used is based on Rayleigh scatter ing using an Optical Backscatter Reflectometer (OBR). The OBR provides a large number of strain sensor s with high spatial and strain accuracy with a plain optical fiber. DFOSs prove to be the most suitable technology for this test due to their capability to cover large areas with an important amount of strain data. Moreover, the technology of Fiber Bragg Grating sensors has been also applied for the monitoring of the cabin structural integrity. This type of sensors enable a continuous monitoring during the pressurization tests. 24 FBG sensors in four fiber optic circuits were installed on cabin surface . This test allows to correlate distributed and discrete fiber optic sensors and evaluate their damage detection capabilities. Several static pressure tests were conducted in order to detec t structural damage and static loads at the cabin structure.

Patricia F. Díaz-Maroto

Papers

Structural Health Monitoring in Composite Structures by Fiber-Optic Sensors

Buckling detection of an omega-stiffened aircraft composite panel using distributed fibre optic sensors

Distributed Fibre Optic Sensors for Stiffened Skin Panels Strain Monitoring and Failure Mode Detection

Free-edge delamination location and growth montoring with an embedded distributed fiber optic network

Strain monitoring on a composite aircraft cabin with fiber optic sensors