In-service Structural Health Monitoring Research Articles

Life test result on 4 channel VCELs chip used in 28Gb/s data transfer in space application

For aeronautic applications, in-service Structural Health Monitoring (SHM) is a now established as a key technique to assess the performance and the integrity of composite aircraft structures for real-time in situ monitoring. For space industry, the Assembly Integration and Test including Electrical Ground Support Equipment is an essential tool enabling a satellite or space launcher developer to integrate and validate the electrical functions of its spacecraft before the launching process. In addition, satellite operators and worldwide SatCom manufacturers are willing to improve the on-board processing capacity, functionality and flexibility. In this context, opto-transceiver technology becomes a major building block regarding the highlighted aeronautic, space and ground telecommunication needs.In this context, the paper deals with the robustness evaluation of 850 nm GaAs VCSEL chip on Submount (CoS) used to 4 channel transceivers at 28 Gbit/s. The reliability investigation has been evaluated for two different manufacturers of VCELs and we perform the robustness of the technology regarding the final environment to define the limit usable for space applications. In this paper, we present the first technology of VCSELs. The main failure mechanisms have been defined to establish degradation laws associated. This last action is useful to define the ad equation between technology chosen and mission profile. These results are using for RYTHMS (ReliabilitY of opto-Transceivers for Health Monitoring Systems) Clean sky 2 project.

Study on damage identification of High-Speed railway truss bridge based on statistical steady-state strain characteristic function

The localization and quantification of damage in bridge structures have always been a challenging problem, especially for high-speed railway truss bridges with intricate structural forms and complex load conditions. To address this issue, this paper proposes a novel damage indicator based on strain characteristic functions, and integrated it into an automated framework for real-time damage detection which is particularly suitable for in-service structural health monitoring applications. To evaluate the effectiveness of this approach, a study is conducted using a finite element digital twin model to simulate the structural responses at critical areas of high-speed railway truss bridges under train-induced loads. The proposed damage indicator of the model was used to investigate and evaluate different types of bridge damage, reflecting both the location and severity of damage in high-speed railway truss bridges. The results suggest that the proposed damage indicator, when used with a distributed sensor system, shows promise in accurately locating and measuring damage in high-speed railway truss bridges. In addition, for the bridge health monitoring system with sparse sensor distribution, this indicator can provide a probability assessment of various damage conditions of a high-speed railway truss bridge. This information could potentially assist in the process of conducting in-service maintenance of a high-speed railway truss bridge.

Bonding quality monitoring of Carbon Fiber Reinforced Plastics bonded structures by fiber Bragg gratings

This paper deals with a new method for the continuous and remote monitoring of the bonding quality of Carbon Fiber Reinforced Plastics (CFRP) bonded structures. The proposed approach is based on the use of Fiber Bragg Gratings (FBGs) sensors and relies on the dependence of their temperature-induced strain sensitivity, generated by local inductions of (even small) thermal changes, on the quality of the bonding. Bonding composite laminates is a structural solution often adopted in aeronautics. The presence of disbonded areas during production is normally investigated with nondestructive tests (NDT), generally based on ultrasonic techniques (UT). However, these techniques do not always allow to detect areas with poor bonding quality (insufficient adhesion forces). To simulate such situations, some specimens were used for the experimental tests consisting of CFRP flat skins partially overlapped in a bonded area. To reproduce various bonding conditions, the specimens were bonded with different adhesive laying techniques. Experimental results here reported demonstrate that the FBGs thermal sensitivity can change significantly depending on the quality of the bonding in proximity of their position and can increase up to ∼ 50 % if the FBG is positioned in proximity of areas of the composite laminates presenting a disbonding or defects. The proposed monitoring approach is useful for in-service structural health monitoring of aircraft, supporting the NDT methods usually included in maintenance manuals.

Damage detection in non-planar carbon fiber-reinforced polymer laminates via electrical impedance tomography with surface-mounted electrodes and directional sensitivity matrices

Carbon fiber reinforced polymers (CFRPs) have emerged as promising alternatives to traditional metals and alloys in weight-conscious applications such as aerospace due to their superior strength-to-weight properties. However, these materials are vulnerable to complex and difficult-to-predict sub-surface damages. Therefore, damage detection modalities are important for assuring the safety of CFRP-based structures and components. Particularly for in-service structural health monitoring (SHM), it would be desirable to utilize an inherent property of these materials, such as electrical conductivity, as a means of detecting and localizing damages. In this respect, electrical impedance tomography (EIT) is a promising SHM modality. Studies to-date involving the application of EIT to CFRPs are limited due to the challenges associated with high electrical anisotropy. Furthermore, most studies have only considered flat laminates with unrealistic edge-placed electrodes. In this paper, we advance the state of the art by studying the application of EIT to non-planar CFRP geometries using surface-mounted electrodes. Additionally, we present a modified EIT sensitivity matrix formulation which intrinsically incorporates the electrical anisotropy of the material by forming the sensitivity matrix via three approaches – with respect to i) a scalar multiple of the conductivity tensor, ii) the in-plane conductivity, and iii) the through-thickness conductivity. It was found that both through-hole and impact damages can be adeptly identified with a combination of surface-mounted electrodes and a sensitivity matrix formed with respect to either a scalar multiple of the conductivity tensor or the in-plane conductivity. The results presented here are an important step towards the transition of EIT-based diagnostics to actual CFRP structures.

Fatigue life estimation of structures under statistically and spectrally similar variable amplitude loading

A new fatigue life prediction framework provides an improved life prediction under statistically and spectrally similar irregular variable-amplitude loading for a notched beam model. It enables the cumulative damage rule to account for the load sequence effects by modifying the probability density function of the stress-amplitude history through (1) identification of overloads based on the rainflow-counting algorithm; (2) analytical characterization of the overload retardation effects; and (3) correction to the damage rule using overload amplitude rate characterization. The fatigue lives estimated from experimentally acquired and synthetically generated load-time histories are compared to the ones generated from simulations that qualitatively reproduce the fatigue lives in physical experiments. The notable improvement in prediction accuracy outperforms the Palmgren–Miner’s rule and power-spectrum-based life estimation. The demonstrated application to the field acceleration data substantiates its use for in-service structural health monitoring and damage prognosis. This framework does not require a priory knowledge of the applied load, and it can be applied to other engineered structures with known structural and defect properties.

A Brief Overview and Future Perspective of Unmanned Aerial Systems for In-Service Structural Health Monitoring

A Brief Overview and Future Perspective of Unmanned Aerial Systems for In-Service Structural Health Monitoring

Analysis of barely visible impact damage severity with ultrasonic guided Lamb waves

Barely visible impact damage is one of the most common types of damage in carbon-fibre-reinforced polymer composite structures. This article investigates the potential of using ultrasonic guided Lamb waves to characterise the through thickness severity of barely visible impact damage in thin carbon-fibre-reinforced polymer structures. In the first step, a laser Doppler vibrometer was used to capture the full damage interaction of the wavefield excited by a piezoelectric actuator. Damage-scattered wavefield for four different severities were studied to find the best parameters for characterising the severity of damage. To reduce the overall acquisition time and size of data collected using the laser Doppler vibrometer, the measured signals were reconstructed from a singular broadband chirp response using a post-processing algorithm. From the full wavefield analysis obtained at a wide range of toneburst frequencies, the results showed that barely visible impact damage severity could be characterised using ultrasonic guided Lamb waves and that the mode, dominant at lower frequencies, gave better results than the mode. In the second step, the parameters for characterising the damage severity were applied to a sparse network of transducers as an in-service structural health monitoring methodology. The damage was successfully detected and located. In addition, the transducer path close to the predicted damage location was utilised to successfully quantify the damage severity based on the proposed damage index.

A smart multi-functional printed sensor for monitoring curing and damage of composite repair patch

A novel multifunctional diagnostic sensor is developed as a cost-effective, in-service structural health monitoring system for determining the initial quality of curing of a bonded composite repair patch and assessing its long-term durability on composite structure. The proposed multi-functional sensor technology involves the creation of a ‘tailor-to-order’ 2D conductive patterns onto step-sanded repair surface of composite repair patch using inkjet printing. In employing this methodology, bondline quality during curing and in service was successfully assessed via impedance spectroscopy and resistance change measurements, respectively. The ability of this technology to effectively monitor the integrity of the bondline and the extent of damage in real-time was investigated by subjecting the scarf-repaired carbon fiber reinforced polymer panels to 3-point bending fatigue and low-velocity impact tests. The obtained results were compared with those of transient infrared thermography and ultrasound inspection techniques, thus validating the proposed method.

Structural identification and evaluation for SHM applications

Optimal condition monitoring, maintenance and management of the existing civil infrastructure have been identified as a critical factor in ensuring and raising the economic productive capacity. Direct in-service structural health monitoring and vibration behaviour measurement data are essential to provide more accurate performance assessment and capacity estimation. The main focus of this special issue, organised by Australian Network of Structural Health Monitoring (ANSHM), is to present recent developments and applications of innovative techniques for structural modelling, structural identification with uncertainties, data interpretation for structural performance prediction and reserve capacity evaluation.

Development of piezoelectric thick-film sensors to be embedded into adhesively bonded joints

Techniques have been developed for making a unique type of piezoelectric thick-film strain sensor in the form of a ‘piezoelectric adhesive’ to be embedded into a bonded joint. Lead zirconate titanate (PZT) powder is used as the active ingredient of the piezoelectric composite and epoxy resin as the matrix. The sensors have been fully characterised (i.e. type of piezoelectric powder, the surface area of the sensor, its thickness, weight fraction and the poling field) through an extensive testing programme in order to determine the optimum formulation for enhancing the sensitivity of the sensor. One of the main challenges was to effectively incorporate those sensors into the adhesive layer of a lap joint without greatly affecting the joint's mechanical properties. This application would allow the direct measurement of the stress field in a joint along the bond area, aiming at the optimisation of the adhesively bonded joint design as well as serving as an in-service structural health monitoring device. In the present work, the sensors were tested using the dynamic four-point bending test and preliminary results demonstrate the feasibility of this approach, achieving a satisfactory performance of the piezoelectric sensors applied on a metal substrate.

Development of Low Frequency High Temperature Ultrasonic Transducers for In-service Monitoring of Pipework in Power Plants

In this paper, a low-frequency (20-100kHz) long-range ultrasonic guided wave transducer for in-service structural health monitoring of high temperature pipework in power plants has been developed. The transducer is designed for continuous operation at a target temperature of 350°C. This paper describes the methodology followed to develop and test the high temperature transducer. The method includes selection of the suitable piezoelectric material and characterisation of their high temperature properties for the desired thickness-shear mode of vibration to validate its performance at the target temperature. The assembly procedure established for manufacturing transducer is discussed. Ultrasonic performance of the developed transducer was examined and has demonstrated its application for in-service monitoring of pipework operating at the target temperature.

In-service structural health monitoring of a full-scale composite horizontal tail

In-service structural health monitoring (SHM) technologies are critical for the utilization of composite aircraft structures. We developed a Lamb wave-based in-service SHM technology using built-in piezoelectric actuator/sensor networks to monitor delamination extension in a full-scale composite horizontal tail. The in-service SHM technology combine of damage rapid monitoring (DRM) stage and damage imaging diagnosis (DID) stage allows for real-time monitoring and long term tracking of the structural integrity of composite aircraft structures. DRM stage using spearman rank correlation coefficient was introduced to generate a damage index which can be used to monitor the trend of damage extension. The DID stage based on canonical correlation analysis aimed at intuitively highlighting structural damage regions in two-dimensional images. The DRM and DID stages were trialed by an in-service SHM experiment of CFRP T-joint. Finally, the detection capability of the in-service SHM technology was verified in the SHM experiment of a full-scale composite horizontal tail. Experimental results show that the rapid monitoring method effectively monitors the damage occurrence and extension tendency in real time; damage imaging diagnosis results are consistent with those from the failure model of the composite horizontal tail structure.

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

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.

Mechanical behavior and health monitoring by Acoustic Emission of unidirectional and cross-ply laminates integrated by piezoelectric implant

Recent progress in sensor technologies, signal processing and electronics has made it possible to fulfill the need for the development of in-service structural health monitoring (SHM) systems. This study presents a health monitoring of composite materials integrated by piezoelectric sensor using Acoustic Emission (AE) technique. A series of specimens of composite laminates with and without piezoelectric implant were subject to three-point bending in static and creep tests while continuously monitoring the response by the AE technique. The analysis and observation of AE signals lead to the identification of the acoustic signatures of damage mechanisms in composite laminates. The mechanical behavior of composites with and without integrated sensor shows no difference in the form. The incorporation of piezoelectric sensor influences specially the fracture load and causes low degradation of mechanical properties of materials. One of the major differences between the two types of materials (with and without embedded sensor) is the intense acoustic activity in the integrated material.

In-service Inspection and Structural Health Monitoring for Safe and Reliable Operation of NPPs

In nuclear power plants, continuous efforts are made to ensure that plants are operated in safe, reliable and economical manner. While utmost care is taken during design, construction and commissioning of the structures, systems& components (SSCs), continued healthiness has to be ensured during operation phase in accordance with the design intent. This is primarily achieved through the establishment of a comprehensive life management programme of surveillance, condition monitoring, periodic In-service inspections (ISI) and maintenance, the purpose of which is to ensure that required safety margins are maintained for all important SSCs throughout plant service life. In any industry, the important mechanisms which lead to SSCs degradation are general corrosion, Flow accelerated corrosion, erosion, thermal effects, fatigue and mechanical wear & fretting. In a Nuclear Power Plant, in addition to these, degradation also occurs due to irradiation and creep. Therefore SSCs need to be designed with due consideration of all such degradation mechanisms. During operation phase, the limiting conditions for operation and requirements of surveillance & condition monitoring for all components important to safety are documented in Technical Specification. A plant specific ISI manual defines and elaborates the ISI internal inspection methods and acceptance criteria. The ISI requirements are decided taking into account best industry practices and operational experience and meet the requirements of existing codes and standards. Both these documents are approved by AERB. In pressurized heavy water reactors, major components covered in ISI programme are coolant channels, feeder pipes, steam generators, heavy water heat exchangers, pressure boundary components, relief valves etc. The inspection methods adopted include use of UT, ECT, DPT and visual inspection. In addition to functional checks for some of the components, ferrography, vibration monitoring and thermography etc are also utilized for rotating equipment. The SSCs where unacceptable indications are revealed by ISI are repaired, replaced or isolated. The base line data generated during Pre-Service inspection is used for comparing and trending of observations. The observations made during ISI are subjected to thorough review and analysis by qualified experts to obtain assurance that unacceptable degradation in component quality is not occurring and it remains fit for service. The containment system of a NPP plays a crucial role in minimizing dose to the public in case of an accident situation. The integrity of Primary and Secondary Containment is assessed by conducting integrated leak rate tests in every Biannual Shut Down. The concrete structure is also subjected to NDT checks on a specified frequency to ensure continued healthiness. Indian nuclear power plants have accumulated an operating experience of more than 380 reactor years of operation. Over the years, the programmes for surveillance, condition monitoring and in-service inspections have been improved significantly and match with best in the industry. As a result, there has been no major age related failure in any of the important SSCs and thus continued safe and reliable operation of NPPs is assured. This paper brings out the ISI & health monitoring methodology adopted at Indian NPPs, to ensure safe and reliable operation.

In-line ultrasonic monitoring for sediments stuck on inner wall of a polyvinyl chloride pipe.

This research verified the applicability and effectiveness of the ultrasonic monitoring of sediments stuck on the inner wall of polyvinyl chloride (PVC) pipes. For identifying the transmittance of acoustic energy and the speed of sound in the PVC material, the pulse-echo ultrasonic testing was conducted for PVC sheets of different thicknesses. To simulate the solidified sediment, the hot melt adhesive (HMA) was covered on the inner wall of the PVC pipe in different heights. From the experiment, the speeds of sound in the PVC and the HMA materials were obtained as about 2258 and 2000 m/s, respectively. The thickness of the materials was calculated through the signal processing such as taking the absolute value and low pass filtering, the echo detection, and the measurement of the time of flight. The errors between actual and measured thicknesses of PVC sheets were below 5%. In the case of the substance stuck on the inner wall, the errors were below 2.5%. Since the pulse-echo ultrasonic inspection is available on the outer surface and its measurement accuracy was over 95%, it can be an efficient and effective in-service structural health monitoring for the sediment on the wall of PVC pipes.

Strain measurement for composite tubes using embedded, fiber Bragg grating sensor

Composite tube structures have been extensively used in automotive, aerospace and aircraft industries. In addition, the fiber Bragg grating (FBG) sensor, a distinctive non-destructive tool, has begun to play an important role in structural health monitoring (SHM). This study aims to prove the feasibility of the FBG sensor integrated into a composite tube structure. Uniform FBG sensors are separately embedded in circumferential and axial directions of a glass fiber-reinforced plastic (GFRP) tube to detect the strain changes under loading. Two experiments, an inner pressure test and an outer concentrated force test, are conducted to investigate the deformation characteristics of the composite tube. The strain measurement results from the FBG sensors are compared with those of resistance strain gauges and FEM analysis. The results demonstrate that the FBG sensor can be successfully used for in-service structural health monitoring of the composite tube.

POTENTIAL OF BRAGG GRATING SENSORS FOR AIRCRAFT HEALTH MONITORING

The increased requirement to operate military platforms and aerospace structures beyond their designed life imposes heavy maintenance and inspection burden on aircraft operators and owners. In-service structural health monitoring is potentially a cost-effective approach by which service usage information can be obtained and knowledgeable decisions can be made. Advanced sensor technology, such as optical fibres, are expected to provide existing and future aircraft with added intelligence and functionality, reduced weight and cost, enhanced robustness and performance. This paper furthers the understanding of technical and practical issues related to full implementation of a fibre optic sensor based structural health monitoring system for aerospace and military platforms. It also reports experimental findings on the use of fibre Bragg grating sensors for measurement of parameters relevant to aircraft structural monitoring and smart structures; with an emphasis on the suitability of multifunctional fibre optic sensor system. Experimental evaluations revealed that Bragg grating sensors correlate well with conventional sensors technology for temperature, stain, crack growth and cure monitoring and were insensitive to pressures up to 300 psi. These sensors were determined to have minimum impact on the structural integrity when embedded parallel to host fibres into composite laminates. Recommendations on the implementation and integration of these sensors into a structural health monitoring system are also provided.

A real-time active smart patch system for monitoring the integrity of bonded repair on anaircraft structure

There currently exists a need to develop a cost-effective, in-service structural healthmonitoring (SHM) system for determining the initial quality of a bonded repair andassessing the long-term durability of the bonded repair on an aircraft structure. In thispaper, a real-time active smart patch system (SPS) based on SMART layer technology isintroduced for monitoring the integrity of bonded repairs. First, an overview of the SPS isgiven for typical metal and composite repairs. To illustrate the capability of the SPS, threeapplications are presented: (1) monitoring of the cure progress of the bonded repairadhesive, (2) detection of the initial artificial disbond between the compositepatch and the metal structure, and (3) monitoring of damage in and around abonded repair during fatigue cycling. The results show that, through the use of areal-time active SPS approach of using sensors placed in, on or around the repair,the initial quality and long-term durability of the repair can be evaluated andmonitored.