Brillouin Optical Correlation Domain Analysis in Composite Material Beams

In this paper, we discuss the dynamics of beams with multiple delaminations. The objective of the study is to use the dynamic response of the delaminated beams in health monitoring of composite structures. As a first step, we present experimental results on the vibrations of a cantilever beam with multiple delaminations. These experiments are conducted by exciting the beam with piezoelectric actuators. The response is detected by using a high-speed video camera to study the existence of delamination opening modes in a beam with multiple delaminations. Then, an analysis of natural frequencies and mode shapes, for a delaminated beam, is presented by using analytical techniques. The measured dynamic responses of the beam with multiple delaminations are also presented. The experimentally detected nonlinear response is discussed. Introduction Delaminations or interlayer cracks and impact damages are typical defects observed in composite structures. The presence of these defects changes the dynamic characteristics of the structure . Some of the specific changes in the observed dynamic characteristics include changes in natural frequencies, changes in the mode shapes and changes in damping ratios. These changes can be used to identify the existence, location and magnitude of a delamination or an impact damage, before they can grow to their critical sizes. Studies, to date, are concerned with the dynamics of beams and plates with a single delamination. In this paper, we specifically discuss the structural dynamics of a beam with multiple delaminations. This information can be used to develop health-monitoring techniques for composite beams with multiple delaminations. In an earlier paper, Luo and Hanagud studied the effects of defects, like saw-cuts, impact damage and delamination, on the structural dynamic characteristics of composite beams. They have shown that perturbations in natural frequencies and modes of a beam, due to defects, can be used for health monitoring of structures. In particular, it has been shown that an integral equation that considers perturbations in curvature modes and natural frequencies can be very effectively used to detect local defects. To distinguish delaminations from other types of monitored (identified) defects Luo and Hanagud have discussed nonlinear effects like superharmonic response in the dynamic response of a harmonically excited delaminated beam. They considered only a single delamination. In this paper we study beams with multiple delaminations. More specifically, we discuss dynamic response and the changes in natural frequencies, mode shapes due to multiple delaminations in a composite beam. One of the earliest models for vibration analysis of composite beams with delaminations was proposed by Ramkumar et al. This model simply used four Timoshenko beams connected at the delamination edges to model a composite beam with one throughwidth delamination. The predicted frequencies, based on this model, were consistently lower than the results of experimental measurements. Wang et al. improved the analytical solution by including coupling between flexural and axial vibrations of the delaminated sublaminates. Using an isotropic beam with splits and the classical beam model, they found that the calculated natural frequencies were closer to experimental results. With similar considerations, Nagesh Gummadi and Hanagud formulated a finite element solution for arbitrary composite beams. In the finite element models, they considered a classical composite beam model as well as the beam model with high order shear deformations. Mujumdar and Suryanarayan have proposed a model which imposed a constraint between the delaminated sublaminates to force them to have the same flexural displacement. This model was unable to * Ph.D. Student, School of Aerospace Engineering † Professor, School of Aerospace Engineering, AIAA Member Copyright © 1999 The American Institute of Aeronautics and Astronautics Inc. All rights reserved

Structural health monitoring (SHM) of composite materials is critical for ensuring the reliability and longevity of high-performance engineering systems. This review comprehensively examines the advancements, challenges, and applications of flat optical fibre sensors (FOFS) for real-time strain monitoring in composite structures. Traditional electrical strain gauges and piezoelectric sensors face limitations in multiplexing, electromagnetic interference (EMI), and integration within composite layups. In contrast, FOFS offer unique advantages, including high spatial resolution, compatibility with composite manufacturing processes, and immunity to EMI. This paper analyses the working principles of FOFS, their fabrication techniques, integration methodologies, and signal interrogation systems. Case studies from aerospace, civil engineering, and renewable energy sectors underscore their practical efficacy. Challenges such as signal attenuation, temperature cross-sensitivity, and long-term durability are critically evaluated. The review concludes with future directions, including nanotechnology-enhanced sensors and machine learning-driven data analytics.

A case study is presented in which the first steps are made towards the development of a structural health monitoring (SHM) data fusion framework. For this purpose, a composite single-stiffener panel is subjected to compression-compression fatigue loading (R = 10). The carbon-epoxy panel contains an artificial disbond of 30 mm, which was created using a Teflon insert during manufacturing and placed between the skin and the stiffener foot. Under the applied fatigue load, the disbond is expected to grow and its propagation is monitored using two SHM techniques, namely acoustic emission (AE) and Rayleigh-scattering based distributed fiber optic strain sensing. Four AE sensors are placed on the skin, thereby allowing for disbond growth detection and localization. On each stiffener foot, fiber optic sensors are surface-bonded to monitor the growth of the disbond under the applied fatigue loading. The distributed strain measurements are used to localize and monitor the disbond growth. The strength of each technique is utilized by fusing the data from the AE sensors and the fiber optic sensors. In this manner, a data-driven approach is presented in which a data fusion of the different techniques allows for monitoring the damage in the stiffened panel on multiple SHM levels, including disbond growth detection and localization.

Optical fiber sensors have attracted considerable attention in health monitoring of aerospace composite structures. This paper briefly reviews our recent advancement mainly in Brillouin-based distributed sensing. Damage detection, life cycle monitoring and shape reconstruction systems applicable to large-scale composite structures are presented, and new technical concepts, smart crack arrester and hierarchical sensing system, are described as well, highlighting the great potential of optical fiber sensors for the structural health monitoring (SHM) field.

A Brillouin optical time domain reflectometer (BOTDR) is a fiber optic strain distributed sensing system which can measure strain along arbitrary regions of an optical fiber. We have used this system for structural health monitoring of composite structures, such as racing yachts. Although the BOTDR is useful to measure strain distributions of a large area efficiently, it is difficult to accurately measure strain changing sharply within the length of the spatial resolution. The resolution depends on the width of the pulse light emitted from the BOTDR and it is 2 m for the pulse width of 20 ns. In this paper, we propose a new technique to improve the spatial resolution of a commercially available BOTDR in strain measurements. This technique is based on analysis of the Brillouin gain spectrum along a sensing fiber. We confirmed experimentally that the BOTDR using 20 ns pulse could measure strain which has a rectangular distribution of 0.3 m in the sensing fiber.

In the present work a fiber-optic loop-sensor is designed and tested for possible applications in structural health monitoring of composite materials. It is known that bending an optical fiber beyond a critical curvature leads to loss of optical power through the curved region. The optical power loss depends on the radius of curvature of the loop. The optical power can be measured by a photodetector and a change in the power due a change to the curvature can be measured. In the present research optical fiber-optic loop-sensors are developed that can exploit this concept. Single-mode optical fiber sensors having different loop radii, from 6-10 mm, are fabricated and calibrated for applied strain on the loop. The calibration is carried out using a 0.098 N load cell and a computer controlled translation stage having 50 nm step resolution. Results show that the sensors provide highly repeatable curves for loading and unloading cycles. Smaller loop radii lead to higher optical power losses, resulting in higher sensitivity. Calibration results show that such sensors can be used in structural health monitoring applications. In this approach the coating and cladding of optical fibers are maintained intact; therefore, the sensors are robust and can withstand several composites fabrication processes.

Νέα προσέγγιση στην παρακολούθηση της αναπτυσσόμενης βλάβης υπό μηχανική φόρτιση σε ινώδη σύνθετα υλικά με μήτρα ενισχυμένη με νανοσωληνίσκους άνθρακα

The application of MEMS and nanotechnology (MNT) to the field of structural health monitoring (SHM) is a fairly recent development. The recent change in this focus for MNT has been driven by the need to expand the applications for much of the technologies that were developed in the late 1990s. In addition, many companies desire to expand beyond their target high volume market segments of automotive, wireless communications, and computer peripherals, since these market segments were not as lucrative as first predicted. Most of the aerospace structural health monitoring developmental activity has been sponsored by agencies of the U.S. Government, which serves to pace the examination of these newer technologies to some degree. With that said, efforts are underway by companies such as Acellent Technologies and Blue Road Research to explore various MNT structural health monitoring approaches. The MNT under test include embedded piezoelectric sensors, MEMS accelerometers, time domain region sensors, and topical and embedded single and multi-axis fiber optic Bragg grating sensors. The promise of MNT for the SHM market segment is very enticing. The many wireless communication developments and miniaturization developments of the past five years is very attractive to the SHM community, especially those that are able to reduce the cost and complexity of integration. The main challenge for the community is one of selective integration. That is, certain pieces may be appropriate for SHM systems and certain pieces may not be. The better companies will chose wisely and put forth an approach that can be seamlessly integrated into the larger structure. For over a decade, Blue Road Research has been developing technologies aimed at structural health monitoring of both composite and non-composite parts, through the use of single and multiaxis fiber optic Bragg grating sensors. These sensors are 80 to 120 microns in diameter making them smaller than the diameter of a human hair. Multiaxis fiber optic sensors are able to measure pressure, temperature, axial and transverse strain, chemical properties, corrosion, as well as transverse strain gradients. This technology is easily embedded in between the various layers of the composite structure, during manufacture, without compromising the structural integrity, in order to verify manufacturing parameters during the cure cycle and well as monitor the on-going condition of the composite structure throughout its life time. This paper reviews some of the technical work that has been accomplished during the past two years; specifically the embedding of fiber optic sensors into various composite structures in order to be able to conduct in situ non-destructive evaluation of the curing process and the service life of the component. The fiber optic technology has been developed to the point that it is at a TRL of 6.

Composite structures are strong and light weight, and have great potential in enhancing the structural strength of Unmanned Air Vehicles (UAVs). In this research, we present some preliminary results of structural health monitoring (SHM) for composite structures using an unpowered wireless ultrasound system. First, we have built a testbed, which consists of ultrasonic sensors, composite specimens, and data acquisition electronics. Second, we have performed extensive experiments to collect data. An 8-sensor array was used to detect and localize defects in composite plates. Third, we have implemented defect detection and localization algorithms. We have shown that, with an 8-sensor array, we can detect and localize small defects. Fourth, we have performed an experiment to demonstrate that wired and wireless data matched well both in time domain and in frequency domain.

With the growing size of large-scale structures, composite materials are being increasingly used due to their high strength-to-weight ratio. As structures get larger, so too does the necessity for a robust health monitoring system; a well-founded and reliable method of doing so is by monitoring acoustic emissions. However, such a method encounters significant problems when employed on composite structures; such as high dispersion of extensional modes, propagation distance-dependent wave velocity and highly subjective characteristic frequencies of failure modes. Ultrasonic guided waves offer the abilities of location and severity detection to a more accurate level, whereas acoustic emission involves a more efficient passive approach. By analysis of ultrasonic waves in a carbon-fibre-reinforced-polymer (CFRP) plate, information is show here that will provide the stepping stones for the development of a robust health monitoring system for composite structures using ultrasonic guided waves. Advanced signal processing is used to show information on selected features of an acoustic burst as it propagates through the composite material, as well as how damage affects these features. It was found that both the energy of the burst and the first asymmetric mode are affected by the orientation of fibres in a composite, although in differing ways. However, the time of flight of an acoustic burst is unaffected. Subsequently, damage affects all of the above mentioned features, and this knowledge can be utilised to identify damage. Finally, the one-dimensional propagation of a wave along the length of the fibres is tested against various statistical-based models to showcase the potential of this method for use in modelling the propagation of an acoustic wave in a composite material.

Highly resolved distributed monitoring of strain and temperature can be easily achieved by means of fiber-optic Brillouin-based sensors. The majority of available Brillouin systems relies on Brillouin Optical Time Domain Analysis (BOTDA) techniques in which pulsed pump and/or signal waves are exploited, allowing a spatial resolution fundamentally restricted to about 1 meter [1]. Brillouin Optical Correlation-Domain Analysis (BOCdA) schemes have also been proposed where pump and probe are instead subjected to synchronous random phase modulation which guarantees an efficient Brillouin amplification confined only in a narrow section (even less than 1 cm) of the fiber under test (FUT), corresponding to the correlation peak [2, 3]. BOCDA architectures typically employ a standard push-pull Mach-Zehnder modulator (MZM) to shift the probe frequency in the Brillouin amplification range by means of the dual sideband (DSB) amplitude modulation of the probe wave. Narrow optical passband filtering, performed for example by using a temperature-controlled fiber Bragg grating, is also required in order to select only the probe sideband experiencing the Stokes Brillouin amplification [1-3]. In this paper we present a BOCDA solution, exploiting the capabilities of a nested I/Q MZM, usually employed for fiber optical communications with complex formats modulation, not only to obtain the required frequency detuning, but also to generate a single sideband (SSB) probe modulation [4] with a significant attenuation of the anti-Stokes probe sideband and a complete suppression of the optical carrier. This approach enables to effectively remove the gain-loss balance of the two sidebands, allowing a precise observation of the Stokes Brillouin gain spectrum (BGS) in order to derive the Brillouin Frequency Shift (BFS) and thus the temperature and strain information, without the need of any demanding optical filtering.

Polymer optical fiber (POF) sensors have the unique ability to measure high strain distributed along the fiber up to 40% using the optical time-domain reflectometry (OTDR) technique. Both, standard PMMA fibers and perfluorinated (PF) graded-index (GI) POF are introduced and evaluated in potential use and applicability. Further, distributed length change measurement based on cross-correlation analysis of the characteristic fiber signature of PF POF is introduced. We conclude the advances in distributed POF strain sensors technology with respect to application in structural health monitoring. Special focus is on the sensor integration into technical textiles for health monitoring of geotechnical structures and masonry structures. Measurement results of sensor-equipped textiles in different model tests are presented, displacement of soil and cracks in retrofitted masonry structures are detected and evaluated.

Structural health monitoring (SHM) is widely used to examine the structural integrity of a structure to improve safety considerations, minimize maintenance cost, and avoid sudden breakdowns that might occur under various loadings under service conditions. Fiber reinforced polymer (FRP) composites are widely used in aerospace, automotive, marine, energy, infrastructure, armor, and biomedical applications due to their high specific stiffness and strength, high degree of dimensional and thermal stability, and good resistance to corrosion. Thus, SHM of the damages developed in FRPs during service conditions is very crucial. However, traditionally used sensors for SHM in composites provide limited sensitivity and might negatively affect the structural integrity. Therefore, a self-sensing approach by evaluating the electrical resistance change (ERC) of FRPs has been successfully used by many researchers. The purpose of this chapter is to describe the ERC techniques using various nanoparticles such as graphene, carbon nanotube, expanded graphite, and carbon black.

Damage quantification in composite laminates based on sparsity-difference estimation of the normal time–frequency representation

Curing quality monitoring and loading detection of composite structures with embedded capacitive sensor array