This paper presents a method for inverting limited discrete strain measurements in rod-like structures into global, three-dimensional displacement. The method takes advantage of a material basis reference frame and, using local linearization of the derived material basis evolution equations, finds exact local basis solutions for the material basis frame, and computes strains from that solution. Continuity among local regions provides closure to the problem, and a continuum of displacement is obtained. The method is validated via finite element simulation and experiment.

Fiber Bragg gratings are capable of accurately determining the strain placed on an object. Imbedding a mesh of gratings into a substrate allows strain to be determined over a larger area. Using a high speed interrogator the incremental steps of the deformation are detectable. Interrogators only operate over a specific wavelength region. Using more gratings increases the resolution of the mesh but also allows the reflection peaks to overlap. Less data is lost when the closer together the peaks can be identified. Dynamic detection of the deformation is possible by reconstructing the data gathered from the grating mesh.

Conventional interrogators for optical sensors such as grating spectrometers can hardly be miniaturized without loss of spectral resolution. Hence, to provide a high spectral resolution they are typically bulky, expensive, and demand special operation conditions such as air-conditioning or vibration-damped supports. The way around these problems is to use an optical filter with a spectral transmission gradient as the dispersive element. In contrast to a grating-based spectrometer, device dimensions and spectral resolution of the interrogator are then decoupled. In our contribution we provide a brief overview over advantages and drawbacks of this interrogation approach and present an interrogator for the read-out of fiber Bragg gratings.

We present a novel sensing scheme for the determination of wavelength shifts in optical non-destructive evaluation. Gold nanoparticle arrays, which show plasmonic activity are used as optical filters in this approach. The lithographically fabricated arrays are modified by a photochemical treatment to manufacture gradients of gold nanoparticle sizes. The localized surface plasmon resoncance wavelengths (LSPRs) of individual spots in the array are therefore altered. These lateral nanoparticle size gradients translate into a laterally varying extinction filter which in combination with a spatially selective detector allows the detection of wavelength shifts. The approach can be used to miniaturize spectral analyzers to generate highly integrated sensor systems.

Fibrous composite materials are widely used especially in the aerospace technology. Low-velocity impact damage is frequently encountered in fibrous composite materials used in the aerospace technology. It is usually taking place during operation as well as during scheduled repairs. Flaws caused by low-velocity impact are extremely dangerous as they often develop to delaminations. It is obvious, that inexpensive and efficient damage inspection during the service life of these structures is crucial for their safe function. A combined thermographic and electrical inspection method is presented in this paper. Low velocity impact damage at different energy levels is applied on Carbon Fibre Reinforced Polymers (CFRPs). For reference, both plain and CNT modified matrix CFRPs were inspected. Modulated electrical current is injected in order to thermally stimulate the employed materials. Pulsed phase thermography is employed for the identification of the induced impact damage. Along with thermography, an electrical potential mapping technique was used to map the applied artificial damage at the very same materials. In this case, electrical current is injected through the bulk material via electroplated electrical contacts. The map of the electrical potential field of the bulk material is characteristic of its internal structural state. Conventional ultrasonics (C-scan) were employed to validate the results of both thermographic and electrical techniques.

In this paper, we investigate the fusion of imaging data from pulsed phase thermography (PPT) with local temperature data obtained from embedded fiber Bragg grating (FBG) sensors for non-destructive evaluation of composite structures. We use the square pulse heating applied for the PPT imaging as the input thermal wave. In addition, the role of the local microstructure surrounding the FBG on the measured wavelength shift as a function of temperature is derived analytically. Fusing the FBG wavelength response with the PPT data at the corresponding pixel and depth is shown to provide a unique characterization of the local material condition.

In this research, fiber Bragg grating temperature sensors are embedded in composites to detect highly localized temperature gradients in the composite structures. The ultimate goal is to rapidly detect high energy electromagnetic radiation incident on the surface of a composite. A secondary goal is to use the sensors as a diagnostic tool to determine the optimal composite materials or structures that are the least susceptible to high energy radiation damage. Initial measurements of the thermal response will be discussed. Additional tests using an array of sensors will be presented, and the impact of strain on the sensor response is considered.

In distributed fiber optic sensing system, single-ended double-pulse input is used to strengthen Brillouin backscattered light by increasing the population of acoustic phonons. The first pulse (pump pulse), has a wide pulsewidth (tens or hundreds of us) and is used to generate a nonlinear population of acoustic phonons in the sensing fiber. Whereas the second pulse which (probe pulse), has a different central wavelength and a much narrower pulsewidth (several ns), and is emitted into the sensing fiber with a controlled time delay to absorb the generated abundant acoustic phonons, so that strong Anti-Stokes light can be generated. In this way, Brillouin backscattered light can be detected easily, leading to high signal to noise ratio and better spatial resolution (less than 1 m), as well as good temperature and strain resolution, and longer sensing distance.

In this work, novel statistical features were proposed for the design of statistical mode sensors. These new features were compared with the known statistical features in literature. Experiments were conducted and analyses were implemented using proposed statistical features. Moreover, statistical features were compared in terms of different performance parameters. After that, Artificial Neural Networks (ANNs) with sensor fusion, intelligent sensor architecture was proposed to predict the measured values of statistical mode sensors. Using sensor fusion with ANNs, statistical mode sensors can be calibrated and fault tolerance of the sensor can be decreased, hence more reliable sensors can be designed.

Fibre optic strain sensors are increasingly used in very different technical fields. Sensors are provided with specifications and must sometimes be prepared and modified to meet measurement requirements. Even if the performance of delivered sensor is well specified, the sensor's strain characteristics and the performance of applied strain sensors can seriously differ from the virgin sensor’s performance. The contribution will therefore focus on issues that can decrease or even deteriorate the sensor function. This leads to unreliable measurement results; ways to overcome these problems will be discussed. In order to proof the stability of the sensor function, experimental investigation/validation is needed. Important aspects how to come to reliable strain measurements and how to validate strain measurements of applied sensors are considered. Experimental facilities to reveal weaknesses in the sensor use will be presented.

SHM of composite structures is an emerging technology due to several benefits in terms of higher safety and security of human life, better material understanding, safe money. Fiber-optic sensing technologies (FOS) are already proved to be reliable in the family of NDT methods. The most frequently used are fiber-optic Bragg grating (FBG) sensors. However, several drawbacks of FBG cause a permanent improvement in the field of FOS. Within this paper the authors will confront the fiber-optic low-coherence interferometry against FBG taking into account experimental results simultaneously obtained by investigation of PEEK probe on stress by applying the three-point loading test.

A chirped long period grating (CLPG) has been demonstrated for directional flow sensing and for monitoring the cure process of a UV-cured epoxy resin. The CLPG exhibited response to refractive indices both lower and greater than that of the fibre cladding. The asymmetry in the CLPG led to the attenuation bands changing shape depending on the resin flow direction. The response of the CLPG and of a uniform period long period grating to the cure process were correlated with the refractive index change of the resin measured by a fibre optic Fresnel based refractometer.

Infusion, flow and cure of RTM6 resin in carbon fibre reinforced preform have been monitored using a variety of multiplexed fibre optic sensors. Embedded fibre optic Fresnel sensors and tilted fibre Bragg gratings (TFBG) of relatively high tilt angle (3o) monitored resin infusion, flow and cure. Temperature was measured by TFBGs of relatively small tilt angle (1.5o). Fibre Bragg gratings (FBG) fabricated in highly linearly birefringent (HiBi) fibre monitored the development of transverse strain during cure. The results obtained from the various sensors in monitoring the resin infusion and flow were comparable, and were in good agreement with visual observations.

The need for a protecting guard for the popular Ceramic Matrix Composites (CMCs) is getting A lot of attention from most engine manufacturers and aerospace companies. This because the CMC has a weight advantages over standard metallic materials and more performance benefits. They are also commonly porous material and this feature is somewhat beneficial since it allows some desirable infiltration. They further undergo degradation that typically includes coating interface oxidation as opposed to moisture induced matrix which is generally seen at a higher temperature. Variety of factors such as residual stresses, coating process related flaws, casting conditions, may influence the strength of degradation. The cause of such defects which cause cracking and other damage is that not much energy is absorbed during fracture of these materials. Therefore, an understanding of the issues that control crack deflection and propagation along interfaces is needed to maximize the energy dissipation capabilities of layered ceramics. These durability considerations are being addressed by introducing highly specialized form of environmental barrier coating (EBC) that is being developed and explored in particular for high temperature applications that is greater than 1100 °C [1, 3]. The EBCs are typically a multilayer of coatings and are in the order of hundreds of microns thick. Thus, evaluating components and subcomponents made out of CMCs under gas turbine engine conditions is suggested to demonstrate that these material will perform as expected and required under these aggressive environmental circumstances. Progressive failure analysis (PFA) is being performed to assess the crack growth of the coating under combined thermal and mechanical loading conditions. The PFA evaluation is carried out with the Genoa [4] software using a full-scale finite element model to account for the average material failure at the microscopic level. The roadmap for the analysis involves the following steps: (1) Reverse engineering of fiber and matrix properties from lamina properties, (2) Determination of residual stresses as a result of thermal cycling, (3) Assessment of degradation of material strength during cycling, and (4) Damage tracking and fracture using physics based failure criteria to determine life of the specimen under service load. Results related to crack growth; behavior and life assessment of the coating at the interface of the EBC/CMC will be presented and discussed.

The measurement of response spectra from FBG sensors enables detailed visualization of the local strain field at the location of the sensor. In this paper we apply a dynamic, full-spectral interrogator to FBG sensors embedded in adhesive lap joints. We demonstrate that as fatigue damage is initiated in the lap joint, spectral distortion appears in FBG sensors embedded in the adhesive. This distortion is measured during vibration loading of the joint. This paper focuses on the demonstration of unique spectral signatures and their correlation to the progression of fatigue induced damage in an adhesively bonded joint.

This paper reports recent developments on high-temperature, multi-element integrated ultrasonic transducers (IUTs). The multi-element IUTs are fabricated from a sol-gel route, where piezoelectric films are deposited, poled and machined into an array of 16 elements. Electrical wiring and insulation are also integrated into a practical, simple high-temperature assembly. These multi-element IUTs show a high potential for structural health monitoring at high temperatures (in the 200-500°C range): they can withstand thermal cycling and shocks, they can be integrated to complex geometries, and they have broadband and suitable operating frequency characteristics with a minimal footprint (no backing needed).

Acoustic emission (AE) technique is applied in materials to evaluate the internal fracturing condition. Apart from the number of signals which are correlated to the density of cracks, qualitative features of the waveforms shed light in the fracturing mode. This is due to the fact that the emissions depend on the motion of the crack sides. The fracture process of materials includes shift between modes and therefore, real-time characterization of the dominant mode supplies information on the current condition and poses an early warning before failure. Although a lot of work has been done on AE characterization of damage, the work on welded components is scarse. In this study aluminum plates are cross-welded and loaded until fracture in fatigue experiments at different load levels. Their acoustic activity is recorded along with mechanical parameters. The study of the AE rate relatively to the applied load, and qualitative parameters like the frequency content can be used to study the evolution of the crack under the different modes.

This work deals with the nondestructive evaluation (NDE) of the fracture behavior of ceramic matrix composite (CMCs) materials combining infrared (IR) thermographic and acoustic emission (AE) characterization. IR thermography as a non-destructive, real-time and non-contact technique, allows the detection of heat waves which are generated by the thermo-mechanical coupling and the intrinsic energy dissipated during mechanical cyclic loading of the sample. Three different thermographic methodologies, based on measuring the surface temperature, the sum of the principal stresses and the intrinsic dissipated energy respectively, were applied in order to monitor the crack initiation and propagation, the crack growth rate (Da/DN) and to assess rapidly the fatigue limit of cross-ply SiC/BMAS composites. The thermographic results on crack growth rate were found to be in agreement with measurements obtained by the conventional compliance method. Simultaneously, AE monitoring was taking place recording any type of cracking event from matrix cracking to fiber fracture and pull-out. AE event rate, as well as qualitative indices like the rise time and peak frequency reveal crucial information allowing the characterization of the severity of fracture in relation to the applied load. Additionally, rapid assessment of the fatigue limit of CMCs composites was also attempted by the AE. Testing a specimen at different load levels for predetermined blocks of cycles shows that the AE acquisition rate keeps low for loads below the fatigue limit, while it increases abruptly for higher levels. The thermographic assesment of fatigue limit is in total agreement with the AE results enabling the reliable evaluation of the fatigue limit of the material by testing just one specimen. The application of combined NDE techniques is proved very valuable for benchmarking purposes while the sensitivities of the methods act complementarily to each other providing a very detailed assessment of the damage status of the material in real time.

Ultrasonic sensing performance of a large area piezoceramic coating with inter digital electrodes is investigated. The piezoceramic coating is prepared using slurry coating technique and poling under DC field. A wide frequency band of operation is achieved by considering the variation in the number of inter-digital electrodes. The piezoceramic coatings with inter-digital electrodes have been characterized for guided wave sensing at various different frequencies by determining its piezoelectric coefficient. Sensitivity in the strain levels is studied for various combinations with different numbers of inter-digital electrodes.

The development of techniques for the health monitoring of the rotating components in gas turbine engines is of major interest to the NASA’s Aviation Safety Program. As part of this on-going effort an experiment utilizing a novel optical Moiré based concept is being planned on a subscale turbine engine disk as a means of demonstrating a potential strain measurement and crack detection technique. Moiré patterns result from the overlap of two repetitive patterns with slightly different spacing. When this technique is applied to a rotating disk, it has the potential to allow the detection of very small changes in spacing and hence radial growth in a rotating disk due to a flaw such as a crack. This investigation is a continuation of previous efforts undertaken in 2011-2012 for the demonstration of this concept. The initial demonstration was attempted on a subscale turbine engine disk and was inconclusive due to the lack of radial growth experienced by the disk during operation. For this next phase of testing a new subscale turbine engine disk made out of Aluminum has been fabricated and improvements are being made to better demonstrate the concept. The experiment will involve laser etching a circular reference pattern onto the subscale engine disk and operating it at speeds up to 12 000 rpm as a means of optically monitoring the Moiré created by the shift in patterns created by the radial growth of the disk due to defects such as a crack. Testing will first be done on a clean defect free disk as a means of acquiring baseline reference data. A notch will then be machined on to the disk to simulate a crack and the testing will be repeated for the purposes of investigating the Moiré optical detection concept. Displacement data will be acquired using external blade tip clearance and shaft displacement sensors as a means of confirming the optical data and validating other sensor based crack detection techniques. In addition, a novel technique to measure strain using a cross-correlation imaging technique will also be attempted during this experiment. The results from this technique will be analyzed and compared to the data yielded from the Moiré experiment.

The NASA Aviation Safety Program has a high interest in the development of fault detection techniques for gas turbine engines in an effort to detect flaws in key engine components before failure. A cross-correlation imaging technique, which builds off of the principles of Background Oriented Schlieren (BOS), is investigated as a possible technique to optically measure a localized strain field resulting from an induced crack on a subscale turbine engine disk. As performed in previous crack detection studies, a notch will be machined on to the disk to simulate a crack; the disk will be studied under static conditions at 0 rpm (unloaded condition) and at 12 000 rpm (loaded condition). Spinning the cracked disk at high speeds induces a load, resulting in a radial growth of the disk in the flawed region and hence, a localized strain field. Similar to BOS, two data images will be acquired: a reference image of the cracked disk will be acquired at the unloaded condition, while a secondary image will be acquired of the flawed disk at the loaded condition. When imaging the cracked disk under static conditions, it is thought that the disk will appear undistorted. However, during rotation the cracked region will grow radially, thus causing the high-contrast random background adhered to the disk to appear deflected. Due to the small amount of deflection during rotation, it will be necessary to work towards the development of an optimal background that will be sensitive enough to detect the fault of the subscale turbine engine. Therefore, different background, sizes, shapes, and densities will be investigated in an effort to reach the desired crack detection sensitivity. The corresponding image displacements between the two acquired disk images will be calculated using existing PIV cross-correlation algorithms, as is the case with BOS. It will also be attempted to map the observed deflection into a strain field, which will be compared to finite element analysis and external sensor data.

Optical measurement technologies, such as optical coherence tomography (OCT), becomes increasingly import in many fields of nondestructive testing. In this paper we present an automated high speed swept source OCT system with a scan rate of 100.000 A-scans per second. A MEMS based light source leads to optimal system integration and a cost reduction of more than 50\%. Thanks to the combination with a table top robot, automated wide range measurements over an area of up to 400mm in square are possible. Therefore our OCT system can be used as a tool for testing scenarios in research, for failure analyses and semi-automated production processes.

We demonstrate the implementation of a long period fiber grating into our scanning near-field optical microscope (SNOM) with a transmission probe. The LPG is used to excite radial polarization states in an index-tailored fiber that can couple efficiently to similar plasmon polaritons (SPPs). The SPPs allow for higher throughput and better field confinement at the tip of an apertureless probe. The goal is to increase the feasibility and reliability of SNOM measurement for nondestructive (optical) evaluation of nanoscale devices and nanostructured materials. The method will be further combined with Raman spectroscopy to acquire more physical information from the optical signals.

We present recent investigations on time-resolved measurements on alterations in the temporal luminescence decay of up-conversion phosphors induced by electron beam treatment. As a consequence of the tight regulations concerning sterility in the food and medical sectors a reliable and nondestructive in situ evaluation of the sterilization process faces a huge demand. Our technique is based on placing a suitable marker material (based on rare-earth phosphors) inside or as a coating on the packaging material. Upon electron irradiation the marker material changes its luminescent properties as a function of applied energy dose. The energy dependence is verified by time-resolved measurements on different up-conversion materials.

Monitoring intracellular ice formation (IIF) as well as understanding the cellular freezing process at temperatures nearly at -40°C is beneficial to the study of cryopreservation. This paper discusses the use of optical absorption spectroscopy to examine the thermal changes occurring in cells as they reach cryogenic temperatures. The presented arrangement employs a 0.6 m all-silica steering-wheel photonic crystal fiber (SW-PCF) filled with 20uL of di-ionized water that is fusion spliced with a 1.07 m single mode fiber (SMF). Unlike our previous cryogenic arrangement, a water circulated cooling system consisting of a cold plate with an attached radiator promotes operation temperatures of nearly – 40 °C. Measured absorption characteristics were consistent with standard spectra for water vapor at cryogenic temperatures.

Slab-coupled optical fiber sensors (SCOS) provide an effective alternative for electric field sensing applications requiring small, minimally intrusive electric-field sensors. One such application involves testing the effectiveness of electronic shielding as a means of safeguarding electronic systems from electromagnetic interference. SCOS use resonant coupling of light between an optical fiber and electro-optic slab waveguide. Although the small sensor size is advantageous, it also lowers its sensitivity. This paper presents improvements to SCOS sensitivity based on changing the crystal cut and orientation of the slab waveguide which enhances sensitivity due to a combined increase in the effective electro-optic coefficient and electric field penetration by exploiting a tangential boundary configuration.

A solid-core photonic crystal fiber (SC-PCF) with a microstructured all-silica steering wheel lattice containing a 3 μm diameter core is used to identify dye-free monoclonal antibodies (mAbs) of very small volumes <0.1uL for concentrations ranging from 0.1mg/mL to 〖1×10〗^(-12) mg/mL. The samples used in this study are mAbs, which are useful in detecting the presence of the botulism toxin. Human IgG antibodies (Southern Biotech Inc.) served as the “coating” antibody to detect an enzyme-linked mAb (Southern Biotech Inc.) - the “antigen”. The spectra for mAb samples linked with enzymes were compared to control samples without enzymes and conventional ELISA measurements.