Cost effective and reliable damage detection is critical for the utilization of composite materials in structural applications. Non-destructive evaluation techniques (e.g. ultrasound, radiography, infra-red imaging) are available for use during standard repair and maintenance cycles, however by comparison to the techniques used for metals these are relatively expensive and time consuming. This paper presents part of an experimental and analytical survey of candidate methods for the detection of damage in composite materials. The experimental results are presented for the application of modal analysis techniques applied to rectangular laminated graphite/epoxy specimens containing representative damage modes, including delamination, transverse ply cracks and through-holes. Changes in natural frequencies and modes were then found using a scanning laser vibrometer, and 2-D finite element models were created for comparison with the experimental results. The models accurately predicted the response of the specimems at low frequencies, but the local excitation and coalescence of higher frequency modes make mode-dependent damage detection difficult and most likely impractical for structural applications. The frequency response method was found to be reliable for detecting even small amounts of damage in a simple composite structure, however the potentially important information about damage type, size, location and orientation were lost using this method since several combinations of these variables can yield identical response signatures.

Increasing niche applications, growing international markets, and the emergence of advanced rotorcraft technology are expected to greatly increase the population of helicopters over the next decade. In terms of fuselage fatigue, helicopters show similar trends as fixed-wing aircraft. The highly unsteady loads experienced by rotating wings not only directly affect components in the dynamic systems but are also transferred to the fixed airframe structure. Expanded use of rotorcraft has focused attention on the use of new materials and the optimization of maintenance practices. The FAA's Airworthiness Assurance Center (AANC) at Sandia National Labs has joined with Bell Helicopter andother agencies in the rotorcraft industry to evaluate nondestructive inspection (NDI) capabilities in light of the damage tolerance of assorted rotorcraft structure components. Currently, the program's emphasis is on composite rotor hubs. The rotorcraft industry is constantly evaluating new types of lightweight composite materials that not only enhance the safety and reliability of rotor components but also improve performance and extended operating life as well. Composite rotor hubs have led to the use of bearingless rotor systems that are less complex and require less maintenance than their predecessors. The test facility described in this paper allows the structural stability and damage tolerance of composite hubs to be evaluated using realistic flight load spectrums of centrifugal force and bending loads. NDI was integrated into the life-cycle fatigue tests in order to evaluate flaw detection sensitivity simultaneously wiht residual strength and general rotor hub peformance. This paper will describe the evolving use of damage tolerance analysis (DTA) to direct and improve rotorcraft maintenance along with the related use of nondestructive inspections to manage helicopter safety. OVeralll, the data from this project will provide information to improve the producibility, inspectability, serviceability, and cost effectively of rotorcraft components.

Flywheel energy storage devices comprised of multilayered composite rotor systems are being studied extensively for utilization in the international space station. These composite material systems were investigated with a recently developed ultrasonic resonance spectroscopy technique. The system, UltraSpec^TM, employs a swept frequency approach and performs a fast Fourier transform (FFT) on the frequency spectrum of the response signal. In addition, the system allows for equalization of the frequency spectrum, providing all frequencies with equal amounts of energy to excite higher order resonant harmonics. Interpretation of the second FFT, along with equalization of the frequency spectrum, offers greater assurance in acquiring and analyzing the fundamental frequency, or spectrum resonance spacing. The range of frequencies swept in a pitch-catch mode was varied up to 8 MHz depending on the material and geometry of the component. Single and multilayered material samples, with and without known defects, were evaluated to determine how the constituents of a composite material system affect the resonant frequency. Amplitude and frequency changes in the spectrum and spectrum resonance spacing domains were examined from ultrasonic response of a flat composite coupon, thin composite rings, and thick composite rings. Also, the ultrasonic spectroscopy responses from areas with an intentional delamination and a foreign material insert, similar to defects that may occur during manufacturing malfunctions, were compared to those from defect free areas in thin composite rings.A thick composite ring with varying thickness was tested to investigate the full thickness resonant frequency and any possible bulk interfacial bond issues. Finally, the effect on the frequency response of naturally occuring single and clustered voids in a composote ring was established.

Composite flywheels are being considered as a replacement for chemical batteries aboard the International Space Station (ISS). Due to the serious consequences of a failure in a space environment, extensive testing of the flywheel systems must be conducted prior to flight certification. In addition to standard material testing and characterization, non destructive evaluation (NDE) of the rotors must be preformed to identify processing flaws and to understand the damage progression in a rotor under load. This paper describes the development of a scanning ultrasonic spectroscopy system for the non destructive evaluation of composite flywheels. Emphasis is placed on the novel resonance spectroscopy method that was investigated and its integration into a scanning system. Results of a scan of a composite ring standard and Plexiglas cylinder with known defects are presented and compared to ultrasonic pulse-echo c-scans.

A structural assessment by integrating finite-element methods (FEM) and a nondestructive evaluation (NDE) of two flywheel rotor assemblies is presented. Composite rotor A is pancake like with a solid hub design, and composite rotor B is cylindrical with a hollow hub design. Detailed analyses under combined centrifugal and interference-fit loading are performed. Two- and three-dimensional stress analyses and two-dimensional fracture mechanics analyses are conducted. A comparison of the structural analysis results obtained with those extracted via NDE findings is reported. Contact effects due to press-fit conditions are evaluated. Stress results generated from the finite-element analyses were corroborated with the analytical solution. Cracks due to rotational loading up to 48 000 rpm for rotor A and 34 000 rpm for rotor B were successfully imaged with NDE and predicted with FEM and fracture mechanics analyses. A procedure that extends current structural analysis to a life prediction tool is also defined.

The effect of matrix toughness on the fatigue life of polymer matrix composites using plain woven carbon fabrics (pw-CFC) was studied. In order to vary the matrix toughness without changing the inherent cohesion properties such as adhesive strength between matrix and fibers, two different curing agents (acid anhydride and amine types) were used. Static tensile and tension/tension fatigue cyclic loads were applied to pw-CFC specimens. It was observed that the fatigue life was significantly affected by matrix toughness. During the fatigue tests, damage progression was observed intermittently by using a thermo-elastic stress analyzer (TSA). The stress re-distribution occurs due to fatigue damage progression. TSA can identify such stress re- distribution by means of detecting surface temperature amplitude. Highly fatigue-damaged area of pw-CFC was localized if the matrix toughness was high, although moderately damaged area grew all over the specimen. The experimental results indicate that the fatigue life and damage of pw-CFC are strongly governed by matrix toughness.

This work aims at presenting two electromagnetic methods for non-destructive evaluation of delaminations in graphite- epoxy composites: the eddy current holography and the measurement of field reflected by delamination. The obtained results permit to get delamination images that are compared to the results obtained by ultrasonic C-scan procedure which is considered as reference method.

Until recently, thermographic methods for NDE have generally been quantitative, relying heavily on operator interpretation of image data. Although quantitative methods have been developed, they have generally required a priori knowledge of the sample physical properties, or identification of a defect free region within the field of view. Recent advances in pulsed thermography allow reference-free measurement of defect size, sample thickness and material properties without operator intervention or a priori knowledge of sample properties. An essential component of these advances are new signal processing methods based on both the spatial and temporal thermal response of the sample surface temperature to an instantaneous heat pulse. These methods provide a significant reduction in noise and blurring due to lateral diffusion of heat, and effectively increase the maximum penetration depth and spatial resolution beyond that of conventional thermography.

The goal of this work is to develop a multi-sensor nondestructive evaluation (NDE) approach to characterize aluminum alloy airframe structures under polymeric corrosion protective coatings. Two main efforts are highly relevant: (1) studying different degradation processes in the polymers to estimate the coating performance in service; and (2) detecting and quantifying early stages of corrosion beneath an intact coating. To address these tasks we employed acoustic and thermographic NDE techniques, especially Scanning Acoustic Microscopy and Fan Thermography. SAM can be utilized to map either coating or interface properties (C-scans). The method revealed potential to determine the curing quality of the coatings. It was also possible to detect small corrosion pits under delaminated areas. Furthermore, we evaluated the reflections of surface waves, which are generated and detected by the same probe. This provided an additional tool to examine the substrate/coating interface. Thermography was applied to detect corrosion under the coatings and sites of delamination. Fan Thermography (hot air heating) made it possible, to observe sites of decreasing adhesion over longer time periods. Both acoustic and thermographic results were correlated to electrochemical mapping of corrosion activity which was obtained by Scanning Vibration Electrode Technique (SVET).

Composite sandwiches have been used widely in flight controls of aircraft for many years; solid laminates have also begun to appear in primary structures such as the empennage. In their normal service life, composite parts may suffer damages and require repair and post-repair inspection. Nondestructive inspection is also needed for many of the rebuilt and refurbished parts in the maintenance, repair and overhaul industry. This paper describes the development of fieldable nondestructive inspection methods and instruments for composite structures and their repairs. For composite sandwiches the method developed is an instrumented tap test using the Computer Aided Tap Test (CATT) system. For repairs in solid laminates, the method used is ultrasonic pulse-echo C-scan using the Dripless Bubbler. The CATT system maps out the repaired region and produces an image of the local stiffness. Such images reveal voids and unbonds in a repair as areas of anomalously low stiffness; it also maps out areas of increased stiffness due to core potting and splicing. A number of examples of composite repairs inspected with the CATT system will be described. For engineered flaws in solid laminate repair panels from Boeing, scan images obtained with the Dripless Bubbler as a function of depth will be shown.

In this paper, a study based on the numerical simulation is performed to investigate the possibility of a new NDE system - Tapping Sound Analysis (TSA). Through the investigation of existing coin tap method, coin tap method cannot be used alone for the detection of defects inside general structure. TSA detects the existence of defects inside the laminated composite structures by comparing tapping sound with pre-computed sound data of healthy structures. Tapping on the structures is modeled as impact problem and solved using finite element method. Calculation of sound is formulated based on the coupled finite element and boundary element method. To extract the features from the tapping sound, wavelet packet transform is utilized. Numerical simulation of tapping sound and feature extraction scheme show that the tapping sound can be used in the identification of defects of laminated composites.

High frequency Non-Contact Ultrasonic (NCU) analysis of materials has been a persistent dream of materials engineers and scientists. However, a natural impediment to NCU is the exorbitant acoustic impedance mismatch between air and the test materials. The magnitude of this obstacle is further exacerbated in the Megahertz frequencies, necessary for high resolution, detectability, and sensitivity. This air- material acoustic impedance barrier has been successfully broken by virtue of our phenomenally high transduction piezoelectric transducers (from 100 kHz to 5 MHz) and equally high sensitivity non-contact analyzer (> 150 dB dynamic range and nano-second time-of-flight accuracy). It is now possible to transmit ultrasound in virtually all materials and all states of matter (with the exception of vacuum) without any contact with them. Unlike the conventional contact ultrasound, with NCU, characterization of materials is easy in the early stages of their formation and processing. This includes powders, green ceramics, consolidated powder metals, composite pre-pregs, and polymerization process. In this paper we provide a systematic introduction to the significant of materials characterization, NCU transducers and an analyzer. Also provided are a number of examples of NCU analysis of a wide range of materials, including their internal and surface characteristics. Materials are characterized for thickness, velocity (density and mechanical properties), microstructure, delaminations, defects, etc.

Acoustography is a full field, large area ultrasonic imaging method where a novel, wide area acousto-optic (AO) sensor is employed to form ultrasonic images similarly to real-time x-ray imaging. The AO sensor converts ultrasound directly into a visual image due to the inherent acousto-optic property of a proprietary mesophase material contained in the AO sensor. The AO sensor also offers exceptionally high pixel resolution, as a continuous layer of the mesophase material, with sensing molecules on the order of 20 Angstroms in size, senses the ultrasound. This paper will report on progress being made under a SBIR project to develop acoustography as an efficient and economical alternative to conventional point-by-point ultrasonic scanning (e.g. A-scan, C-scan).

Current requirements for structural integrity, product safety, process feedback control and smart structures require methods for monitoring material properties and component performance to improve quality control and decrease maintenance costs. In most applications the onset of damage occurs at the component surface due to plastic deformation, fretting, corrosion, or crack nucleation. Therefore, the first step in nondestructive evaluation of a component is a thorough evaluation of the surface conditions. White light interferometry provides a fast, non- contact method of characterizing the 3D surface topography with a 3 nm vertical resolution. Thus far, white light interferometry has not been widely utilized as a nondestructive inspection tool. In this paper, the applicability of white light interferometry for monitoring damage progression for applications that are surface condition sensitive will be examined. These applications include corrosion initiation and progression, fretting damage characterization, combustion byproduct characterization and surface crack detection. For each application, a method for obtaining a quantitative measure of damage and a metric that can be used to determine remaining component life will be discussed.

In the pulse-echo NDT approaches, which use ultrasonic or electromagnetic pulse to detect damages in structures, a common task is to locate the echo pulses and to analyze characteristics of them. This paper discusses a physical wavelet method for pulse-echo NDT approaches. The physical wavelet is used to stimulate the test object. The time and frequency localization feature makes this kind of signal suitable for detection and localization of damages. Because the input to the structure is a wavelet, the measured signal can be seen as a combination of wavelets with different delay, scales and weights. It is proposed to model the measured signal by several wavelet atoms and to use parameters of the model as features of the signal. Algorithms for the proposed method are developed. Features of the measured waveform are estimated first by iterative matching and then by gradient search for optimizing. Numerical verification of above method was carried out. The simulated structure is a concrete beam to be inspected by electric time-domain reflectrometry method. By using a transmission line model, damages of different sizes, locations and types are simulated. It is shown that the simulated damages can be well described by this wavelet stimulating and modeling method.

This work consists in applying the crystals theory and mechanical waves propagation in wood. The wood when is considered as a body of orthotropic symmetry satisfies Hooke's law in its tensor form. Therefore, from the dynamical point of view the elastic constant are expressed by means of Christoffel's equation and can be determined using the ultrasound wave propagation of different polarization through wood. To obtain the constants is necessary the measurement of longitudinal and shear waves in different directions. The experiment results show that in some cases the exact shear wave velocities are very difficult to measure due to waves superposition . In this work the elastic constants (three moduli of elasticity and three shear moduli) Pinus radiata D. Don growing in Chile by ultrasound trasmission techniques are estimated.

This paper describes a unique, disk spin simulation system currently being utilized at NASA Glenn Research Center. The system allows for precision controlled spin tests that can facilitate the application of various sensing technologies for in-situ detection of rotor damage. In addition, the disk spin simulation system has the capability for elevated temperatures up to 540°C (1000°F). The current rotor used to simulate a bladed disk consists of a 46 cm(18 in.) diameter, titanium disk with 30 machined gear teeth. The gear design imitates the blades of a compressor or turbine disk. Operating speeds for the system can reach 1000 revolutions per minute. This allows the system to achieve circumferential velocities paralleling those seen in actual aircraft engines. For this study, a new, innovative capacitive sensing system was used to monitor blade tip clearance (i.e., gear tooth clearance). In turn, the sensor information was employed to calculate the change in the center of mass of the rotor system. T he capacitive sensor and corresponding software were analyzed by attaching a localized weight at numerous positions on the disk. Upon calculating the change in the center of mass, the sensitivities of the sensor and software were established. In the end, it is hoped that by studying the motion and position of blades as well as the change in the center of mass of a rotor system, it may be feasible to identify alterations due to damage (e.g., cracks) eitehr in the blades or the disk itself.

A thermal imaging NDE method has been developed for nondestructive characterization of early stages of fatigue damage. The method is based on evaluation of the thermal effects induced in a material by a short-term mechanical loading. The mechanical loading causes in addition to thermoelastic temperature change, an increase due to heat dissipation that depends upon the microstructure of the material in a characteristic manner. The origin of this heat dissipation is the mechanical damping process. Utilizing the initial temperature rise due to a short-term mechanical loading, the dissipated energy per cycle was evaluated as a thermal parameter. This new thermal NDE parameter allows a quantitative characterization of the mechanical hysteresis, without the need for calibration to eliminate influences of thermal boundary conditions. The measurement of the thermal NDE parameters has been performed on Ti-6Al-4V dog-bone specimens, fatigued in low cycle fatigue (LCF) as well as in high cycle fatigue (HCF) experiments. Characteristic dependence of the NDE parameters on the already accumulated fatigue damage has been observed. The advantage of the thermal method is the applicability to components under service conditions because of simplicity, rapid measurements (a few seconds) and the ability of locally resolved evaluations.

There is a need to analyze locomotive wheels for flank cracks in a non-destructive manner in order to prevent catastrophic failures. Flaw, shape, and size are desired parameters in establishing the quality of commercial tires. A variety of defects such as voids, inclusions, surface and internal cracks, or the like, must be discerned in order to prevent failure. This paper exhibits and compares the benefits of a number of different techniques used for flaw detection. Non-destructive evaluation techniques used consist of a magnetic particle, dye penetrant, eddy current, electro-magnetic acoustic transducer (EMAT), and longitudinal and shear wave ultrasonic inspection. The techniques vary in their ability to ascertain the flaw characteristics. Surface, sub-surface, and internal defects were visualized using the various methodologies. Magnetic particle, dye penetrant, and eddy current inspection techniques are viable methods for looking at surface flaws. Depending on the penetration depth, sub- surface flaws were also detectable via these methods. EMAT and ultrasonic transducer methods can be used to find surface, subsurface, and internal flaws based on the configuration utilized.

This paper contains a description of an unique wave that was created in a two plate system separated by a layer of water. First, a Lamb wave was created in the upper plate by placing a transducer on a wedge on top of the plate at an appropriate angle and frequency. This wave was created to act like quasi-Rayleigh (surface) waves on both surfaces of the plate. The wave on the bottom surface of the upper plate then leaked through the water into the upper surface of the lower plate. We will show both experimentally and theoretically that the wave on the surfaces of the plates in contact with water leak constructively to create a leaky-wave that can travel great distances.

The cracking and failure in ceramic substrates during the laser drilling process has been acknowledged as a major problem by designers and manufacturers in the electronic component industries. The cracking and failure is due to large localized thermal stresses within the narrow heat-affected zone on the ceramics. Although the knowledge of the stress distribution in the ceramic substrate is important in understanding and solving the cracking/failure problem, it is impossible to measure the stress directly. The physical parameters of the laser drilling process such as temperatures or displacements, which can be directly related to stresses, can however be measured. That is why, in this research, an electronic speckle pattern interferometer (ESPI) system was designed and used to take speckle pattern images of the ceramic surface during the laser drilling process. Using commercial software, the speckle fringe images were image processed to quantify whole-field transient out-of-plane displacement measurements. A deformation history of the ceramic surface during the laser shaping process with millisecond temporal resolution was obtained, restricted only by the camera frame rate, camera resolution and laser power available. A finite difference model was developed to compare the deformation measurements with the predicted strain calculations. The experimental study and the analysis show that the designed in-situ electronic speckle pattern interferometer system provides an excellent experimental basis for whole- field, transient deformation measurements of ceramic substrates during the laser drilling process.

Fibre reinforced plastics offer high specific mechanical properties (performance vs. weight ratio). Thus during the last decade, they have been increasingly used as components in engineering structures and particularly for aeronautical applications. Ageing, load-transfer, and off-axes behaviour of composites are directly dominated by the viscoelastic matrix properties linked to cure process. So, there is a growing need for sensors, which provide real-time, in-situ monitoring of the manufacturing process. This study proposes to follow the cure mechanism of an epoxy-amine resin using simultaneously three sensors embedded in the material: The fibre-optic sensor is based on the measurement of angular distribution of light transmitted through an optical fibre inside the cured polymer. The original siloxane cladding is removed from the central part of the fibre. Then a sample of curing epoxy is placed around the stripped region. It is thus possible to monitor the refractive index variation of the polymer. Frequency dependant dielectric measurements provide a sensitive in situ sensor able to access to the electrical conductivity and complex permittivity of the surrounding medium. The conductivity parameter related to the ionic mobility is linked to the polymerisation advancement. The ultrasonic waves are generally used for global characterisation of mechanical properties. For in situ applications, a piezoelectric implant is embedded in the structure during its processing and the rheological properties of such a material-system can be monitored. The parameters, determined simultaneously with these three techniques, allow to understand the different steps of the epoxy cure regarding molecular motion, viscosity, density and their consequences on the mechanical properties of the material.

Fabrication strains during cure can lead to warpage or spring-back, which presents difficulties in the assembly of composite structures and residual stresses due to differences of thermal expansion coefficients between fiber and matrix in cooling stage can have intense effects on the mechanical properties of the composite product. The quality of the final composite part is directly affected by cure time, temperature, and pressure. The knowledge of cure process is very helpful to obtain high quality composites at reduced producing cost. Therefore, sensors capable of monitoring the cure process are desired and fiber optic sensors are the good candidate for cure monitoring of composite materials. In this paper, we present the simultaneous monitoring of the fabrication strain and temperature during the composite cure process by using fiber optic sensors. Fiber Bragg grating/extrinsic Fabry-Perot interferometric (FBG/EFPI) hybrid sensors are used to monitor those measurands. The characteristic matrix of the sensor is analytically derived and measurements can be done without sensor calibration for each experiment. A wavelength-swept fiber laser is utilized as a light source. FBG/EFPI sensors are embedded in graphite/epoxy composite laminates at different direction and different location. We perform the real time monitoring of fabrication strains and temperatures at two points of composite laminates during cure process in an autoclave. Through this experiment, we can provide a basis for the efficient smart processing of composites.

For the acceleration of the osteointegration processes of the metals implants we deposit on their surface the biokompotible ceramic coatings on the basis of hydroxyapatite. However such coatings have a certain deficiency connected with the absence of the necessary strength characteristics for a such kind of the implant. That's why it actual to create the coatings having beside biological compatibility the necessary strength and springy- elastic properties. We have developed the method of the receiving of the new biocompatible coatings with gradient structure over width on the titanium substrate. The essence of the developed method is in plasma coatings deposition within beforehand given supply of the powder consisting of two components (oxyde aluminum and hydroxyapatite) in the process of the deposition. It's showed that the received gradient coatings are the mixture of the crystals Al₂O₃ and HA, the concentrations of which change over the width. The topological investigation of the surface and the coating cross-sections was performed from which the chemical composition distribution over width was studied and found the absense of the chemical interaktion between Al₂O₃ and HA. By regulation of the distribution of the initial components over the coating widths it is possible to set the phase comsposition and crystal sizes in the biocompatible coatings. In the process of etching in the solutions imitating the physiological ones it is found the formation of the pores having dendritic structure. The performed investigations show the possibility of the usage of the developed coatings in medicine particularly in stomatolgoy.

Frequency multiplier in X-ray region has not been reported. This paper proposes to design the x-ray frequency multiplier, which is based on recent discovery of anti- Compton effect of X-rays from polyethylene terepthalate and low density polyethylene. Electronic system of most of the polymeric atom get excited and some electrons recoil when it is irradiated with x-ray in Compton process. When recoil electron transfers its energy partly or fully to incident x-ray photon it produces anti-Compton effect. At high scattering angle about 160°, the wavelength of anti-Compton scattered radiation decreases by 0.047A0 and there is enhancement of frequency. Fixing another polymer crystal at the calculated position of anti-Compton line, the wavelength of re-scattered radiation at 160° can further be decreased by 0.047A⁰ or thereby enhancing the frequency. This principle of repeated scattering can be used to multiply the X-ray frequency. In general, x-ray absorption depends approximately on Z. while maximum possible diffracting power is proportional to only Z², where Z is the atomic number. Thus the crystals of polymer are expected to form better monocromator for designing XFM. The production of high energetic and low intense x-ray by XFM may be used in radiotherapy.

Shot peening is a well-known method for extending the fatigue life of metal components by introducing near-surface compressive residual stresses. The capability to nondestructively evaluate near-surface residual stress would greatly aid the assurance of proper fatigue life in shot-peened components. This paper describes our work on near-surface residual stress measurement by an ultrasonic surface wave method. In this method, a variation of ultrasonic surface wave speed with shot peening intensity is measured. Since the effective wave penetration depth is inversely related to the excitation frequency, the method has the potential to provide the stress-depth profile. The paper presents results from an ultrasonic characterization study of shot peened Al-7075 and Waspaloy surfaces. Rayleigh wave velocity measurements by a V(z)-curve method were made on smooth and shot peened samples using line-focus ultrasonic transducers. Several factors were found to contribute to the surface wave velocity measurements: surface roughness, near-surface grain reorientation (texture), dislocation density increase, and residual stress. In this paper we estimate quantitatively the effects of each factor and discuss how these effects can be separated and accounted for during residual stress measurement.