The dynamic response of a cracked Jeffcott rotor passing through the critical speed with constant acceleration is investigated analytically and numerically. The nonlinear equations of motion are derived and include a simple hinge model for small cracks and Mayes' modified funciton for deep cracks. The equations of motion are integrated in the rotating coordinate system. The angle between the crack centerline and the shaft vibaiton (whirl) vector is used to determine the clsoing and opening of the crack, allowing one to study the dynamic response with and without the rotor weight dominance. Vibration phase response is used as one of possible tools for detection the existence of cracks. The results of parametric studies of the effect of crack depth, unbalance eccentricity orientation with respect to crack, and the rotor acceleration on the rotor's response are presented.

Systems Planning and Analysis, Inc. (SPA) has developed a novel statistical approach to estimating the remaining useful life of aircraft components based on known usage monitoring data. The analysis technique is known as the Remaining Useful Life Estimation (RULE) methodology. The basic premise of RULE is to determine conservative predictions for the component loads and fatigue life values from Monte Carlo simulations based on a desired component reliability. Then, as the aircraft's usage is monitored, the component life can be calculated with a known reliability based on the conservative predictions generated by the Monte Carlo simulation. The RULE methodology, which has been successfully tested on small-scale analytical problems, is ideally suited to be integrated into both rotorcraft and fixed-wing aircraft. Furthermore, modifications to the technology may prove to be applicable to wide variety of health and prognostic problems.

The use of giant magnetoresistive (GMR) sensing elements in inductive sensors permits low frequency operation for materials characterization and defect detection in aerospace and engineering materials. This offers a substantially increased depth of sensitivity over conventional eddy-current sensing coils and also allows new measurement capabilities, such as the non-contact remote monitoring of temperature and stress variations through material layers. This paper provides an overview of the Meandering Winding Magnetometer (MWM) drive winding constructs that incorporate GMR based sensing elements. The sensors are designed so that the magnetic field distribution created by the primary winding and the resulting response of sensing elements can be accurately modeled. Representative applications to be described include (1) detection and imaging of 3% material loss in a 6.4-mm (0.25-in.) thick aluminum plate, (2) monitoring of temperature variations of an aluminum plate located behind another 6.4-mm thick aluminum plate with an air gap between the plates, and (3) independent measurements of stress (through magnetic permeability measurements) in a steel plate located behind an aluminum plate with an air gap between the plates.

A computer vision inspection system, named Edge of Light TM (EOL), was invented and developed at the Institute for Aerospace Research of the National Research Council Canada. One application of interest is the detection and quantitative measurement of “pillowing” caused by corrosion in the faying surfaces of aircraft fuselage joints. To quantify the hidden corrosion, one approach is to relate the average corrosion of a region to the peak-to-peak amplitude between two diagonally adjacent rivet centers. This raises the requirement for automatically locating the rivet centers. The first step to achieve this is the rivet edge detection. In this study, gradient-based edge detection, local energy based feature extraction, and an adaptive threshold method were employed to identify the edge of rivets, which facilitated the first step in the EOL quantification procedure. Furthermore, the brightness profile is processed by the derivative operation, which locates the pillowing along the scanning direction. The derivative curves present an estimation of the inspected surface.

Multi-frequency techniques are widely adopted for eddy current testing. One of the advantages of these techniques can be deduced from the skin depth formula (formula available in paper) where delta is the standard depth of penetration at excitation frequency f, with the other two parameters, mu and sigma, related to material properties. Thus, an inspection can be performed at several depths into the material with the simultaneous use of multiple frequencies. To investigate the potential of a multi-frequency eddy current technique (MFECT) for corrosion quantification, an experiment was carried out on a two-layered fuselage lap joint splice. Two data fusion approaches, namely Bayesian inference and multiresolution analysis, are investigated in this study to fuse eddy current images of different frequencies. The corrosion types are classified based on the percentage of material loss. The estimated thickness results, based on the fusion processes, are compared with accurate thickness maps obtained from teardown X-ray inspection data.

In this paper an experimental and theoretical investigation of the applicability of the time-reversal concept to guided waves in plates, where the waves are dispersive and of multi-modes. It is shown that although temmporal and spatial focusing can be achieved through time reversal, the dispersive behavior of the flexural waves renders it impossible to exactly reconstruct the waveform of the original excitation. Nevertheless, the temporal and spatial focusing allows the development of a synthetic time-reversal array method for a distributed network of sensors and actuators. This new method, which overcomes the limitation of the conventional phased array method that operates under pulse-echo mode, can considerably enhance the signal strength, thus reducing the number of sensors and actuators required to achieve a given signal-to-noise ratio.

Strong, lightweight, temperature-resistant ceramic matrix composite (CMC) materials such as carbon fiber reinforced silicon carbide (C/SiC) are being developed for use in reusable launch vehicles. C/SiC coupons were developed to investigate damage behavior due to tensile and fatigue testing. In order to describe the nature of damage in this material a nondestructive evaluation technique that can detect damage progression is necessary. This study determines acousto-ultrasonics’ (AU) capabilities and limitations for the detection of damage in these composites. AU parameters were evaluated for two sets of C/SiC coupons prior to interrupted fatigue testing. In addition, a single coupon was tested with two different loading configurations. The statistical significance of several AU parameters is determined for characterizing this composite material. Ten AU waveforms were collected along the gauge length of the C/SiC coupons prior to tensile and fatigue testing. Three operators collected the waveforms from each set of coupons to check repeatability. These waveforms were processed with an analysis routine that calculates AU parameters such as ultrasonic decay rate, the first moment of the power spectrum (M0), and the centroid of the power spectrum (fc). The results will recommend the most repeatable AU parameters and loading configuration for future evaluation of C/SiC components.

Acousto-ultrasonics (AU) is a NDE technique that utilizes two ultrasonic transducers to interrogate the condition of a test specimen. The sending transducer introduces an ultrasonic pulse at a point on the surface of the specimen while a receiving transducer detects the signal after it has passed through the material. The aim of the method is to correlate certain parameters of the detected waveform to characteristics of the material between the two transducers. The waveform parameter of interest is the attenuation due to internal damping for which information is being garnered from the frequency domain. The parameters used to indirectly quantify the attenuation are the ultrasonic decay rate as well as various moments of the frequency power spectrum. For the most part, AU is used to gage the damage state of materials subjected to various mechanical or environmental loads. The AU technique has been applied to polymer matrix composites, ceramic matrix composites, metal matrix composites as well as metallic alloys. Historically, AU has been a point by point, manual technique with waveforms collected at discrete locations and post-processed. Data collection and analysis of this type limits the amount of detail that can be obtained. Also, the manual movement of the sensors is prone to user error and is time consuming. This paper discusses an automated AU scanning system recently developed and assembled at NASA Glenn Research Center. The paper will include a description of the hardware and software systems as well as the techniques for data reduction and presentation. In order to demonstrate the system capabilities, AU scan results for a SiC/SiC composite panel are presented.

Epoxy resins are essential to the fabrication of carbon fiber reinforced composites (CFRCs). This paper investigates laser generated ultrasound in epoxy resins using three pulsed lasers: A TEA CO₂, a fundamental Nd:YAG and a XeCl excimer. In the low power thermoelastic regime, the laser beam causes the surface of the sample to expand rapidly, in times that are comparable to the rise time of the laser pulse. In non-metals the phenomenon is dominated by the optical absorption depth, which is a function both of the properties of the material and the laser wavelength, and for epoxy resins, varies from a few microns to several millimeters. Compared to the thermoelastic source in metals, a bigger volume of the material is affected, the temperature rise is less and the amplitude of the longitudinal wave is greater. This condition is referred to as "a buried thermoelastic source". In CFRCs, the superficial layer of epoxy resin (typically 50-100 microns thick) plays an important role to the generation mechanism. At the Nd:YAG wavelength the epoxy is transparent and acts as a constrained layer. At the TEA CO₂ and the XeCl excimer wavelengths both the epoxy and the underlying fibers absorb strongly. Experiments were carried out on CFRC and pure epoxy resin samples, comparative results and efficiency graphs are presented.

Many small composite parts undergo manual pulse-echo scan because 1) the set-up time for and automated scan is unjustifiably long and 2) the automated scan does not provide the flexibility to cope with frequent angle changes in a complex geometry part. Manual scans can be time consuming, laborious, and are prone to errors due to operator fatigue and subjectivity. What is required is a full-field ultrasonic inspection system analogous to real time radiography that allows the operator to perform ultrasonic inspection by manipulating the part under a systems field of view. In this paper, we will present an acoustography-based ultrasonic inspection system developed under a SBIR (Small Business Innovation Research) award that is bringing this vision to into reality. Acoustography is the ultrasonic analog of radiography and photography. A unique, wide area 2D detector, called acousto-optic (AO) sensor, is used to directly convert ultrasound into visual images; much like a fluorescent screen is able to convert x-rays into visual images. It offers the potential for providing the NDT engineer with a large field of view (e.g. 6”x 6” or larger) and a capability to inspect complex shaped parts in real time.

The present article reports a technique to measure velocity of an organic film deposited on a homogeneous substrate, wherein the thickness of the film and the diameter of the measured area of the specimen are in the order of a few microns. A thinly sliced human kidney was selected as an example of an organic film. The thickness of the specimen was substantially 3 μm. For the substrate, fused quartz was used because its elastic properties are known and stable. The spherical acoustic lens was used to determine the position for measurement. The frequencies at 400 MHz and 600 MHz were used for the measurement and the visualization, respectively. The generation of the Rayleigh waves under the above conditions was simulated by numerical calculations based onthe wave propagation theory for layered media.

A new fiber-optic vibration sensor has been developed and applied to structural health monitoring. The sensor is based on new finding that frequency of light wave transmitted through an bended optical fiber is shifted by vibration at the bended region. The principle can be explained that Doppler's effect of the light wave which changes its direction in the optical fiber. Several configurations of the sensor have been designed, and very high sensitivity has been achieved in the extremely wide frequency band by applying laser Doppler velocimetory. Practically numberless sensing points can be arranged on a single optical fiber, and regional monitoring which covers large area of the structures can be achieved. The performance of the sensor is examined experimentally by applying to detection of AE signals and elastic wave propagation in the composite material and adhesive joint test specimens. The experimental results show that the sensitivity is almost equivalent to PZT sensor and that failure of composite materials and debonding in the adhesive joint can be detected successfully. The durable, low-cost and high-sensitive sensor can show a new scope to structural health monitoring in the very variety of applications, for examples, composite aerospace structures, energy plants, piping system, infrastructures such as bridges and tunnels, surface and underground facilities. University and industry collaboration initiated a new business of new NDE for health monitoring and diagnostics.

The use of integrated Lamb wave sources (piezoelectric transducers) is known as a possible way of performing integrated, on-line health monitoring. Either omnidirectional (circular) or quasi-unidirectional (bar-shaped) transducers can be used. However, both of them have their own drawbacks, which makes them not optimal. A much more satisfying solution could be the use of phase-delayed multi-element arrays to perform angular steering of the emitted Lamb wave beam. In this paper both the proper conditions and the limitations for the applicability and performance of Lamb wave beam steering using integrated piezoelectric arrays are established. Then experimental damage detection capabilities using this principle are demonstrated.

Ultrasonic inspection uses sound waves of short wavelength and high frequency to detect flaws in materials. The pulse of ultrasonic energy that is reflected by a discontinuity such as a void, delamination, inclusion or any other type of imperfection is highlighted as flaw in the ultrasonic image. Usually an ultrasonic signal that is reflected from the material contains multiple interfering microstructure echoes, with random amplitudes and phases. The interference noise produced by the unresolvable scatterers is randomly distributed throughout the material, thus reducing the diagnostic value of the ultrasonic images. In the process of developing structural health monitoring techniques is proposed to improve the signal-to-noise ratio in the ultrasonic B-scan image. Wavelet Packet Transforms are used for decomposing the signal to obtain maximum detail inforaiotn about the flaws. Once the input signal (B-scan image) is split into different frequency channels, a selection of useful information about flaw is done based upon the statistics of the detail images. An adaptive thresholding procedure is employed to extract the flaw information from the selected detail images. The method has been verified with reasonable accuracy in prediction disbonds in composite patch repairs of aging airframes. The advantage of this method is that it gives the flaw location that is easier to interpret with less ambiguity. The amount of data being processed is less thus reducing the complexity of processing. The method proved successful in locating the delaminations along the length and width of the composite patch. The procedure has also been applied to detect damage in multiple locations.

Ceramic matrix composites are being considered as candidate materials for high temperature aircraft engine components to replace the current high density metal alloys. The current Ceramic Matrix Composites (CMC) are engineered material composed of coated 2D woven high strength fiber tows and melt infiltrated ceramic matrix. Matrix voids are common anomalies generated during the melt infiltration process. The effects of these matrix porosities are usually associated with a reduction in the initial overall composite stiffness, and an increase in the thermal conductivity of the component. Furthermore, the role of the matrix as well as the coating is to protect the fibers from the harsh engine environment. Hence, the current design approach is to limit the design stress level of CMC components to be always below the first matrix cracking stress. In this study, the effects of matrix porosity on the initial component stiffness and the onset of matrix cracking are analyzed using a combined NDE/Finite-Element Technique. The Computed Tomography (CT) is utilized as the NDE technique to characterize the initial matrix porosity's locations and sizes in various CMC test specimens. The Finite Element is utilized to calculate the localized stress field around these pores based on the geometric modeling of the specimen's CT results, using image analysis and geometric modeling software. The same specimen was also scanned after tensile testing to a maximum nominal stress of 150 MPa to depict any growth of the previous observe voids. The post test CT scans depicted an enlargement and some coalescence of the existing voids.

This paper discusses recent advances in modeling and simulation of an artificial neural system and simulation of wave propagation for designing structural health monitoring systems. An artificial neural system was modeled using piezoceramic nerves and electronic components. Wave propagation in a panel is modeled using classical plate theory and a closed-form solution of wave propagation and reflection is obtained. Equations representing a half sine input similar to a projectile impact or a tone burst excitation were added to the existing algorithm that predicts the response of the artificial neural system due to impulse inputs. Firing switches have been modeled in the simulation to predict the sequential firing of the neurons as the waves pass over them. Also, new active fiber sensors have been designed for use in the artificial neural system. Simulated responses of the artificial neural system are shown in this paper and indicate that large neural systems can be formed with hundreds of sensor nodes. Experiments were performed to study a small neural system on a glass fiber panel. Waves were induced in the panel due to a lead break to simulate a crack and due to an impact from an impact hammer. Testing showed the location of a crack could be determined within the unit cell of the neural system for an orthotropic panel.

This paper discusses the development of continuous Active Fiber Composite sensors to detect damage in composite materials. Continuous sensors contain multiple interconnected sensor nodes that can be integrated into an artificial neural system as an array of sensor nerves. Continuous sensors have demonstrated a possibility of damage detection in large structures when used as a part of Artificial Neural System. The advantage of this passive health monitoring approach is that the sensor system is highly distributed and uses parallel processing allowing large structures to be monitored for damage using a small number of channels of data acquisition. In the paper, the continuous sensor system is modeled and simulated by solving the elastic response of a plate and the coupled piezoelectric constitutive equations. The model and simulation allow the sensor system to be optimized for a particular material and plate size. The simulation predicts that acoustic waves representative of damage growth can be detected anywhere in the plate using a simple artificial neural system. To improve the sensitivity of the continuous sensor, unidirectional active fiber composite sensors were built from piezoceramic ribbon preforms. Manufacturing of the active fiber composite sensors is discussed in the paper. The continuous sensors were evaluated in a realistic test to show their ability detect acoustic emissions caused by damage to a composite material. The sensors were mounted on narrow glass fiber plates and tested to failure in a mechanical test machine. Results from the experiments are also presented.

Quadrupole Resonance (QR) has recently been shown to be a feasible method for the non-contact measurement of strain in polymeric fiberglass-reinforced composites. Tiny crystals of a QR active additive are embedded into the composite or are applied as part of a surface coating. Strains in the composite are transferred to the additive crystals. These crystals can be interrogated via radio frequency pulses provided by a single-sided radio frequency coil. Thus, the additive crystals give rise to a strain dependent QR frequency response. The QR frequency and line width from composites containing additive are found to be sensitive parameters for the measurement of tensile strain. The QR active additive that was embedded in the composite matrix was found to be inert and non-intrusive.

Since fiber reinforced polymer composites are becoming increasingly popular, nondestructive damage detection in these materials has become an important issue. Real time structural health monitoring using Fiber Optic Polarimetric Sensors (FOPS) in an interesting field of research in recent years. This paper presents the performance of FOPS using three different optical sensing fibers: low birefringence (low-bi), high birefringence (high-bi) and an ordinary single mode polarization maintaining fiber. The static and dynamic response of these fibers are experimentally evaluated under different conditions. The experiments were conducted on composite structures by embedding the optical fibers into specimen. Composite materials are extensively used in aerospace applications. The sensitivity of all fibers under different conditions is discussed for static and dynamic loading.

This paper presents a Novelty-based detection technique to identify core material properties of honeycombs and cellular structures. A numerical model (FE) representing full scale and/or reduced size of the cellular solid is used to generate transmissibilities between topological points at cells in different locations, with a statistical Gaussian distribution of the core material property target variable. The numerical set of transmissibilities is then used in a Novelty detection framework to find Euclidean and Mahalanobis distances from analogous data from a point excitation experimental test carried out with SLV.

This paper presents the dynamic characteristics and damage detection of an aircraft wing panel using a scanning laser vibrometer. The panel has an irregular shape with side lengths 16.44" x 14.82" x 11.10" x 5.38" x 14.22", different values of thickness (0.059" to 0.110"), and seven ribs on its backside. An in-house finite element code GESA is used to model the panel using 528 DKT plate elements and to obtain mode shapes and natural frequencies, and Operational Deflection Shapes (ODS) are measured using a scanning laser vibrometer. Results show that numerical dynamic characteristics agree well with the experimental ones. Six defects are created in the panel, including four small nuts glued on the backside and two small slots cut by electron discharge machining. Detection of the six defects is performed using the distributions of RMS velocities under high-frequency broadband periodic chirp excitations provided by a PZT patch and damage locating curves obtained by processing experimental ODSs using a newly developed BOudnary Effect Evaluation (BEE) method. The BEE method is non-destructive and model-independent; it processes experimental ODSs to reveal local boundary effects caused by defects. Experimental results show that the six small defects in the panel can be pinpointed using the approach.

Wire integrity has become an area of concern to the aerospace community including DoD, NASA, FAA, and Industry. Over time and changing environmental conditions, wire insulation can become brittle and crack. The cracks expose the wire conductor and can be a source of equipment failure, short circuits, smoke, and fire. The technique of using the ultrasonic phase spectrum to extract material properties of the insulation is being examined. Ultrasonic guided waves will propagate in both the wire conductor and insulation. Assuming the condition of the conductor remains constant then the stiffness of the insulator can be inferred by measuring the ultrasonic guided wave velocity. In the phase spectrum method the guided wave velocity is obtained by transforming the time base waveform to the frequency domain and taking the phase difference between two waveforms. The result can then be correlated with a database, derived by numerical model calculations, to extract material properties of the wire insulator. Initial laboratory tests were performed on a simple model consisting of a solid cylinder and then a solid cylinder with a polymer coating. For each sample the flexural mode waveform was identified. That waveform was then transformed to the frequency domain and a phase spectrum was calculated from a pair of waveforms. Experimental results on the simple model compared well to numerical calculations. Further tests were conducted on aircraft or mil-spec wire samples, to see if changes in wire insulation stiffness can be extracted using the phase spectrum technique.

The principle of infrared thermography is presented briefly. And delamination defects of honeycomb aluminum composites were inspected by infrared thermography technique. Testing result showed that infrared thermography was a rapid, effective nondestructive evaluation method for delamination defects of composite materials. The optimal testing time for different defects was different. And the testing speed was higher than two screens per minute for all delamination defects of honeycomb aluminum composites. Rapid automatic heating modes, automatic synchronous scanning of heating source and infrared detector, and intelligentized defect identification system will be the important developing aspects of infrared thermography testing technique.

Advanced high performance materials and components such as CFRP, GFRP and Smart Structures require improved testing techniques. The first part of our contribution deals with nonlinear vibrometry as a defect selective non-destructive testing method. This method uses higher harmonics (which are generated only at defects) to locate the defect by scanning across the surface of the sample with a laser interferometer. For input coupling of the elastic wave both an external (like ultrasound welding converters) or internal (integrated piezo actuators) excitation source can be used. The external detection tools are a microphone or a scanning laser vibrometer. With this technique, we characterized Smart Structures made of aerospace materials and composites with embedded piezoelectric actuators. The next part is about health monitoring techniques and diagnostics where integrated elements are used for excitation and detection. Thus, we monitored the transfer function over a large frequency spectrum and especially its changes caused e.g. by defects. Changes in the properties of structures by fatigue, impacts, and thermoplasticity have been successfully observed. Also the changes in reinforced plastics under tensile stress have been monitored. The results were correlated with destructive measurements. For health monitoring we also present the impedance analysis of embedded piezo ceramic sensors. A defect causes changes in the modal response of the hole structure and that effect can be detected using the phase angle of the electric impedance of the piezo element. Additionally some types of defects cause a non-linear behavior of the structure which was verified by extracting higher harmonics as a reaction to sinusoidal single frequency excitation.

This work deals with the processing of GPR (ground penetrating radar) signals for AP (anti-personnel) mine detection. It focuses on two steps in this processing, namely the deconvolution of the system impulse response, and the extraction of target features for classification. The objective of the work is to find discriminant and robust target features by means of time-frequency analysis. Deconvolution is an ill-posed inverse problem, which can be solved with regularization methods. In this paper a deconvolution algorithm, based on the iterative v-method, is proposed. For discriminant feature selection the Wigner distribution (WD) is considered. Singular value decomposition (SVD) along with the concept of the center of mass as the most robust feature are used for feature extraction from the WD. The proposed normalized time-frequency-energetic features have a good discriminant power, which doesn't degrade with increasing object depth.

The analysis of pertrubations on the thermal signature of the soil is a powerful tool for the detection of the presence of buried objects on the soil from measured infrared images but, by itself, gives litle insight in the nature of the detected targets. In this paper, we will present a method for the detection of surface and shallowly buried land mines in infrared images based on a 3D thermal model of the soil. This model will be used to detect perturbations on the expected behavior that will lead to the assumption of the presence of unknown buried objects. Next, we will outline a procedure that makes use of the theory on inverse problems in order to extract information of the natuer of the detected targets and to infer whether they actually correspond to land mines or not.

The detection of buried landmines is an important problem in regions where an army conflict has occurred. In particular, antipersonnel plastic mines cannot be detected with classical techniques, such as metal detectors. So a very promising detection technique based on a thermal model of the soil is applied to detect this kind of mines, in which infrared (IR) images of the soil are used. The core of this technique is the solution of the heat transfer process in the soil and at the soil-air interface, which is a very time consuming process. To overcome this problem we propose an analog circuit which can solve the equations that model the system reducing time cost by taking advantage of the inherent massive parallelism of the circuit. The description of the equations is made with VHDL--AMS and then an automatic synthesis tool generates a circuit which solves the equations.

A fighter aircraft has been instrumented with piezo sensors on a carbon fibre reinforced plastic main landing gear door. During ground handling and standard test flights, the piezo sensor output was recorded on a flight test instrumentation tape recorder with a frequency range from 30 kHz to 250 kHz. Parallel to the flight test, a number of laboratory impact tests have been performed on a separate CFRP main undercarriage door with the same sensor suite. The measured background noise level has been compared to the impact spectra from the laboratory tests. The comparison shows at least a difference of 30 dB between the impact and the background noise.

With the increasing use of advanced composite materials in aircraft, automobiles, military hardware, and aerospace composites (such as rocket motorcases) a sizable need for composite health assessment measures exist, particularly where there is risk of failure due to high mechanical and thermal stresses. For most epoxy-based laminate composites, even low-momentum impacts can lead to "barely visible impact damage" (BVD), corresponding to a significant weakening of the composite. This weakening can lead to sudden and catastrophic failure when the material is subjected to normal operating loads. Following the explosion of Delta 241 (IIR-1) on Jaunary 17th, 1997, the failure investigation board concluded that an entire fleet of Graphite Epoxy Motorcases (GEMs) should be instrumented with a health monitoring system. This system would provide continuous structural health data on the GEM from initial acceptance testing through final erection on the launch pad. The result presented here contribute significantly to the understanding of the acoustic properties of the GEM casing, and make a substantial advancement in the theoretical phase of the source location algorithm development. When the system is complete it will continuously monitor the structural health of the GEMs, communicate wirelessly with base stations, operate autonomously for extended periods, and fit unobtrusively on the GEM itself.

Many military platforms such as fighter aircraft are nowadays operated for several decades under sometimes varying missions. Additional requirements resulting from more severe fatigue spectra or extended life for these platforms may require additional means of ensuring structural integrity. It is then important to gain the maximum usage (fatigue life) of aircraft components most efficiently still ensuring structural integrity at all times. Conventional structural health monitoring systems are typically based on loads and usage monitoring. Together with modern non destructive damage detection techniques it could be possible to safely operate even aged platforms. This goal is achieved by periodic examinations in order to ensure that a structural item is free of damage. However, the dismantling of structures for the purpose of non destructive testing can be very costly, time intensive and sometimes harmful to the surrounding structure itself. Therefore integrated, reliable and affordable damage detection techniques are needed to avoid disassembly where economically or technically justified. Especially for well known hot spots an integrated damage sensor could provide an alternative solution to conventional procedures. SWISS (Smart Wide area Imaging Sensor System) is an ultrasonic imaging approach. A small sensor is permanently surface mounted on the component that is to be monitored. Typically the sensor is activated on ground and interrogated via cables that are built into the platform. These sensors facilitate the examination of the internal structure of a subcomponent. The ultrasonic beam is electronically controlled in order to scan the most critical areas from a fixed position. Functionality aspects as well as practicability issues of such a technology had to be addressed and solved. As a result of this study, simulated fatigue tests on a real complex fitting structure have proven the reliability of the imaging ultrasonic sensor under laboratory conditions for more than 60000 simulated flight hours without problems and that the high volume coverage proved to be beneficial for detecting even unexpected damages.

In this work, numerical modeling of tapping sound was introduced and verified by comparison with experimental results. Closeness of numerical results and experimental results were shown by feature indices. According to the comparison of numerical results and experimental results, sound print (tapping sound data of healthy structure) can be successfully obtained through present numerical modeling tapping sound. Also, an automatic tapping device (called a mouse robot) was developed for the efficient application of tapping sound analysis to realistic structures. To show the performance of automatic tapping device, a realistic composite structure was manufactured and tested. Test results showed the usefulness of automatic tapping device and performance of tapping sound analysis.

The replacement of chemical batteries with composite flywheels offers many potential advantages in space applications. Before such flywheels can be successfully employed, it is imperative to ensure their integrity using NDE techniques. Previously, the use of traditional C-scan for the NDE of flywheels was compared to Scanning Ultrasonic Spectroscopy. However, both C-scan and scanning ultrasonic spectroscopy are point-by-point inspection techniques, and are thus inherently limited in inspection speed. In this paper, the application of Acoustography for the NDE of flywheels is reported. Acoustography provides an efficient and economical alternative to point-by-point ultrasonic scanning; in this approach a novel, wide area (AO) sensor is employed to provide full-field, real time ultrasonic images similar to x-ray imaging, significantly decreasing the required inspection time. Side by side images generated using Acoustography and traditional C-scan techniques for Plexiglas cylinder and a composite ring standard (both with known defects) are presented.

An important way of increasing the payload in a reusable launch vehicle (RLV) is to replace heavy metallic materials by lightweight composite laminates. Compared to metallic materials, composite laminates are a relatively new class of materials and therefore require more attention to ensure the safety and reliability when they are used. Among various parts and systems of the RLV, this study focuses on tanks containing cryogenic fuel. Historically, aluminum alloys have been used as the materials to construct fuel tanks for launch vehicles. To replace aluminum alloys with composite laminates or honeycomb materials, engineers have to make sure that the composites are free of defects before, during, and after launch. In addition, the performance of the composite structures needs to be evaluated constantly. In recent years, the impedance-based health monitoring technique has shown its promise in many applications. A major advantage of this technique is that the procedure is nondestructive in nature and does not perturb the properties and performance of the materials and structures. This paper reports the results of applying the impedance-based nondestructive testing technique to the damage identification of composite laminates at cryogenic temperature. These materials have potential application for fuel tanks in future RLV’s. Regular and single-crystal piezoceramic sensor/actuators are tested to assess their performance under cryogenic temperature.

Space vehicles require high performance thermal protection systems (TPS) that provide high temperature insulation capability with lower weight, high strength, and reliable integration with the existing system. Carbon-carbon panels mounted with bracket joints are potential future thermal protection systems with light weight, low creep, and high stiffness at high temperatures. However, the thermal protection system experiences a very harsh high-temperature and aerodynamic environment in addition to foreign object impacts. Damage or failure of panels without being detected can lead to catastrophe. Therefore, knowledge of the integrity of the thermal protection system before each launch and reentry is essential to the success of the mission. The objective of the study is to develop a built-in diagnostic system to assess the integrity of TPS panels as well as to lower inspection and maintenance time and costs. An integrated structural health monitoring system is being developed to monitor the TPS panels. The technology includes investigation of the loosening of bolts which connects TPS panels to the supporting structure, and potentially, identifying the location of damage on the panel caused by external impacts from micrometeorites and other objects. The first generation prototype was manufactured and tested in an acoustic chamber which simulated a re-entry environment to investigate the feasibility of the health monitoring system focusing on its survivability and sensitivity. The preliminary results were very promising. Based on the test results, the second generation design was proposed to improve the performance of the first generation design. To put a reliable and accurate decision on the diagnostics of the TPS panels, an advanced algorithm was developed with the aid of a wavelet transform technique.