Showing papers in "Journal of Nondestructive Evaluation in 2009"

Carbon nanotube sensing skins for spatial strain and impact damage identification

Abstract: Impact damage, excessive loading, and corrosion have been identified as critical and long-term problems that constantly threaten the integrity and reliability of structural systems (e.g., civil infrastructures, aircrafts, and naval vessels). While a variety of sensing transducers have been proposed for structural health monitoring, most sensors only offer measurement of structural behavior at discrete structural locations. Here, a conformable carbon nanotube-polyelectrolyte sensing skin fabricated via the layer-by-layer technique is proposed to monitor strain and impact damage over spatial areas. Specifically, electrical impedance tomographical (EIT) conductivity mapping techniques are employed to offer two-dimensional damage maps from which damage location and severity can be easily and accurately quantified. This study deposits carbon nanotube-based sensing skins upon metallic structural plates with electrodes installed along the plate boundary. Based on boundary electrical measurements, EIT mapping captures both strain in the underlying substrate as well as damage (e.g., permanent deformation and cracking) introduced using an impact apparatus.

Modal Parameter Identification of Hagia Sophia Bell-Tower via Ambient Vibration Test

Abstract: Many of historical structures have degenerated in time by environmental effects, earthquakes, and winds because of the inadequate preservation. The preservation of historical heritage is considered a fundamental issue in the cultural life of modern societies. The protective measures can be supplied if the actual behaviour of the structures is known. The paper presents the results of ambient vibration test and operational modal analysis carried out on the historical masonry bell-tower of the Hagia Sophia church in Trabzon, Turkey. The bell-tower is about 23 m high and dates back to the XIII century. The study includes also the initial analytical model of the tower constituted by the geometrical survey. The experimental measurements are performed using two measurement setups in different times. In the first setup twelve uniaxial accelerometers are used, while in the second setup four triaxial accelerometers with one uniaxial reference are used with the aim of determining the bending and torsional mode shapes as well as natural frequencies and modal damping ratios of the tower. The analytical model of the tower is developed by using solid brick elements, and a relatively large number of finite elements have been used in the model to obtain a regular distribution of mass. The first five natural frequencies and corresponding mode shapes are determined from both theoretical and experimental modal analyses and compared with each other. A good harmony is attained between mode shapes, but there are some differences between natural frequencies. The sources of the differences are introduced in terms of variations in the elasticity modulus of walls, cracks on upper walls, and boundary conditions on base level.

Local Damage Detection with the Global Fitting Method Using Mode Shape Data in Notched Beams

Abstract: Damage in a structure alters its dynamic characteristics such as frequency response functions and modal parameters. The present study extends the ‘gapped smoothing method’ for identifying the location of structural damage in a beam by introducing the ‘global fitting method.’ The procedure uses only the mode shape data obtained from a damaged structure with an assumption that the undamaged structure is homogeneous and uniform and the damage size is small. The sensitivity of damage detection algorithm is evaluated using a finite element analysis (FEA) of a few beams having a notch. Structural irregularity index (SSI) was used to identify the locations and size of damage. The ability to detect damage was enhanced by averaging SSI over a few modes. A statistical procedure was applied to identify damage with respect to background noise. A methodology and quantitative criteria was developed to select the optimum excitation grid spacing. Numerical results showed that the present method can detect both narrow (13 mm width) and wide damage (126 mm width) associated with less than 3% local thickness reductions. Experimental results validated the numerical results and detected the depth to thickness ratio about 41% and 35% for the wide and narrow notch beams, respectively. The present method showed improved resolution on detecting the location and size of damage in a beam over the previous methods using mode shape data reported in literature.

Characterization of Flaws Embedded in Externally Bonded CFRP on Concrete Beams by Infrared Thermography and Shearography

Abstract: Carbon fiber-reinforced polymer (CFRP) has become an important material to rehabilitate deteriorating concrete structures. The quality of the bond between the externally-bonded CFRP and concrete elements is crucial to durability and structural integrity of the rehabilitated concrete structure. However, flaws between CFRP and concrete interfaces can reduce the effective contact area significantly and therefore, the overall bond strength at these interfaces. Such flaws are not readily noticed by naked eyes, but can be detected and quantified non-destructively and effectively by using full-field and non-contact infrared thermography (IRT) and laser shearography. Flaw and sound areas presented within the FRP-concrete interfaces manifest very different thermal decay and mechanical fingerprints. In this paper, we report a study which tested, with IRT and shearography, the sizes and shapes of 17 round-shaped and artificial flaws embedded in 6 CFRP-concrete specimens; and then compared the results with the actual flaw sizes and shapes. In general, the results show that IRT underestimated the actual flaw sizes by 6.1% on average, whilst shearography overestimated the actual flaw sizes by 9.4% on average. These results demonstrated that both IRT and shearography are promising non-destructive evaluation techniques that can be used to define flaw boundaries and determine flaw sizes within the CFRP-concrete composites.

Yarn Diameter and Linear Mass Correlation

Abstract: This paper focuses on the determination of the statistical correlation between yarn diameter and yarn linear mass. The experimental methods employed are based on optical analysis and on image processing techniques applied to electron microscope images. Several different cotton yarns were examined over a wide range of yarn linear masses. The results indicate that diameters predicted by the relationship commonly quoted in the literature can be as much as 62% smaller than those experimentally observed.

Utilization of Ultrasonic Measurements for Determining the Variations in Microstructure of Thermally Degraded 2205 Duplex Stainless Steel

Abstract: The aim of this study is to investigate the changes in attenuation of longitudinal waves and velocity of shear wave modes for assessing variations in the microstructure of thermally degraded 2205 duplex stainless steel samples that were aged isothermally at 700°C and 900°C for different time intervals. The evaluation of material microstructural changes such as phase transformation and second phase precipitation which are related to material properties is of primary importance to ensure quality of components. This paper presents evidence that indicate that the attenuation coefficient of the longitudinal mode is sensitive to gradual microstructural changes produced by the aging treatments. The gradual changes in microstructure are associated with reduction of impact properties. On the other hand, for samples aged at 700°C, the velocity of the fast mode of shear wave show changes at high aging times and does not show changes at early aging times were the materials properties are already degraded.

Damage Mode Identification for the Clustering Analysis of AE Signals in Thermoplastic Composites

Abstract: An objective analytical procedure for the investigation of damage mechanisms in the thermoplastic self-reinforced polyethylene (UHMWPE/PE) composites under quasi-static tensile load has been established, using Unsupervised Pattern Recognition (UPR) technique for the clustering task of Acoustic Emission (AE) signals. Focus is on the correlating between the obtained classes and their specific damage mechanisms. This was carried out by waveform visualization and Fast Fourier Transform analysis. Pure resin and fiber bundles were tested to collect typical waveforms of matrix cracking and fiber fracture respectively, in order to label the signal classes in the composites. The evolution process of various damage mechanisms in the composites revealed that the correlating method was effective. The AE characteristics of different damage modes found out in this study can be used as the reference for identifying unknown AE signals in the UHMWPE/PE composites. The established procedure is also potential in the investigation of failure mechanisms for composite materials with UPR technique.

Broadband Poly(vinylidene fluoride-trifluoroethylene) Ultrasound Focusing Transducers for Determining Elastic Constants of Coating Materials

Abstract: This paper develops broadband Poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] focusing transducers and its surface wave measurement system for determining the elastic constants of coating layers. The measurement is based on a defocussing measurement method and V(f,z) analysis of the focusing transducers. Few P(VDF-TrFE) focusing transducers are successfully fabricated in laboratory and the wave measurements can cover a wide frequency range of 4–120 MHz with great accuracy. Brass and glass substrates electroplated with nickel coating layer with thickness ranging from 15 to 60 μm are tested in this work. Dispersion curves sensitivity analysis is carried out to decide the best approach for inversely determination of coating elastic properties. A searching method based on the downhill simplex algorithm and numerical calculation on waves in a layered half-space model is used for inversely determining the elastic constants of coating layer. The results show good agreement with reported data. Measurement accuracy and potential applications for other types of nondestructive evaluation of the focusing transducers and measurement system are addressed.

Structural Properties of Doped GaN on Si(111) Studied by X-Ray Diffraction Techniques

Abstract: X-ray diffraction (XRD) is a non-destructive technique which is widely used in material characterization, particularly to determine structure, crystalline quality and orientation of samples. Two representative samples are used in this work, these samples are sample I (Si-doped GaN/AlN/Si) nominally consisted of 284 nm AlN followed by 152 nm of Si-doped GaN, and sample II (Mg-doped GaN/AlN/Si), grown with 194 nm AlN followed by 136 nm of Mg-doped GaN. Both doped GaN films were investigated by high resolution X-ray diffraction (HRXRD) with rocking curve (RC) measurement around the symmetrical (0002) and asymmetrical (10 \(\overline{1}\) 2) diffraction peaks. The phase analysis result revealed that monocrystalline GaN was obtained. The XRD pattern show sharp and well separated (000l) reflections of doped GaN and AlN indicating complete texture with GaN[0 0 0 2] ∥ AlN[0 0 0 2] ∥ Si[1 1 1]. From the HRXRD RC ω/2θ scans of (10 \(\overline{1}\) 2) and (0002) plane, we determined both a and c lattice parameters of the doped GaN. The symmetrical and asymmetrical RC full width at half maximum (FWHM) of doped GaN were obtained.

Improving Source Location Accuracy of Acoustic Emission in Complicated Structures

Abstract: To determine failure locations is a practical effort in many engineering applications. A proper selection of signal wave velocity is critical for the implementation. Current practice in AE using a constant velocity model has a serious disadvantage that may introduce significant error and slow the speed of convergence, and it becomes even worse when the material is in form of complicated structure. In this paper, we proposed a variable velocity model to address this disadvantage. The variable velocity approach is validated by static tests (pencil lead break) and dynamic tests (fatigue tests) performed on a cemented total hip arthroplasty (THA) that has four material layers. Compared to the constant velocity model, the variable velocity approach reduces source location errors in both static (from 7.1 mm to 4.7 mm) and dynamic tests. The results indicated that the proposed variable velocity approach has significant advantages over the constant velocity approach when microcracks occur in a complicated structure.

Sizing Cracks in Power Plant Components Using Array Based Ultrasonic Techniques

Abstract: New techniques developed using the array transducer, to image and size the bottom-surface and near-through-wall crack-like defects in thick carbon steel components are discussed. Three studies are reported here including (a) Optimal beam steering angle for focused and unfocused inspection using phased array method for bottom-surface crack sizing. (b) A front wall correction algorithm for sizing of near-through-wall crack-like defects. (c) A small aperture technique for sizing of near-through-wall crack-like defects. A Finite Difference Time Domain (FDTD) based simulation was used to study and verify the experimental observations. The application of time domain scheme relative arrival time technique (RATT), to measure the size of the near-through wall crack-like defects for the leak before break (LBB) criterion, was also investigated and found to be insufficient. A conventional SAFT algorithm was used for improving the sizing using the small aperture technique.

X-ray Diffraction Based Residual Stress Measurements for Assessment of Fatigue Damage and Rejuvenation Process for Undercarriages of Aircrafts

Abstract: Assessment of fatigue damage during the service life of any component is important to ensure its continued integrity and predict the remnant life of the component. This is important to reduce the overall life cycle cost of the components. A component undergoing fluctuating stresses experiences fatigue damage and this is one of the major causes of failure of engineering components. Accumulation of fatigue damage takes place in undercarriages of aircrafts due to fluctuating stresses experienced after each landing. The accumulated fatigue damage has been assessed by carrying out residual stress measurements at stress critical regions of the undercarriages using X-ray diffraction technique. In the undercarriages, high compressive residual stresses are introduced as part of fabrication process, to enhance the fatigue resistance. These compressive residual stresses get redistributed due to the localized plastic deformation and become tensile with the increase in number of landings. The life of the undercarriages is extended by employing a rejuvenation treatment to overcome the surface tensile residual stresses, by first removing the material from stress critical regions, followed by shot peening treatment which introduces surface compressive stresses, thus enabling continued use of the undercarriages. The additional thickness provided at the design stage enables removal of fatigue damaged surface layers without affecting the overall structural integrity. The residual stress redistribution in stress critical regions of the struts of the undercarriages was measured and found to match qualitatively well with the values predicted from FEM based simulations.

A Remote, Non-destructive Laser Ultrasonic Material Evaluation System with Simplified Optical Fibre Interferometer Detection

Abstract: An optical non-contact ultrasonic testing system is presented. It uses broadband surface acoustic wave impulses generated with a cylindrical lens focused laser line source. The detection of the ultrasound is achieved by a simple and yet effective design of optical fibre interferometer which provides good sensitivity and manoeuvrability. The effectiveness of the fibre interferometer is demonstrated by accurately measuring the broadband surface waves on various common metal structures. The measurement system’s ability for surface material evaluation is also demonstrated from fitting experimental surface wave dispersion curves with theoretical simulations. This measurement system can test small localised areas and miniature samples that were previously difficult to examine.

Thermographic Monitoring of Aluminium Foaming Process

Abstract: A novel method for measuring the temperature distribution and evolution of metal foams in the molten state is proposed. Foamable AlSi9 precursor material containing 0.6 wt% TiH2 was foamed, kept at high temperatures and solidified while its temperature distribution was monitored by a thermographic camera. Free foaming and foaming inside a closed mould were carried out and direct and screened IR monitoring have been tested. Different heating conditions were applied giving rise to homogeneous and inhomogeneous temperature distributions. The effect of oxidation was studied on a piece of pure aluminium for reference purposes. The error sources of the measured temperature were analysed. Direct monitoring of foams was shown to be associated to serious problems with quantitative temperature measurement, while screened monitoring yielded promising and accurate quantitative results.

Application of L cr Waves to Evaluate Residual Stresses in the RIM of Railroad Forged Wheels

Abstract: The process of manufacturing wrought railroad wheels consists of forging heated blocks of steel, which are machined, heated, and quenched to reach the correct level of the mechanical properties to be used in railroads. The process generates compressive hoop stresses in the wheel rim. In field conditions, the original stress pattern can be changed mainly by overheating, which is caused by severe braking. The change from compressive stresses in the rim to traction can facilitate crack propagation. Indeed, even a small crack can propagate suddenly, causing a derailment. This work presents a new approach to evaluate the stresses generated by braking heating. It is based on the acoustoelastic effect, which relates the wave speed of ultrasonic waves to the strain state of the part. The method uses longitudinal critically refracted waves and a simplified system based on ultrasonic commercial products. The results show that the method is a stress sensitive and high resolution way to improve the safety of railroad wheels, making it a suitable alternative to be used at maintenance shops.

Nondestructive Identification of Knot Types in Hand-Made Carpet. Part I: Feature Extraction from Grey Images

Abstract: Knot (pile) formation has a very important role in evaluation and rating of hand-made carpets. Visual identification of carpet structural elements is impossible even for expert people. This research presents a nondestructive method for recognition of carpet knotting structure. It is based on X-ray radiography (mammography). Available X-ray beams create visible images from the internal structure of object in two dimensions. Primary radiological images are taken by radiography from hand-made carpets. They are then converted to structural recognizable images by employing different image processing techniques.

Acoustic Emission Study on Effect of Stress Triaxiality on Damage Micromechanisms in Near Alpha Titanium Alloy

Abstract: Acoustic Emission (AE) studies have been performed on smooth and notched specimens of a near alpha titanium alloy under monotonic loading condition to study the effect of stress triaxiality on damage micromechanisms. The damage in the notched specimen was found to be of cleavage type as against the ductile type of damage in smooth specimen. This difference was distinguished in the AE energy evolution with notched specimen producing higher energy rate with lower total energy than that of smooth specimen.

Techniques for Neutron Stress Determination with High Spatial Resolution

Abstract: A number of sub-surface strain measurement problems require spatial resolutions of the order of 10−1 mm to be meaningful. The use of neutron diffraction can be both time effective and otherwise advantageous compared to synchrotron diffraction if measurement directions and beam shapes are chosen such that scattered intensity is maximized. The first point refers to those directions and specimen orientations for which the gage volume orientation can be kept constant. They can be chosen such that preferred orientation boosts the diffracted intensity. The second point refers to the shaping of the incident beam for maximizing the illuminated volume in the material even for curved specimens. Guidelines and limitations of such high spatial resolution measurements are discussed.

Non-Destructive Evaluation of Grain Structure Using Air-Coupled Ultrasonics

Abstract: Cast material has a grain structure that is relatively non-uniform. There is a desire to evaluate the grain structure of this material non-destructively. Traditionally, grain size measurement is a destructive process involving the sectioning and metallographic imaging of the material. Generally, this is performed on a representative sample on a periodic basis. Sampling is inefficient and costly. Furthermore, the resulting data may not provide an accurate description of the entire part's average grain size or grain size variation. This project is designed to develop a non-destructive acoustic scanning technique, using Chirp waveforms, to quantify average grain size and grain size variation across the surface of a cast material. A Chirp is a signal in which the frequency increases or decreases over time (frequency modulation). As a Chirp passes through a material, the material's grains reduce the signal (attenuation) by absorbing the signal energy. Geophysics research has shown a direct correlation with Chirp wave attenuation and mean grain size in geological structures. The goal of this project is to demonstrate that Chirp waveform attenuation can be used to measure grain size and grain variation in cast metals (uranium and other materials of interest). An off-axis ultrasonic inspection technique using air-coupled ultrasonics has been developed to determine grain size in cast materials. The technique gives a uniform response across the volume of the component. This technique has been demonstrated to provide generalized trends of grain variation over the samples investigated.