A Procedure for Detecting Hidden Surface Defects in a Thin Plate by Means of Active Thermography

Let \(\varOmega _\epsilon \) be a metallic plate whose top inaccessible surface has been damaged by some chemical or mechanical agent. We heat the opposite side and collect a sequence of temperature maps \(u^\epsilon \). Here, we construct a formal explicit approximation of the damage \(\epsilon \theta \) by solving a nonlinear inverse problem for the heat equation in three steps: (i) smoothing of temperature maps, (ii) domain derivative of the temperature, (iii) thin plate approximation of the model and perturbation theory. Our inversion formula is tested with realistic synthetic data and used in a real laboratory experiment.     

Evaluation of the role of transverse reinforcement in confining tension lap splices in high strength concrete

Reports on the very brittle and splitting mode, of failure of tension lap splices anchored in high strength concrete (HSC) specimens and the lower normalized bond stress\(\left( {{u \mathord{\left/ {\vphantom {u {\sqrt {f'_c } }}} \right. \kern-\nulldelimiterspace} {\sqrt {f'_c } }}} \right)\) for high strength than for normal strength concrete, instigated several research projects aiming at recommending mechanisms to provide confinement and ductility for bars or splices anchored in HSC. The state-of-the-art report of ACI Committee 363, ACI 363R-84, defines high strength concrete as concrete with compressive strength above 6,000 psi (41.4 MPa).The main objective of the research reported in this paper was to assess the effect of transverse reinforcement on bond strength and ductility of the mode of failure of tension lap splices anchored in HSC beam specimens with nominal concrete compressive strength 55 MPa (8,000 psi), and to recommend a transverse reinforcement design requirement related to the ACI transverse reinforcement index K_tr.To meet the objective fifteen full-scale HSC beam specimens were tested. No pozzolanic material such as silica fume or fly ash was added to achieve high strength to avoid any effect such material might impose, on structural behavior. Each beam was designed with bars spliced in a constant moment region at midspan. The variables used in the investigation were bar size, and the amount of transverse reinforcement placed in the splice region.     

A Quantitative Approach to Active Thermographic Inspection for Material Loss Evaluation in Metallic Structures

Abstract

This paper presents an approach to the inverse geometry problem pertaining to the characterization of material loss due to corrosion damage in metallic structures. It is based on a simplified geometric representation which admits, in the context of a restricted time frame, a simple analytical form for the contrast response evolution. The inverse problem is then solved by means of a deterministic optimisation scheme applied to a suitably defined objective function. Experimental trials on specimens containing simulated corrosion damage have shown that estimates for residual thickness obtained on the basis of this approach are both precise and robust.     

ESPI and Digital Speckle Correlation Applied to Inspection of Crevice Corrosion on Aging Aircraft

A recent advance in the nondestructive inspection of crevice corrosion is reported. Crevice corrosion is one of the most hazardous defects that threaten the integrity of aging aircraft. Two optical techniques, electronic speckle pattern interferometry (ESPI) and digital speckle correlation (DSC), are employed to reveal the existence of crevice corrosion, which is usually undetectable from outside by visual inspection. The ESPI is configured to measure out-of-plane displacement. Both thermal loading and vacuum loading are used. The anomaly of fringe patterns reveals the existence of crevice corrosion hidden between two metallic plates. Not only the location of defects, but also the approximate size can be estimated. The digital speckle correlation technique takes advantage of laser speckle properties. The speckle decorrelation due to out-of-plane displacement in the region with crevice corrosion is larger than the region without defects. This difference is calculated by a digital correlation algorithm, which reveals the crevice corrosion. These two techniques can be used alternatively for field inspection of aircraft. While digital speckle correlation offers a simple, rapid approach to qualitatively detect defects, ESPI can be used as a supplementary tool if a more sensitive and quantitative evaluation is required.     

A Quantitative Approach to Active Thermographic Inspection for Material Loss Evaluation in Metallic Structures

This paper presents an approach to the inverse geometry problem pertaining to the characterization of material loss due to corrosion damage in metallic structures. It is based on a simplified geometric representation which admits, in the context of a restricted time frame, a simple analytical form for the contrast response evolution. The inverse problem is then solved by means of a deterministic optimisation scheme applied to a suitably defined objective function. Experimental trials on specimens containing simulated corrosion damage have shown that estimates for residual thickness obtained on the basis of this approach are both precise and robust.     

Chemical volume diffusion coefficients for stainless steel corrosion studies

Chemical diffusion coefficients for chromium in austenitic and ferritic steels are determined using diffusion couples studied by electron probe microanalysis techniques. The average chemical volume diffusion coefficient, for the composition range 14 to 28 at % chromium, for ferritic AISI 446 in the temperature range 800 to 1000° C is: $$\tilde D = 0.15\left( {\frac{{ + 0.54}}{{ - 0.12}}} \right) \exp \left( {\frac{{ - 210( \pm 15)}}{{RT}}} \right) cm^2 \sec ^{ - 1} $$ and for austenitic AlSl 310 in the temperature range 800 to 1200°C is: $$\tilde D = 0.27\left( {\frac{{ + 1.04}}{{ - 0.22}}} \right) \exp \left( {\frac{{ - 246( \pm 16)}}{{RT}}} \right) cm^2 \sec ^{ - 1} $$ whereR is in kJ K^?1 mol^?1 Good agreement is found with existing data for ferritic steels but the data are more scattered in the austenitic case. Diffusion data from diffusion couples are thought to be more realistic than those obtained from tracer work for the purpose of predicting diffusion-controlled corrosion behaviour.     

Cerium-based coating for enhancing the corrosion resistance of bio-degradable Mg implants

Recently there has been interest in employing degradable metallic implants for internal fixation in bone fracture healing. The major purpose of using degradable implants is to avoid a second surgery for implant removal when bone healing has completed. However, the corrosion rate of Mg in vivo is too high. Thus increasing the corrosion resistance of Mg is the key problem to address in the development of degradable Mg implants. One possible route is by way of surface treatment, which would lower the corrosion rate at the initial phase of bone healing, the period during which the implant provides mechanical support for the broken bone. In the present study cerium oxide coating was prepared on pure Mg by cathodic deposition in cerium nitrate solution followed by hydrothermal treatment. The coated samples were characterized by SEM, EDS and XRD. The corrosion resistance in Hanks’ solution (a simulated body fluid) was studied using polarization method and electrochemical impedance spectroscopy (EIS). The corrosion resistance of cerium oxide coated Mg in Hanks’ solution at 37 °C and pH 7.4 was higher than that of bare Mg by about two orders of magnitude.     

Numerical study of particle-size distributions retrieved from angular light-scattering data using an evolution strategy with the Fraunhofer approximation

An algorithm is presented based on an evolution strategy to retrieve a particle size distribution from angular light-scattering data. The analyzed intensity patterns are generated using the Mie theory, and the algorithm retrieves a series of known normal, gamma, and lognormal distributions by using the Fraunhofer approximation. The distributions scan the interval of modal size parameters 100 < or = alpha < or = 150. The numerical results show that the evolution strategy can be successfully applied to solve this kind of inverse problem, obtaining a more accurate solution than, for example, the Chin-Shifrin inversion method, and avoiding the use of a priori information concerning the domain of the distribution, commonly necessary for reconstructing the particle size distribution when this analytical inversion method is used.     

High temperature characteristics of Ir–Ta coated and aluminized Ni-base single crystal superalloys

High temperature oxidation and hot corrosion properties of Ir–Ta coated and aluminized Ni-base superalloys are presented. An Ir–Ta binary alloy, proposed as a novel metallic bond coat material, was coated on a Ni-base single crystal superalloy TMS-75 using electron beam physical vapor deposition, followed by a conventional low activity Al pack cementation process. Cyclic oxidation tests and hot corrosion tests revealed that these Ir–Ta coated and aluminized specimens showed reasonably good oxidation and hot corrosion resistance. In addition, it was found that the formation of TCP phases is suppressed by the presence of the Ir–Ta enriched layer. These results indicated that the Ir–Ta alloy system is promising as a new metallic bond coat material for high temperature structural materials.     

Assessment on failure pressure of high strength pipeline with corrosion defects

An accurate prediction on the failure pressure of line pipe is very important in the engineering design and integrity assessment of oil and gas transmission pipelines. This paper analytically investigates the failure pressure of line pipes with or without corrosion defects, and focus on the high strength steels. Based on von Mises strength failure criterion, a classic strength failure criterion, the failure pressure of end-capped and defect-free pipe p_M is theoretically deduced with the strain hardening material. In order to derive a general solution for corrosion defect assessment of high strength pipelines, an extensive series of finite element analyses on various elliptical corrosion defects was performed. Finally, a new formula for predicting the failure pressure of corroded pipe in the material of high strength steels is formulated, based on the FE models and p_M, and is validated using 79 groups of full-scale burst test data, which contain the low, middle and high strength pipeline. The results indicated that the proposed formula for predicting the failure pressure is closely matches the experimental data for the high strength steels.     

Evaluation of general corrosion on pipes using the guided wave technique

The torsional mode of a guided wave, T(0,?1), is capable of detecting features and defects in pipelines, especially in the cases of coated and buried pipes. However, it is hard to find or locate general corrosion, since the signals of shallow corrosion easily merge into the noise signal. The situation worsens when an inspector sets the array ring of the guided wave transducer directly on a general corrosion area of the pipe and serious energy attenuation of the detecting signal occurs. The goal of this study is to investigate the effects of the above-mentioned general corrosion on guided wave tests, as determined by the finite element method, experiments, and site inspection cases. The results show that the deeper the corrosion depth or the higher the operating frequency, the larger the attenuation rate of the guided wave; also, the higher the operating frequency, the fewer the coherent signals caused by the general corrosion. In the case of detecting localized severe corrosion inside a section of general corrosion on a pipe, the results show that a higher operating frequency reduces the coherent noise of general corrosion, and gives an obvious reflection signal of the local severe corrosion that leads to a good measurement being obtained.     

Aerosol properties from spectral extinction and backscatter estimated by an inverse Monte Carlo method

The feasibility of using a generalized stochastic inversion methodology to estimate aerosol size distributions accurately by use of spectral extinction, backscatter data, or both is examined. The stochastic method used, inverse Monte Carlo (IMC), is verified with both simulated and experimental data from aerosols composed of spherical dielectrics with a known refractive index. Various levels of noise are superimposed on the data such that the effect of noise on the stability and results of inversion can be determined. Computational results show that the application of the IMC technique to inversion of spectral extinction or backscatter data or both can produce good estimates of aerosol size distributions. Specifically, for inversions for which both spectral extinction and backscatter data are used, the IMC technique was extremely accurate in determining particle size distributions well outside the wavelength range. Also, the IMC inversion results proved to be stable and accurate even when the data had significant noise, with a signal-to-noise ratio of 3.     

Vortex Interaction with Mesoscopic Surface Cavities in Superconductors

The conformal mapping method is used to study the problem of flux line interaction with surface cavities having cylindrical profile and characteristic size , i.e., within mesoscopic scale, where λ is the penetration length. It is shown that the metastable states are achieved when the dimensions of the surface irregularities do not exceed the coherence length ξ. Our study shows that the surface barrier may vanish at some weak point at which the surface irregularities have mesoscopic scales. On the other hand, a remarkable decrease in the surface barrier occurs when the surface defects size . Our results are compared with the available experimental data and theoretical results.     

Novel hydroxyapatite/tantalum surface coating for metallic dental implant

The aim of this study was to design and produce a novel surface composite coating on metallic substrate in order to improve the biocompatibility of metallic dental implant and the bone osteointegration simultaneously.Stainless steel 316L (SS) was used as a metallic substrate and a novel double-layer hydroxyapatite/tantalum (HA/Ta) coating was prepared on it. Tantalum coating was made using physical vapor deposition process and HA coating was produced using plasma-spraying technique on it. X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were utilized to investigate the coating characterization. Electrochemical polarization tests were performed in two types of physiological solutions at 37 ± 1 °C in order to determine the corrosion behavior of the coated and uncoated specimens as indication of biocompatibility.The results indicated that the decrease in corrosion current density was significant for HA/Ta coated specimens and was much lower than the value obtained for uncoated 316L SS. The novel double-layer HA/Ta composite coating could improve the corrosion resistance and thus the biocompatibility of 316L SS dental implant.     

Influence of pH on ZnO nanocrystalline thin films prepared by sol–gel dip coating method

ZnO nanocrystalline thin films have been prepared on glass substrates by sol?Cgel dip coating method. ZnO thin films have been coated at room temperature and at four different pH values of 4, 6, 8 and 10. The X-ray diffraction pattern showed that ZnO nanocrystalline thin films are of hexagonal structure and the grain size was found to be in the range of 25?C45?nm. Scanning electron microscopic images show that the surface morphology improves with increase of pH values. TEM analysis reveals formation of ZnO nanocrystalline with an average grain size of 44?nm. The compositional analysis results show that Zn and O are present in the sample. Optical band studies show that the films are highly transparent and exhibit a direct bandgap. The bandgap has been found to lie in the range of 3 $\boldsymbol\cdot$ 14?C3 $\boldsymbol\cdot$ 32?eV depending on pH suggesting the formation of ZnO nanocrystalline thin films.     

Inverse approach for estimating boundary properties in a transient fin problem

A solution methodology is proposed for an inverse estimation of boundary conditions from the knowledge of transient temperature data. A forward model based on prevalent time-dependent heat conduction fin equation is solved using a fully implicit finite volume method. First, the inverse model is formulated and accomplished for time-invariant heat flux at the fin base, and later extended to transient heat flux, base temperature and average heat transfer coefficient. Secondly, the Nusselt number is then replaced with Rayleigh number in the forward model to realistically estimate the base temperature, which varies with respect to time, based on in-house transient fin heat transfer experiments. This scenario further corroborates the validation of the proposed inverse approach. The experimental set-up consists of a mild steel \(250 \times 150 \times 6\, \hbox {mm}^3\) fin mounted centrally on an aluminium base \(250 \times 150 \times 8\, \hbox {mm}^3\) plate. The base is attached to an electrical heater and insulated with glass-wool to prevent heat loss to surroundings. Five calibrated K-type thermocouples are used to measure temperature along the fin. The functional form of the unknown parameters is not known beforehand; sensitivity studies are performed to determine suitability of the estimation and location of sensors for the inverse approach. Conjugate gradient method with adjoint equation is chosen as the inverse technique and the study is performed as a numerical optimization; subsequently, the estimates show satisfactory results.     

Transverse Magnetoresistance Behaviors of Thin Polycrystalline Bismuth Films

The magnetoresistance (MR) properties of several thin polycrystalline bismuth films have been measured over a wide temperature interval (0.42K T 292 K) and a magnetic field range (0 T B 45 T). In most cases, the magnetic field was oriented in the transverse direction, with the field parallel to the substrate of the film and also perpendicular to the current direction. These MR results are different from those in either the perpendicular or parallel field orientations. The anomalous behavior of the transverse magnetoresistance can be explained considering partially diffused scattering of the carriers at the top and bottom surfaces of the films. The data are fitted using a phenomenological model, based upon the theory of Way and Kao and also using the two carrier expressions of Pippard and Fawcett (P-F).     

Improved FEM model for defect-shape construction from MFL signal by using genetic algorithm

In-line inspection of ferromagnetic gas or oil pipe lines having pipe wall defects is typically accomplished using magnetic flux leakage (MFL) technique. An efficient modelling and computational scheme for forward model, during the process of solving inverse problems in magnetostatic non-destructive evaluation using finite-element method is presented. The shape, size and place of defect are determined considering the nonlinearity of the pipe material using genetic algorithm as the optimisation technique. It is shown that the reduced model improves the FE computations significantly. The methodology for construction of defect shapes from particular MFL signals has been explained     

DEVELOPMENT OF PLASMA SPRAYED LUBRICANT FREE SLIDE PLATES WITH CERAMIC COAT FOR RAILWAY USE

Ceramic coated slide plates were developed in order to ensure smooth sliding of the tongue rail and tight contact with stock rail. The 0·5 mm thickness thin metallic bonded layer was synthesized by spraying ceramic powders using conventional plasma spraying technique. Coated materials were mostly Al₂O₃ ceramic including metallic materials. It is concluded field tests that coated plates satisfied endurance specifications, corrosion, wear and load specifications without any maintenance of ceramic coated plates. Since field tests have been successfully performed, more than 10,000 slide plates are being used in Japan by Shin-kansen Rail Way and private railway companies.     

High temperature characteristics of Ir–Ta coated and aluminized Ni-base single crystal superalloys

High temperature oxidation and hot corrosion properties of Ir–Ta coated and aluminized Ni-base superalloys are presented. An Ir–Ta binary alloy, proposed as a novel metallic bond coat material, was coated on a Ni-base single crystal superalloy TMS-75 using electron beam physical vapor deposition, followed by a conventional low activity Al pack cementation process. Cyclic oxidation tests and hot corrosion tests revealed that these Ir–Ta coated and aluminized specimens showed reasonably good oxidation and hot corrosion resistance. In addition, it was found that the formation of TCP phases is suppressed by the presence of the Ir–Ta enriched layer. These results indicated that the Ir–Ta alloy system is promising as a new metallic bond coat material for high temperature structural materials.