Dorian Minkov

NDE of degradation of thermal barrier coating by means of impedance spectroscopy

Abstract Nondestructive evaluation of thermal barrier coating (TBC) degradation during service operation has received a wide attention for service life prediction of advanced gas turbines. In this work, TBC on nickel base superalloy degraded at various degrees by thermal aging is investigated. Particularly, TBC and Metal–Chromium–Aluminum–Yttrium alloy (McrAlY) (M indicates iron (Fe), cobalt (Co), nickel (Ni) or these combinations) bond coating interface where a detrimental reaction takes place and forms various reaction products during thermal aging, is studied in detail. Formation kinetics and physical properties of the reaction layer are evaluated by means of Impedance Spectroscopy (IS). Specimens aged at higher temperature and a longer aging time shows larger impedance. The impedance behavior of TBC and bond coating is found to be markedly changed by the formation of the reaction layer. Physical properties and thickness of the reaction layer are evaluated using the change of the impedance. A mechanistic interpretation of the cause of change in the physical properties and IS behavior are described.

Study of crack inversions utilizing dipole model of a crack and Hall element measurements

Abstract A method is proposed for computing the depth and width of simple surface cracks based on dipole model of a crack. Surface cracks with different widths cut mechanically in SS400 steel specimens are investigated. Hall voltage distributions are measured by a Hall element sliding on the specimen’s surface along a line parallel to the direction of the applied magnetic field, perpendicular to the long axis of the crack, and halving its length. The performed crack inversions show that the depth of the crack can be determined with 2% error, provided, that the crack length and width are measured independently. When the crack width is unknown, the depth error is within 12%, but the width error can be as large as 30%.

Method for sizing of 3-D surface breaking flaws by leakage flux

Abstract A method is proposed for sizing of 3-D surface breaking flaws with complex profiles. The method can be used for either magnetic materials or conductive materials when a magnetic field or electric current are distributed homogeneously inside the sample. The distribution of the intensity of the leakage magnetic field or electric field has to be measured at a constant distance above the surface of the sample in a direction perpendicular to the long axis of the flaw. The method is based on a dipole model of the flaw which predicts the intensity of the leakage field and on minimisation of the error between the measured distribution of the intensity of the leakage field and the corresponding theoretical distribution which changes with the size of the flaw. Computations are made for sizing of flaws with different shapes and sizes while it is assumed that the density of the charge at the walls of the flaw can vary. It is shown that the method allows fast distinguishing of the shape of a particular flaw where the sample contains several flaws with different shapes and the sizes of these flaws are known. Fast determination of the size of a particular flaw is achieved when the sample contains flaws with the same shape and different but known sizes. Also, the size of the flaw and the density of the charge at the walls of the flaw could be computed precisely, independently of the initial approximation of the minimisation procedure, when the shape of the flaw is known. The speed of the minimisation process increases with the decrease of the measurement distance from the surface of the flaw. Experimental requirements for precise flaw sizing are discussed.

Inspection of metal surface containing cracks by small antennas

In order to realize advanced nondestructive inspection technique characterized by high sensitivity and high accuracy, a new technique has been developed for detection and characterization of surface cracks in metals, using small loop antenna or dipole antenna. The principle of the new technique is presented. Experiments are performed on detection and characterization of artificial surface cracks on the paramagnetic and ferromagnetic materials, and a fatigue crack is detected in a 316 stainless steel specimen.

Study of the dipole model of a crack

The dipole model of a crack (DMC) is applied for a first time assuming that the distribution of the surface density of magnetic charge at the crack walls m along the depth of a surface crack is not constant, but is described by a second order polynomial. New analytical expression is derived for the z-component of the intensity of the leakage magnetic field (ZILMF) in this case. DMC inversion is performed using Hall sensor measurements of ZILMF by varying the coefficients of the second order polynomial which leads to determining the depth distribution of m. Additional inversions are performed assuming correspondingly constant and linear depth distribution of m and the results and the accuracies of the different inversions are compared. It is shown that m is significantly larger at the tip of the crack with respect to its mouth which indicates that the depth distribution of m should not be considered to be constant. Instead, the distribution of the surface density of magnetic charge at the crack walls m along the depth of the crack can be represented by a polynomial of first or second order, and the distribution of the corresponding ZILMF is described by the new analytical expression. These developments might make DMC inversions a viable alternative of Eddy-current inversions.The dipole model of a crack (DMC) is applied for a first time assuming that the distribution of the surface density of magnetic charge at the crack walls m along the depth of a surface crack is not constant, but is described by a second order polynomial. New analytical expression is derived for the z-component of the intensity of the leakage magnetic field (ZILMF) in this case. DMC inversion is performed using Hall sensor measurements of ZILMF by varying the coefficients of the second order polynomial which leads to determining the depth distribution of m. Additional inversions are performed assuming correspondingly constant and linear depth distribution of m and the results and the accuracies of the different inversions are compared. It is shown that m is significantly larger at the tip of the crack with respect to its mouth which indicates that the depth distribution of m should not be considered to be constant. Instead, the distribution of the surface density of magnetic charge at the crack walls m alo...

Sizing of 3-D surface cracks with varying width by using leakage magnetic field and Hall element probe

A method is developed for computing the sizes of surface cracks in magnetic materials. The method is applicable to a crack with varying width along the crack depth, and is based on measuring leakage magnetic field by Hall element probe. New analytical expressions are derived for the z-component of the intensity of the leakage magnetic field of a surface crack with linearly varying width. Computations are performed for sizing of several such cracks with complex cross-sections, and unknown shapes. The computed results show that reasonably precise sizing can be achieved for all of the investigated cracks.

Trading System Applied to Large Mutual Fund Company

Personal trading of our private portfolios is required for decreasing losses, and improving performance. Spending one hour per day with our portfolios is sufficient to achieve good results. A system is developed for both medium term, and short term trading. The system is applied to trading of mutual funds in a large mutual fund company.

QNDE of degradation in thermal barrier coatings using impedance spectroscopy

Thermally grown oxide (TGO), formed at the interface between 8 wt% yttria stabilized zirconia (YSZ) top coating and MCrAlY bond coating, is characterized by impedance spectroscopy (IS) technique, sensitivity analysis, and inverse analysis. It is possible to understand the impedance spectrum obtained from the IS technique using sensitivity analysis. Furthermore, the thickness and the physical properties of TGO could be estimated by inverse analysis.

Sizing of 3-D Surface Breaking Flaws from the Distribution of Leakage Field

The electrical methods for sizing of surface breaking flaws in conductive materials are divided in two groups: DC methods and AC methods. Most of these methods are based on potential drop techniques where electrical current passes inside the sample, and information about the sizes of the flaw is extracted from measurements of the potential drop between two electrodes which contact the surface of the sample.