Hiroyuki Fukutomi

Analysis of Wave Propagation for the TOFD Method by Finite Element Method: Optimization of Test Configuration and Proposal of a New TOFD Method

Wave propagation in an austenitic stainless steel specimen with a slit open to the lower surface is modeled using finite element method, with consideration of the effects of scattering at grain boundaries. The distribution of energy of a diffracted longitudinal wave shows that this wave has the strongest energy directly above the slit. As a consequence of this result, a new testing method is proposed. The new method is verified using austenitic stainless steel specimens. Experimental results show that the new method is applicable to austenitic stainless steel with high accuracy, to which the time of flight diffraction method (TOFD) is not applicable.

A novel crack reconstruction method for steam generator tube ECT with noise sources outside

In eddy current testing of steam generator tubes, the signals obtained are always noised by many factors because of the complex environment. Some of these noise factors are caused by the support plates, the expansion part of the tube and the tube sheets outside. Previous works show that the noises of support plates can be reduced by a multi-frequency method. However, the elimination of noises from the expansion part of the tube is proved impossible using the multi-frequency method only. A new signal processing method is introduced in this paper by applying a differential filter using the two dimensions scan data. Signal noise ratio (SNR) is increased significantly, even a small signal from outer defects can be recognized. Furthermore, a new method for the crack reconstruction from these noised signals has been developed with a fast signal prediction and the conjugate gradient algorithm considering the outside ferromagnetic structures such as support plates and tube sheets. The electromotive force signals ...

PREDICTION OF RECEIVED SIGNALS IN ULTRASONIC TESTING BY FINITE ELEMENT METHOD COMBINED WITH GEOMETRICAL OPTICS THEORY

In order to predict received signals for ultrasonic testing with smaller memory and higher speed than traditional FEM, a hybrid numerical method, which combines geometrical optics theory (GT) and FEM, is developed. To realize the hybrid method, a new absorbing boundary condition (ABC) is proposed. Numerical results show that the proposed ABC is independent of propagation direction, frequency and wave mode. Then, an approach based on the reciprocity theorem is derived to calculate received signals. Various computation examples by the hybrid method are given, whose results are compared with those by experiments or traditional FEM. They are in good agreement with each other. Moreover, the number of nodes and computation time by the hybrid method are 1/3 and 1/10 of those by traditional FEM, respectively.

Low frequency eddy current inspection of wall-thinning of large pipes by bobbin coils

This study firstly discussed effect of three coils orientations on sensitivity of low frequency eddy current testing for inspecting wall-thinning of large pipes through experimental signals. One specimen made of carbon steel was prepared. Hollows with different depth were introduced on the bottom of the specimen. Results of experiments showed that sensitivity of low frequency eddy current testing would be best if axes of bobbin coils are both parallel to surface of a pipe. Further experimental data showed that usage of coils shield could not improve the sensitivity of inspection. Subsequent simulations were performed to reveal the mechanism.

PREDICTION OF B SCOPE IMAGES FOR ULTRASONIC TESTING BY GEOMETRICAL THEORY OF DIFFRACTION

A high speed simulation software to predict ultrasonic B scope images from crack‐like defects in a plate is developed using the geometrical theory of diffraction. Generalized equations are derived to calculate diffraction and specular reflection echoes, which are effective even if beam paths are longer than 0.5 skips. Moreover, interpolation formulae are proposed to modify equations of diffraction coefficient to remove singularities and numerical results show that these formulae work well. B scope images obtained by these equations are in good agreement with those by experiments. B scope images can be obtained in about 10 seconds on a personal computer with clock frequency of 1.5 GHz and RAM of 512 MB.

A New Ultrasonic Inspection Technique for Flaw Depth Sizing Based on Short Path of Diffraction

This paper describes a new tip diffraction technique, short path of diffraction (SPOD), for accurate depth sizing of flaws on the surface opposite to the inspection surface in which a set of an angle beam and a longitudinal 0-degree transducers is used as a transmitter and a receiver. In this technique, flaw depth is just calculated from the time difference between two echoes of a longitudinal diffracted wave at the tip of a flaw, which arises after an incident wave from the angle beam transducer impinges on the flaw. One echo is related to a wave traveling directly from the tip to the receiver, and the other is a wave which is reflected at the back wall and subsequently travels to the receiver. The two echoes are detected by the longitudinal 0-degree transducer located right above the flaw. Incident angles and incident points have no effect on determining flaw depth in this technique, therefore, more accurate and efficient flaw sizing becomes possible for the proposed technique. The new technique will be demonstrated by applying to depth sizing of slits and stress corrosion cracks which are machined into welded specimens made from austenitic stainless steel.Copyright

Finite Element Modeling of the Eddy-Current Inspection of Land-Based Gas Turbine Blades

There has been a growing need in recent technology of gas turbines in combined cycle to assess the remaining life of high temperature components. It is also required that the nondestructive assessment be more accurate in maintaining combined cycle plants. This paper describes the use of a simulator solving forward and inverse problems under eddy current testing, in parametric studies of inspection parameters for 1100°C-class gas turbine blades in terms of test probe capabilities, frequencies and signal interpretation. The simulator is based on a differential formulation constructed with a magnetic vector potential and a 3-dimensional edge-based finite-element modeling method. Its features are forming coils and defects independent of a whole finite element model, very fast eddy current response predictions, and identifications of electromagnetic properties. Using the simulator, optimal sensor types and test frequencies are determined in terms of assessment of degradation, selectability between surface-breaking and subsurface cracks, reconstruction of crack profiles, and detection of multiple cracks.Copyright