Sizing a surface-breaking crack in a complex geometry component, for example a steam turbine's solid rotor shaft, is a challenging inspection task. This paper deals with experimental work for the evaluation of the Time-of-Flight Difftaction (TOFD) technique for sizing of surface-breaking cracks in such a complex geometry component. The experimental results show that with the help of the mathematical model explained here, TOFD can size such cracks with reasonable accuracy.

Krishnan Balasubramaniam

Department of Mechanical Engineering

C V Krishnamurthy

Department Of Physics

Crack initiation

geometry

Mathematical models

Nondestructive examination

ROTORS

Shafts (machine components)

Steam turbines

COMPLEX GEOMETRY COMPONENT

SURFACE-BREAKING CRACK

TIME-OF-FLIGHT DIFFRACTION (TOFD) TECHNIQUE

Cracks

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Insight: Non-Destructive Testing and Condition Monitoring

This article presents an analytical approach for simulation of ultrasonic diffracted wave signals from cracks in two-dimensional geometries based on a novel Huygens–Fresnel Diffraction Model (HFDM). The model employs the frequency domain far-field displacement expressions derived by Miller and Pursey in 2D for a line source located on the free surface boundary of a semi-infinite elastic medium. At each frequency in the bandwidth of a pulsed excitation, the complex diffracted field is obtained by summation of displacements due to the unblocked virtual sources located in the section containing a vertical crack. The time-domain diffracted wave signal amplitudes in a general isotropic solid are obtained by standard Fast Fourier Transform (FFT) procedures. The wedge based finite aperture transducer refracted beam profiles were modelled by treating the finite dimension transducer as an array of line sources. The proposed model is able to evaluate back-wall signal amplitude and lateral wave signal amplitude, quantitatively. The model predicted range-dependent diffracted amplitudes from the edge of a bottom surface-breaking crack in the isotropic steel specimen were compared with Geometrical Theory of Diffraction (GTD) results. The good agreement confirms the validity of the HFDM method. The simulated ultrasonic time-of-flight diffraction (TOFD) A-scan signals for surface-breaking crack lengths 2 mm and 4 mm in a 10 mm thick aluminium specimen were compared quantitatively with the experimental results. Finally, important applications of HFDM method to the ultrasonic quantitative non-destructive evaluation are discussed. © 2014 Elsevier B.V.

Ultrasonics

Quantitative simulation of ultrasonic time of flight diffraction technique in 2D geometries using Huygens–Fresnel diffraction model: Theory and experimental comparison

Experimental probability of detection (POD) and probability of sizing (POS) curves are developed for the detection and sizing of surface-breaking cracks in components having complex geometry. A manual ultrasonic time of flight diffraction (TOFD) inspection system has been used to develop these POD and POS curves. Initially, it was assumed and subsequently verified that signal responses have a normal distribution. The effects of probe angle and probe center-to-center spacing on both POD and POS were observed. Unlike the PODs previously reported for conventional ultrasonic amplitude based flaw sizing techniques, the PODs associated with TOFD in the range of defect sizes considered here, exhibit a decreasing trend for a fixed set of experimental parameters. It is important to emphasize that the curves obtained in this work are specific to the geometry, specimen dimensions, set of notches and the specific probes used in the experiment. However, when using a manual TOFD inspection system, these curves can be useful for planning the risk-based inspection of components having complex geometry (such as a steam turbine rotor shaft). © 2009 Elsevier Ltd. All rights reserved.

NDT and E International

Reliability assessment of manual ultrasonic time of flight diffraction (TOFD) inspection for complex geometry components

The basic aim of this current research is to reliably detect and size defects in complex geometry welds using the well-known ultrasonic time of flight diffraction (TOFD) technique. A complex geometry weld specimen with various artificial defects was designed and fabricated. Different combinations of probe angles and probe center spacings were used in experiments for determining the optimum one. TOFD models were also developed for sizing the defects and experimentally verified. Copyright © 2009 by ASME.

Journal of Pressure Vessel Technology, Transactions of the ASME

Detection and sizing of defects in complex geometry weld by manual ultrasonic time of flight diffraction inspection

Ultrasonic time of flight diffraction (TOFD) for sizing defects is based on the time of flight of the diffracted echo that is generated when a longitudinal wave is incident on a crack tip. This technique has the limitation during near-surface inspection due to signal superposition. Here, this limitation is overcome by using the shear wave-diffracted signal (instead of longitudinal wave) and hence called S-TOFD. Experiments were conducted on samples with defect tip closer to the surface of a flat plate sample to illustrate the utility of the S-TOFD technique. An increase in the flaw sizing accuracy, by using the shear wave-diffracted echoes from the tip and through the application of a signal processing technique (ESIT), was demonstrated. © 2006 Elsevier Ltd. All rights reserved.

Shear-wave time of flight diffraction (S-TOFD) technique

It is necessary to size the cracklike defects accurately in order to extend the life of thin-walled (< 10 mm) components (such as pressure vessels) particularly for aerospace applications. This paper discusses the successful application of ray techniques to simulate the ultrasonic time-of-flight diffraction experiments for platelike structures. For the simulation, the diffraction coefficients are computed using the geometric diffraction theory. The A and B scans are simulated in near real time and the different experimental parameters can be interactively controlled due to the computational efficiency of the ray technique. The simulated results are applied to (1) defect signal identification for vertical defects, (2) inspection of inclined defects, and (3) study the effect of pulse width or probe frequency on experimental results. The simulated results are compared with laboratory scale experimental results. Copyright © 2005 by ASME.

Ray based model for the ultrasonic time-of-flight diffraction simulation of thin walled structure inspection

The ultrasonic Time-of-Flight Diffraction (TOFD) technique is a well developed technique for sizing defects in thick sections (thickness >10 mm). Attempt has been made here to extend this technique for thin sections (6-10 mm). An automated defect sizing algorithm using the Embedded Signal Identification Technique (ESIT) was developed for separating partially superimposed signals often encountered in thin sections and the results were compared with the manual sizing method. Both EDM notches and more realistic fatigue cracks in thin section were used to evaluate the proposed technique.

Journal	Insight: Non-Destructive Testing and Condition Monitoring
ISSN	13542575
Open Access	No