The ultrasonic Time-of-Flight Diffraction (TOFD) technique is a well-known technique for defect sizing. This technique has been applied to thick sections (>15 mm). The application of the TOFD technique to thin sections like pressure vessels and piping requires simulation of this technique. Simulation gives ideas about the expected results from experiments where real experiments are not possible or are difficult to conduct. Further, simulation helps its to derive the optimum choice of experimental parameters. This paper discusses the application of the finite element technique to simulate the ultrasonic time-of-flight diffraction (TOFD) technique. The diffracted and reflected signals in TOFD techniques for vertical and inclined defects were simulated using plane strain elements. The simulated results are compared with the experimental observations. FEM simulation of wave propagation in complex joints such as a T-joint with embedded flaws is also discussed.

This article attempts to relate ultrasonic second harmonic generation to dislocation structures in fatigued materials. By focusing on the fact that asymmetric dislocation motion is required to generate the second harmonic, a theoretical derivation of the non-linearity parameter (ratio of the amplitude of second harmonic to square of the amplitude of the fundamental) from dislocation pile-ups is presented. In order to verify the theory, harmonic generation was measured along a failed fatigue sample of DA718, a nickel base superalloy that exhibits planar slip, and consequently dislocation pile-ups. TEM studies along the length of the failed fatigue sample revealed a banded dislocation structure, composed of pile-ups near the fracture zone and a sparse dislocation network closer to the grip region. The non-linearity parameter was found to increase by between 90% and 140% from the grip region to the fracture zone and this correlated well with the calculations based on theoretical expressions derived here.

Ultrasonic forms an important tool in industrial health and process monitoring. In general, ultrasonic wave propagation studies are carried out in a stationary medium. But in certain scenarios, medical application included, it is essential to understand ultrasound path in a non-stationary medium. In this paper, we quantify the influence of a moving medium on the propagation of ultrasound through a finite element implementation of a modified wave equation. Analytical profile for the velocity of the moving medium is mapped over the spatial domain. Results for the beam drift due to the moving medium for both normal and angular incidence of the wave are presented.

This paper aims at developing a numerical model for guided wave propagation in plates and the interaction of modes with defects using Finite Element Modeling (FEM). Guided waves propagate as extensional, flexural and torsional waves. Theoretically, these modes are infinite in number, but only some of these propagate and the others are attenuated. The dispersion curves for a structure reveal the plausibility of these modes. In this paper, FEM is used to examine interaction of first few symmetric and anti‐symmetric modes independently with the cracks of various sizes in a plate. A time‐frequency representation of the acquired guided wave mode signals will be discussed to show the mode sensitivity with crack size.

Numerical simulation of ultrasonic wave propagation using methods such as finite element or finite difference is computationally expensive particularly when (a) structural dimensions are high, (b) inspection at higher frequencies (due to short wavelengths), and (c) in complex materials that are not isotropic. This paper discusses a numerical technique, which is similar to FEM, but works in frequency domain and has advantage of more accurate results in quick computational time called the spectral element method (SEM). When the second order partial differential wave equation transformed to frequency domain by Continuous Fourier Transform, the wave equation transforms to ordinary differential equation (ODE) that has exact solution. This paper discusses simulation of Lamb wave modes and Time of Flight Diffraction Technique in isotropic plates.

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.

It is necessary to size the cracklike defects accurately in order to extend the life of thin-walled (<10mm) 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.

Numerical simulation of wave propagation and its interaction with defects in a long range (1 m) thin sections (10 mm) using the regular finite element method is very costly, since it requires very fine meshes to accurately capture response signals. Spectral element method (SEM) is a numerical technique that solves the dynamic problem in frequency domain. SEM has been applied for detection of vertical and horizontal defects. This paper discusses application of SEM to any arbitrarily orientated crack in long range pressure vessels. The simulation of applications of Lamb wave for detection and sizing using TOFD technique is demonstrated. The advantage of the current formulation is quick detection of defects in thin bodies of any orientation using Lamb waves and sizing the same using TOFD techniques are illustrated.

It is difficult to accurately size the defects that are oriented at an angle (that is not normal to the wave) using conventional amplitude based ultrasonic techniques. Since Time of Flight Diffraction (TOFD) is based on the diffraction of ultrasound at defect edges, defect sizing using this technique is amplitude independent. However, most of the TOFD based assessment relies on manual sizing, whose accuracy depends on quality of image and the operator's experience. Also, the utilization of TOFD for sections less than 15 mm has reportedly several difficulties. In this paper, we report our attempts to size the vertical and inclined defects using an in-house TOFD system built to inspect thin sections (6-10 mm). To improve sizing, automated defect sizing techniques termed Embedded Signal Identification Technique (ESIT) and Point Source Correlation Technique (PSCT) were developed. A ray tracing based model was also developed for a) optimizing the experimental parameters for thin sections, b) interpreting the received signals. Experiments were conducted on 10 mm thick samples with EDM defects and 6-7 mm welded maraging steel samples. The results obtained using manual and automated techniques were compared. Our comparisons lead us to believe that the automated defect sizing techniques can provide accurate and reliable results for thin sections.

This paper proposes a technique for automatic discontinuity location and sizing using the ultrasonic time of flight diffraction technique. Here, the crack tips are modeled as point sources of diffracted waves in a homogenous, isotropic medium. The diffraction arcs are modeled using a ray based approach and the modeled arcs are correlated with the experimental B-scan data. The points of high correlation provide information about the location of the crack tips. A statistical echo separation procedure to isolate the diffraction arcs in the B-scan image is discussed. This paper also addresses the issue of application of this time of flight diffraction technique to a thin section (less than 12 mm [0.47 in]), wherein the echoes from the various sources (lateral wave, back surface reflection, diffraction from crack tips and so on) interfere with each other, making it difficult to identify diffracted signals from the discontinuity tips.

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 (6-10mm). 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.

Conventional ultrasonic inspections techniques for fatigue characterization are based on linear elasticity. Studies have shown that NDE methods based upon sound velocity and attenuation are more sensitive and reliable after crack formation i.e. 100% fatigue. In recent years nonlinear acoustics/ ultrasonics has been suggested as a new approach for the effective evaluation of a wide range of material degradation. In this paper we present results from studies on Titanium samples to quantify LCF, using second harmonics generated during nonlinear ultrasound inspections. Samples were tested to failure and inspected with 5 MHz signals for harmonics. The nonlinearity parameter (β) is evaluated from the measurements of the fundamental and second harmonics. Over 80% change in the nonlinearity parameter is observed between the gage and the grip sections of the LCF sample, where a conventional ultrasound velocity measurement shows a negligible variation