The Scattering Response of a Flat-Bottom Hole

The flat-bottom hole is one of the oldest reference/calibration standards in the field of ultrasonic nondestructive evaluation (NDE). It has been used for both calibration of ultrasonic test equipment sensitivity and for the generation of distance-amplitude correction (DAC) curves [1]. Flat-bottom holes are also useful for equivalent flaw sizing applications since they can represent the response, at normal incidence, of ideal “perfect” scatterers, such as flat cracks

Defect Sizing Using Distance-Gain-Size Diagrams for Flat-Bottomed Holes in a Solid: Theoretical Analysis and Experimental Verification

Although there are a number of potential pitfalls, the classical method of relating defect area to echo amplitude is still the most widely used method to size defects using ultrasonic pulse-echo techniques. In 1959 Krautkramer [1] was the first to introduce a set of curves (DGS diagrams) showing the variation of echo amplitude with range and target size. As Krautkramer made clear, such curves are dependent on transducer pulse shape. For the very far field he gave theoretical results assuming a fluid-like medium of propagation, but he had to resort to a large number of experimental measurements to construct the near field portion of the curves. Well known problems in using DGS diagrams include the sensitivity of echo amplitudes to target angular and lateral alignment and the need to construct a new set of curves for each transducer pulse shape. Furthermore, when sizing targets in solids there are likely to be errors if curves constructed assuming a fluid medium are used. In 1987, McLaren and Weight [2] gave an impulse-response method to predict echo amplitudes for arbitrary target position in the field and for any transducer pulse shape. Normally-aligned, flat-ended cylindrical targets and a fluid medium were assumed. More recently, Schmerr and Sedov [3,4] have calculated single frequency DGS diagrams for flat-bottomed holes (FBH’s), for both direct and water coupling, but the holes are assumed to be in a fluid-like material. Their method takes account of diffraction and refraction effects but not mode conversion. A more exact treatment of the effect of a solid medium of propagation on DGS diagrams has been given by Sumbatyan and Buyove [5] who developed DGS diagrams for disc-like targets using a boundary element method to solve the elastodynamic equations, but again, only for the case of continuous sinusoidal waves. One disadvantage of such an approach is that the calculations can be rather time consuming

Experimental Study on Hidden Corrosion/Delamination Detection with Ultrasonic Guided Waves

Hidden corrosion detection is critical in the aerospace industry. Occurring on the inside surfaces or at the interfaces of an aircraft’s skin, the corrosion must be detected from the outside surface. Surface waves are, therefore, not suitable for detecting such defects/failures. Ultrasonic bulk wave methods can be used to detect the corrosion caused thinning in the wall or a delamination of a structure [1,2]. However, since the method is based on point-by-point testing, it becomes a tedious time consuming procedure for large area inspection. Guided wave methods are being developed to tackle this problem [3–6]. An experimental study of hidden corrosion/delamination detection in single/multiple layer aluminum plates is conducted with specially selected ultrasonic guided wave modes. Both corrosion simulation specimens by machine cutting, and real corrosion specimens by electrochemical processing, Two-layer specimens have been prepared with such corroded sheets to form an artificial interface corrosion/delamination. Various wave modes are subsequently generated on these specimens to examine the implications of thinning on mode cutoff, group velocity changes, mode frequency shifts, and transmission and reflection amplitudes. Finally, a practical problem of skin to honeycomb core delamination detection with guided waves is also addressed.</p

Air-Coupled Ultrasonic Transducers for the Detection of Defects in Plates

In order to minimise the problems due to the acoustic impedance mismatch between solids and air, the non destructive testing of materials using ultrasonic transducers generally requires either contact transducers or immersion transducers to be used [1]. Air-coupled transducers however would be very advantageous for testing structures which must be not contaminated with couplant and also for all in-situ industrial applications. Although the propagation of ultrasonic waves from laser generation [2] involves air-coupling, the difficulties due to the experimental set-up of this technique and the financial investment it implies are two major disadvantages

Nondestructive Testing System to Assess Lack-Of-Bond in Brazed Generator Coils by Ultrasonic Retro-Reflection

Margetan et al. investigated the problem of assessing the integrity of diffusion bonds using reflected ultrasound at oblique incidence [1,2]. They presented a quasi-static distributed spring model to derive the ultrasonic reflectivity of an imperfectly-bonded interface as a function of frequency and angle of incidence. The results were then incorporated in a model for the corner reflection from a diffusion-bonded joint between two butting plates. Rose also studied the ultrasonic reflectivity of diffusion bonds and utilized it for quantitatively characterizing defective joints [3, 4]. Angel and Achenbach investigated the reflection of ultrasonic waves by an array of microcracks [5]

Four Transducer Ultrasonic Array for Detecting and Sizing Defects in Plate and Pipe Materials

Ultrasonic pulse-echo techniques are widely used for detection and sizing of defects. However, studies over recent years have shown that the detection and sizing capability of many of the widely accepted ultrasonic techniques do not provide accuracy required to assure safety, reliability or maintainability. While certain types and orientations of defects can be detected, others may go undetected. Even after detection, studies have shown that the ability to size defects is far from accurate. [1,2,3

Two Dimensional Pseudo-Wiener Filtering in Ultrasonic Imaging for Nondestructive Evaluation Applications

This paper deals with the use of a two dimensional pseudo-Wiener filter for ultrasonic image enhancement. Experimental results are presented to demonstrate the effectiveness of the technique for the improvement of the lateral resolution and image enhancement of ultrasonic images in materials such as graphite/epoxy composites and stainless steel. The difficulties encountered in the implementation of the filter will be delineated. Methods of overcoming some of these ‘implementational hurdles’ will be suggested

A Real-Time Saft System Applied to the Ultrasonic Inspection of Nuclear Reactor Components

In 1982 Pacific Northwest Laboratory began activity under the sponsorship of the U.S. Nuclear Regulatory Commission to implement SAFT technology in a field usable system. The University of Michigan had previously laid the groundwork by performing extensive research related to the development of the SAFT algorithm in the area of ultrasonics and the investigation of ways to improve the computation time [1,2]. The task given PNL was to deploy the results of this research effort by developing an instrument that would perform in-service inspection of nuclear reactor components using the SAFT-UT algorithm.</p

Quantitative Ultrasonic Characterization of Metal Matrix Composite Fiber/Matrix Interfacial Failure

The transverse properties of unidirectional titanium matrix composites (TMCs) are dominated by the fiber/matrix interfacial properties, residual stresses and the matrix mechanical response. Nimmer et al. [1] have pioneered the research on the role of the interface when the composites are under transverse loading. In their work, a characteristic “knee” has been observed in the transverse tensile stress-strain curve of a Ti-6A1-4V/SCS-6 composite. This “knee” occurs well below the stress level at which the matrix yields extensively. The comparisons of experimental results with finite element modeling indicate that the “knee” is due to the failure of a weak interface under the transverse loading