Showing papers on "Time-of-flight diffraction ultrasonics published in 1993"

Depth measurement of short cracks with an acoustic microscope

Abstract: The depth of short cracks (70–200 μm surface length) has been measured with an acoustic microscope by utilizing the nondestructive time-of-flight diffraction technique (TOFD). The depth measurements were first carried out in the transparent polymer polystyrene, thus allowing a comparison between the acoustical values and direct optical measurements: the agreement in the results was better than 95%. The depth of a 70 μm long crack in an aluminium alloy was then measured, demonstrating the application of the technique to metals.

Image Processing for the Measurement of Crack Depth using the Scanning Acoustic Microscope

Abstract: The ability to measure the depth of surface breaking fatigue cracks, especially while they are still in the short crack stage, is of great importance for the study of crack growth behaviour.1–3 Time-resolved scanning acoustic microscopy offers the possibility to accomplish such depth measurements in a nondestructive manner,4–6 and thus the ability to actually monitor crack growth throughout the fatigue process. In time-resolved acoustic microscopy pulses of very short temporal extent are used to excite the SAM lens, so that echoes from the individual scattering sites within a specimen can be separately identified in the received signal.7–9 The measurement of crack depth therefore relies on the behaviour of the crack tip as a scattering site, with the depth being determined by measuring the arrival time of the signals caused by diffraction of the acoustic waves at the tip. Although the crack tip diffraction signal is much weaker in strength than specular scattering, it has been successfully used to measure longer cracks (>1 mm) at lower frequencies (< 10 MHz), where the technique is referred to as Time-of-Flight Diffraction (TOFD).10,11

Small size ultrasonic sources for time-of-flight diffraction tomography

Abstract: Time-of-flight diffraction techniques are particularly suited for NDT inspections of metal structures because they are reliable and accurate. This paper describes a tomographic imaging system based on time-off-light diffraction with point-like sources. The design of efficient point-like sources for standard immersion probes is outlined. Applications of this imaging technique for defect detection and sizing are reported and compared to simulated images.

Determination of short crack depth with an acoustic microscope

Abstract: For the prediction of the lifetime of any component, subjected to alternating stresses, the knowledge of the growth behavior of defects is essential. Most methods of monitoring the propagation of short cracks are confined to measuring the length of the crack on the surface [1]. The depth of the crack must be determined indirectly, assuming the shape of the crack. Acoustic waves, on the other hand, offer the possibility of measuring the depth directly, since acoustic waves can penetrate into the material. This allows the measurement not only of the growth behavior of fatigue cracks on the surface, but also changes of the crack geometry inside the specimen. Current applications of direct acoustic monitoring of crack growth have been developed for cracks of the order of millimeters. One acoustic depth measurement technique is the Time-of-Flight-Diffraction (TOFD) technique [2–4], which is based on timing measurements of the scattered signals from the defect. Our investigations are concerned with the application of TOFD technique for the depth measurement of short cracks (70–200 μm in surface length) using a scanning acoustic microscope (SAM) [5–6]. Depth measurements were first carried out on cracks in the transparent material polystyrene. This allows a direct comparison between acoustic and optical depth measurements. Subsequently, the depth of fatigue cracks in an A1 alloy were measured, and the acoustic measurements were compared with direct measurements of the crack geometry by sectioning the crack.