Detection Of Subsurface Defects Research Articles

Scanning acoustic microprobe analysis for testing solid state materials

A survey of analytic techniques for subsurface inspection by means of acoustic reflective microscopy is presented. The measurement of the acoustic material signature is carried out using a computer-assisted scanning acoustic microscope operating at frequencies between 100 and 400 MHz. Thickness measurements of surface layers and the detection of subsurface defects with descriptive parameters are pointed out. The examination of stress states in the surface comprises methods for measuring the change in the surface acoustic wave velocity with applied stress. Some typical results obtained for solid state materials are shown. Advantages and limitations of alternative measuring techniques are discussed.

VISION APPLICATIONS IN THE FISHING AND FISH PRODUCT INDUSTRIES

This paper surveys the applications of vision to fish sorting, fish fillet sorting and detection of surface and sub-surface defects (such as worms and bones). It stresses the specific implementation context, needed performances, illumination, detection principle, as well as vision algorithms. Also analyzed are the optical properties of fish. Examples of results are given.

Detection of subsurface defects using phase conjugate waves and forced vibration

In four-wave mixing in BSO crystal, a periodic forced vibration is applied to both the object under test and the standard object to provide signal and reference waves, respectively. The system images subsurface defects of the test object by converting the difference of vibration between the test and the standard objects into image intensity. The principle and basic experimental results are given.

Thermal-wave detection and characterisation of sub-surface defects

Results of the analysis of air-gap and thermal-contact resistance defects are presented. The analysis is illustrated by model predictions of the influence of such defects in a number of important coated and uncoated materials. Experimental results of defects in steel and aluminium coated steel samples are presented and compared with theory. These results show the importance of the lateral extent of the defect and the presence of contact points within the defect.

The converging-surface-acoustic-wave technique: anaylsis and applications to nondestructive evaluation

Converging surface-acoustic waves (SAW) are generated by irradiating the inspected material with an annular-shaped pulsed laser beam. The converging-SAW pulse arrival is detected by a laser interferometer focused on the center of the annulus, where the converging effect produces a strong amplification of the ultrasonic pulse. This technique can be applied either to the detection of defects or to the characterization of the material by measuring the SAW velocity or attenuation. In this paper we present an analysis of the converging-wave propagation in order to explain some features of the detected signal, such as its shape and amplitude for different positions of the probing beam. A comparison with the signal intensities expected for a diverging as well as a collimated SAW is also presented. Applications of this technique to the characterization of anisotropic materials as well as to the detection of subsurface planar defects are presented and discussed.

Photothermal interferometry for nondestructive subsurface defect detection

A symmetrical interferometer as thermal expansion detector was developed for photothermal nondestructive material analysis. Phase and amplitude exchange phenomena have been observed, when introducing an electronic reference phase shifter mixed to the interferometer signal. Experimental and theoretical results obtained with this system are presented.

Photoacoustic characterization of subsurface defects in plasma-sprayed coatings

The photoacoustic detection of subsurface defects in metallic plasma coatings on metallic substrates has been achieved. The investigation is performed with samples that contain well-known artificial defects (Teflon, alumina or boron nitride) whose thermal properties are sufficiently different from those of the coatings (aluminium or Ni-Cr alloy) or the metallic substrates. The experimental results are interpreted in terms of a theoretical model for the layered structure of the sample. Moreover, the photoacoustic effects of the defect may be characterized by a thermal resistance at the coating-substrate interface. This study enables us to determine different characteristics of the coatings such as thermal properties, thickness and adherence to the substrate, in spite of the effects of the surface roughness on the photoacoustic signal.

Stereoscopic depth analysis by thermal wave transmission for nondestructive evaluation

Thermal wave transmission probing is used for nondestructive remote subsurface flaw detection in metal. It is shown that depth information of subsurface structure is obtained when two thermal wave sources are operated simultaneously in a stereoscopic arrangement.

Subsurface flaw detection in metals by photoacoustic microscopya

The scanning photoacoustic microscope (SPAM) is used in both the conventional and phase-contrast modes to detect a well-characterized subsurface flaw in Al. The physical mechanism is that of thermal diffusion, with a subsurface probe depth and flaw resolution length of approximately one thermal-diffusion length. Comparison of the dependences of the photoacoustic signal upon chopping frequency from the different regions of the sample confirm that the differential signal from the flaw corresponds to a transition from thermally thick to thermally thin boundary conditions. Experimental results are in good agreement with calculations based upon a three-dimensional thermal-diffusion model.