Faster and more accurate testing : Green, J.E. Jr. Advanced Materials and Processes, Vol. 131, No. 1, pp. 72, 75–76, 79–80 (Jan. 1987)

Abstract

Scanning acoustic microscopy (SAM) is a modern, powerful technology for the visualization of the internal structure of materials of a different nature. It uses high frequency ultrasonic waves to get information from inside inhomogeneities and therefore is a sensitive, precise and safe method for materials characterization. It is successfully used in scientific and research practice but has the greatest potential as a technology for nondestructive quality evaluation. This chapter describes the methodology and different aspects of ultrasonic inspection performed on a variety of material samples using a scanning acoustic microscope working in the range of 10–400 MHz as an example. The samples come from different areas of material sciences and were intentionally chosen as containing defects specific to each case.Compared to the widely used nondestructive technique and medical ultrasound, SAM operates with higher frequencies and at relatively short distances. The acoustic beam emitted by the transducer passes through a coupling liquid and is reflected back by inhomogeneities in the sample structure. The strength of the reflected signal is determined by the change in characteristic acoustic impedance at such an inhomogeneity. The reflections captured in the digitized received signal (A-scan) represent a 1-D property distribution along the acoustical beam. The ultrasonic image is digitally constructed from a series of A-scans acquired during lateral scanning, when the acoustic microscope scans with a mechanical motion of a single acoustic lens along the sample surface and as a result generates (collects) 3-D data about the micromechanical properties of the object in this particular volume. The main types of images obtained by acoustic microscopes are B-scan and C-scan. More complex data processing algorithms produce enhanced images that can also be attributed to these two categories. A B-scan represents the 2-D distribution of amplitudes along the axial and the lateral coordinates (vertical cross-section). A C-scan represents the 2-D distribution of amplitudes in a lateral plane at a certain time delay after the initial pulse has been emitted from the transducer (horizontal cross-section). In addition to such efficient digital characterizations of internal microstructure, it was demonstrated that SAM technology can effectively detect, evaluate, and classify a wide range of different imperfections: porosity in aluminum casting, undersized and cavernous resistance spot welds, low penetration depth of laser weld seam, insufficient or damaged area of adhesive bonding of metals and fiber-reinforced composites, biocomposites and biomaterials, etc. The precision and value of the obtained acoustic images are dependent on frequency and particular methodology, and in some industrial application cases could even be proven by destructive testing. The samples come from different areas of material sciences and were intentionally chosen as containing defects specific to each case.