Acoustic micro imaging uses a moving transducer that pulses ultrasound into materials and receives the return echoes from material interfaces. Images made from the echo signals may show anomalies such as delaminations or other cracks, since gaps send back stronger echoes than well-bonded interfaces.

Figure 1. Solid-to-solid material interfaces typically reflect a portion of the ultrasonic pulse as a usable signal.

In manufacturing that involves bonded layers of polymers, ceramics, or metals, engineers often check bond quality by making images of internal interfaces. They may also image a single layer between interfaces to search for voids or cracks within it. When destructive physical analysis is planned, the acoustic image shows exactly where to section the sample.

Imaging a Multi-layer Composite

In an acoustic micro imaging system, the ultrasonic transducer and other hardware provide the reflection-mode or transmission-mode data about the sample being examined, but it is the software that does the work of manipulating that data to solve specific problems.

In the case reported here, acoustic micro imaging was used to determine whether a particular laminated composite material could withstand the various stresses that it would encounter in service. The material, imaged at Sonoscan’s headquarters and applications laboratory in Elk Grove Village, IL, was a 4.9 mm thick graphite epoxy composite having 27 layers of fine fibers laid down at 0°, 90°, +45° and -45°.

For this application, a small hole was drilled vertically through the sample, and the hole was used to mill a long narrow channel. The acoustic micro imaging system could then gather data concerning the internal damage caused by these two operations.

Acoustic Micro Imaging Hardware

Figure 2. An acoustic image of graphite-epoxy composite. The white regions are internal defects.

The imaging system’s transducer raster-scans one flat surface of the sample while pulsing ultrasound ranging in frequency from 5 MHz to 400 MHz into the sample and receiving the return echoes. The speed of ultrasound through most production materials is so high that the pulse-echo function can be carried out at each of several thousand x-y coordinates per second as the transducer moves across the sample.

The transducer itself contains two primary elements: a piezoelectric crystal that generates the pulses and collects the incoming echoes, and a spherical lens that focuses the pulse. As the transducer scans the sample, it is coupled to the top surface of the sample by water or another fluid, since ultrasound at these frequencies is propagated poorly or not at all through air.

In samples that consist of layers of solid materials, pulsed ultrasound is reflected by the interfaces between different materials (see Figure 1). The essentially flat material interfaces in this sample are the air-filled delaminations caused by the destructive test. All gaps (delaminations, voids, cracks) reflect virtually 100% of the ultrasonic pulse, unlike solid-to-solid interfaces, where a portion of the pulse crosses the gap and travels deeper (see gray arrow in Figure 1). In the acoustic images of this study’s particular composite material (see Figure 2), epoxy-graphite interfaces are represented by gray (reflections of modest amplitude), and gaps are represented by white (reflections of very high amplitude).