Ultrasound imaging is widely used in laboratories for nondestructive materials testing and in health care as a noninvasive diagnostic tool. But for it to work, transducers and an acoustic-coupling liquid must touch the specimen. This rules out testing of such materials as uncured plastics, green ceramics and powered metals, materials that are continuously rolled on a production line, or those with a large surface area to be scanned.
But the advent of highly sensitive piezoelectric transducers and an inexpensive precision positioning system help overcome these limitations.
Ultrasound imagers from SecondWave Systems, a division of Ultran Laboratories Inc., Boals-burg, Pa., measure a sample's thickness, density, mechanical properties, and defects, as a transducer moves above its surface. This task is challenging because an extremely large acoustic impedance mismatch between air and sample can attenuate ultra-sound waves by seven orders of magnitude compared with water as the transmission media.
This is where the better transducers come in. The updated transducers operate in air. They lose just 12 to 40 dB of sensitivity over a frequency range of 50 kHz to 5 MHz.
In one configuration, a CNC gantry from Techno-Isel, New Hyde Park, N.Y., accurately positions the transducer over a test article. This makes possible the ultrasound imaging of large surface areas in the familiar C-Scan mode. C-Scan mode or pulse-echo gives planar images of samples at a specific depth.
Airtech 4000 imagers use the two-axis linear motion control platforms to scan a 50 54-cm area. The gantry drive consists of 16-mm ball screws and 4-mm double recirculating ball bearings. The arrangement in standard form is good for travel speeds from 0.0125 to 500 mm/sec at a 0.0125-mm resolution. Custom configurations can push speeds to greater than 2.5 m/sec.
The other system, called iPass, works statically or in a scanning configuration. It can monitor the trend of any measured parameter (thickness, velocity, time-of-flight, attenuation, and density) as a function of its variation from one point to another. This feature is particularly good for linear imaging or for applications where a product is in motion. The system provides two independent trend plots that represent two different locations on a sample. Trend-plot speed can be synchronized with that of the moving object or material.
In-air ultrasound imaging needs no sample preparation, is nonhazardous, and works on all states of matter except plasma and vacuum. The imagers work in either a direct-transmission or direct-reflection mode. The direct-transmission mode characterizes sample thickness, velocity, density, defects, and microstructure. A discontinuity or defect in a sample attenuates more of the transmitted ultra-sound energy than does a defect-free region. This property permits the detailed imaging of a sample's interior features. A direct-reflection mode characterizes surfaces because reflectivity of ultrasound energy relates directly to surface roughness.
Typical uses for noncontact ultrasound imaging
Aircraft/aerospace composites; Space Shuttle thermal-protection tiles; green ceramics and powder metals; porous materials and foams; rubbers, tires, and plastics; wood, lumber, asphalt and reinforced concrete; food and pharmaceuticals.
Delaminations in multilayered, particulate and fibrous materials; proximity and dimensional analysis; measurements of anisotropy and heterogeneity; surface profiling, chemical corrosion, crystallization and polymerization; liquid and gas-flow metering; imaging of surface and internal features of materials; viscosity of liquids; texture and microstructure of granular and cellular materials; level sensing and proximity analysis; bone-density (osteoporosis) measurements, applied and residual stresses; high temperature, pressure and radiation environments.