The pulse velocity can be used to determine a number of fundamental material properties. For instance, if the density and Poisson's ratio of the material are known, the pulse velocity can be used to calculate the elastic modulus. Techniques also exist and are documented in national standards (e.g., BS1881:203 for concrete) for using the pulse velocity to calculate material strength and moisture content. Such techniques are often used by pre-cast concrete manufacturers and some refractory manufacturers.
Using just the transit time measurement, it is possible to detect cracks, voids, delaminations and other defects with relative ease. These tests rely on the fact that the speed of the ultrasound is much higher in the test material than it is in air. Any air in the signal path (due to cracks or voids, for example) causes a reduction in the pulse velocity and thus an increase in transit time. This effect produces good results for large cracks, as well as microcracking induced by thermal shock or freeze-thaw cycling.
The instrument detects the first part of the ultrasonic signal to arrive at the receiving transducer. In a "good" or high-quality material, the ultrasonic signal travels in a straight line from the transmitting transducer to the receiving transducer. Testing known quality samples easily establishes a reference measurement for good material. If a much larger transit time (i.e., lower velocity) is measured on the same sample, it is a clear indication that the signal has had to travel around or through some defect.
When using the technique on refractories, the material must be relatively viscous (thick) to cope with the rough surface. A thick grease or petroleum jelly is often used in concrete applications, but this can present problems for ceramic products because of the absorption of the couplant into the product, staining of the product and difficulties handling the couplant. To avoid these problems, many refractory manufacturers use other couplant materials. For example, Dyson Precision Ceramics uses a thin rubber disc (cut from sheets of the material used to attach dentures) fixed to the transducer face. Other companies use modeling clay or similar materials, which stay attached to the transducer, deform sufficiently to give a good acoustic coupling to the material under test, and can easily be replaced if required. They leave no stain or mark on the product and create no mess. In still other applications, small transducers with replaceable rubber tips are used to ensure adequate contact between the transducers and the product being analyzed. Since ceramic manufacturers must often perform many tests (possibly several hundred) per day, having a reusable couplant is invaluable.
However, even with a good couplant, other possible test variables can exist. For instance, different operators might apply different levels of pressure to the transducers, which can vary the thickness of the coupling layer and thereby reduce the distance traveled by the ultrasonic pulse. This can easily be overcome by ensuring that adequate training is given to operators, and/or by using an instrument that features built-in zeroing or calibration functions.
The choice of transducer is also an important consideration. Frequencies used for ceramic materials are generally in the 150-500 kHz range, which correspond to wavelengths of around 200 mm to 16 mm. The wavelength determines the size of the smallest defect that can be detected, so it is important to select an instrument that is capable of operating within this range. The physical size of the transducers can also be a factor in some applications, since the transducers must be able to form a bond with the material to achieve accurate analyses.
Dyson's Stoke-on-Trent facility has also used UPV testing* to determine the moisture content of large batts before firing. (If the batts contain too much moisture during firing, they can be damaged or destroyed due to steam build-up.) The plant carried out a series of tests in which the batts were analyzed during drying to produce a graph of ultrasonic velocity compared to moisture content. From this graph, operators on the factory floor were able to obtain an instant assessment of moisture content and determine whether the batts were dry enough to enter the firing process. As a result, waste from damaged products has been reduced significantly.
The PUNDIT instrument, developed and supplied by CNS Farnell Ltd., Borehamwood, Hertfordshire, UK.
For more information about ultrasonic pulse velocity testing, contact CNS Farnell Ltd., Elstree Business Centre, Elstree Way, Borehamwood, Hertsford, WD61RX England, UK; (44) 20-8238-6900; fax (44) 20-8238-6901; e-mail email@example.com ; or visit http://www.cnsfarnell.com .