Quality with a Pulse
Ultrasonic pulse velocity (UPV) testing is used to analyze the quality and elastic properties of a material. Originally developed for testing concrete, the method was soon adapted for use with other materials and has been successfully applied to a range of ceramic products-including tile, refractory bricks and blocks, and kiln furniture-as well as graphite and timber. Although early UPV instruments were primarily designed for lab applications, later versions were developed to be light, portable and easy to use in production environments. Today, UPV testing is widely used in refractory plants throughout the world, and the advantages of this method over traditional testing techniques are likely to further increase its application.
Detecting Defects
The technique requires an electronic instrument and a pair of ultrasonic transducers. The instrument generates a high-voltage, short-duration electrical pulse (up to 1200 volts for 1.5 microseconds), which is converted by one transducer into an ultrasonic signal. The other transducer detects the ultrasonic signal and converts it back into an electrical signal. The instrument then measures the time required for the pulse to travel between the two transducers-this is known as "transit time" and is usually measured in microseconds. From this information, the system can easily calculate the pulse velocity using the equation v = d/t, where v is velocity, t is transit time and d is the signal path length.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.
Understanding the Variables
A minor drawback of the UPV method is that a good mechanical "bond" is required between the test material and the transducers. Failure to achieve a sufficient bond can result in erroneous test results-even a tiny pocket of air between the transducer and the product will produce the same effect as a crack in the product. To overcome this problem, some form of coupling material is used, usually one that has similar acoustic properties to the material under test and that can deform in such a way as to fill the microscopic gaps between the transducer and the product (similar to the gel used in medical ultrasound applications).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.
Applying the Technique
Companies that understand how to use the technique and are aware of the variables can reap significant advantages from integrating UPV testing into their manufacturing operations. For example, Dyson Industries Ltd. in the UK operates several divisions at a number of different sites, many of which use UPV testing to ensure the quality of their products. At the Stoke-on-Trent location of the Dyson Ceramic Systems division, which manufactures a range of kiln furniture, UPV testing* is being successfully used to detect cracks in fired samples. The setters are tested in three positions for transit time, and a limit is established for determining whether a crack is present. A typical set of results is shown in Table 1. As seen in the table, the transit time is almost doubled in the case where a crack is present, allowing operators to easily identify parts that contain defects.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.
Improving Overall Operations
In an increasing number of refractory and ceramic applications, the ultrasonic pulse velocity testing technique has been used with positive results. UPV testing has enabled users to improve their production processes, increase the integrity and quality of their products, and reduce scrap and reject rates-thereby saving both time and money. In today's economy, such bottom-line benefits are difficult to ignore.Author's Acknowledgements
The author would like to thank John Gallimore, Dyson Ceramic Systems, and Paul Chevolleau, Dyson Precision Ceramics, for their assistance with this article.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 sales@cnsfarnell.com ; or visit http://www.cnsfarnell.com .