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Traditionally, the quality of magnesite-chrome brick is assessed by measuring its cold crushing strength (CCS), which is a slow and costly process compared to nondestructive testing. However, nondestructive testing methods do not always provide results that correlate well to CCS.
The impulse excitation technique (IET), as implemented by the Buzz-o-sonic nondestructive testing system, can be used to measure the sonic velocity in the longitudinal direction of the brick, and these measurements correlate reasonably well with CCS. Weaker brick can then be identified and removed from the general population.
The overall goal is to improve manufacturing processes such that no sorting out of “bad” product is necessary. Steps include:
- Use non-destructive testing methods to correlate to key quality criteria
- Accomplish 100% testing if problems are evident
- Learn how to change the process to produce fewer weaker bricks
Test ProceduresMagnesite-chrome brick was tested by RHI Canada at its Bécancour plant using both the standard destructive CCS test and the nondestructive IET with the Buzz-o-sonic testing system. The impulse excitation technique is described in ASTM standards E1876 and C1259. Generally, a sample (usually of uniform cross-section) is struck with a small hammer that causes the sample to vibrate at its natural frequencies.
Since a sample can vibrate in different modes, the locations of the strike and vibration measurement are important. For sonic velocity measurements, the longitudinal mode of vibration is most commonly used because of the following simple relationship:
v = 2.L.f (1)
where v = sonic velocity, L = length, and f = fundamental longitudinal resonant frequency.
Thus, the longitudinal vibrations were excited and measured using the setup shown in Figure 1. Note that, for practical reasons, two supports were used rather than the ASTM E1876/C1259 recommendation of one support placed at the mid-length. The longitudinal vibrations were then measured using the Buzz-o-sonic testing system and converted to sonic velocities using Equation 1.
ResultsA correlation between the CCS and sonic velocity is shown in Figure 2. Because of the good correlation, it was possible to identify the weakest brick as those with a sonic velocity of 1800 m/s or lower. As a result, though one in five samples is still crushed, one line of brick has been tested using the Buzz-o-sonic testing system rather than CCS since September 2008.
When a result below the sonic velocity limit is observed, the standard destructive CCS test is performed before releasing or recycling the product. Nevertheless, the reduction in CCS testing has been significant-approximately 25% for some brands in 2008.
Thus, by replacing some CCS testing on a line of magnesite-chrome brick with the nondestructive IET test, the RHI Canadian Bécancour plant is saving around 10 destructive CCS tests per month. It was possible to reduce the number of CCS testing by about 25% on some products, with the advantage of an improved level of quality control. The savings will increase as more products are tested nondestructively.
For more information regarding IET, contact BuzzMac International LLC at 6720 North Sidney Place #104, Glendale, WI 53209; call (414) 352-5419; fax (253) 540-9798; e-mail firstname.lastname@example.org; or visit www.buzzmac.com. RHI Canada can be reached at 7000 Rue Yvon Trudeau, Bécancour, QC G9H 2V9; call (819) 294-9961; fax (819) 294-2494; or visit www.rhi-ag.com.