Strona 2_redak - Instytut Agrofizyki im. Bohdana DobrzaÅskiego ...
Strona 2_redak - Instytut Agrofizyki im. Bohdana DobrzaÅskiego ...
Strona 2_redak - Instytut Agrofizyki im. Bohdana DobrzaÅskiego ...
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97<br />
The sample holder proper for particular tested material is placed in the body of the<br />
apparatus and thoroughly filled. The holder is closed with the cover and prescribed air<br />
pressure is applied that is adjusted with the valve and kept constant for 30 seconds.<br />
Then the pressure is decreased to zero and the perforated plate removed to open the<br />
outlet. The pressure gauge is turned on, and pressure in the sample holder increased<br />
till the moment of collapse of an arch when material flow commences. The<br />
measurement cycle from consolidation to outflow is repeated four t<strong>im</strong>es (at least) for<br />
one level of consolidation pressure. Results obtained with the tester were compared<br />
with results of the Jenike method for l<strong>im</strong>estone powder and baking soda. Part of the<br />
tests showed fairly good agreement of results of the two methods, while another part<br />
was in disagreement. The authors concluded that the tester may be successfully<br />
applied for comparison of different lots of the same material [134].<br />
3.1.4. Uniaxial tester of POSTEC<br />
Led by premises s<strong>im</strong>ilar to those reported by other designers of powder testers,<br />
researchers of the Norwegian institution POSTEC proposed their own solution. The<br />
tester is a type of indirect shear apparatus, namely uniaxial, and its description here is<br />
quoted after Maltby and Enstad [102]. In uniaxial compression test a compressive<br />
failure strength s<strong>im</strong>ilar to the unconfined yield strength 1 c may be determined directly<br />
as a function of the consolidation stress 1 1 . The measurement is taken in a fraction of<br />
t<strong>im</strong>e required by other testers. Due to the consolidation procedure used, the tester does<br />
not measure the flow function, and therefore should not be used for silo design unless<br />
a correction factor is introduced. The procedure used assures the scatter of the results<br />
is min<strong>im</strong>al, which together with the rapidity with which such measurements can be<br />
made, makes the test perfectly suitable for quality control purposes. In the POSTEC<br />
tester (see fig. 13.4) the sample is confined in a slightly conical die and consolidated<br />
by the piston moving vertically down. The flexible membrane is stretched between<br />
the outer surface of the piston and inner surface of the lower part of the die. A layer of<br />
lubricant is spread between the flexible membrane and the die wall. The die is filled<br />
upside down and closed with a tight cover, turned upside down again and placed into<br />
the guiding device of the tester. The sample is consolidated by moving the piston until<br />
the predetermined value of 1 1 is reached, corresponding to a strain 0 1 . After a predetermined<br />
t<strong>im</strong>e of sample stabilization the die is raised up and compressive failure<br />
strength is measured with the piston moving down. The max<strong>im</strong>um value 1 c is reached<br />
before the sample falls apart.<br />
The apparatus allows for direct observation of the forming of failure surface that<br />
theoretically forms an angle of . ƒ + 3 with the horizontal plane. More complicated<br />
shape of failure surface points indicates that the sample was not tall enough.<br />
Using precise procedure for examination of BCR-l<strong>im</strong>estone, Maltby and Enstad