According to the standardized procedures for silo design, the horizontal to vertical stress ratio is a key parameter for the correct estimate of stresses acting on silo walls. To date no standardized equipment is available for the measurement of such ratio. In this paper the state of the art is presented and discussed. A new split cell for the measurement of the horizontal to vertical stress ratio was built. It is a significant improvement of another apparatus, previously developed by the same authors, with the aim of simplifying as much as possible the design of the cell and the procedure to be applied to carry out the measurement. This new version of the cell can overcome the effects of the friction on the cell lid and on the cell bottom, without the use of the twisting action that was necessary in the former cell. The new cell was tested with three different non-compressible powders and three different compressible powders in wide range of vertical stresses between 1 and 16 kPa. Results indicate that the cell can provide repeatable and consistent results with standard deviations within few points percent for vertical stresses in the most usual range between 3 and 16 kPa. Larger standard deviations of results are found at lower stresses, in particular with the most free-flowing materials, but measurements still provide internal consistency of results. Experiments show no significant effect of the twisting action on the lid at high consolidation stresses, while some effect of the twisting is visible a low powder consolidation levels. Results of the vertical to horizontal stress ratio are compared with the most used theoretical and empirical equations to relate the ratio to the angle of internal friction. It is shown that incompressible materials are closer to the internal state of incipient shear during the tests and that, consistently, the theoretical expression relating the stress ratio to the effective angle of internal friction provides satisfactory results. Instead, compressible powders are apparently away from a condition of internal shear and, therefore, empirical and more conservative expression for the stress ratio provide better predictions.
A new split cell for the measurement of the horizontal to vertical stress ratio of powders
Barletta, Diego;Poletto, Massimo
2019-01-01
Abstract
According to the standardized procedures for silo design, the horizontal to vertical stress ratio is a key parameter for the correct estimate of stresses acting on silo walls. To date no standardized equipment is available for the measurement of such ratio. In this paper the state of the art is presented and discussed. A new split cell for the measurement of the horizontal to vertical stress ratio was built. It is a significant improvement of another apparatus, previously developed by the same authors, with the aim of simplifying as much as possible the design of the cell and the procedure to be applied to carry out the measurement. This new version of the cell can overcome the effects of the friction on the cell lid and on the cell bottom, without the use of the twisting action that was necessary in the former cell. The new cell was tested with three different non-compressible powders and three different compressible powders in wide range of vertical stresses between 1 and 16 kPa. Results indicate that the cell can provide repeatable and consistent results with standard deviations within few points percent for vertical stresses in the most usual range between 3 and 16 kPa. Larger standard deviations of results are found at lower stresses, in particular with the most free-flowing materials, but measurements still provide internal consistency of results. Experiments show no significant effect of the twisting action on the lid at high consolidation stresses, while some effect of the twisting is visible a low powder consolidation levels. Results of the vertical to horizontal stress ratio are compared with the most used theoretical and empirical equations to relate the ratio to the angle of internal friction. It is shown that incompressible materials are closer to the internal state of incipient shear during the tests and that, consistently, the theoretical expression relating the stress ratio to the effective angle of internal friction provides satisfactory results. Instead, compressible powders are apparently away from a condition of internal shear and, therefore, empirical and more conservative expression for the stress ratio provide better predictions.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
