Computer controlled shear cell for measurement of flow properties of particulate materials

doi:10.1016/S0032-5910(97)03230-0

Powder Technology

Volume 92, Issue 2, 15 July 1997, Pages 135-146

https://doi.org/10.1016/S0032-5910(97)03230-0 Get rights and content

Abstract

A shear tester for powders that allows the determination of a dynamic yield locus from a single test was built and tested. The principle of the computer controlled shear cell (CCSC) is that dynamic yield states can be maintained in a test cell over a range of normal and shear stresses in the same test. This leads directly to a dynamic yield locus. With its flexible design, the CCSC can also be used as a Jenike shear cell or a direct shear cell. The performance of the CCSC used as a dynamic yield locus tester (YLT) was compared with the CCSC used as a Jenike shear cell. Wheat flour and sugar were the test materials. The yield loci obtained with the CCSC-YLT using fine cohesive material, wheat flour, were not statistically different (at the 95% confidence interval) from the yield loci obtained with the CCSC-Jenike, while there were marginal differences (at the 95% confidence interval) between the testers for the less cohesive material, granulated sugar. Design of the CCSC also eliminated the need for trial and error in determining the number of twists needed for critical consolidation of powder mass. In addition, the duration of the test and amount of material needed to generate a dynamic yield locus using the CCSC are reduced by 3/4 and 2/3, respectively, compared with those required for the CCSC used as a Jenike shear cell.

Introduction

In many applications involving handling and storage of bulk solids such as tableting, particle size reduction, mixing, packaging and quality control, it is important to be able to design bulk solids handling systems to precise engineering specifications. To design a reliable system, using rational principles, it is necessary to characterize the flow properties of the bulk solids.

Several test devices have been used to measure the flow properties of bulk solids. Some of the commonly used testers are: the Jenike shear cell, the direct shear box, the rotational shear cell, and the triaxial tester [1]. The Jenike shear cell is widely used in industry [2]; however, a significant amount of time is needed to obtain results which are often not sufficiently reproducible. The direct shear box is mostly used to test soils and few published data for other powders are available. The direct shear box cannot be used to determine the flow function. The rotational shear cell [3]was designed in an attempt to improve the measurement of bulk solids flow properties. Although available on the market, the rotational shear cell is not yet in wide use. In the triaxial tester, a new elastic membrane is needed for each test, which increases the cost of testing. Other testers such as the biaxial [4]and cubical [5](true triaxial) testers involve complex design and testing methodology that entail time intensive tests. With the exception of the rotational shear cell, of which an automatic version is now available [3], the testers mentioned above require laborious test procedures to measure the yield locus.

Some work has been done towards making test procedures simpler and faster 6, 7, 8, e.g. with the development of testers which measure the yield locus from a single test. Such testers have been referred to as `constant volume' testers. However, published results either do not include comparisons with accepted standards, or the methods published also involve complicated test procedures.

Therefore, there is a definite need to improve upon the design of the constant volume tester and test procedure, and to compare its performance with testers that are commonly in use. In this study, the constant volume tester developed is referred to as the computer controlled shear cell (CCSC).

Section snippets

Review of literature

A yield locus is a curve joining several points on a shear stress versus normal stress plot which represent stress states that lead to flow in powder samples of the same initial bulk density 9, 10. Shear tests are common in powder technology to characterize the flowability of granular materials and powders [9]. Kamath et al. [1]discussed several testers which are widely accepted for measuring flow properties of powders in industry. Their results indicate that no single tester is suitable for

Objectives

The specific objectives of this research were:

(i) to adapt an existing design of a constant volume tester for powders with which dynamic yield loci can be obtained directly from single tests;

(ii) to develop a simple operating procedure for a computer controlled dynamic yield locus shear cell and test the shear cell's performance; and

(iii) to compare the performance of the computer controlled shear cell (CCSC) used as dynamic yield locus tester (YLT) with that of the CCSC used as a Jenike shear

Characterization of test material physical properties

`All-purpose' wheat flour and granulated sugar were selected for use as test materials for a number of reasons. They are inexpensive, readily available, and are of great importance to the food industry. Also, they are examples of particulate food materials which are, respectively, fairly and slightly cohesive [1]. Most importantly, much work has been done on these powders and, consequently, shear test data are available for comparison of results. The load values and ranges used (Table 1) were

Data from the CCSC used as a Jenike shear cell

Fig. 3Fig. 4 show typical data from the CCSC used as a Jenike shear cell for wheat flour and sugar, respectively, including data from sample preparation. The data generated by the CCSC include: shear force and stress, normal force and stress at the top of the specimen, normal force and stress at the bottom of the specimen, and lid displacement over the entire duration of the test (lid displacement data were recorded as increasing in the downward direction, i.e. with decrease in cell volume).

Summary, conclusions and assessment

The computer controlled shear cell (CCSC) was successfully used to obtain dynamic yield loci from single tests. In addition, by selecting different options in the computer program, the CCSC could be used as a dynamic yield locus tester (YLT), a Jenike shear cell or a direct shear cell.

Conclusions drawn from this study are as follows.

1. Within the 95% confidence interval, there were no significant differences in the yield loci results between the CCSC used as a Jenike shear cell and the CCSC

References (18)

S. Kamath, V.M. Puri, H.B. Manbeck and R. Hogg, Flow properties of powders using four testers — measurement, comparison...
A.W. Jenike, Gravity flow of bulk solids, Utah Eng. Exp. Stn. Bull. 108,...
I.A.S.Z. Peschl, New developments in the field of shear test equipment and their application in industry, Proc. Int....
J. Schwedes, Measurement of flow properties of bulk solids, Proc. First Int. Particle Technology Forum, Denver, CO,...
S. Kamath and V.M. Puri, Measurement of powder flow constitutive model parameters using cubical triaxial tester, Powder...
H. Tsunakawa and R. Aoki, Measurement of the failure properties of granular materials and cohesive powders, Powder...
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