Tensile strength of cohesive powders

Abstract Measurement and prediction of cohesive powder behaviour related to flowability, flooding or arching in silos is found to be very challenging. Previous round robin attempts with ring shear testers did not furnish reliable data and have shown considerable degrees of scatter and uncertainty in key measurements. Thus studies to build a reliable experimental database using reference materials, are needed in order to evaluate the repeatability and effectiveness of shear testers and the adopted methodologies. In this paper, first we study the effect of particle size on the yield locus for different grades of limestone (calcium carbonate). We use the nonlinear Warren Spring equation to obtain the values of cohesion C, tensile strength T, and the shear index n. We recover linear (n = 1) yield loci for d 50 > 70  μm with respectively small C. For smaller fractions ( d 50 = 2.2 μ m), the powder is strongly cohesive with considerable non-linearity ( 1 n 2 ). Then we compare the values of the parameters C , T and n obtained from two different shear testers (Schulze and Brookfield PFT), thereby demonstrating the validity of the Warren Spring equation. The differences encountered are fortunately not leading to a great deviation in terms of cohesion C and tensile strength, T although further experiments with a variety of cohesive powders are needed to confirm or rebut this point. Finally, we compare the values of the tensile strength obtained by a tranverse experimental method using the Ajax tensile tester and found a very good agreement.