There are a number of techniques available for characterising powder flow behaviour, most of which are carried out under quasi-static conditions. Quasi-static methods are commonly used to define the conditions for the flow initiation, but they are inadequate for characterising the shear strain rate sensitivity of powder flow. The number of devices available to capture powder behaviour under dynamic conditions is indeed very limited. The very few commercially available instruments all require a large sample, which is not desirable for early stages of product development. We have recently proposed the use of ball indentation for characterising flow resistance of cohesive powders. The technique can be applied at very low consolidation stress levels (much less than 1 kPa) and requires only a small sample quantity, typically a few mm³. Previous work showed a good correlation of flowability with established methods under quasi-static conditions. Therefore, an attempt is made to extend the method to dynamic conditions and investigate the sensitivity of the stresses on the shear strain rate for a range of powders, including glass ballotini, α-lactose monohydrate, calcium carbonate (Durcal) and limestone. The results show the existence of a threshold boundary above which the flow resistance experienced by the penetrating ball becomes dependent on the shear strain rate. This is in line with the trends reported in the literature. Therefore, the ball indentation technique has the potential to be used to assess powder flowability at high strain rates.

有许多表征粉末流动特性的技术，其中大多数是在准静态条件下进行的。准静态方法通常用于定义流动开始的条件，但是它们不足以表征粉末流的剪切应变速率敏感性。可以用来捕捉动态条件下粉末行为的设备的数量确实非常有限。极少数市售仪器都需要大量样品，这对于产品开发的早期阶段是不希望的。我们最近提出使用球压痕来表征粘性粉末的流动阻力。该技术可以在非常低的固结应力水平（远小于1 kPa）下应用，并且只需要少量的样品，通常只有几毫米³。先前的工作表明在准静态条件下流动性与既定方法之间具有良好的相关性。因此，试图将这种方法扩展到动态条件，并研究应力对各种粉末的剪切应变率的敏感性，这些粉末包括玻璃巴洛蒂尼，α-乳糖一水合物，碳酸钙（Durcal）和石灰石。结果表明存在阈值边界，在该阈值边界之上，穿透球所经历的流阻变得取决于剪切应变率。这与文献报道的趋势一致。因此，球压痕技术有潜力用于评估高应变速率下的粉末流动性。