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Use of jump-tests for the characterization of the viscoplastic behavior of pharmaceutical powders during compaction

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Strain rate sensitivity (SRS) refers to how the compaction kinematics influence powder behavior during compaction and the resulting tablet properties. Various physical phenomena, such as viscoelasticity or viscoplasticity, can elucidate SRS. This study specifically examines the aspect of viscoplasticity, focusing on products known for exhibiting minimal viscoelasticity. Initially, monotonic compression tests at different speeds revealed that viscoplasticity was not constant throughout the entire compaction process. Subsequent Jump tests indicated the presence of transient phenomena during speed changes, reminiscent of dynamic strain aging described in the literature. The inclusion of extended dwell times during Jump tests further revealed stress relaxation in all products, even at slow speeds. This observation was corroborated by prolonged relaxation experiments lasting 10 hours. Consequently, for viscoplasticity, even compactions conducted at very slow speeds should be regarded as dynamic processes.
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