Investigations into the dynamics of penetration tests in digital fabrication

Precise control of fresh concrete properties is crucial for effective extrusion-based 3D printing. In particular, the evolution of yield stress over time is of interest as it determines the structural build-up of concrete and thus its ability to support subsequent layers during printing. Penetration tests are a standard method to analyze the yield stress and its evolution. However, the duration required to reach a steady state flow around the measurement tip, which is essential to evaluate yield stresses, is rather long. Our research focuses on a novel test setup to reach an accelerated steady state flow during probe penetration. A coupled fast-to-slow penetration approach was developed, leading to the rapid establishment of steady state flow. Experiments were performed using a clay mortar with a consistency comparable to that of cementitious mortars, as typically used in 3D printing. The clay mortar was observed to exhibit an almost constant consistency over time, allowing for experiments that exclude the effect of changing material. The observed variations in forces required for fast and slow penetration tests indicate different underlying friction mechanisms. The force in fast penetration likely overcomes the sliding friction associated with dynamic yield stress, while slow penetration is linked to static friction and static yield stress. This distinction and the establishment of steady state flow is important for understanding material behavior and penetration test dynamics under different stress conditions, in particular for materials for 3D printing applications.