GEOMECHANICAL CHARACTERIZATION OF GYPSUM-BASED 3D PRINTED MATERIALS (Invited Presentation)

3D printing of geo-materials has the potential to enhance mechanical interpretations of natural rocks by generating engineered samples with reproducible microstructures and tunable mechanical properties. For geomaterials, sample-to-sample heterogeneity that plagues the consistency of rock physics testing can be overcome by 3D printing method. In this work, we used a powder-based 3D printing technique to print three point bending (3PB) and cylindrical core samples for tensile failure and compression testing, respectively. Bassanite powders react with a proprietary water-based binder to form a mix of gypsum and bassanite. Due to the printing process, two major directional properties caused by the powder depositional direction and binder spray direction dominate the mechanical properties of printed materials. For 3PB testing, our experimental results with post fracture roughness characteristics and flow property estimate clearly demonstrated a fundamental link among fracture toughness, roughness and mineral fabric, which enables optimization of fracture design to enhance permeability based on detail knowledge of sample mineralogy. For cylindrical samples, various printing directions and binder amount were varied to evaluate their impact on mechanical properties. In addition, samples were stored in different humidity conditions under warm temperature (50 degree C), ambient, and 80% of relative humidity. Unconfined compression testing results show that the samples printed perpendicular to the loading direction are stronger than those printed parallel to the loading and at 45 degree. Micro-CT images of the tested samples were analyzed to evaluate the effect of printing quality on testing reproducibility and post-failure characteristics such as fracture surface roughness. Overall, strong anisotropic properties of peak strength and wave velocities were observed. Due to the reaction of the printed material with water vapor, dry samples had stronger UCS strengths than samples stored under 80% relative humidity. Advantages and disadvantages of power-based 3D printing for mechanical testing will be discussed and attempts at acoustic emission analysis during 3PB testing with machine learning techniques will be presented.

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