Einfluss der Kornpackung auf das bruchmechanische Verhalten kohlenstofffreier grobkörniger Feuerfestwerkstoffe
Zusammenfassung der Projektergebnisse
The project invstigated the influence of the minimum particle size, distribution of the fines, aggregate size distribution and sintering temperature on the fracture mechanical behavior and acting mechanisms. It was shown on the one hand that for large samples with a higher brittleness, the specific fracture energy increased with the strength for a higher sintering capability and a higher bulk density giving increased strengths. On the other hand, it was shown on smaller samples with a reduced brittleness that an exaggerated sintering capability leads to coarsening in the matrix which leads to a high amount of small inter-connected flaws in the matrix. These flaws cause a longer macrocrack path, but also a reduced strength. Additionally, a high amount of aggregates, i.e. a near-to-dense aggregate packing, increases macrocrack deflection and enables also grain bridging especially if the delamination gap size is small. The background of the study were previous results on the thermal shock resistance, where samples (cross section 25 × 25mm2) with a reduced fines content (and sintering capability) and a near-to-dense aggregate packing had very low strength drops over five thermal shocks. It was hypothesized that crack deflection was increased and that the dense aggegate packing enabled grain bridging and friction between crack faces. The objective of this project was to investigate the reasons, focusing on the fracture mechanical behavior and microstructural changes. As the sintering capability seemed to have an effect on the thermal shhock resistance, the project focused furthermore on its influence adjusted by the sintering temperature, the amount (distribution) and the minimum particle size of the fines. The bricks were manufactured prior to the project from batches designed according to a modified Kawamura particle size distribution model featuring a minimum particle size and distribution moduli for the matrix and aggregate fractions. From these bricks within the project then, samples for the tests were prepared. The main experiments were microstructural analyses by scanning electron microscopy, standard wedge splitting tests accompanied by digital image correlation to analyze the specific fracture energy in dependence on the investigated parameters and miniaturized wedge splitting tests accompanied by DIC that were conducted under an optical microscope to investigate the acting fracture mechanical mechanisms. For the standard wedge splitting tests, it turned out that the specific fracture energy increased with an increasing strength which was high for high densities after sintering. The densities were high for a higher amount of fines of smaller size (improved compaction during forming; increased sintering capability), for a near-to-dense aggregate packing and a higher sintering temperature (sintering capability). The results from the miniaturized wedge splitting test samples with reduced brittleness differed. The results showed that comparable values of the specific fracture energy G′ f can be obtained by samples that behave microstructurally completely different as hence other mechanisms are dominating the behavior: A coarsened matrix as a result of a too high sintering capability (low minimum particle size and high sintering temperature) causes a high amount of small crack-like voids within the matrix. These inter-connected flaws cause a longer macrocrack path. Despite a small fracture process zone, hence, a comparable G′ f was reached. Delaminated aggregates can support energy consumption if the gap size is small so that friction between aggregate and matrix surfaces is enabled. A high amount of aggregates, i.e. a near-to-dense aggregate packing, increases macrocrack deflection and enables also grain bridging. The results of the miniaturized wedge splitting test could generally explain the thermal shock behavior of the bars tested previously. A future study could investigate the reason for this circumstance as either the more comparable sample size (and thus brittleness) or different cracking behavior during thermal shock (causing many cracks on the surface simultaneously) compared to during the wedge splitting test (forcing a single macrocrack) are possible causes.
Projektbezogene Publikationen (Auswahl)
-
ParSD — Tool to design and analyze particle size distributions. Software X, Vol. 15, 100753
Fruhstorfer, Jens
-
Analyzing Crack Deflection Behavior of Refractories by Digital Image Correlation. AIP Conference Series, 2023
J. Fruhstorfer, D. Gruber and H. Harmuth
-
Influence of the Fine Fraction and Sintering on Selected Isotropic and Anisotropic Bulk Properties of Uniaxial Compacts. Interceram, Vol. 71, p. 38–47
Fruhstorfer, Jens; Wagner, Moritz; Gruber, Dietmar & Harmuth, Harald
