טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentAlexander Belenky
SubjectDynamic Flexural Strength of Advanced Ceramics and its
Relation to Structural Defects
DepartmentDepartment of Mechanical Engineering
Supervisor Full Professor Rittel Daniel
Full Thesis textFull thesis text - English Version


Abstract

Advanced ceramic materials are used in many cutting edge engineering applications. Quasi-static strength measurements, specifically tensile, are a very delicate issue for these materials (due e.g. specimen's gripping and alignment), aspects that have not been solved for dynamic testing yet. Therefore, this research focuses firstly on developing a simple methodology to measure the dynamic flexural strength of brittle materials. The proposed technique is based on 1-point impact experimental setup with (unsupported) small beam specimens. A simple numerical (finite element) model was used to assess the validity of the proposed experimental technique and the influence of specimen’s geometry on the stress distribution during a typical test. The expected failure pattern was also identified numerically by implementing a maximum principle stress criterion into the simulations. Two batches of 99.5% commercial alumina beam specimens were used to assess static and dynamic flexural strength of this material and further validate the proposed experimental technique. In the first batch, specimens of the same size and surface roughness conditions were used, in a large sample size, thus allowing an estimation of basic statistical parameters for the evaluated material. The main observation from this batch is that the flexural strength of this material is distinctly rate-sensitive, a point that was explained by fracture surface porosity levels corresponding to the different loading regimes. In the second batch, specimens of the same size and systematically varying surface roughness conditions were used, in order to assess the influence of the latter on the static and dynamic strength of this material. It was found that there is a minor increase in static flexural strength for polished specimens. However, for the dynamic loading, no clear correlation between surface roughness and the flexural strength was found. A thorough microstructural and fractographic examination reveals the presence of bulk (surface) pore-like flaws that are not obliterated by the polishing process, thus governing the failure process. The identification of the role of surface flaws is expected to clarify the discrepancy found in the literature as to the influence of surface roughness on the mechanical properties of brittle materials.