Strengths of Sapphire Crystals
THE strength of single crystals of entirely brittle materials should depend on the presence of stress raising flaws. Internally these could be formed by static dislocation arrays, voids or inclusions of foreign material. On the surface the flaws could be formed by mechanical damage or by surface steps as suggested by Marsh1.
Strengthening Sapphire by Compressive Surface Layers
Compressive surface layers were formed on sapphire rods. The methods included glazing and quenching, formation of Al2O3–Cr2O3 solid solution surface layers, and formation of mullite and calcium aluminate surface layers. The presence of the compressive stresses was demonstrated by slotted rod tests. Substantial improvements in flexural strength and thermal shock resistance were observed. The highest average flexural strength was obtained by glazing and quenching and was 340 000 psi. The strengths were measured in the ``as‐treated'' and ``as‐treated and abraded'' conditions. The abraded samples retain a substantial portion of the improved strength.
Using Activity Theory in Developing Instructial Tools for Project Management Studies
Competence and skills of the project manager are significant to project success. The skills needed in project managersâ work cannot be learned only by reading the books or a lecture hall; one learns them by practice. Therefore, an important challenge for educational institutions is to develop pedagogical practices that allow students to participate in working life projects and to confront real-life problems. Project-based learning (PBL) offers a model that enables students to practice the skills and competences needed in working life projects by utilizing real-world work assignments in time-limited projects. Using PBL method alone does not necessarily guarantee learning result. In order to be successful, PBL method requires effective and competent supervision and guidance of students as well as appropriate tools for instruction. In this study the concepts from activity theory (AT) are applied to development tools for supervising project-based learning.