Structural Observations in Fatigued Specimens

Abstract

The resistance of structural ceramics, such as oxides, carbide or nitrides to exposure of cyclic loading determine their life. The cyclic deformation process starts with dislocation movements at the microscopic level forming persistent slip bands which become the nucleus of short cracks. The fatigue fracture occurs at three stages, namely (I) initiation, (II) propagation and (III) final rupture. The structural damage in components can occur in stress range far below the static material strength. The damage is not reversible and do not recover when rested (while not in service). Progression markings, known also as beach marks are observed on fractured surface. Striations represent the advance of the crack front by one load application in many ductile metals, whereas beach marks locate the position of the crack front when repetitive. Among the typical fracture surface markings for brittle materials are Wallner lines, which are striations from crack propagation along the crystal. Deformation by dislocations is the basic understanding in ambient temperature in alloys requiring some plasticity. High temperature deformation also of ceramics by dislocation explains plastic deformation. Thus, alumina at high temperature deforms by dislocation mechanism. Also, twin deformation is involved. MgO single crystal can even deform up to a few percent plastic strain due to dislocation processes which depend on parameters such as pressure, temperature and strain rate. Further, dislocations are involved in deformation of partially and fully stabilized zirconia, B₄C and Si₃N₄ as proven by TEM.