Macroscopic and Microscopic Residual Stresses in Ceramics Due to Contact Loading
The paper deals with microplastic deformation and macroscopic residual stresses in silicon nitride ceramics due to contact loading and their effect on the near-surface strength. Depth-resolving X-ray residual stress measurements on shot peened samples detect high microplastic deformation and macroscopic compressive residual stresses up to 1.3 GPa at the surface decreasing to zero within 10 pm. These high compressive stresses enhance the load bearing capacity in the ball-on plate test up to 50 % compared to polished samples.
Strength Properties of Surface-Machined Components of Structural Ceramics
The high quality required for special components of structural ceramics can only be achieved by an additional fine-machining. Since processes like lapping or grinding strongly influence the service behaviour of the components and in addition cause a considerable part of the manufacturing costs there is a need for parameter studies in order to optimize the resulting quality and reduce the costs of the machining process. Therefore, the strength-controlling surface effects have been investigated using a newly developed method for the determination of micro-strains and residual stresses by x-ray diffraction techniques. The resulting information on the defect situation in the surface layer can be modeled by assuming hypothetical cracks. When the additional information on the distribution of the residual stresses caused by machining and the strength properties obtained from bending tests are taken, as a consequence, the effects of damage processes and residual stresses on the strength of ceramics can be quantified by the methods of fracture mechanics. Typical case studies will be discussed in which different grinding and lapping procedures are applied to specimens of silicon nitride and alumina.
Neutron Residual Stress Measurements on Rail Sections for Different Production Conditions
Rail sectioning with subsequent neutron diffraction experiments has been used to assess residual stresses in the rails. In this study we present the results of neutron stress measurements performed at the NIST Center for Neutron Research (NCNR) on rail sections from rails that were produced under various conditions. Specifically, these are air-cooled, air-cooled and roller-straightened, head-hardened and head-hardened and roller-straightened. More significantly, a head-hardened and roller-straightened rail was also studied after service to elucidate evolution of the service-induced residual stresses. In the latter case both a transverse-cut slice and the central region of a 0.53 m long piece were studied. Measurements on this piece are the first in which triaxial stresses have been determined for an intact rail. Neutron strain measurements with 3×3×3 mm3 spatial resolution were successfully employed for transversally cut slices to verify the difference in the stress state depending on the production process. Although examination of slices allows determination of only two-dimensional stresses in the plane of the slice, additional measurements on obliquely-cut slices, which were also carried out, and utilization of FEM gives the possibility of reconstructing the full triaxial stress distribution. Together, these approaches provide a better understanding of rail fabrication and the possibility of improving the durability and safety of rails in the future.Copyright © 2004 by ASME