טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentBrizmer Victor
SubjectElastic-Plastic Contact of a Sphere and a Flat under
Combined Normal and Tangential Loading
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Emeritus Izhak Etsion
Full Thesis textFull thesis text - English Version


Abstract

The behavior of an elastic-plastic contact between a deformable sphere and a rigid flat under combined normal and tangential loading is investigated theoretically. The normal preloading is assumed under full stick contact condition leading to the formation of a junction that can support additional tangential load. Sliding inception is treated as a plastic yield failure mechanism, which allows static friction modeling under highly adhesive conditions. Several contact parameters such as: junction tangential stiffness, static friction force and static friction coefficient are extensively investigated. It is found that at low normal preloads the static friction coefficient decreases sharply with increasing normal preload, in breach with the classical laws of friction. As the normal preload further increases the static friction coefficient approaches a constant value that is about 0.3 for many material properties combinations.

The evolution of the contact area (junction growth) of an elastic-plastic preloaded spherical contact subjected to an additional tangential loading prior to sliding inception is also investigated theoretically in the present work. The model is based on full stick contact condition while the sliding is treated as a plastic yield mechanism. It is found that the contact area grows in an asymmetric way during the tangential loading but remains almost circular (with a very small elongation in the direction of sliding) prior to sliding inception. The physical mechanism of junction growth is extensively investigated. The relative junction growth is found over a wide range of dimensionless normal preloads along with corresponding relative growth in the normal interference.

The theoretical results obtained in the present work correlate well with some limited experiments that were performed with copper balls in contact with a hard sapphire flat.