טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentMalamut Sergio
SubjectCreep of Spherical Contact under Combined Normal and
Tangential Loading
DepartmentDepartment of Mechanical Engineering
Supervisors Professor Emeritus Izhak Etsion
Dr. Yuri Kligerman
Full Thesis textFull thesis text - English Version


Abstract

A theoretical model is developed to study the effect of dwell time on the junction growth during tangential loading and on static friction of a creeping polymer sphere in contact with a rigid flat under full stick contact condition. A rapid normal loading into the elastic-plastic contact of a creeping polymer sphere is followed by a rest period. During this period (dwell time) creep takes place causing increase of the contact area and stress relaxation. At the end of the dwell time a rapid increasing tangential loading is applied to the flat till sliding inception occurs. During the tangential loading step, further increase of the contact area (junction growth) and reappearing of a plastic volume in the sphere take place. The reappearing of the plastic volume and its increase during the tangential loading step decreases the tangential stiffness of the sphere. Sliding inception, which determines the end of the tangential loading step, is defined as the moment when the tangential stiffness of the sphere decreases significantly compared to its initial tangential stiffness (ITS). The minimum tangential force causing that sliding inception is defined as the static friction force. The friction coefficient is defined as the ratio of the friction force over the normal preload. An increase of the static friction with increase of the dwell time is clearly demonstrated and explained.