Ph.D. Thesis Defense by Chung-Hyun Goh

(Drs. David L. McDowell and Ward O. Winer, co-advisor)

"Crystallographic Plasticity in Fretting of Ti-6Al-4V"

Abstract

Fretting fatigue is a damage process associated with extremely small relative surface displacements, on the order of microns, in the presence of normal pressure and stick-slip traction fields across the contact. Since fretting fatigue can lead to serious fatigue life shortening in engineering components, several analysis methods have been developed to quantify fatigue damage. However, it is difficult to accurately capture the physical characteristics of fretting fatigue damage due to the complexity of a large number of parameters and variables.

Continuum crystal plasticity theory, which can account for the heterogeneity of discrete grains and their crystallographic orientation distribution, was successfully applied to model fretting fatigue contacts with the implementation of an ABAQUS User MATerial (UMAT) subroutine. This study is the first of its kind to apply this modeling tool to this industrially significant problem. Since the plastic deformation associated with fretting contact occurs over subsurface distances comparable to microstructure dimensions, such a modeling effort is necessary to extend understanding of fretting processes and mechanisms.

This study has focused on the relation of plasticity in the surface and subsurface fields to loading conditions and coefficient of friction as well as the influence of several factors affecting fretting fatigue crack formation and small crack growth using crystal plasticity theory. In spite of its simplified 2-D character, the planar triple slip model used in this study appropriately captures the essence of granular heterogeneity and crystalline orientation distribution.

Clearly, a number of interesting findings have emerged from this work that have implications for future development of models for fretting fatigue. The representation of microstructure is in many respects inescapable from a practical standpoint since fretting is intrinsically a boundary layer phenomena occurring over microstructure length scales. Also, the joint experimental and crystal plasticity modeling should proceed to understand the nature of friction in the slip zones more completely, as this is a weak link in essentially all existing models for fretting fatigue. In summary, crystal plasticity provides a stimulus to revisit the framing of fretting fatigue crack formation concepts.