טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentWeissman Avi
SubjectSurface Damage Evolution under Combined Bending-Torsion
Fatigue Loading
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Emeritus Eli Altus
Full Thesis textFull thesis text - English Version


Abstract

Micro phenomena are crucial for understanding how fatigue damage evolves. Knowing which parameters contribute to the damage may bring to earlier failure prediction or even avoiding.

The majority of fatigue failures have a multiaxial state of stresses. In spite of enormous studies on multiaxial fatigue, there is still no agreed failure criterion. The main reason is the multiplicity of parameters relating multiaxial fatigue which led to more than 30 different criterions. An approach, combining micro and macro aspects, may bypass this difficulty by collecting information from the micro phenomena before crack initiation.

In this work a micro-macro connection is suggested through observations on known fatigue micro phenomenon called Persistent Slip Bands (PSB). Surface morphology of AL6061-T6 round specimens has been investigated. The specimens were subjected to a combined torsion-bending state of alternating loads in different ratios.

Specimens were designed with a unique geometry in purpose for producing maximum information. By creating a changing radius along the rod, a critical surface region was achieved, in which similar Von-Mises stress acted. From the macro point of view, a critical crack can initiate at any location on this surface. Yet, only one crack becomes critical and causes the final failure.

The surface of the specimens was investigated, at different stages of the cyclic loading, by using a White Light Interferometer scanner. The scans revealed PSBs in different directions. Some of them were observed adjacent to the critical cracks and some in areas were no crack occurred.

The directions of the PSB, adjacent to the critical crack were measured. A relation was found between them and the orientation of the maximum macro shear stress, dictated by the torsion-bending ratio. This relation did not hold for ratios higher than a value of 0.6 and a sharp drop was received.

A theoretical explanation for the relation found was suggested, referring to the structure of matter and a multiaxial fatigue failure criterion. The importance of this finding was the integration it enables between macro stress, which is easily known, and a micro phenomenon which is measurable.

The sharp drop in the relation raised some possible explanations. It seemed that a mechanism different from PSBs creates the critical crack in high torsion-bending ratios. This may imply that the "hidden" mechanism acts beneath the outer surface of the material.