טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentOfer Abraham
SubjectDual Shakedown Bounds of Fatigue Wear during Fluctuating
Shear
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Emeritus Tirosh Jehuda
Full Thesis textFull thesis text - English Version


Abstract

The wear phenomenon has been recognized as particles removal from (at least) one surface while rubbing against other surface. Due to such wear, many mechanical parts may lose their durability and reliability by, for example, increasing clearances between moving components. The fatigue wear is considered as a possible cause for such hazardous failures in kinematical motions of machineries (sleeves, cylinders, etc.). Understanding the sources which are 'responsible' for this kind of fatigue wear and implementing restrictions to the allowable loads may lead eventually not only for longer life-time of machines (which is an economical issue) but also for safer human life with daily-used machineries (i.e. cars, planes, etc.) that are exposed to detrimental surface fatigue failures.

By viewing (via sets of Lab. Experiments) the main parameters which govern the fatigue wear, we have attempted to search analytical methods to understand their source. The approach of our investigation has been based on the dual shakedown theorems from theoretical plasticity, apparently, lightly used hitherto in facing such problems.

Wear in general is roughly categorized by four major processes: Abrasive wear, Adhesive wear, Fatigue wear and Corrosive wear. Herewith, we focus our research only on fatigue wear, attributed on continuous fluctuating contact stresses with relative sliding between the contacting bodies.

Naturally, there is a paramount need to assess in advance what is the highest tolerable stress-amplitude which keeps the material safe in sustaining 'infinite' number of cycling shear-stresses. By doing so we may be assured that prescribed stress amplitudes are beneath the material endurance limit.

Therefore, the main issue is to formulate the highest safe loads that avoid fatigue wear. The fatigue wear mechanism is postulated to be a result of over-accumulation of plastic strain in the contacting material asperities, called 'ratcheting'. The bounds will be applied on the shear stress amplitudes based on the shakedown theorems: the static shakedown (due to Melan,1936) and the kinematic shakedown (due to Koiter, 1960). The expressions of these dual bounds are rigorously formulated and compared to self-made (and others) fatigue-wear experiments. The experimental work is performed by our own, self-designed apparatus, based on the pin-on-disk concept but constructed to measure sensitively on-line shear-stresses.