טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentLavi Erel
SubjectAdaptation and Extention of the Unified Model for
Fatigue of Metals
DepartmentDepartment of Mechanical Engineering
Supervisors Professor Menachem Weiss
Professor Reuven Katz
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Fatigue problems have been a major issue in engineering design. Due to insufficient knowledge in the field, especially by design engineers, often too heavy and therefore too expensive structures are being built. In rare cases disastrous fatigue failures occur. Numerous studies by respected researchers have been published and many aspects of the phenomenon have been modeled. Two main approaches to fatigue have been applied, the classical approach and the fracture mechanics approach. The classical approach models fatigue as a function of external alternating stresses and the experimental measurements of the number of loading cycles till failure. The results of these experiments are displayed as S-N curves. The classical approach does not include any measureable parameters that can enable to describe continuously the state of the fatigued specimen. The more modern fracture mechanics approach is based on the Stress Intensity Factor (SIF) as a function of crack shape, length, loading stresses, and specimen geometry.  Experiments have shown a link between the crack propagation rate and the amplitude of SIF under cyclical loading. This phenomenon has been modeled empirically by Paris and by others. Fracture mechanics uses the Paris’ law to predict the crack propagation rate in every cycle of loading until failure occurs. The fracture mechanics approach can be implemented only when the initial crack length is known and generates acceptable predictions when the crack length is relatively large. For the crack initiation stage no quantitative crack size predictions have been accepted. A practical comprehensive method for fatigue life evaluation is essential for design engineers and one such approach is introduced in the current study.

Earlier versions of a comprehensive model that is applicable to the domain of both the classical and fracture mechanics approaches to fatigue have been introduced by Weiss and his team. A wider and inclusive model has been developed in the current research. The whole fatigue domain is defined and used for better presenting the wide combination of the many available fatigue parameters.  The two main parameters that vary under cyclic loading are the loading stress function and the crack length. Thus the state of the specimen in the fatigue domain can be plotted on a diagram with these two parameters as axes, and we will call it the “fatigue diagram”. This diagram is divided by three stress values - the ultimate tensile stress Su, the yield stress Sy and the fatigue limit Se,  and by two lines of constant Stress Intensity Factors - the threshold value ∆Kth and the fracture toughness value K1C, into six fatigue zones. Each of the fatigue domain zones represents a different fatigue behavior resulting from the combination of different fatigue mechanisms. For crack propagation prediction in each zone special equations have been introduced, including two materials parameters.  An important contribution of this research is the extension of the unified model to analytically account for the effects of surface finish on fatigue life. The proposed calculation procedures enable design engineers to better tackle the fatigue problems that they encounter in their work.