Fatigue theory survey and comparison for assessing high cycle fatigue

The approaches to fatigue theory are typically grouped as Fatigue Analysis Techniques and Failure Mechanics Techniques. Fatigue Analysis Techniques are often referred to as Safe-Life techniques, where the material is assumed to be structurally defect-free until failure (crack, yield, etc.). In these approaches, no attempts are made to use a damaged (cracked, or yielded) component regardless of how benign the damage may be. Consequently, these approaches never generated inspection criteria or fault monitoring techniques and nearly all failures were sudden and unexpected. The fatigue analysis safe- life techniques include Stress-Life (the oldest, 1800s, where stress is considered the dominant fatigue parameter) and Strain-Life (circa 1960, where strain is considered the dominant fatigue parameter). Stress-life is used primarily for long life (high cycle, low load) applications while Strain-life typically is used for predicting initiation of a failure mechanism (cracking or yielding), and is typically used for short (low cycle, high load) fatigue life assessment. These theories are predominantly experimentally based.

Realizing that failures continue to occur even if strict Fatigue Analysis techniques are applied, competing fatigue approaches have appeared, the most notable that of Fracture Mechanics (circa 1950), with the alternative to the safe-life approach being Damage Tolerance. This approach is diametrically opposed to the safe-life approaches. Here it is assumed that all materials have defects, which eventually lead to failure if the defect grows to a critical value. Fracture Mechanics attempts to describe life in terms of the propagation of cracks based upon the stress concentration and plastic flow at the edge of the crack in a more theoretically-based manner. However, fracture mechanics is also very dependent upon experimental data and correlation. Typically, fracture mechanics is more conservative than safe-life/fatigue analysis, since Fatigue Analysis considers the life prior to the initiation/nucleation of a crack and subsequent failure, while fracture mechanics predicts life remaining after an assumed initial crack size. The establishment of an inspection interval process is the major accomplishment of Fracture Mechanics. Other competing and more recent fatigue theories include the Strain Energy Density (SEO, 1980s) failure criterion, Continuum Damage Mechanics (circa 1970), Critical Plane Theory (1973, rev. 1988), and Holistic Design approaches. The objective of this thesis is to summarize and compare fatigue theories as they may apply to turbine engine component life assessment, i.e. on an engineering level as opposed to a theoretical one. The emphasis is in identifying an approach, or methodology, which can be implemented economically by any structural analyst, not just one who has an enormous budget, a materials laboratory, and a PhD in material behavior. Such a methodology requires making the most out of the information available, and yet must be based on realistic physics and have at least a limited predictive capability. This thesis will describe in detail each of the broad elements of fatigue alluded to above, highlighting the assumptions, strengths and weaknesses of these approaches. A comparison of these techniques for evaluating high cycle fatigue potential for turbine engine components will be made. Other factors (some controversial) pertinent to the fatigue problem will be described. Finally, an approach for assessing the potential for fatigue will be described from a test and evaluation perspective.