Wednesday, September 10, 2008

What is Fatigue

Fatigue is a technical term that elicits a degree of curiosity. When citizens read or hear in their media of another fatigue failure, they wonder whether this has something to do with getting tired or "fatigued" as they know it. Such is not the case.

One way to explain fatigue is to refer to the ASTM standard definitions on fatigue, contained in ASTM E 1150. It is difficult, if not impossible, to carry on intelligent conversations if discussions on fatigue do not use a set of standard definitions such as E 1150. Within E 1150, there are over 75 terms defined, including the term fatigue:
"fatigue (Note 1):
the process of progressive localized permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations (Note 2). Note 1--In glass technology static tests of considerable duration are called `static fatigue' tests, a type of test generally designated as stress-rupture. Note 2--Fluctuations may occur both in load and with time (frequency) as in the case of `random vibration'." (Ref 2). The words in italics (emphasis added) are viewed as key words in the definition. These words are important perspectives on the phenomenon of fatigue:

· Process
· Progressive
· Localized
· Permanent structural change
· Fluctuating stresses and strains
· Point or points
· Cracks or complete fracture
The idea that fatigue is a process is critical to dealing with it in design and to the characterization of materials as part of design. In fact, this idea is so critical that the entire conceptual view of fatigue is affected by it! Another critical idea is the idea of fluctuating stresses and strains. The need to have fluctuating (repeated or cyclic) stresses acting under either constant amplitude or variable amplitude is critical to fatigue. When a failure is analyzed and attributed to fatigue, the only thing known at that point is that the loads (the stresses/strains) were fluctuating. Nothing is necessarily known about the nucleation of damage that forms the origin of fatigue cracks. Design for Fatigue Prevention In design for fatigue and damage tolerance, one of two initial assumptions is often made about the state of the material. Both of these are related to the need to invoke continuum mechanics to make the stress/strain/fracture mechanics analysis tractable:· The material is an ideal homogeneous, continuous, isotropic continuum that is free of defects or flaws.· The material is an ideal homogeneous, isotropic continuum but contains an ideal cracklike discontinuity that may or may not be considered a defect or flaw, depending on the entire design approach. The former assumption leads to either the stress-life or strain-life fatigue design approach. These approaches are typically used to design for finite life or "infinite life." Under both assumptions, the material is considered to be free of defects, except insofar as the sampling procedure used to select material test specimens may "capture" the probable "defects" when the specimen locations are selected for fatigue tests. This often has proved to be an unreliable approach and has led, at least in part, to the damage-tolerant approach. Another possible difficulty with these assumptions is that inspectability and detectability are not inherent parts of the original design approach. Rather, past and current experience guide field maintenance and inspection procedures, if and when they are considered.The damage-tolerant approach is used to deal with the possibility that a crack-like discontinuity (or multiple ones) will escape detection in either the initial product release or field inspection practices. Therefore, it couples directly to nondestructive inspection (NDI) and evaluation (NDE). In addition, the potential for initiation of crack propagation must be considered an integral part of the design process, and the subcritical crack growth characteristics under monotonic, sustained, and cyclic loads must be incorporated in the design. The final instability parameter, such as plane strain fracture toughness (KIc), also must be incorporated in design. The damage-tolerant approach is based on the ability to track the damage throughout the entire life cycle of the component/system. It therefore requires extensive knowledge of the above issues, and it also requires that fracture (or damage) mechanics models be available to assist in the evaluation of potential behavior. As well, material characterization procedures are needed to ensure that valid evaluation of the required material "property" or response characteristic is made. NDI must be performed to ensure that probability-of-detection determinations are made for the NDI procedure(s) to be used. This approach has proved to be reliable, especially for safety-critical components.The above approaches often are used in a complementary sense in fatigue design. The details of all three approaches are discussed in this Volume.The fatigue process has proved to be very difficult to study. Nonetheless, extensive progress on understanding the phases of fatigue has been made in the last 100 years or so. It now is generally agreed that four distinct phases of fatigue may occur (Ref 3, 4):· Nucleation· Structurally dependent crack propagation (often called the "short crack" or "small crack" phase)· Crack propagation that is characterizable by either linear elastic fracture mechanics, elastic-plastic fracture mechanics, or fully plastic fracture mechanics· Final instabilityEach of these phases is an extremely complex process (or may involve several processes) in and of itself. For example, the nucleation of "fatigue" cracks is extremely difficult to study, and even "pure fatigue" mechanisms can be very dependent on the intrinsic makeup of the material. Obviously, when one decides to pursue the nucleation of cracks in a material, one has already either assumed that the material is crack-free or has proved it! The assumption is the easier path and the one most often taken. When extraneous influences are involved in nucleation, such as temperature effects (e.g., creep), corrosion of all types, or fretting, the problem of modeling the damage is formidable. In recent years, more research has been done on the latter issues, and models for this phase of life are beginning to emerge.