XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX''"> When to Use Which Method?
Of the three fatigue methods used to predict life, it is important to understand when to use which. This will become more evident as you proceed through this manual and work each exercise. As a quick answer to this question, the following guidelines are presented.
S-N (Total Life)
• Long life fatigue problems where there is little plasticity since the S-N method is based on nominal stress
• Components where crack initiation or crack growth modeling is not appropriate, e.g., composites, welds, plastics, and other non-ferrous materials
• Situations where large amounts of pre-existing S-N data exist
• Components which are required by a control body to be designed for fatigue using standard data such as MIL handbook data.
• Spot weld analysis and random vibration induced fatigue problems
Crack Initiation
• Mostly defect free, metallic structures or components
• Components where crack initiation is the important failure criterion - safety critical components
• Locating the point(s) where cracks may initiate, and hence the growth of a crack should be considered
• Evaluating the effect of alternative materials and different surface conditions
• Components which are made from metallic, isotropic ductile materials which have symmetric cyclic stress-strain behavior
• Components that experience short lives - low cycle fatigue - where plasticity is dominant
Crack Growth
• Pre-cracked structures or structures which must be presumed to be already cracked when manufactured such as welds
• Pre-prediction of test programs to avoid testing components where cracks will not grow
• Planning inspection programs to ensure checks are carried out with the correct frequency
• To simply determine the amount of life left after crack initiation
• Components which are made from metallic, isotropic ductile materials which have symmetric cyclic stress-strain behavior