AULA_Teorico-Pratica sobre Fadiga nos materiais.pdf

pwp fadiga

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2. Material subjected to repeated stress cycles may fail even when the peak stress is well below the tensile strength,
or even below that for yield. Fatigue data are measured and presented as curves.
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4. has a small value, between 0.07 and 0.13
Coffin’s exponent, , is much larger than Basquin’s; typically it is 0.5.
These laws adequately describe the fatigue failure of unnotched components cycled at constant amplitude
about a mean stress of zero.
But what when the mean stress is not zero?
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5. Goodman’s law is good enough for preliminary design, but is not that good;
tests replicating service conditions are essential if the application is a safety-critical one.
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6. A perfectly smooth sample with no changes of section or surface serrations or roughness, and
containing no inclusions, holes, or cracks would be immune to fatigue provided or do not exceed its
yield strength. But that is a vision of perfection that, even with the greatest care (and the greatest
care is taken), is unachievable. These blemishes, small as they are, are deadly.
Holes, change of section, cracks, and surface scratches concentrate stress so that, even when the
sample as a whole remains elastic (the “high-cycle” regime), local plasticity occurs. The damage this
creates accumulates, finally developing into a tiny crack.
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7. During the tensile part of a cycle, a tiny plastic zone forms at the crack tip (see notes on Fracture toughness), stretching it
open and thereby creating new surface. On the compressive part of the cycle, the crack closes again and the newly-
formed surface folds forwards, advancing the crack. Repeated cycles make it inch forward, leaving tiny ripples on the crack
face marking its position on each cycle. These “striations” are characteristic of a fatigue failure and are useful, in a
forensic sense, for revealing where the crack started and how fast it propagated.
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8. In low cycle fatigue, the stresses are higher and the plastic zone larger, as on the right of Figure 4; it may be so
large that the entire sample is plastic. The largest strains are at the crack tip where plasticity now causes voids to
nucleate, grow, and link, just as in static fracture.
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14. Fatigue striations in 2024 aluminum
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