{"id":30535,"date":"2021-07-08T11:57:03","date_gmt":"2021-07-08T11:57:03","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=30535"},"modified":"2023-09-19T07:40:57","modified_gmt":"2023-09-19T07:40:57","slug":"fatigue-life-s-n-curve-woehler-curve","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-engineering\/metals-what-are-metals\/failure-modes-of-materials\/fatigue-of-material-fatigue-failure\/fatigue-life-s-n-curve-woehler-curve\/","title":{"rendered":"Fatigue Life \u2013 S-N Curve – Woehler Curve"},"content":{"rendered":"
In materials science, fatigue<\/strong> is the weakening of a material caused by cyclic loading<\/strong>, resulting in progressive, brittle, and localized structural damage. Once a crack has been initiated, each loading cycle will grow the crack a small amount, even when repeated alternating or cyclic stresses are of an intensity considerably below the normal strength. The stresses could be due to vibration or thermal cycling. Fatigue damage is caused by:<\/p>\n A fatigue crack will not initiate and propagate if any of these three is not present. The majority of engineering failures are caused by fatigue.<\/p>\n Although the fracture is of a brittle type, it may take some time to propagate, depending on the intensity and frequency of the stress cycles. Nevertheless, there is very little warning before failure if the crack is not noticed. The number of cycles required to cause fatigue failure at a particular peak of stress is generally quite large, but it decreases as the stress increases. For some mild steels, cyclical stresses can be continued indefinitely provided the peak stress (sometimes called fatigue strength) is below the endurance limit value. A good example of fatigue failure is breaking a thin steel rod or wire with your hands after bending it back and forth several times in the same place. Another example is an unbalanced pump impeller resulting in vibrations that can cause fatigue failure. The type of fatigue of most concern in nuclear power plants is thermal fatigue. Thermal fatigue can arise from thermal stresses produced by cyclic changes in temperature. Large components like the pressurizer, reactor vessel, and reactor system piping are subject to cyclic stresses caused by temperature variations during reactor startup, changes in power level, and shutdown.<\/p>\n <\/p>\n The American Society for Testing and Materials defines fatigue life, Nf<\/sub>, as the number of stress cycles of a specified character that a specimen sustains before failure of a specified nature occurs. Fatigue life<\/strong> is affected by cyclic stresses, residual stresses, material properties, internal defects, grain size, temperature, design geometry, surface quality, oxidation, corrosion, etc. For some materials, notably steel and titanium, there is a theoretical value for stress amplitude below which the material will not fail for any number of cycles, called a fatigue limit<\/strong>, endurance limit<\/strong>, or fatigue strength<\/strong>.<\/p>\n\n
Fatigue Life \u2013 S-N Curve<\/h2>\n