Is Paris law valid for Low cycle fatigue? is there any limitation on number of cycle for Paris law study for crack growth?
If your question is "Is Paris Law applicable to a crack initiating and growing during low cycle fatigue", the answer is NO. Cracks initiated during low cycle fatigue will be very small and are largely dependent upon the microstructural conditions of the material. Their growth rate will be in non-steady state (follow crystallographic orientations of the grains) and will be within the regime I of the crack growth Vs delK plot. In order to apply Paris Law, the crack should reach a steady state condition (crack ought to grow sufficiently longer) and you will not have sufficient material to accommodate such long crack in LCF specimens.
Paris data is represented over a range of crack growth rate (typically 10-4 to 10-8 mm/cycle), and not in terms of number of cycles. Please refer to ASTM 647 standard to learn more.
To add to M. Sivaprasad's answer, if we consider an analogous concept of low-cycle fatigue (which applies only to fatigue-initiation) and apply it to fatigue crack propagation, then a low number of cycles would be needed to grow an existing crack up to final failure. One could then assume that to induce such rapid failure, a high stress intensity factor range would be required. Depending on the actual value, you might be near the upper limit of the range of applicability of the Paris law, or you might be in the crack propagation rate acceleration region of the crack propagation curve.
up till now what i have studied is.....Fatigue life study is of two....1, low cycle fatigue and 2, high cycle fatigue. I want to study behaviour in low cycle fatigue, mean i want to apply less cycles till complete failure and study whether we can predict fatigue life of such component using paris law. (keeping da/dN high)....Actually i was interested to know that is there any restriction on number of cycles for applying paris law? minimum number of cycles......
Dear Abid,
At low cycle fatigue for mild steel both Paris's and Forman's formulations fail to satisfactorily characterize the combined FCP rates of mild steel . Answer is no.
Refer article given below.
Article Fatigue crack propagation in mild steel
Hello, this research is about fatigue crack growth in mild steel. Hope it would be helpful.
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