How does the SN-curve definition via universal slope work?

This functionality of Specialist Durability allows estimating an SN-curve from the type of material and the tensile strength. The method makes use of estimations found in reference [1] below.

For some material types two different SN-curves are offered, depending on which recommendation (found in literature) you would like to follow. The ones based on the FKM Guideline follow a more conservative approach.

For each material the endurance limit (σw) is derived from the ultimate tensile strength (σz). With the number of cycles at the endurance limit and the slope k, the SN-curve is completely defined.

Here are the formulas used in the software:

Steel:
	σw = 0.5*σz	(if σz <= 1400 N/mm2
	σw = 700 N/mm2	(if σz >= 1400 N/mm2
	ND = 5*106	k = 5

Steel (FKM conservative):
	σw = 0.385 * σz +30 N/mm2
	ND = 106	k = 5

Iron casts:
	σw = 0.4 * σz	(with σz <= 500 N/mm2
	ND = 5*106	k = 5

Iron casts (FKM conservative):
	σw = 0.4 * σz	(with σz <= 500 N/mm2
	ND = 5*106	k = 5

Aluminum:
	σw = 0.4*σz	(if σz <= 325 N/mm2
	σw = 130 N/mm2	(if σz >= 325 N/mm2
	ND = 108	k = 5

Magnesium:
	σw = 0.35*σz	(if σz <= 325 N/mm2
	σw = 130 N/mm2	(if σz >= 325 N/mm2
	ND = 108	k = 5

Aluminum/magnesium (FKM conservative):
	σw = 0.22 * σz
	ND = 108	k = 5

Titan:
	σw = 0.55*σz	(if σz <= 1130 N/mm2
	σw = 620 N/mm2	(if σz >= 1130 N/mm2
	ND = 5*106	k = 5

Copper/nickel:
	σw = 0.4*σz	(if σz <= 750 N/mm2
	σw = 300 N/mm2	(if σz >= 750 N/mm2
	ND = 108	k = 5

References

The formulas labeled FKM conservative are derived from [2].

All other formulas are from [3], [4], and [5].

[1]: Radaj, D.; Vormwald, M. Ermüdungsfestigkeit, 3. Auflage; Springer-Verlag Berlin Heidelberg 2007

[2]: FKM-Guideline: Analytical Strength Assessment of Components in Mechanical Engineering, 5th edition, VDMA-Verlag, Frankfurt/M. 2003

[3]: Fuchs, H.O.; Stephens, R.J. Metal Fatigue in Engineering; John Wiley, New York 1980

[4]: Juvinall, R.C. Engineering Considerations of Stress, Strain and Strength; McGraw-Hill, New York 1967

[5]: Sonsino, C.M.; Hanselka, H. Betriebsfeste Bemessung von Bauteilen aus Magnesium; Konstruktion 53 (2001) 12, 55-58

Learn more

Defining SN-curves

Logarithmic nature of fatigue

Determination of SN-curves

Modifying factors of the SN-curve

Mean stress effects

Variable amplitude loading

Stress-life analysis in Specialist Durability

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