The s-n curve/Woehler curve

S-n curve said before the material fracture are the sum of the load change.According to DIN 50100 under constant amplitude loading (also known as the S - N test) fromHigh cycle fatigue testDraw, and divided intoLow cycle fatigue K,Limited fatigue life ZandHigh cycle fatigue DThis a few areas.

According to the cycle number N divided these areas

Cycle number is 100-30000 is a low cycle fatigue
Cycle number about 2000000 is limited fatigue life
High cycle fatigue cycle number is infinite

From the s-n diagrams, you can read the largest number of specific load amplitude of load change.It depends on the material properties, force and load types (pulse compression load, pulse tensile load or alternating load).

Smooth material sample the s-n curve of stress ratio (R = 1)

In our example, the nominal stress amplitude S_a.And the number of cyclesNUsing logarithmic method.in双对数表示中，有限寿命疲劳区域代表一条直线。生成的曲线指定为The s-n curve.

Divided into the low cycle fatigue, limited fatigue life and high cycle fatigue s-n curve

S-n curve description:

R_mStatic strength (for hereThe tensile strength)
s._a.Nominal stress amplitude
s._{The aD}High cycle fatigue strength
N allow cycle number
N_DEdge load cycle number
N_GCycle number threshold
K low cycle fatigue and low cycle fatigue strength
Z limited fatigue life/limited life fatigue strength
D high cycle fatigue/high cycle fatigue strength

Low cycle fatigue

Low cycle fatigueK.The number of load cycles range of about 10⁴To ten⁵.

Low cycle fatigue strengthLow cycle fatigue (LCF) testDetermination.in这个范围中，材料和部件所受的应力达到在循环过程中发生塑性变形的程度，并且材料在早期阶段失效。Coffin-Manson模型通常用于更详细的表示。

Cracking the sample in a quarter cycle load is called static strength, can also passTensile testDetermination.

Static strength of s-n curve (low cycle fatigue)

Limited fatigue life

Limited fatigue lifeZIs the cycle number 10⁴To 2, 10⁶Between the range (depending on the material).inLimited fatigue life范围内，试样总是达到失效标准条件（如裂纹或断裂）。

Limited life fatigue strengthHigh cycle fatigue (HCF) testDetermination.After the test, the test result is a load on the number of load cycles.

Limited fatigue life curve

In double logarithmic said, the s-n curve is almost straight.The curve (slope - k) is called limited fatigue life curve.

High cycle fatigue

High cycle fatigueDSaid materials during cyclic loading has no obvious signs of fatigue or failure cases, able to withstand the stress limit.In high cycle fatigueHigh cycle fatigue testDuring the measurement.

In a high cycle fatigue area, the limited cycle number is determinedN_G.If the sample is reached the limited cycle number before failure, is regarded as a "failure".inHigh cycle fatigue testPeriod, able to withstand more than 1000000 times, without material is regarded as fatigue fracture.

High cycle fatigue concept significantly lower than the allowed stress of the concept of static.

Within the scope of high cycle fatigue s-n (Woehler curve process is divided into three categories:

S-n curve of horizontal process: ferritic steel often appear obvious high cycle fatigue strength or fatigue strength for the long term
S-n curve to decline further smaller Angle: often occur in austenitic steel or aluminum
After initial level process, the s-n curve in about 10⁸Falls in circulation after: internal defects lead to crack of below the surface

Determination of s-n curve related standards

DIN 50100 - control fatigue test load -- metal samples and components of constant load cycling test execution, and evaluation
ASTM E466-15 - metal material axial force control constant amplitude fatigue test standard procedures
ISO 1099 - metal materials - fatigue test - axial force control method
DIN EN 6072 - aerospace series - metal materials - test methods - constant amplitude fatigue test