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Fatigue Index

Metal Fatigue and Endurance



Introduction

Metal fatigue results from the progressive and localized structural damage that occurs when a material is subjected to cyclic loading and results in failure at stress levels which are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.    Fatigue considerations are important because the consequent failure is generally sudden and at a stress level much lower than the strength values determined for normal tensile tests.

Important notes..
The information below is for guidance only .

Evaluating the fatigue strength to be used for component design should be carried out using validated material information and with careful consideration of all factors relevant to the stress locations. The links below the table provide more detailed information on fatigue design.

Symbols

SU = Ultimate tensile strength

Sn = The fatigue strength (tensile/compressive) is the maximum completely reversed stress under which a material will fail after it has experienced the stress for a specified number of cycles. (The strength is accompanied by the number of cycles)

Sn = The fatigue strength (shear) is the maximum completely reversed stress under which a material will fail after it has experienced the stress for a specified number of cycles. (The strength is accompanied by the number of cycles)

S'n = The maximum completely reversed (tensile/compressive) stress for which it is assumed that the material will never fail regardless of the number of cycles. This is a Pseudo value and includes identification of number of cycles (n)

Se = Endurance limit. This property , for primarility ferrous materials and titanium, is the strength value at which tha material will not fail whatever the number of cycles

Ses = Endurance limit.(shear) This property , for primarility ferrous materials and titanium, is the strength value at which tha material will not fail whatever the number of cycles

S'e = Endurance limit (tensile/compressive) related specifically to rotating test specimens

S'es = Endurance limit (shear) related specifically to rotating test specimens

See = Modified endurance/fatigue limit.(tensile/compressive) : Se which has been modified by correction factors.

Ses = Modified endurance/fatigue limit (shear )

Fatigue Properties

Fatigue properties of materials are generally determined by producing Wohler /S-N Plots.   These are simply plots with stress as the vertical axis and log (number of complete stress reversals) as the horizontal axis.   A number of material specimens are tested and the points at which they break are plotted on the S-N curve.

It is a useful property of steel (and titanium) that when the stress level fall below a certain value the specimen is effectively never likely to fail.   Generally other materials do not exhibit this effect.

The fatigue strength is the maximum completely reversed stress under which a material will fail after it has experienced the stress for a specified number of cycles. (The strength is accompanied by the number of cycles).    ..Fatigue Strength (fixed number of cycles) = Sn

The Fatigue limit is the maximum completely reversed stress for which it is assumed that the material will never fail regardless of the number of cycles. Fatigue Limit = S'n

Experiments have shown little direct relationship between the fatigue limit and the yield strength ,ductility etc.   However some relationship between the fatigue limit and the tensile strength Su has been established for unotched polished specimens tested using the rotating beam method.   This method loads the specimens by reversed bending.

S'e = 0,5 Su

for Wrought Steels where Su < 1400mpa

S'e = 690MPa

for Wrought Steels where Su > 1400MPa

S'e = 0,5 Su

for Titanium

S'e = 0,4 Su

for cast steel and cast iron

S'n = 0,38 Su

for magnesium casting and wrought alloys (based on 10 6 cycle life)

S'n = 0,35 Su ->0,5 Su

for nickel alloys (based on 10 8 cycle life)

S'n = 0,25 Su ->0,5 Su

for copper based alloys (based on 10 8 cycle life)

S'n = 0,38 Su

for for wrought aluminium alloys up to a strength of 280 MPa (based on 5 x 10 8 cycle life)

S'n = 0,16 Su

for for cast aluminium alloys up to a strength of 350 MPa (based on 5 x 10 8 cycle life)


Note: The fatigue limit S'n a is pseudo limit based on a number of stress cycles this applies to the engineering metals which will eventually fail at some time if subject to continuous reversing /repeated stress cycles.   Ferrous metals and titanium can operated continuously without failure at stress levels at or below the stress limit S'e.

Note;All of the above relationships are based on a 50% survival life.

The fatigue limit for reversed axial load of a polished,unnotched specimen is about 15% lower than that for reversed bending.

The fatigue limit for torsional testing of polished unnotched specimens is

  • S'es is about 0,58 x the fatigue limit in reversed bending for steel.
  • S'es is about 0,8 x the fatigue limit in reversed bending for cast iron.
  • S'ns is about 0,48 x the fatigue limit in reversed bending for copper.

The above values are all experimentally derived under relatively ideal conditions.
These values should be modified using factors that take into account actual operating conditions.

Approximations for endurance limits for three types of loading for steel are as follows

  • Bending S'e is about 0,5 Su
  • Axial S'e is about 0,45 Su
  • Torsion S'es is about 0,29 Su
  • .
Some Fatigue Values

Notes:
N = Normalised, H & T = Hardened and Tempered
I have not been able to obtain good fatigue information, the following values should be used as guidance.
Contact material suppliers direct for quality fatigue information...

Material Specification....... Treatment Fatigue
Strength
MPa
107Cycles
Ultimate
Strength
MPa
Fatigue
Ratio
Sn'/Su
Carbon
Steel
BS 970 /
070M20
070M26
080M30
080M40
 
N
N
N
H&T
 
193
201
232
278
 
400/430
430/500
460/500
620/780
 
0.45/0.48
0.40/0.47
0.46/0.50
0.37/0.45
Low Alloy
Steel
BS 970/;
En22
722M24
653M31
976M33
 
H&T
H&T
H&T
H&T
 
525
293
432
486
 
772/850
850/1080
770/1000
950/1050
 
0.61/0.68
0.27/0.35
0.43/0.56
0.46/0.54
Alloy
Steel
BS 970/
150M19
150M19
709M40
735A50
 
N
H&T
H&T
H&T
 
250
325
480
650
 
540
700
1000
1500
 
0,46
0,53
0,48
0,43
Wrought
Aluminium
N3 Not Heat
Treated
5 x 107cycles
48
55
70
 
110
130
175
 
0,44
0,42
0,40
Wrought
Aluminium
H9 Heat
Treated
5 x 107cycles
80
85
 
155
240
 
0,52
0,35
Titanium  
Grade 1
Grade 9
  107cycles
193
373
 
345
740
 
0,56
0,50
Brass  
ISO CuZn30
ISO CuZn30
 
Heat Treated ?
Heat Treated ?
107cycles
105
145
 
365
525
 
0,28
0,276

Useful Links..
  1. MATWEB ... Vast source of material data including some fatigue values
  2. Fatigue Tests on Welded Joints ... Useful informative report download
  3. Fatigue Design & Evaluation Committee of the SAE ... Includes a accessible database of fatigue reports on SAE steels
  4. Efatigue.com... Excellent Source of Fatigue data. (Based on US materials )
  5. Fatigue Basics -epi_eng ...Useful set of notes

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Last Updated 06/03/2013