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
S_{U} = Ultimate tensile strength
S_{n} = 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)
S_{n} = 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)
S_{e} = 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
 S_{es} = 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
 S_{ee} = Modified endurance/fatigue limit.(tensile/compressive) : S_{e} which has been modified by correction factors.
S_{es} = Modified endurance/fatigue limit (shear )

Fatigue Properties
Fatigue properties of materials are generally determined by producing Wohler /SN
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 SN 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) = S_{n}
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 S_{u}
 Axial S'_{e} is about 0,45 S_{u}
 Torsion S'_{es} is about 0,29 S_{u}
.

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 10^{7}Cycles 
Ultimate Strength MPa 
Fatigue Ratio S_{n}'/S_{u} 
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 10^{7}cycles 48 55 70 
110 130 175 
0,44 0,42 0,40 
Wrought Aluminium 
H9 
Heat Treated 
5 x 10^{7}cycles 80 85 
155 240 
0,52 0,35 
Titanium 
Grade 1 Grade 9 

10^{7}cycles 193 373 
345 740 
0,56 0,50 
Brass 
ISO CuZn30 ISO CuZn30 
Heat Treated ? Heat Treated ? 
10^{7}cycles 105 145 
365 525 
0,28 0,276 
