Fatigue considerations are important because the consequent failure is generally sudden and at a stress level much lower than the ultimate stress.
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 <> |
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
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> 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
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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 |
