In this article we will discuss about:- 1. Mechanism of Fatigue Failure 2. Characteristics of Fatigue Failure 3. Fatigue or Endurance Limit 4. Theories of Fatigue 5. Effect of Temperature on Fatigue.
Mechanism of Fatigue Failure:
Invariably fatigue failure begins as irregularities on the surface of metals, which act as stress raisers, and at points of high stress or stress concentration. The basic mechanism in fatigue is “slip”.
Commercial metals are composed of aggregation of small crystals with random orientations. It has been indicated by the experiments that some crystals in a stressed-piece of metal reach their limit of elastic action sooner than others owing to their unfavourable orientation, which permits slip to occur.
Also distribution of stress from crystal to crystal within a piece of stressed metal is probably uniform, and when a piece is subjected to cyclic stress variation, the constituent particles tend to move slightly with respect to one another. This movement finally weakens some minute elements to such an extent that it ruptures and microscopic cracks or series of such cracks originate on the surface of the piece.
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In the zone of failure a stress concentration develops and with successive repetitions of stress the fracture spreads inward from this nucleus across the entire section. Ultimately, the unaffected portion of the section is reduced to a small core no longer capable of sustaining the applied load, and the specimen breaks in two parts.
It is, therefore, evident that fatigue failure is neither sudden nor hidden. For this reason, fatigue is often referred to as “progressive fractures”. This sequence of events makes it clear that fatigue is a result of cumulative process involving slip. High temperature increases the mobility of atoms, facilitating greater slip and deformation before fracture.
Highly localised stress is also developed at:
(i) Abrupt changes in cross-section,
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(ii) Base of surface scratches,
(iii) Root of a screw thread,
(iv) Edge of small inclusion of foreign substances, and
(v) Minute blow hole etc.
Characteristics of Fatigue Failure:
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Fracture caused by fatigue is of brittle nature, even in ductile materials.
There is no observable plastic deformation of the part or the whole of the material. The regions corresponding to the progressive and final (sudden) fractures can be easily identified in freshly broken sections. One region is smooth and polished, while the other is jagged and rough.
While the fracture is progressing, the severed portions of the section rub and hammer against each other every time the alternation of loading closes the microcrack. This process ends up by smoothing out any roughness produced by the crack propagation. However, the roughness does not disappear in the core, because here the break occurs under a single load application in the last cycle.
Since fatigue is essentially a surface initiated phenomenon, the condition of the surface is particularly important. Also since fatigue occurs by slip, any structural condition that can inhibit slip would be useful for long fatigue life.
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In order to secure satisfactory fatigue life it is necessary to observe the following points in design procedure:
(i) Modification of the design to avoid stress concentration by eliminating sharp recesses and severe stress raisers.
(ii) Precise control of the surface finish by avoiding damage to surface by rough machining, punching, stamping, shearing etc.
(iii) Control of corrosion and erosion or chemical attack in service and prevention of surface decarburisation during processing or heat treatment.
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(iv) Surface treatment of the metal.
Fatigue or Endurance Limit:
The shape of the curve is of much significance to engineering results. For metals such – as mild steel and titanium the curve becomes horizontal at certain stress. This stress is called “fatigue limit” or “endurance limit”. Below this stress, the specimen does not fail or fracture, i.e., the material will not fail even after infinite number of stress cycles. This implies that if the material is loaded to a stress below the fatigue limit, it will not fail, no matter, how many times the stress is applied.
Non-ferrous metals like magnesium, aluminium and copper alloys do not have true fatigue limit, because the curve never becomes horizontal. The curve slopes downward with increasing number of cycles. For such non-ferrous metals, the term fatigue strength is used to specify the ‘fatigue properties’.
“Fatigue strength” is the stress that will cause fracture after certain specified number of cycles say 106 or 107 or 108; the number of cycles are arbitrarily taken.
Theories of Fatigue:
Although several theories of fatigue have been put forward, yet we shall discuss briefly the following theories:
1. Orowan theory.
2. Fatigue limit theory.
3. Wood’s theory
4. Dislocation movement theory.
1. Orowan Theory:
According to this theory, the metal is considered to contain small, weak regions along which the slip occurs easily. The metal also contains inclusions (impurities) which act as notches and cause stress concentration (around inclusion). These regions are treated as plastic regions in elastic matrix and they experience increase in stress even when the repeated applied stress is constant.
This increase in stress causes plastic strain in the weak region, this plastic strain increases with the increase in the applied stress on the specimen. When the plastic strain in the weak region exceeds beyond certain value, a crack is formed. This process is repeated again and again until a large crack is formed.
2. Fatigue Limit Theory:
According to this theory the localised strain ageing affects the fatigue properties to a large extent. Some metals have well defined fatigue limit while others do not show fatigue limit. The metals which undergo strain ageing have well defined fatigue limits. A specimen with less amount of nitrogen and carbon will have less tendency for strain ageing and S-N curve will not show fatigue limit for this specimen. Heat treated steels show definite fatigue limit.
3. Wood’s Theory:
The strain direction, in fatigue, is reversed again and again. The slip produced by fatigue consist of slip bands which are the slip movements of the order 10-8 mm in length and 10-6 and 10-5 mm in height. When there is a strain in one single direction, the steps that appear are simple. When the loading is cyclic, the slip bands tend to group as notches or ridges.
The notch acts as starting point for fatigue crack. The notch with a very small root radius, acts as stress raiser and crack slowly propagates through the material, and after sometime a stage will come when the remaining (sound) portion of cross-section is too small to bear the maximum load. At this point sudden failure occurs. The final fracture is usually of cleavage type.
4. Dislocation Movement Theory:
The dislocation moves during fatigue and consequently a localized deformation occurs in the slip bands. This deformation is called extrusions (i.e., some metal is expelled out of the surface). The extrusions are generally accompanied by intrusions (inverse of extrusion). These extrusions and intrusions (these appear as early as 1/ 10th of the whole life of specimen and have the height of 10-5 to 10-6 mm) are responsible for the initiation of crack.
Effect of Stress Concentration of Fatigue:
Majority of the machine parts contain keyways, screw threads, holes, press fits, fillets etc. These geometrical irregularities act as stress raisers and reduce the fatigue strength greatly. Therefore, these irregularities must be reduced by careful design. Surface roughness also causes stress concentration.
Another class of stress raisers, called metallurgical stress raisers, consists of inclusion, decarburisation, local overheating due to grinding, porosity etc.
The effect of stress raisers on fatigue is studied by preparing a specimen with a V or circular notch.
When the specimen is loaded, the notch has the following effect:
(i) A triaxial state of stress is produced,
(ii) There is stress concentration at the root of notches,
(iii) Stress gradient is set up from the root of the notch to the centre of the specimen. Owing to stress concentration at the root, a crack is developed.
Corrosion Fatigue:
“Corrosion fatigue” is the simultaneous action of fatigue (cyclic stress) and corrosion (chemical attack). Corrosion alone produces pits on the metal surface. These pits act as stress raisers and reduce fatigue strength. When corrosion attack and loading occur simultaneously the reduction in fatigue strength is much more and crack propagates at much faster rate. The fatigue strength goes on reducing in corrosive medium.
The corrosion fatigue can be minimised by the following ways:
1. Elimination of the stress raisers by careful design.
2. Using the material having corrosion resistance properties (rather than conventional fatigue properties).
3. Providing metallic and non-metallic coatings in order to avoid contact with corrosive environments.
4. Adding corrosion inhibitors to reduce corrosive attack.
Nitriding imparts more fatigue strength in corrosive medium.
Effect of metallurgical variables on fatigue properties:
Fatigue properties can be greatly improved by:
(i) Careful design.
(ii) Reduction in stress concentration.
(iii) Intelligent use of residual compressive stresses.
(iv) Controlling certain metallurgical variables.
The ratio of fatigue limit (or fatigue strength at 108 cycles) to the ultimate strength is called “fatigue value.” Fatigue limit of cast steel and wrought steel is about 50 percent of ultimate tensile strength. For copper, nickel and magnesium, the fatigue ratio is 0.35.
Some facts regarding effect of metallurgical variables on fatigue properties are as follows:
1. Fatigue limit increases when percentage martensite formed during quenching is more.
2. Fatigue strength of non-ferrous metals and annealed steel increases with decreasing grain size.
3. Fatigue limit increases with decreasing tempering temperature for quenched and tempered steels.
4. Fatigue limit of eutectoid steel increases with decreasing isothermal reaction temperature.
5. Almost all fatigue failures in transverse direction start at non-metallic inclusions.
6. Fatigue limit in transverse direction of steel forging is only 60-70 percent of longitudinal fatigue limit.
7. Fatigue limit in transverse direction is increased when non-metallic inclusions are eliminated by vacuum melting.
Effect of Temperature on Fatigue:
A. The fatigue strength, below room temperature, increases with decreasing temperature. There is no sudden change in fatigue properties at temperature below ductile to brittle transition temperature. Fatigue failure at room temperature is associated with vacancy formation and condensation.
B. Fatigue strength decreases with increase in temperature above room temperature. But, only for steel, fatigue strength is maximum at 400 to 600°C (due to strain ageing of steel).
When temperature is increased above room temperature creep comes into play. Creep increases with increasing stress.
C. At high temperature, the fatigue properties are dependent upon frequency of stress application.
D. The finer the grain size, the better the fatigue properties at low temperature. At high temperature where creep predominates, coarse grain materials have high strength. The materials which have creep strength will have high fatigue strength at high temperature.
E. Compressive residual stresses improve fatigue properties at room temperature. These stresses will be annealed out at high temperature and hence there will be reduction in fatigue properties.