ASTM defines fatigue life, Nf, as the number of stress cycles of a specified character that a specimen sustains before failure of a specified nature occurs.[1]
Characteristics of fatigue
Fracture of an Aluminium Crank Arm. Dark area: slow crack growth. Bright area: sudden fracture.
The process starts with dislocation movements, eventually forming persistent slip bands that nucleate short cracks.
Fatigue is a stochastic process, often showing considerable scatter even in controlled environments.
The greater the applied stress range, the shorter the life.
Fatigue life scatter tends to increase for longer fatigue lives.
Damage is cumulative. Materials do not recover when rested.
Some materials (e.g., some steel and titanium alloys) exhibit a theoretical fatigue limit below which continued loading does not lead to failure.
In recent years, researchers (see, for example, the work of Bathias, Murakami, and Stanzl-Tschegg) have found that failures occur below the theoretical fatigue limit at very high fatigue lives (109 to 1010 cycles). An ultrasonic resonance technique is used in these experiments with frequencies around 10–20 kHz.
High cycle fatigue strength (about 103 to 108 cycles) can be described by stress-based parameters. A load-controlled servo-hydraulic test rig is commonly used in these tests, with frequencies of around 20–50 Hz. Other sorts of machines—like resonant magnetic machines—can also be used, achieving frequencies up to 250 Hz.
Low cycle fatigue (typically less than 103 cycles) is associated with widespread plasticity; thus, a strain-based parameter should be used for fatigue life prediction. Testing is conducted with constant strain amplitudes at 1–5 Hz.
1843: Joseph Glynn reports on fatigue of axle on locomotive tender. He identifies the keyway as the crack origin.
1848: Railway Inspectorate report one of the first tyre failures, probably from a rivet hole in tread of railway carriage wheel. It was likely a fatigue failure.
1849: Eaton Hodgkinson is granted a small sum of money to report to the UK Parliament on his work in ascertaining by direct experiment, the effects of continued changes of load upon iron structures and to what extent they could be loaded without danger to their ultimate security.
1854: Braithwaite reports on common service fatigue failures and coins the term fatigue.[4]
1870: Wöhler summarises his work on railroad axles. He concludes that cyclic stress range is more important than peak stress and introduces the concept of endurance limit.[2]
Micrographs showing how surface fatigue cracks grow as material is further cycled. From Ewing & Humfrey (1903)
1903: Sir James Alfred Ewing demonstrates the origin of fatigue failure in microscopic cracks.
1910: O. H. Basquin proposes a log-log relationship for SN curves, using Wöhler's test data.
1945: A. M. Miner popularises A. Palmgren's (1924) linear damage hypothesis as a practical design tool.
1961: P. C. Paris proposes methods for predicting the rate of growth of individual fatigue cracks in the face of initial scepticism and popular defence of Miner's phenomenological approach.
1970: W. Elber elucidates the mechanisms and importance of crack closure in slowing the growth of a fatigue crack due to the wedging effect of plastic deformation left behind the tip of the crack.
High-cycle fatigue
Historically, most attention has focused on situations that require more than 104 cycles to failure where stress is low and deformation primarily elastic.
The S-N curve
In high-cycle fatigue situations, materials performance is commonly characterised by an S-N curve, also known as a Wöhler curve. This is a graph of the magnitude of a cyclical stress (S) against the logarithmic scale of cycles to failure (N).
S-N curves are derived from tests on samples of the material to be characterised (often called coupons) where a regular sinusoidal stress is applied by a testing machine which also counts the number of cycles to failure. This process is sometimes known as coupon testing. Each coupon test generates a point on the plot though in some cases there is a runout where the time to failure exceeds that available for the test (see censoring). Analysis of fatigue data requires techniques from statistics, especially survival analysis and linear regression.
Probabilistic nature of fatigue
As coupons sampled from a homogeneous frame will manifest variation in their number of cycles to failure, the S-N curve should more properly be an S-N-P curve capturing the probability of failure after a given number of cycles of a certain stress. Probability distributions that are common in data analysis and in design against fatigue include the lognormal distribution, extreme value distribution, Birnbaum-Saunders distribution, and Weibull distribution.
Complex loadings
Spectrum loading
In practice, a mechanical part is exposed to a complex, often random, sequence of loads, large and small. In order to assess the safe life of such a part:
Reduce the complex loading to a series of simple cyclic loadings using a technique such as rainflow analysis;
Create an histogram of cyclic stress from the rainflow analysis;
For each stress level, calculate the degree of cumulative damage incurred from the S-N curve; and
Combine the individual contributions using an algorithm such as Miner's rule.
Miner's rule
In 1945, M. A. Miner popularised a rule that had first been proposed by A. Palmgren in 1924. The rule, variously called Miner's rule or the Palmgren-Miner linear damage hypothesis, states that where there are k different stress magnitudes in a spectrum, Si (1 ? i ? k), each contributing ni(Si) cycles, then if Ni(Si) is the number of cycles to failure of a constant stress reversal Si, failure occurs when:
\sum_{i=1}^k \frac {n_i} {N_i} = C
C is experimentally found to be between 0.7 and 2.2. Usually for design purposes, C is assumed to be 1.
This can be thought of as assessing what proportion of life is consumed by stress reversal at each magnitude then forming a linear combination of their aggregate.
Though Miner's rule is a useful approximation in many circumstances, it has two major limitations:
It fails to recognise the probabilistic nature of fatigue and there is no simple way to relate life predicted by the rule with the characteristics of a probability distribution.
There is sometimes an effect in the order in which the reversals occur. In some circumstances, cycles of low stress followed by high stress cause more damage than would be predicted by the rule. It does not consider the effect of overload or high stress which may result in a compressive residual stress. High stress followed by low stress may have less damage due to the presence of compressive residual stress.
Paris' Relationship
Anderson, Gomez and Paris[5] derived relationships for the stage II crack growth with cycles N, in terms of the cyclical component ?K of the Stress Intensity Factor K
\frac {da} {dN} = C (\Delta K)^m
where a is the crack length and m is typically in the range 3 to 5 (for metals).
This relationship was later modified (by Forman, 1967http://ijd.sagepub.com/cgi/reprint/15/1/89.pdf) to make better allowance for the mean stress, by introducing a factor depending on (1-R) where R = min. stress/max stress, in the denominator.
Low-cycle fatigue
Where the stress is high enough for plastic deformation to occur, the account in terms of stress is less useful and the strain in the material offers a simpler description. Low-cycle fatigue is usually characterised by the Coffin-Manson relation (published independently by L. F. Coffin in 1954 and S. S. Manson 1953):
?f' is an empirical constant known as the fatigue ductility coefficient, the failure strain for a single reversal;
2N is the number of reversals to failure (N cycles);
c is an empirical constant known as the fatigue ductility exponent, commonly ranging from -0.5 to -0.7 for metals.
A similar relationship for materials such as Zirconium, used in the nuclear industry, is due to W.J. O'Donnell and B. F. Langer (Nuclear Science and Engineering, vol 20, pp 1-12, 1964).
Fatigue and fracture mechanics
The account above is purely phenomenological and, though it allows life prediction and design assurance, it does not enable life improvement or design optimisation. For the latter purposes, an exposition of the causes and processes of fatigue is necessary. Such an explanation is given by fracture mechanics in four stages.
Crack nucleation;
Stage I crack-growth;
Stage II crack-growth; and
Ultimate ductile failure.
Factors that affect fatigue-life
Cyclic stress state. Depending on the complexity of the geometry and the loading, one or more properties of the stress state need to be considered, such as stress amplitude, mean stress, biaxiality, in-phase or out-of-phase shear stress, and load sequence,
Geometry. Notches and variation in cross section throughout a part lead to stress concentrations where fatigue cracks initiate.
Surface quality. Surface roughness cause microscopic stress concentrations that lower the fatigue strength. Compressive residual stresses can be introduced in the surface by e.g. shot peening to increase fatigue life. Such techniques for producing surface stress are often referred to as peening, whatever the mechanism used to produce the stress. laser peening and ultrasonic impact treatment can also produce this surface compressive stress and can increase the fatigue life of the component. This improvement is normally observed only for high-cycle fatigue.
Material Type. Fatigue life, as well as the behavior during cyclic loading, varies widely for different materials: E.g. composites and polymers differ markedly from metals.
Residual stresses. Welding, cutting, casting, and other manufacturing processes involving heat or deformation can produce high levels of tensile residual stress, which decreases the fatigue strength.
Size and distribution of internal defects. Casting defects such as gas porosity, non-metallic inclusions and shrinkage voids can significantly reduce fatigue strength.
Direction of loading. For non-isotropic materials, fatigue strength depends on the direction of the principal stress.
Grain size. For most metals, smaller grains yield longer fatigue lives, however, the presence of surface defects or scratches will have a greater influence than in a coarse grained alloy.
Environment. Environmental conditions can cause erosion, corrosion, or gas-phase embrittlement, which all affect fatigue life. Corrosion fatigue is a problem encountered in many aggressive environments.
Temperature. Higher temperatures generally decrease fatigue strength.
Design against fatigue
Dependable design against fatigue-failure requires thorough education and supervised experience in structural engineering, mechanical engineering, or materials science.
There are three principal approaches to life assurance for mechanical parts that display increasing degrees of sophistication:
Design to keep stress below threshold of fatigue limit (infinite lifetime concept);
Design (conservatively) for a fixed life after which the user is instructed to replace the part with a new one (a so-called lifed part, finite lifetime concept, or "safe-life" design practice);
Instruct the user to inspect the part periodically for cracks and to replace the part once a crack exceeds a critical length. This approach usually uses the technologies of nondestructive testing and requires an accurate prediction of the rate of crack-growth between inspections. This is often referred to as damage tolerant design or "retirement-for-cause".
Stopping fatigue
Fatigue cracks that have begun to propagate can sometimes be stopped by drilling holes, called drill stops, in the path of the fatigue crack.[6] This is not recommended as a general practice because the hole represents a stress concentration factor which depends on the size of the hole and geometry. There is thus the possibility of a new crack starting in the side of the hole. It is always far better to replace the cracked part entirely. Several disasters have been caused by botched repairs to cracked structures, such as JAL 123.
Material change
Changes in the materials used in parts can also improve fatigue life. For example, parts can be made from better fatigue rated metals. Complete replacement and redesign of parts can also reduce if not eliminate fatigue problems. Thus helicopter rotor blades and propellers in metal are being replaced by composite equivalents. They are not only lighter, but also much more resistant to fatigue. They are more expensive, but the extra cost is amply repaid by their greater integrity, since loss of a rotor blade usually leads to total loss of the aircraft. A similar argument has been made for replacement of metal fuselages, wings and tails of aircraft.
On May 8, 1842 one of the trains carrying revellers on their return from Versailles to Paris, having witnessed the celebrations of the birthday of Louis Philippe, derailed and caught fire. Though the resulting conflagration mutilated the dead beyond recognition or enumeration, it is estimated that 53 perished and around 40 were seriously injured.
The derailment had been the result of a broken locomotive axle. Rankine's investigation of broken axles in Britain highlighted the importance of stress concentration, and the mechanism of crack growth with repeated loading.
De Havilland Comet
Metal fatigue became apparent to aircraft engineers in 1954 after three de Havilland Comet passenger jets had broken up in mid-air and crashed within a single year. Investigators from the Royal Aircraft Establishment at Farnborough in England told a public enquiry that the sharp corners around the plane's window openings (actually the forward ADF antenna window in the roof) acted as initiation sites for cracks. The skin of the aircraft was also too thin, and cracks from manufacturing stresses were present at the corners. All aircraft windows were immediately redesigned with rounded corners.
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