20001549402000200660 (1961 words) Essay


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Material Engineering And Metallurgy Research2420096000
Material Engineering And Metallurgy Research
Ashik Mohd. Saifudeen
Ashik Mohd. Saifudeen
Material fatigue is a phenomenon where structures fail when subjected to a cyclic load. This type of structural damage occurs even when the experienced stress range is far below the static material strength. Fatigue is the most common source behind failures of mechanical structures.

The process until a component finally fails under repeated loading can be divided into three stages:
During a large number of cycles, the damage develops on the microscopic level and grows until a macroscopic crack is formed.

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The macroscopic crack grows for each cycle until it reaches a critical length.

The cracked component breaks because it can no longer sustain the peak load.

For certain applications, the second stage cannot be observed. A microscopic crack instead grows rapidly, causing sudden failure of the component. The details of the last two stages are usually considered within the topic of fracture mechanics. The term fatigue applies mainly to the first stage. There is, however, some overlap between the disciplines and the measured number of cycles to fatigue often includes the last two stages as well. Because the largest part of the component’s life is spent before it is possible to observe a macroscopic crack, most designs aim to avoid ever encountering such damage.
Consider a metal wire. When bent downwards, bending stress induced is in the wire cross section. There will be tension at top area and compression at bottom area. When wire is at equilibrium there will not be any stress on wire cross section. When wire is bending upwards there will be compression at top and tension at bottom.

The same phenomenon can happen for axle of this motor where it is undergoing fluctuating stress due to gravity effect of this mass. A rail wheel when it is in contact with the track produces a high contact stress, but when the wheel rotates stress gets relieved. When it comes back to original position again contact stress arises. So this also is a case of fluctuating stress case. Againthiswill lead to fatigue failure if we do not design it carefully. Same is the case with a gear pair. Herethecontact stressarisingat contact point fluctuates with time.

Ifstress induced at a point with respect to time is traced, it will vary as a fluctuating stress with time.Initially the point will have positive stress, after that zero, then negative stress. The same cycle repeats again and again. Such fluctuating stress is root cause of fatigue failure. When such fluctuating load act on a material it will initiate something called micro crack. This crack will begin to grow with fluctuating load and over time it will cause an abrupt failureunlike failure due to static load.

The graphshows stress variation at a point is plotted on stress vs. timegraph.

The stress varies between amaximumstress, and aminimum stress, during a load cycle. In the field of fatigue, the variation in stress is often defined using thestressamplitude, and themean stress. Further, variables defining thestressrange,andtheR-valueare frequently used to describe a stress cycle. The relation between the different fatigue stress variables is:

Fatigue analysis is not always based on a stress response. This branch, however, has historically received much attention since the majority of research has been performed in regimes where stress-based models are useful.

Based on the number of load cycles needed to produce a crack, it is customary to make a distinction between low-cycle fatigue (LCF) and high-cycle fatigue (HCF). The limit between the two is not distinct, but it is typically of the order of 10,000 cycles. The physical rationale is that in the case of HCF, the stresses are low enough that the stress-strain relation can be considered elastic. When working with HCF, the stress range is usually used for describing the local state. For LCF, meanwhile, strain range or dissipated energy are common choices.

One of the classical modelsfor fatigueis the so-calledS-N curve. This curve relates the number of cycles until failure (i.e., lifetime), N, to the stress amplitude in uniaxial loading. The general trend is that a longer lifetime is obtained with a decrease in stress amplitude. Usually, the dependence is very strong, so that a decrease of the stress amplitude by 10% can increase the lifetime by 50%. Some materials exhibit a stress threshold in fatigue testing. At stresses below this threshold, known as the endurance limit, no fatigue damage is observed and components can operate for an infinite lifetime. Not all materials have an endurance limit, though. Therefore, they can fail due to fatigue even at low levels of stress.

In multiaxial loading, the directions or locations of the external load vary and thus deform a structure in different directions. This means that at each time instance, a full stress or strain tensor rather than a scalar value must be evaluated.

Solid Line shows material with an endurance limit.

Dotted Line shows material without an endurance limit400000
Solid Line shows material with an endurance limit.

Dotted Line shows material without an endurance limit
A fatigue evaluation requires both a fatigue model and material data. Each model requires a different set of material parameters that can be obtained from material tests. Fatigue testing can be a rather time-consuming process, as a single test can run for many cycles before fatigue is observed. In high-cycle fatigue, for example, a specimen can last for one million cycles before it fails.

Furthermore, the influence of the microstructure on fatigue sensitivity introduces a scattering in the test results. This is caused by the fact that materials are inhomogeneous on the micromechanical level. Take an alloy, for instance, where there are crystallized grains and the grain boundaries cause stress concentrations. In a metal cast, there might even be pores formed during the solidification process. Therefore, on a local scale, the strains may be much larger than the macroscopic average values and dislocations within the crystals could occur.

Because the location of such micromechanical irregularities are more or less randomly distributed, there is a large scattering in the number of load cycles that a certain type of component can be subjected to, even if the external load is well defined. Because of this, a large number of specimens need to be tested before reliable fatigue data is found.

S-N curve for different components for one material.00
S-N curve for different components for one material.

When evaluating test results, it is important to consider statistical effects as well. Here are two examples of such effects:
If two sets of bars with different diameters are tested in tension with the same nominal stress, the larger one appears to have a shorter lifetime. The reason is that, within a larger volume of material, the risk of finding a microscopic defect of a certain size is greater.

If the same type of bar is tested when subjected to both tensile and bending loads, but giving the same peak stress, the one tested due to bending appears to have a longer lifetime. During bending, only a small volume of the material is subjected to the greatest stress.

Additionally, effects such as surface treatment and operating environment will further influence the fatigue strength.

Structures can be designed to reduce or avoid fatigue. Some ways to reduce fatigue are:
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. 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, though the hole is typically less of a stress concentration than the removed tip of the crack. The possibility remains of a new crack starting in the side of the hole. It is always far better to replace the cracked part entirely.

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.

Peening treatment of welds and metal components
Increases in fatigue life and strength are proportionally related to the depth of the compressive residual stresses imparted by surface enhancement processes such as shot peening but particularly by laser peening. Shot peening imparts compressive residual stresses approximately 0.005 inches deep, laser peening imparts compressive residual stresses from 0.040 to 0.100 inches deep, or deeper. Laser peening provides significant fatigue life extension through shock wave mechanics which plastically deform the surface of the metal component changing the material properties.Laser peening can be applied to existing parts without redesign requirements or incorporated into new designs to allow for lighter materials or thinner designs to achieve comparable engineering results.

High frequency mechanical impact(HFMI) treatment of welds
The durability and life of dynamically loaded, welded steel structures are determined often by the welds, particular by the weld transitions. By selective treatment of weld transitions with theHFMI treatment method,the durability of many designs can be increased significantly. This method is universally applicable, requires only specific equipment and offers high reproducibility and a high degree of quality control.

Example of a HFMI treated steel highway bridge to avoid fatigue along the weld transition.00
Example of a HFMI treated steel highway bridge to avoid fatigue along the weld transition.

Deep Cryogenic treatment
The use of Deep Cryogenic treatment has been shown to increase resistance to fatigue failure. Springs used in industry, auto racing and firearms have been shown to last up to six times longer when treated. Heat checking, which is a form of thermal cyclic fatigue has been greatly delayed.

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:
Complex loading is reduced to a series of simple cyclic loadings using a technique such as rainflow analysis
A histogram of cyclic stress is created from the rainflow analysis to form a fatigue damage spectrum;
For each stress level, the degree of cumulative damage is calculated from the S-N curve; and
Theeffect of individual contributions iscombined using an algorithm such as Miner’s rule.


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