FATIGUE FAILURE

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Fatigue in Engineering Components

The above picture shows a damaged crankshaft due to Fatigue

Why is it important to study Fatigue ?

Fatigue is estimated to cause 90% of all metallic failures

(bridges, aircraft, machine components)

FATIGUE IS THE MOST COMMON MODE OF ENGINEERING FAILURE

Fatigue causes material failure and thus component failure

So what is Fatigue ?

There is no universally accepted definition of fatigue !

Most definitions are presumptions at the best.

So before we attempt to define Fatigue, lets get some basics straight

The normal stress-strain curve –

We all have been seeing this curve ever since we were kids !

Life was simple till here – we only needed to know Yield Strength and Ultimate Strength

Ultimate Strength – It is the maximum strength a material can withstand without breaking.

Yield Strength – Below the yield point, a material will deform elastically and will return to its original shape when the applied stress is removed.

The (in)famous S-N curve

All credit goes to this tiny old man, a german scientist

August Wohler(1819-1914) developed SN-curves to understand railcar axle failures.

While investigating, Wohler discovered that cracks formed and slowly grew on an axle surface. The cracks, after reaching a critical size, would suddenly propagate and the axle would fail. The level of these loads was less than the ultimate strength and/or yield strength of the material used to manufacture the axle.

All these failures occurred well below the Ultimate Strength !!!!

Why ? – FATIGUE

Coming back to the definition of Fatigue

Fatigue is the most common mode of engineering failure, as failure occurs well below the yield or ultimate tensile strength of the material. Fatigue itself is difficult to accurately define, but can be presumed as the failure of a material under fluctuating stress, each of which is believed to produce minute amounts of plastic strain.

In other words,

Fatigue is the failure of a metal under repeated (or cyclic) loading under stress. Each cycle of this cyclic stress is believed to produce minute plastic strain. This stress is lower than would lead to failure if applied singularly.

Materials are tested to find the relationship between the applied stress and number of stress cycles to failure.

The results are plotted in the form of what we know as S-N Curve

Fun Fact – Chapter 3 in Reeds Mathematics deals with Logarithms.The above is an example of usage of Logarithms.

Whenever very large numbers are involved, it is very easy to talk in terms of Logs !!

Under fluctuating / cyclic stresses, failure can occur at loads considerably lower than tensile or yield strengths of material under a static load.

(a) Crack nucleation: Fatigue failure begins with the formation of a small crack. This is generally found on external surface of the surface

(b) Crack growth: The crack form on external surface is then developed slowly into the material in the direction roughly perpendicular to the main tensile axis. That results in weakening the strength of material.

(c) Fracture: A fatigue crack advances a small amount during each stress cycle and each incremental of advance is shown on fracture surface as a multiple ripple line.

Fatigue in engineering components

Fatigue failure is almost always sudden and mostly catastrophic

Factors affecting Fatigue

1. Material selection is paramount to all design considerations. Material selection may be limited by any of a number of factors including economic, environmental, and service restrictions. Selecting a material with a high endurance limit is good practice.

2. Stress concentration is another key factor. Essentially, all sharp corners should be made into a radius if at all possible. Sharp corners provide stress concentration and are often responsible for the initial crack.

3. Surface finish is another critical component. Strength of Materials classes teach a very important lesson: in many loading configurations, like bending and torsion, the critical stress is located on the surface.Therefore, a blemish-free surface will generally lend itself to a good fatigue life.

4. Lastly, material discontinuities are inevitable on the microscopic level, but a good forming process will help to reduce them.

Jitesh Godara

3/E, BW Maritime

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