Explain fatigue limit for ferrous and non-ferrous alloys with the help of S-N diagram.

Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 6M, 10M

Year: May 2014, Dec 2014

Fatigue is the weakening of a material caused by repeatedly applied loads. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values that cause such damage may be much less than the strength of the material typically quoted as the ultimate tensile stress limit, or the yield stress limit.

Fatigue occurs when a material is subjected to repeat loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the stress concentrators such as the surface

The amplitude (or range) of cyclic stress that can be applied to the material without causing fatigue failure. Ferrous alloys and titanium alloys have a distinct limit, amplitude below which there appears to be no number of cycles that will cause failure.

Fatigue testing

The fatigue testing machine consists of:

• The electric motor capable of running 10,000 rpm
• A large bearing whose purpose is to relieve the motor having large bending moment
• Coclets to hold the specimen
• A rotating fever arm, subjected to downward force, in order to place specimen in the state of bending.

The fatigue testing can also be conducted using an instrument as shown in figure. The fatigue specimen is gripped on to a motor at one end to provide the rotational motion whereas the other end is attached to a bearing and also subjected to a load or stress. When the specimen is rotated about the longitudinal axis, the upper and the lower parts of the specimen gauge length are subjected to tensile and compressive stresses respectively. Therefore, stress varies sinusoially at any point on the specimen surface. The test proceeds until specimen failure takes place. The revolution counter is used to obtain the number of cycles to failures corresponding to the stress applied.

Increasing of the weight applied to the fatigue specimen results in a reduction in number of cycles to failure. We can then use the experimental results to construct an S-N curve. The fatigue test is normally conducted using at least 8-12 specimens in order to provide sufficient information for the interpretation of fatigue behavior of the tested material. The S-N curve shows a relationship between the applied stress and the number of cycles to failure, which can be used to determine the fatigue life of the material subjected to cyclic loading. High applied cyclic stress results in a low number of cycles to failure. Approximately 320 MPa. However, nonferrous alloys such as some alloys of aluminium, magnesium and copper will not normally show the fatigue endurance limit. The slope can be found gradually downwards with increasing number of cycles to failure and shows no horizontal line.

Fatigue ratio =Fatigue strength /Fatigue ratio

S-N curve

S-N curve reveals a substantial difference in fatigue characteristics between ferrous metals (like iron, steel and titanium) and non-ferrous metals (like aluminum, magnesium and copper). Ferrous metals exhibit a "fatigue limit" stress below which they can endure an infinite number of repetitive stress cycles without failing. Non-ferrous metals have no fatigue limit, and will always fail eventually if subjected to enough stress cycles