Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 7M

Year: May 2015

• In materials science, fatigue is the weakening of a material caused by repeatedly applied loads.

• It is the progressive and localised structural damage that occurs when a material is subjected to cyclic loading.

• Fatigue occurs when a material is subjected to repeated 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, persistent slip bands (PSBs), and grain interfaces.

• Eventually a crack will reach a critical size, the crack will propagate suddenly, and the structure will fracture.

• The shape of the structure will significantly affect the fatigue life; square holes or sharp corners will lead to elevated local stresses where fatigue cracks can initiate.

• Round holes and smooth transitions or fillets will therefore increase the fatigue strength of the structure.

S-N curve and its precipitation:

S of S-N curve stands for "Stress". That means repeatitive load.

N stands for "Number of cycles to failure".

S describes a vertical axis and N describes a cross axis. Those make a graph below.

• The lins is downward-sloping. We can see from this graph that the more the number of cycles becomes, The smaller stress even causes fatigue fracture.
• However, we can find one line that it won't fracture by getting a load on unlimitedly as long as the load is one constant repeatitive stress.
• It is called Fatigue Limit.