Question

a.What is the difference between endurance limit materials and others?

b.What is an S-N curve? And explain this based on the wing of an aircraft in flight.

c.Why do fatigue cracks only grow during tension and not compression?

d.Tensile stress levels affect the life expectancy of a part - why?

Answer 1

a) Ans :- Endurance limit for a material is defined as the maximum stress in completely reverse bending which can be applied to a material with infinite number of cycles and failure does not occur in the material. The infinite number of cycles are usually taken as 10000000 for calculation purpose. It is important to note that this value of stress is well below the yield strength of the same material in simple uniaxial tensile or compressive loading. So the materials which are subjected to repetitive type of completely reversed loading are required to have good Endurance limit in order to avoid early failure. While other materials can not sustain completely reverse loading so these materials do not fall under the category of Endurance limit materials.

b) Ans:- S( stress) - N (Number of cycles) curves are used to present the test results of a material for which fatigue test and/or Endurance limit test is done. Stress is usually alternating stress which is plotted along on vertical axis (y-axis) and the number of cycles are plotted on horizontal axis( x- axis). Sometimes S- N curves are plotted on logarithmic scale as in case of fatigue life prediction of the wing of an aircraft

Fatigue life of wing of an aircraft is measured using S-N curve. Stress acting on the wing of aircraft is calculated based on bending load acting on it. During flight gust load also act which causes more bending load on the wing. But gust loads are calculated based on empirical formula. Generally S-N curves are usually best for predicting fatigue life of wing of an aircraft.

c) Ans.:- Fatigue cracks grow during tension due to the fact that tensile forces tend to open up the crack while compression tends to close the crack. So when a material is subjected to alternating stresses ( tensile to compressive) the fatigue crack will grow only in tension.

d) Ans.:- As mentioned above fatigue crack growth depends upon tensile stresses so when these crack propagate in the material they lead to failure of the materials.So life expectancy depends upon rth tensile stresses developed in the material.