1. Dr. S & S.S.GHANDHY GOVERNMENT ENGINEERING COLLEGE
3rd year
Mechanical Engineering

2. Topic:- Design Against Fluctuating Loads.
Prepared By:-
Name:- Enrl.No
Gamit Mayur
Panchal Karan
Variya Ankit
Vasava Urvashi
Vansekar Vishal
Bhalgamiya Nilesh
Patel Devansi

3. Fatigue
Introduction :
In several applications, components have to withstand different kinds of load at different times .
Materials subjected to these fluctuating or repeated load tends to show a behavior which is different from what they show under steady loads.

4. Fatigue
It has long been known that a component subjected to fluctuating stresses may fail at stress levels much lower than its monotonic fracture strength, due to a process called Fatigue.
Fatigue is an insidious time-dependent type of failure which can occur without any obvious warning.
It is believed that more than 95 % of all mechanical failures can be attributed to fatigue.

5. Fatigue occurs at stress well within the ordinary elastic range as measured in the static tension test.
Fracture resulting from fatigue is very difficult to predict and hence a good understanding of fatgue behavior is very important.

6. Types of fatigue loading:
1.Completely reversed cycle of stress:
2. repeated stress cycles
3. irregular or random stress cycle:

7. Type of Fluctuating Stresses
max
min 2
Alternating stress
Mean stress
m =
max
min 2 +
Ken Youssefi
MAE dept., SJSU

8. Fatigue properties :
Fatigue life (N): it is total number of cycles are required to bring about final fracture in a specimen at a given stress.
Fatigue life for a given condition is a property of the individual specimen
and is arrived at after testing a number of specimens at the same stress.

9. Fatigue life for P % survival (Np)
It is fatigue life for which P percent of samples tested have a longer life than the rest.
For example, N90 is the fatigue life for which 90% of the samples would be expected to survive
and 10% to fail at a particular stress.

10. Median fatigue life:
it is fatigue life for which 50 % of the population of samples fail
and the other 50 % survive at a particular stress.

11. OR it is the strength of a material for a particular fatigue life.
Fatigue strength (σn)
It is stress at which a material can withstand repeatedly N number of cycles before failure.
OR it is the strength of a material for a particular fatigue life.

12. Fatigue limit or Endurance limit (σE):
it is stress below which a material will not fail for any number of cycles.
For ferrous materials it is approximately half of the ultimate tensile strength.
For non-ferrous metal since there is no fatigue limit.

13. Endurance limit
is taken to be the stress at which it endures, N number of cycles without failure .N is usually taken as
5 x 108 cycles for
non-ferrous metals.

14. Effect of stress concentration 2) Size effect: 3) Surface Roughness:
Factors affecting fatigue:
Effect of stress concentration
2) Size effect:
3) Surface Roughness:
4) Surface Residual Stress:
5) Effect of temperature:
6) Effect of metallurgical variables;

15. Design for Finite Life log .9Sut = log a + b log 103 Sn = a (N)b
log Sn = log a + b log N
Apply conditions for point A and B to find the two constants “a” and “b”
log .9Sut = log a + b log 103
log Se = log a + b log 106 a =
(.9Sut)2 Se b
.9Sut 1 3
log Sn
Kf a = n
Design equation
Calculate Sn and replace Se in the design equation

16. The Effect of Mean Stress on Fatigue Life
Mean stress exist if the loading is of a repeating or fluctuating type.
Mean stress
Alternating stress m a Sy
Yield line
Gerber curve Se Sy
Soderberg line
Sut
Goodman line

17. The Effect of Mean Stress on Fatigue Life Modified Goodman Diagram
Alternating stress m a Sy
Yield line Se
Safe zone C
Goodman line Sy
Sut
Mean stress

18. The Effect of Mean Stress on Fatigue Life Modified Goodman DiagramSy
Yield line
- Syc Se
Safe zone
Sut
Goodman line C
Safe zone
- m Sy
+m

19. The Effect of Mean Stress on Fatigue Life Modified Goodman DiagramSe 1 =
Sut a m +
Infinite life
a = Se nf
Finite life Sn 1 =
Sut a m +
a + m = Sy ny
Yield Se
a + m = Sy ny
Yield C
Safe zone
Safe zone
- m
- Syc Sy
Sut
+m

20. Thank you…