Mechanics of Materials

1. Fatigue failure
MOM- II (LAB)
Muhammad Zaroon
University Of Central Punjab (UCP)-Lahore
Mechanical Engineering.

2. FLOW•BASIC CONCEPT & INTRODUCTION
•FATIGUE IN MATERIALS
•FRACTURE SURFACE
•TYPES OF CYCLIC LOAD
•FATIGUE IN ENGINE CRANKSHAFT
•FATIGUE FAILURE
•EXPERIMENTAL READINGS OF COPPER
•EXPERIMENTAL READING OF MILD STEEL
•DEVIATION FROM TREND
•SIGNIFICANCE (FATIGUE)

3. BASIC CONCEPT
‘FATIGUE’
• Repeated Motion Weakness/tiredness Fatigue
• Example: human legs/ muscles decrease in
output due to constant running

4. Fatigue in Materials
The failure of metal under alternating stresses (CYCLIC
LOADING) is known as Fatigue.
OR
The PREMATURE failure/lowering of SRENGTH of a
material due to repetitive stresses which may be below or
above its yield strength ”

5. Fracture Surface
1) CRACK initiation
2) Crack Propagation
3) Catastrophic rupture
Fracture surface with crack
initiation at top. dull fibrous
texture where rapid failure
occurred after crack achieved
critical size.

6. There are 3 fluctuating
stress-time modes seen in
the figure:
• reversed stress cycle -
symmetrical amplitude
about a mean zero stress
level.
• (b) repeated stress cycle -
Asymmetrical maxima
and minima relative to the
zero stress level;
• variable RANDOM stress
level

7. FATIGUE IN CRANK
SHAFT

8. METAL FATIGUE
Southwest Airlines jet
•metal fatigue caused a hole in the roof (2009).
•caused by the repeated pressurisation and
de-pressurisation of the aircraft cabin.
•Poor Riveting Techniques for Aluminum
Sheets.

9. yield strength

10. Fatigue: S—N curve
(TREND)
S—N curve becomes horizontal at large N (fe and Ti Alloys)
CYCLIC LOADING JUST APPEARS AS CONSTANT
LOADING

11. Experimental Readings
COPPPERCOPPER
Sr. No.
LOAD P (N)
NO. OF
REVOLUTION
(rmp)
STRESS
(MPa)
1 235.44 3600 390.57
2 274.68 518 455.6
3 294.3 650 644.04
ACTUAL TREND : LOAD DIRECT RELATION WITH STRESS
=

12. Experimental Readings
Mild SteelMILDSTEEL
Sr. No.
LOAD P (N)
NO. OF
REVOLUTION
(rmp)
STRESS
(MPa)
1 264 58800
600
2 274 15000
650
3 284 11000
570
4 300 8000
470
Deviation From actual trend  increasing LOAD 
Decreasing Stress

13. Deviation in Mild Steel
LOAD P (N) STRESS
(MPa)
264 600
274 650
284 570
300 470
•Lower Yielding Point
•UN-Equal MASS DISTRIBUTION

14. Improving Fatigue Life
--Method 1: shot peening
put
surface
into
compression
shot
--Method 2: carburizing
C-rich gas
1. Impose a compressive stress
(to suppress surface cracks from growing)
Compression upto depth of roughly ¼ to ½
small (diameters ranging from 0.1 to 1.0 mm), hard particles

15. Improving Fatigue Life
2. Remove stress
concentrators.
By removing sharp edges
bad
bad
better
better

16. THANK YOU FOR
LISTENING