Post on 24-Jan-2017
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Fatigue test & Fatigue failure
MOM- II (LAB)
Muhammad ZaroonUniversity Of Central Punjab (UCP)-Lahore
Mechanical Engineering.
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)
BASIC CONCEPT ‘FATIGUE’
• Repeated Motion Weakness/tiredness Fatigue• Example: human legs/ muscles decrease
in output due to constant running
Fatigue in MaterialsThe 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 ”
Fracture Surface
1) CRACK initiation2) Crack Propagation3) Catastrophic rupture
Fracture surface with crack initiation at top. dull fibrous texture where rapid failure occurred after crack achieved critical size.
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
FATIGUE IN CRANK SHAFT
METAL FATIGUESouthwest 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.
Fatigue: S—N curve (TREND)
S—N curve becomes horizontal at large N (fe and Ti Alloys)CYCLIC LOADING JUST APPEARS AS CONSTANT LOADING
Experimental ReadingsCOPPPER
COPPER
Sr. No. LOAD P (N)
NO. OF REVOLUTION
(rmp)
STRESS (MPa)
1 235.44 3600 390.572 274.68 518 455.6
3 294.3 650 644.04
ACTUAL TREND : LOAD DIRECT RELATION WITH STRESS
=
Experimental ReadingsMild Steel
MILD STEEL
Sr. No. LOAD P (N)
NO. OF REVOLUTION
(rmp)
STRESS (MPa)
1 264 58800 6002 274 15000 6503 284 11000 5704 300 8000 470
Deviation From actual trend increasing LOAD Decreasing Stress
Deviation in Mild SteelLOAD P (N)
STRESS
(MPa)
264 600274 650284 570300 470
•Lower Yielding Point•UN-Equal MASS DISTRIBUTION
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
Improving Fatigue Life
2. Remove stress concentrators.By removing sharp edges
bad
bad
better
better
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