fatigue mechanics of materials
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Fatigue test & Fatigue failure
MOM- II (LAB)
Muhammad ZaroonUniversity Of Central Punjab (UCP)-Lahore
Mechanical Engineering.
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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)
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BASIC CONCEPT ‘FATIGUE’
• Repeated Motion Weakness/tiredness Fatigue• Example: human legs/ muscles decrease
in output due to constant running
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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 ”
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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.
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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
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FATIGUE IN CRANK SHAFT
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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.
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Fatigue: S—N curve (TREND)
S—N curve becomes horizontal at large N (fe and Ti Alloys)CYCLIC LOADING JUST APPEARS AS CONSTANT LOADING
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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
=
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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
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Deviation in Mild SteelLOAD P (N)
STRESS
(MPa)
264 600274 650284 570300 470
•Lower Yielding Point•UN-Equal MASS DISTRIBUTION
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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
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Improving Fatigue Life
2. Remove stress concentrators.By removing sharp edges
bad
bad
better
better
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