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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