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Understanding How Components Fail

Stages of Fatigue Fracture

Fatigue fracture: initiation, propagation, and final rupture

Collection of background data selection of samples

Preliminary examination of failed part (visual examination and record keeping)

Nondestructive testing

Mecanical testing (including ardness and tougness testing)

!election, identification, preservation and cleaning of all specimens

Macroscopic examination and analysis (fracture surfaces, secondary crack, and oter 

surface penomenon)

Microscopic examination and analysis

!election and preparation of metallograpic sections

"xamination and analysis of metallograpic sections

#etermination of failure mecanism

Cemical analyses (bulk, local, surface corrosion products, deposits or coating, and

electron microprobe analysis)

 $nalysis of fracture mecanics

%esting under simulated service conditions (special tests)

 $nalysis of all te evidence, formulations of conclusions, and &riting te report

(including recommendations)

Collection of background data and selection of samples

Potograpic records: $ failure tat appears almost inconse'uential in a preliminary

investigation may later be found to ave serirous conse'uence tus a complete potograpic

record of te investigation can be important

*t is advisable to look for additional evidence of damage beyond tat &ic is

immediately apparent

*t is often necessary to compare failed components &it similar components tat did

not fail to determine &eter te failure &as brougt about by service conditions or &as te

result of an error in manufacturing

*n routine examination of brittle fracture, it is important to kno& if at tat time of te

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accident or failure te prevailing temperature &as lo& and if some measure of sock loading

&as involved

Preliminary examination of the failed part

%e failed part, including all of its fragments, sould be sub+ected to a toroug visual

examination before any cleaning is undertaken !oils and debris found on te part oftenprovide useful evidence in establising te cause of failure or in determining a se'uence of 

events leading to te failure

isual inspection: %e unaided eye as exceptional dept of focus and as te ability

to examine large area rapidly and to detect subtle canges of color and texture !ome of 

tese advantages are lost &en any optical or electron-optical device is used

Nondestructive esting

!everal nondestructive tests are extremely useful in failure investigation and analysis,

particularly magnetic-particle inspection of ferrous metals, li'uid-penetrant inspection,

ultrasonic inspection and eddy-current inspection of material tat conduct electricity

!echanical esting

.ardness testing is te simplest of te mecanical tests and is ofter te most versatile

tool available to te failure analyst

.ardness can be used to provide an approximation of te tensile strengt of steel, and

detect &ork ardening or to detect softening or ardening caused by overeating,

decarburi/ation or carbon pick up

%e failure analyst sould exercise care in interpreting mecanical test results For 

example, te fact tat a material as tensile strengt 0 1 234 belo& te minimum specified

value does not mean tat tis is te prime cause of its failure in service

5eferences:

Failure analysis and prevention 6 $!M olume 22