Preventing Mechanical Failures

8/10/2019 Preventing Mechanical Failures

1/15

Preventing Mechanical Failures - An Introduction to Failure Mode

Identifcation

By Thomas Brown

Failure mode identification is often regarded as a specialized skill requiring years of study

and training to master. However, it is much like vibration analysis. ne does not have to be

able to solve mathematics functions like !aplace transforms or Fourier series to be an

e"cellent vibration analyst. #or does the failure analyst have to solve linear elastic fracture

mechanics problems to be effective.

The ability to recognize a characteristic spectrum pattern allows the vibration analyst to

identify what is happening and the effect on a particular machine. The same may be said of

failure mode identification. $t is a process of comparing surface features of broken parts to

characteristic surface features of known failure modes. This comparative analysis enables

identification of the physical failure mode.

%hether or not a full blown root cause failure analysis or basic component analysis is done,

correct identification of failure modes is essential.

Types of Fractures

Fractures are described in one of three ways& ductile overload, brittle overload and fatigue.

'ach type of fracture has distinct characteristics that allow identification.

Ductile Overload Fractureoccurs as force is applied to a part causing permanent distortion

and subsequent fracture. (s e"cessive force is applied to the part, it bends or stretches. (smore force is applied, it finally breaks.

)uctile fractures are easy to recognize because the parts are distorted. The fracture surface

typically has a dull and fibrous surface. Figure * shows a classic e"ample of a tensile ductile

failure. The narrowing or +necking indicates there has been e"tensive stretching of the

metal. The part has a +cup and cone surface- the sides have roughly a /0 angle.

Figure 1: Ductile Fracture with characteristic distortion and shear lip

Because ductile overload cracks start differently at the molecular level than brittle fractures,

they frequently have a /0 shear lip. The presence of a shear lip is another clue the fracture

was ductile.


2/15

)uctile failure is useful in many situations where bending or distortion absorbs energy. 1teel

highway guard rails are designed to distort and absorb energy before fracture, gradually

slowing the vehicle. ( part that bends gives the operator an unmistakable warning something

is wrong.

Brittle Overload Fractureoccurs when there is little or no distortion of the part before it

breaks. The file pieces in Figure 2 could be put back together in perfect alignment.

Figure 2: Brittle Fracture

Brittle fracture results from the application of e"cessive force to a part that does not have

the ability to deform plastically before breaking. %hen a brittle fracture occurs, there is little

warning. ( high strength bolt breaks suddenly, a glass shatters when it hits the floor, or acast iron bracket breaks without noticeable bending are e"amples of brittle fractures.

Brittle fractures frequently have chevron marks pointing to the origin of the fracture, shown

in Figure 3. The one on the left is like the name implies, a series of chevrons. The chevron

tips point to the origin of the fracture. The chevron marks on the right are fan shaped ridges

radiating from the origin.


3/15

Figure 3: Brittle Fracture Types

The brittle fracture in Figure occurred when a drive shaft suddenly stopped. The universal

4oint fractured, creating the tell5tale chevron marks of a brittle fracture.

Figure 4: Brittle fracture of a universal joint with chevron ar!s pointing to the origin

Fatigue Fracturesare the most common type of fracture. (bout half of all fractures are

fatigue fractures. They are usually the most serious type of failure because they can occur in

service without overloads and under normal operating conditions. Fatigue fractures

frequently occur without warning.

Fatigue fractures occur under repeated or fluctuating stresses. The ma"imum stress in a part

is less than the yield strength of the material. Fatigue fractures begin as a microscopic crack

or cracks that grow as force is applied repeatedly to a part.

Fatigue fractures have several distinct characteristics that make them easy to identify.

1everal distinctive features of a typical fatigue fracture are shown in Figure /& an origin

where a crack started, a fatigue zone and an instantaneous zone. This fatigue fracture

started at the keyway and progressed across the shaft 6the fatigue zone7 until material

remaining was no longer strong enough to support the load and it finally broke 6the

instantaneous zone7.The fatigue zone is unique to fatigue fractures because it is the region

where the crack has grown from the origin to the instantaneous zone. $t may take millions or

billions of cycles for the crack to travel across this zone. %hen the load is high, the number

of required cycles for the part to finally break is low- but if the load is low, the number ofcycles necessary for fracture is much higher.


4/15

Figure ": Features of a typical fatigue fracture: origin# fatigue $one# progression lines andinstantaneous $one%

The presence of progression lines in the fatigue zone is a positive way to identify fatigue

fractures. 8rogression lines also have been called stop marks, arrest marks and beach

marks, all in an effort to describe their appearance. 8rogression lines are visible ridges or

lines that are typical of crack progression across a ductile material. 'ach mark or line is

created when the crack stops. They can be formed by corrosion, changes in load magnitude,

or loading frequency.

1ometimes progression lines are not visible. $f the load doesn9t change or the metal has very

fine grains, they won9t be visible. The fatigue zone will have a uniform fine grained te"ture

like the tension failure of the cylinder rod in Figure :. The instantaneous zone has a coarse

grained or rock candy te"ture.


5/15

Figure &: Fatigue fracture of a cylinder rod without progression lines

There may be some deformation of ductile materials as the final fracture occurs. The final

fracture zone is essentially an overload fracture. $f the material is ductile, deformation may

occur. Brittle materials should not have any gross deformation. Frequently, there is little or

no deformation from the fracture, but the surfaces rub against each other and are damaged

after the final fracture occurs at the instantaneous zone.

Fatigue fractures don9t require high stress, so there is usually very little deformation. $t is

often possible to fit the parts back together in good alignment like the 4ournal in Figure ;.


6/15

Figure ': This journal fatigue fracture could (e fit (ac! together after it fractured%

)tress *oncentrations and +atchet ,ar!s

'very fatigue crack will have at least one and frequently more origins where the crack starts.

$nitiation of a crack occurs because there is a small region where the stress is higher. Higher

stress regions may be caused by change in geometry of the part, such as a keyway, change

in diameter, holes, corrosion pit, or metallurgical flaw.

1tress concentrations have two important characteristics. First is the severity. 1udden

changes in shape, like a keyway, sharp corner, or corrosion pit, will cause a large stress

increase in a very small area. =onversely, a smooth, largeradius will cause a much smaller

stress increase and the part does not fail.

1econd, the number of stress concentrations provides multiple locations 6origins7 for fatigue

cracks to start. >ultiple stress concentrations are frequently caused by corrosion, rough

finish, or welding.


7/15

Figure -: . fatigue fracture with ultiple origins and ratchet ar!s

Types of Force and Fracture

Figure A shows the five types of forces that may be applied to a part&

Figure /: Five types of forces that ay (e applied to a part

Tensionoccurs when a part is pulled at opposite ends. ( bolt is a good e"ample.

Torsionis caused by twisting the ends of a part. Torsion occurs in pump and motor shafts.

Bending occurs when one or both ends of a part are held and a force6s7 is applied at a

point6s7 along its length. Belt tension or misalignment causes bending.


8/15

)hearoccurs when two closely spaced opposing forces are applied across a part. $t often

occurs in bolts and pins.

*opressionoccurs when a part is pushed on both ends.

These forces may be combined in countless ways, but the direction of the fracture will tellwhich one or combination of these forces was present and what force was dominant.

Tension, bending and torsion are the most commonly encountered forces in failures. 8ure

shear, as shown in Figure A, is less frequent and compression failures are rare.

%e frequently discard broken parts before letting them tell us their history. (n e"amination

of broken and damaged parts will yield a wealth of information about their history. The parts

will tell us if they were overloaded, e"posed to corrosive materials, improperly designed,

manufactured or assembled incorrectly, or installed improperly.

Thomas Brown, 8.'. is the principal engineer of #. Tom uses his e"tensive e"perience to analyze machinery and component

failures, provide vibration analysis, and essential reliability skills training.


9/15

Failure Modes: A Closer Look at Ductile and Brittle Overload

Fractures

Follow up to the article, +8reventing >echanical Failures 5 (n $ntroduction to Failure >ode

$dentification 5 Feb?>arch 2*2

Thomas Brown

0s an overload fracture ductile or (rittle This uestion ust (e answered when

analy$ing parts% ,itigating factors that can ipact the answer to this uestion

should (e considered when analy$ing a failed coponent%

>etals are frequently thought of as ductile or brittle. However, they sometimes behave

differently when they fail from an overload. ( ductile metal may act as if it were brittle. (

brittle metal may behave in a ductile manner.

)uctile materials frequently undergo brittle fracture. $nherently, brittle materials rarely crack

in a ductile mode.

The factors that cause these different behaviors include& strength, temperature, rate of

loading, stress concentrations, size and various combinations.

)trength

1trength is the most obvious determinant of a metal9s behavior when it is overloaded. $n

general, soft tough metals will be ductile. Harder, stronger metals tend to be more brittle.

The relationship between strength and hardness is a good way to predict behavior. >ild steel

6($1$ *27 is soft and ductile- bearing steel, on the other hand, is strong but very brittle.The relationship between strength and hardness of steel is shown in Figure *.

Figure 1: )teel ardness vs% )trength
http://reliabilityweb.com/index.php/articles/preventing_mechanical_failures/http://reliabilityweb.com/index.php/articles/preventing_mechanical_failures/http://reliabilityweb.com/index.php/articles/preventing_mechanical_failures/http://reliabilityweb.com/index.php/articles/preventing_mechanical_failures/


10/15

The elongation 6stretch per unit of length7 percentage, usually given as C in 25inch length, is

also a means of 4udging ductility. >ore ductile metals have greater elongation. For e"ample,

the elongation of harder and stronger 3 quenched and tempered steel is about *:C,

while elongation of more ductile hot rolled **@ steel is about 3:C.

There are e"ceptions to this relationship. The most common e"ception is grey cast iron,

which is quite brittle even though it is fairly soft. $ts composition of sharp5edged graphite

flakes creates stress concentrations that override the ductility of the iron.

Teperature

Temperature has a significant affect on the ductility of metals. !ow temperature decreases

ductility, while high temperature increases it. %hen a part is overloaded at low temperatures,

a brittle fracture is more likely to occur. (t high temperatures, a more ductile fracture is likely

to occur.

!ower strength steel 6less carbon and alloys7 maintains ductility 6toughness7 as temperaturedecreases. %hen steel strength increases 6more carbon and alloys7, ductility drops more

quickly as temperature decreases.

The dominant factor causing brittle metals to become more ductile is high temperature.

The steels in the =harpy impact test chart 6Figure 27 show this change.

Figure 2: *harpy 0pact Test *hart

Higher strength steels with carbon above .3C begin to lose ductility 6toughness7 below

room temperature. !ow carbon steels 6.2C carbon or less7 do not begin to lose ductility

until temperatures reach freezing 6320F7.

There are e"ceptions to this relationship. 1tainless steels maintain their toughness at lowtemperatures. However, stainless steels may become work hardened and also lose ductility.


11/15

+ate of oading

%hen an overload happens slowly, there is enough time for microscopic movements in the

metal to occur. The metal deforms plastically before finally breaking. 1udden impact

frequently causes a ductile material to behave in a brittle manner. There is not enough time

for microscopic movements to take place. Brittle behavior is often seen in a catastrophic

failure when the overload is very sudden.

)tress *oncentrations

=hanges in geometry, such as keyways, diameter changes, notches, grooves, holes and

corrosion, result in localized areas where the stress is much higher than in the ad4acent

region of a part.

$n regions where there is no stress concentration, it is easier for microscopic movements to

occur. $n this case, the metal behaves in a ductile manner. ( stress concentration does not

allow microscopic movements, so brittle fracture is more likely.

)i$e

Thin parts are more likely to fail in a ductile manner when overloaded. !arge or thicker parts

will behave more like a brittle metal when overloaded because the geometry does not allow

stress to be evenly distributed. Figure 3 shows the effect of size.

Figure 3: Ductile etals (ehaving ore li!e a (rittle etal

Thin parts will usually have a shear lip or fracture at an angle- this is characteristic of a

ductile fracture. The shear lip becomes smaller as thickness increases and the fracture

becomes more brittle.

0nteractions

Figure summarizes the factors that may be present in an overload failure.


12/15

Figure 4: )uary of factors affecting overload fractures

These frequently occur in many combinations and are sub4ect to many complications in

specific applications. $f they are recognized as trends, they will help guide the analysis.

For e"ample& $f a ductile part has severe stress concentrations from corrosion or improper

machining and receives an impact, the resulting fracture will have features of a brittle

fracture.

The following e"amples illustrate the importance and interplay of these factors.

Brittle fracture of a ductile aterial

The roll 4ournal in Figure / is made from annealed * steel.

Figure ": . (rittle fracture of a journal% . piece was cut out for etallurgical analysis%


13/15

$ts hardness was about *A BH# and elongation 2:C, characteristic of a more ductile metal.

The 4ournal fractured as a fully loaded roll was set into stands using a crane. The brittle

fracture happened because three factors were present&

Stress concentrations - severe

The 4ournal had been repaired- the diameter was decreased and a radius cut at the

location of the failure.

( fatigue crack started in the radius, further increasing the stress concentration.

Rate of loading - high

%hen the 4ournal failed, the roll was being lowered into stands.

Size - large

The diameter of the 4ournal was inches.

Remedy


14/15

Figure &: Bro!en lin! fro a hoist chain

The brittle fracture at the bottom of the link in Figure : occurred immediately after the

fatigue fracture occurred.

The link deformed, indicating it was moderately ductile 63 BH#7. The suddenly increased

load on the remaining side resulted in the brittle fracture. The chevron marks of the brittlefracture are visible in Figure ;.

Figure ': Brittle fracture face

Strength - high

The chain was case hardened with a softer core. Tensile strength was appro"imately

*:, psi.

Rate of loading - high

%hen the first fracture occurred, the entire load was instantaneously transferred to

the remaining side.


15/15

Documents

Preventing Mechanical Failures