Mechanical Failure Modes

BucklingBuckling is the failure of a long, slender column that has been subjected to a compressive,

axial load. As the load is applied, the center of the column span bulges outward, and then

either cracks or yields, depending on the material properties of the specific component.

CorrosionCorrosion is the chemical alteration (generally, but not always, oxidation), of a material due to

environmental exposure to corrosive elements. For example, iron or steel that is exposed to

air can undergo oxidation, forming iron oxide, commonly known as rust.

CreepCreep is the slow deformation of a solid material over time due to applied loads and often

increased temperatures. Creep can result in changes in material properties and part

geometries that can cause failures.

Primary mechanical failure modes

FatigueFatigue is a reduction in the ultimate strength of a material due to cyclic loading of a part.

Even elastic deformations can result in material changes that can reduce the ultimate strength

over a large number of cycles.

FractureFracture begins as a localized micro crack in a part that slowly grows over time, or grows

rapidly when exposed to a large overload. Failure occurs when the crack growth becomes

critical and the part breaks. Crack growth often begins in areas of high stress concentration,

such as corners.

ImpactImpact failure, just as it sounds, is the failure of a part due to impact with or by another

object. A baseball shattering a window is an impact failure.

Primary mechanical failure modes …

RuptureRupture generally occurs in pressure vessels or other containers when the pressure within the

vessel exceeds the strength of a vessel, either globally or locally.

Thermal ShockThermal shock is the result of a component moving quickly from one temperature extreme to

another. For example, brittle materials such as cast iron experience thermal shock if a hot part

is suddenly cooled.

WearWear is the gradual removal of material by two parts rubbing against each other, or

environmental contact with a part, such as water or sand.

YieldingThe yield point is essentially the peak load that the part can hold before the material stretches

apart.

Primary mechanical failure modes …

Metal Fatigue is a process which causes premature irreversible damage or failure of a

component subjected to repeated loading.

Fatigue is a sequence of several complex phenomena encompassing several disciplines:

– motion of dislocations

– surface phenomena

– fracture mechanics

– stress analysis

– probability and statistics

Fatigue takes many forms:

– fatigue at notches

– rolling contact fatigue

– fretting fatigue

– corrosion fatigue

– creep-fatigue

Fatigue is a not a cause of failure, but it leads to the final failure/damage

Fatigue

There are many harmful factors to the materials beyond the scope of strength of materials.

The accumulation of one or several of these factors eventually shorten the service life of

materials. The combined effect of these factors is called "fatigue mechanism". Some common

fatigue mechanisms include:

– Time-varying Loading Fatigue

– Thermal Fatigue

– Corrosion Fatigue

– Surface/Contact Fatigue

– Combined Creep and Fatigue

Definition: Fatigue is “the process of progressive localized permanent structural change

occurring in a material subjected to conditions which produce fluctuating stresses and strains

at some point or points and which may culminate in cracks or complete fracture after a

sufficient number of fluctuations”. (ASTM standard definition)

Fatigue …

Versailles rail accident occurred on May 8, 1842Examination of several broken axles from British railway vehicles by “William John Macquorn

Rankine” showed that they had failed by brittle cracking across their diameters, a problem

now known as fatigue. At the time, there was considerable confusion about the problem.

First fatigue failure

Drawing of a fatigue failure in an axle, 1843. Versailles train disaster

1840’s Fatigue failure of railroad axles – stress concentration at shoulders

1850 – 1860 Wöhler (German), “ The father of systematic fatigue testing” introduced S-N

diagram

1870 – 1890 Gerber and Goodman provide analysis tools to account for superimposed

mean and alternating stresses

1920’s Gough examined slip lines and analyzed the mechanisms of fatigue

1920’s Griffith studied brittle fracture

1930’s Haigh – notch strain analysis

1930’s Almen – shot peening to create residual compressive surface stresses

1945’s Miner – cumulative damage concept to account for stress regime of varying

amplitude (suggested by Palmgren in 1924)

1950’s Irwin introduced stress intensity factor, the basis for linear elastic fracture

mechanics (LEFM)

1960’s Manson and Coffin introduced LCF (low cycle fatigue)

Fatigue in history

Appearance of failure surfaces caused by various modes of loading (SAE Handbook)

Appearance of failure surfaces

Time-Varying Loading• Time-varying Loading Fatigue can be defined as a process caused by time-varying loads

which never reach a high enough level to cause failure in a single application, and yet results

in progressive localized permanent damages on the material.

• The damages, usually cracks, initiate and propagate in regions where the strain is most

severe. When the local damages grow out of control, a sudden fracture/rupture ends the

service life of the structure.

Thermal fatigue• Thermal fatigue is the gradual deterioration ad eventual cracking of a material by alternate

heating and cooling during which free thermal expansion is partially or completely

constrained.

• Constraint of thermal expansion causes thermal stresses which may eventually initiate and

propagate fatigue cracks.

• Mostly thermal fatigue will be classified under low – cycle – fatigue . Because thermal

fatigue cracks usually starts in less than 50000 cycles.

• Thermal fatigue was classified as “thermal fatigue” and “isothermal fatigue”

Fatigue

Corrosion fatigue• Corrosion Fatigue (CF) is the metal cracking caused by combined action of a cyclic loading

and a corrosive environment.

• Classified as Corrosion fatigue and Stress corrosion fatigue. The principal difference is static

stress in corrosion fatigue & alternating/ fluctuating stress in stress corrosion fatigue.

Surface / Contact fatigue• CONTACT FATIGUE is a surface-pitting-type failure commonly found in ball or roller bearings.

• This type of failure can also be found in gears, cams, valves, rails, and gear couplings.

• Contact fatigue differs from classic structural fatigue (bending or torsion) in that it results

from a contact or Hertzian stress state.

• This localized stress state results when curved surfaces are in contact under a normal load.

Fatigue …

Fatigue loadingFatigue loading

Constant amplitudeConstant amplitude

Proportional loading


Non proportional

loading

Non proportional

loading

Variable amplitudeVariable

amplitude



Non proportional

loading

Non proportional

loading

Note: Loading ratio = 1, for proportional loading { ratio of second load to first load}

Fatigue loading


•Loading ratio = 1

•Principal stress axes do not

change over time

•Single set FE result study

can identify critical fatigue

location

•No, cycle counting and

cumulative damage

calculations

Non – proportional loading

•Loading ratio ≠ 1

•Principal stress axes are

free to change between 2

load sets

•Critical fatigue location may

appear in spatial

•Cycle counting, damage

summation need to be done

to identify total fatigue

damage

Fatigue loading …

2σσσ

stress, Mean

minmaxm

2σσσ

stress, gAlternatin

minmaxa

maxmin

σσ R

ratio, Stress

min maxr σ σ σ range, Stress

maσ

σ A

amplitude, Stress

Fatigue loading …

• German engineer August Wöhler conducted the first systematic fatigue investigation. Wöhler

conducted cyclic tests on full-scale railway axles and also on small-scale bending, torsion and

push-pull specimens of several different materials. Typically, the S-N relationship is

determined for a specific value of MEAN STRESS, STRESS RATIO or AMPLITUDE RATIO.

• Most determinations of fatigue properties have been made in completely reversed bending,

i.e. R = -1, by means of the so-called rotating bend test.

• The usual laboratory procedure for determining an S-N curve is to test the first specimen at

a high stress, about two thirds of the static tensile stress of the material, where failure is

expected in a fairly small number of cycles. The test stress is decreased for each succeeding

specimen until one or two specimens do not fail before at least 107 cycles. For materials

which exhibit it, the highest stress at which no failure occurs, a run out, is taken to be the

fatigue limit.

S-N Curve (or) Wöhler curve

S-N Curve (or) Wöhler curveR. R. Moore rotating beam experimental setup.

S-N Curve …

S-N Curve …

• Haigh proposed and conducted series of tests to investigate different combinations of stress

amplitude and mean stress for a given number of cycles to failure.

• The diagram plots the mean stress, both tensile and compressive, along the x-axis and the

alternating constant stress amplitude along the y-axis.

• This is commonly referred as the Haigh diagram.

• Failure appears to be more sensitive to tensile mean stress, than compressive mean stress.

Mean stress influence

Mean stress influence …

SAE Master diagram AISI4340 steel

stress) fracture True- (σ

1 σS

SS : s)1960' (USA, Morrow

1 SS

SS : 1930) (USA, Soderberg

1 SS

SS : 1874) (Germany,Gerber

1 SS

SS :1899) (England, Goodman

f

f

m

e

a

y

m

e

a

2

u

m

e

a

u

m

e

a

• All methods should only be used for tensile mean stress values.

• The Soderberg method is very conservative. It is used in applications where neither fatigue

failure nor yielding should occur.

• For hard steels (brittle), where the ultimate strength approaches the true fracture stress, the

Morrow and Goodman curves are essentially equivalent.

• For ductile steels (σf > Su), the Morrow model predicts less sensitivity to mean stress.

Empirical relations

Applied Stresses• Stress range – The basic cause of plastic deformation and consequently the accumulation ofdamage

• Mean stress – Tensile mean and residual stresses aid to the formation and growth of fatiguecracks

• Stress gradients – Bending is a more favorable loading mode than axial loading because inbending fatigue cracks propagate into the region of lower stresses

Materials• Tensile and yield strength – Higher strength materials resist plastic deformation and hencehave a higher fatigue strength at long lives. Most ductile materials perform better at shortlives

• Quality of material – Metallurgical defects such as inclusions, seams, internal tears, andsegregated elements can initiate fatigue cracks

• Temperature – Temperature usually changes the yield and tensile strength resulting in thechange of fatigue resistance (high temperature decreases fatigue resistance)

• Frequency (rate of straining) – At high frequencies, the metal component may be self-heated.

Factors Influencing Fatigue Life

• Size and shape of the component or structure

• Type of loading and state of stress

• Stress concentration

• Surface finish

• Operating temperature

• Service environment

• Method of fabrication

σe = kakbkckdkekfkgkhσe’

where σe = endurance limit of componentσe’ = endurance limit experimentalka = surface finish factor (machined parts have different finish)kb = size factor (larger parts greater probability of finding defects)kc = reliability / statistical scatter factor (accounts for random variation)kd = operating T factor (accounts for diff. in working T & room T)ke = loading factor (differences in loading types)kf = stress concentration factor kg = service environment factor (action of hostile environment)kh = manufacturing processes factor (influence of fabrication parameters)

Factors effecting the Fatigue life

There are typically three stages to fatigue failure.

1. First, a small crack is initiated or nucleates at the surface and can include scratches, pits,

sharp corners due to poor design or manufacture, inclusions, grain boundaries or

dislocation concentrations.

2. Second, the crack gradually propagates as the load continues to cycle.

3. Third, a sudden fracture of the material occurs when the remaining cross-section of the

material is too small to support the applied load.

Stages of fatigue failure

The total number of cycles to failure is the sum of cycles at the first and the second stages:

Nf = Ni + Np

Nf : Number of cycles to failure

Ni : Number of cycles for crack initiation

Np : Number of cycles for crack propagation

High cycle fatigue (low loads): Ni is relatively high. With increasing stress level, Ni decreases

and Np dominates

Cycles to failure

Fatigue analysisFatigue analysis

FEA basedFEA based

Stress life approachStress life approach

Strain life approachStrain life approach Crack propagation Crack propagation Vibration approachVibration approach

Experimental basedExperimental based

•High cycle fatigue

•Subjected to less

sever loads

•Stress with in

elastic limit

•First fatigue

analysis

•Uses S-N curves

•Low cycle fatigue

•Heavy duty

applications

•Crack initiation life

•Elastic & plastic

strains

•Uses Ɛ-N curves

•Developed in

1960’s

•Fracture

mechanics

•LEFM, EPFM

•Rate of crack

growth

•Life left

•Can combine with

LCF

•Complex analysis

•Resonance effect

•Need dynamic

stress as input

•FRF and PSD

based analysis

Fatigue analysis

• Generally high cycle fatigue involves high frequencies ≥ 1000 Hz (cycles/sec)

• Assumes that all stresses in the component, even local stress, stay below elastic limit at all

time

• Easy to use and simple approach based on S-N curve

• Availability of ample data

High cycle fatigue

• Generally low cycle fatigue involves lower frequencies but using higher forces to test the

plastic properties of a material.

• Strain is the basic cause of fatigue.

• At some point in the component being loaded the strain must be plastic (i.e. non reversible)

for a crack to start.

• This method calculates crack initiation life.

• It is important for situations in which components or portions of components undergo either

mechanically or thermally induced cyclic plastic strains that cause failure within relatively few

cycles.

• LCF is also referred as LLC (life limited components)

Low cycle fatigue