mechanical failure modes
DESCRIPTION
Mechanical Failure ModesTRANSCRIPT
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
Proportional loading
Non proportional
loading
Non proportional
loading
Variable amplitudeVariable
amplitude
Proportional loading
Proportional loading
Non proportional
loading
Non proportional
loading
Note: Loading ratio = 1, for proportional loading { ratio of second load to first load}
Fatigue loading
Proportional 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