Why things fail
fatigue is the weakening of a material caused
by repeatedly applied loads
Macroscopic and microscopic
discontinuities as well as component
design features which cause stress
concentrations (holes, keyways, sharp
changes of direction etc.) are
common locations at which the
fatigue process begins.
Fatigue is a process that has a degree of randomness
(stochastic), often showing considerable scatter even in
well controlled environments.
Fatigue is usually associated with tensile stresses but
fatigue cracks have been reported due to compressive
loads.[7]
The greater the applied stress range, the shorter the life.
Fatigue life scatter tends to increase for longer fatigue
lives.
Fatigue life scatter tends to increase for longer fatigue
lives.
Damage is cumulative. Materials do not recover when
rested.
Fatigue life is influenced by a variety of factors, such as
temperature, surface finish, metallurgical microstructure,
presence of oxidizing or inert chemicals, residual stresses,
scuffing contact (fretting), etc.
Some materials (e.g., some steel and titanium alloys)
exhibit a theoretical fatigue limit below which continued
loading does not lead to fatigue failure.
Cyclic stress state: Depending on the complexity of the geometry
and the loading, one or more properties of the stress state need to
be considered, such as stress amplitude, mean stress, biaxiality, inphase or out-of-phase shear stress, and load sequence,
Geometry: Notches and variation in cross section throughout a part
lead to stress concentrations where fatigue cracks initiate.
Surface quality: Surface roughness can cause microscopic stress
concentrations that lower the fatigue strength. Compressive residual
stresses can be introduced in the surface by e.g. shot peening to
increase fatigue life. Such techniques for producing surface stress are
often referred to as peening, whatever the mechanism used to
produce the stress. Low plasticity burnishing, laser peening, and
ultrasonic impact treatment can also produce this surface
compressive stress and can increase the fatigue life of the
component. This improvement is normally observed only for highcycle fatigue.
Material Type: Fatigue life, as well as the behavior during cyclic loading, varies
widely for different materials, e.g. composites and polymers differ markedly from
metals.
Residual stresses: Welding, cutting, casting, grinding, and other manufacturing
processes involving heat or deformation can produce high levels of tensile residual
stress, which decreases the fatigue strength.
Size and distribution of internal defects: Casting defects such as gas porosity voids,
non-metallic inclusions and shrinkage voids can significantly reduce fatigue
strength.
Air or Vacuum: Certain materials like Metals are more prone to fatigue in air than
in a vacuum. Depending upon the level of humidity and temperature, the lifetime
for metals such as aluminum or iron might be as much as 5 to 10 times greater in a
vacuum. This is mostly due to the effect of the oxygen and water vapour in the air
which will aggressively attack the material and so encourage the propagation of
cracks. Other environments such as oil or seawater may reduce the fatigue life at
an even greater rate.
Direction of loading: For non-isotropic materials, fatigue strength depends on the
direction of the principal stress.
Grain size: For most metals, smaller grains yield longer fatigue lives, however, the
presence of surface defects or scratches will have a greater influence than in a coarse
grained alloy.
Environment: Environmental conditions can cause erosion, corrosion, or gas-phase
embrittlement, which all affect fatigue life. Corrosion fatigue is a problem encountered
in many aggressive environments.
Temperature: Extreme high or low temperatures can decrease fatigue strength.
Crack Closure: Crack closure is a phenomenon in fatigue loading, during which the crack
will tend to remain in a closed position even though some external tensile force is acting
on the material. During this process the crack will open only at a nominal stress above a
particular crack opening stress. This is due to several factors such as plastic deformation
or phase transformation during crack propagation, corrosion of crack surfaces, presence
of fluids in the crack, or roughness at cracked surfaces etc. this will provide a longer
fatigue life for the material than expected, by slowing the crack growth rate
Design against fatigue
Dependable design against fatigue-failure requires
thorough education and supervised experience in
structural engineering, mechanical engineering, or
materials science. There are four principal approaches to
life assurance for mechanical parts that display increasing
degrees of sophistication:
Design to keep stress below threshold of fatigue limit
(infinite lifetime concept);
fail-safe, graceful degradation, and fault-tolerant design:
Instruct the user to replace parts when they fail. Design
in such a way that there is no single point of failure, and
so that when any one part completely fails, it does not
lead to catastrophic failure of the entire system.
Safe-life design: Design (conservatively) for a fixed life after which the
user is instructed to replace the part with a new one (a so-called lifed
part, finite lifetime concept, or "safe-life" design practice); planned
obsolescence and disposable product are variants that design for a
fixed life after which the user is instructed to replace the entire device;
damage tolerant design: Instruct the user to inspect the part
periodically for cracks and to replace the part once a crack exceeds a
critical length. This approach usually uses the technologies of
nondestructive testing and requires an accurate prediction of the rate
of crack-growth between inspections. The designer sets some aircraft
maintenance checks schedule frequent enough that parts are replaced
while the crack is still in the "slow growth" phase. This is often
referred to as damage tolerant design or "retirement-for-cause".
Stopping fatigue
Fatigue cracks that have begun to propagate can sometimes
be stopped by drilling holes, called drill stops, in the path of
the fatigue crack. This is not recommended as a general
practice because the hole represents a stress concentration
factor which depends on the size of the hole and geometry,
though the hole is typically less of a stress concentration
than the removed tip of the crack. The possibility remains
of a new crack starting in the side of the hole. It is always
far better to replace the cracked part entirely.
Material change
Changes in the materials used in parts can also improve
fatigue life. For example, parts can be made from better
fatigue rated metals. Complete replacement and redesign
of parts can also reduce if not eliminate fatigue problems.
Thus helicopter rotor blades and propellers in metal are
being replaced by composite equivalents. They are not only
lighter, but also much more resistant to fatigue. They are
more expensive, but the extra cost is amply repaid by their
greater integrity, since loss of a rotor blade usually leads to
total loss of the aircraft. A similar argument has been made
for replacement of metal fuselages, wings and tails of
aircraft.[19]
High Frequency Mechanical Impact (HFMI) treatment of
welds
The durability and life of dynamically loaded, welded steel
structures are determined often by the welds, particular by
the weld transitions. By selective treatment of weld
transitions with the High Frequency Mechanical Impact
(HFMI) treatment method, the durability of many designs
can be increased significantly. This method is universally
applicable, requires only technical equipment and offers
high reproducibility and a high grade of quality control.
Type of Failure
The major types of failures likely to be encountered by
metals in service are:
A.
Ductile,
B.
Brittle, and
C.
Fatigue fractures
A. Ductile Fracture
Ductile fractures are characterized by tearing of metal
accompanied by appreciable gross plastic deformation. The
microstructure of the fracture surface is quite complex and
may include both transgranular and intergranular fracture
mechanisms. Ductile fractures in most metals have a gray
fibrous appearance and may be flat-faced (tensile overload)
or slant-faced (shear). The specimen usually shows
considerable elongation and possible reduction of crosssectional area as well.
Whether a part fails in a ductile or brittle fashion
depends on the thickness of the part, temperature,
strain rate and the presence of stress-raisers. Most
commonly seen characteristics of ductile failures are:
•
Lateral contraction, or necking;
•
Fracture path in the interior following a generally
flat plane perpendicular to the principal stress direction,
and
•
Tensile stress.
B. Brittle Fracture
Brittle fractures are characterized by rapid crack
propagation without appreciable plastic deformation. If
brittle fractures occur across particular crystallographic
planes they are called Tran crystalline fracture. If along grain
boundaries they are called intergranular fracture. Brittle
fracture is promoted by:
•
thicker section sizes,
•
lower service temperatures, and
•
increased strain rate.
C. Fatigue fracture
Fatigue is a progressive localized permanent structural
change that occurs in a material subjected to repeated or
fluctuating stresses well below the ultimate tensile strength
(UTS). Fatigue fractures are caused by the simultaneous
action of cyclic stress, tensile stress, and plastic strain, all
three of which must be present. Cyclic stress initiates a crack
and tensile stress propagates it. Final sudden failure of the
remaining cross-section occurs by either shear or brittle
fracture. Striations on the crack surface are the classic sign
of fatigue fracture.
High Cycle Fatigue Low Cycle Fatigue Fatigue cracks may
start because of tool marks, scratches, indentations,
corrosion pits and areas of high stress. At the crack tip, the
material is plastic. At a small distance from the crack tip, in
the material is elastic
Low cycle fatigue cracks occur under conditions of high
strain amplitude (with failure in less than about 104 cycles)
whereas high cycle fatigue occurs with low strain amplitude
with failure after a large number of load fluctuations.
Thermal Fatigue cracking is caused by cycling the
temperature of the part in the presence of mechanical
constraint, e.g., rigid mounting of pipe. It could also be
caused by temperature gradients in the part.
Contact Fatigue - Elements that roll, or roll and slide against
each other under high contact pressure are subject to the
development of surface pits or fatigue spalls after many
repetitions of load.
Corrosion pit acting as stress concentrator for fatigue crack
Liquid Metal Embrittlement
Liquid metal embrittlement is the decrease in ductility of a
metal caused by contact with liquid metal. The decrease in
ductility can result in catastrophic brittle failure of a
normally ductile material. Very small amounts of liquid
metal are sufficient to result in embrittlement.
Some events that may permit liquid metal
embrittlement under the appropriate circumstances are
listed below:
Brazing
Soldering
Welding
Heat treatment
Hot working
Elevated temperature service
In addition to an event that will allow liquid metal
embrittlement to occur, it is also required to have the
component in contact with a liquid metal that will
embrittle the component.
Liquid Metal Embrittlement Failures
The liquid metal can not only reduce the ductility but
significantly reduce tensile strength. Liquid metal
embrittlement is an insidious type of failure as it can
occur at loads below yield stress. Thus, catastrophic
failure can occur without significant deformation or
obvious deterioration of the component
Corrosion is is a normal, natural process. Corrosion can
seldom be totally prevented, but it can be minimized or
controlled by proper choice of material, design, coatings,
and occasionally by changing the environment. Various
types of metallic and nonmetallic coatings are regularly
used to protect metal parts from corrosion.
Corrosion Failures
Corrosion is chemically induced damage to a material
that results in deterioration of the material and its
properties. This may result in failure of the
component. Several factors should be considered
during a failure analysis to determine the affect
corrosion played in a failure.
Examples are listed below:
• Type of corrosion
• Corrosion rate
• The extent of the corrosion
• Interaction between corrosion and other failure
mechanisms
Stress corrosion cracking necessitates a tensile stress,
which may be caused by residual stresses, and a specific
environment to cause progressive fracture of a
metal. Aluminum and stainless steel are well known for
stress corrosion cracking problems. However, all metals are
susceptible to stress corrosion cracking in the right
environment.
Corrosion Failures Analysis
Identification of the metal or metals, environment the
metal was subjected to, foreign matter and/or surface layer
of the metal is beneficial in failure determination.
Examples of some common types of corrosion are
listed below:
• Uniform corrosion
• Pitting corrosion
• Intergranular corrosion
• Crevice corrosion
• Galvanic corrosion
• Stress corrosion cracking
Not all corrosion failures need a comprehensive failure
analysis. At times a preliminary examination will
provide enough information to show a simple analysis
is adequate.
High Temperature Failure Analysis
Creep occurs under load at high temperature. Boilers, gas
turbine engines, and ovens are some of the systems that
have components that experience creep
Failures involving creep are usually easy to identify
due to the deformation that occurs. Failures may
appear ductile or brittle. Cracking may be either
transgranular or intergranular
Hydrogen Embrittlement
When tensile stresses are applied to a hydrogen embrittled
component it may fail prematurely. Hydrogen
embrittlement failures are frequently unexpected and
sometimes catastrophic. An externally applied load is not
required as the tensile stresses may be due to residual
stresses in the material. The threshold stresses to cause
cracking are commonly below the yield stress of the
material.
High strength steel, such as quenched and tempered steels
or precipitation hardened steels are particularly susceptible
to hydrogen embrittlement.
Very small amounts of hydrogen can cause hydrogen
embrittlement in high strength steels. Common
causes of hydrogen embrittlement are pickling,
electroplating and welding, however hydrogen
embrittlement is not limited to these processes.
Hydrogen embrittlement is an insidious type of failure
as it can occur without an externally applied load or at
loads significantly below yield stress. While high
strength steels are the most common case of
hydrogen embrittlement all materials are susceptible.
Ductile Fractures
Ductile fracture is characterized by tearing of metal
and significant plastic deformation. The ductile
fracture may have a gray, fibrous
appearance. Ductile fractures are associated with
overload of the structure or large discontinuities.
Ductile and Brittle Metal Characteristics
Ductile metals experience observable plastic deformation
prior to fracture. Brittle metals experience little or no
plastic deformation prior to fracture
The fracture modes (dimples, cleavage, or intergranular fracture) may be seen on
the fracture surface and it is possible all three modes will be present of a given
fracture face
Brittle Fractures
Brittle fracture is characterized by rapid crack
propagation with low energy release and without
significant plastic deformation. The fracture may have
a bright granular appearance. The fractures are
generally of the flat type and chevron patterns may be
present.
Residual Stresses
Residual stresses can be sufficient to cause a metal part
to suddenly split into two or more pieces after it has been
resting on a table or floor without external load being
applied. While this is not a common occurrence,
experienced people in the metal working industry have
witnessed this phenomenon. While there may be
additional factors causing this to occur residual stresses
help explain these occurrences.
Residual stresses are stresses that are inside or locked into a
component or assembly of parts. The internal state of stress
is caused by thermal and/or mechanical processing of the
parts. Common examples of these are bending, rolling or
forging a part. Another example are the thermal stresses
induced when welding.
Stress Concentration
The image at the right shows the concept of stress concentration
at the root of the notch on the left. The image on the right
shows a uniform stress field represented as imaginary uniform
stress lines that concentrate in intensity at the root of the
notch. Stress concentrations may permit failure modes to occur
more quickly in fatigue, stress corrosion cracking, creep
Marine Corrosion
Marine corrosion includes the immersion of components in
a seawater, equipment and piping that use seawater or
brackish water, and corrosion in marine atmospheres.
Exposure of components can be continuous or intermittent.
Ships, marinas, pipelines, offshore structures, desalination
plants, and heat exchangers are some examples of systems
that experience marine corrosion.
Corrosion of a component, such as a bolt, can vary markedly
depending on if it is simply in a marine atmosphere, a splash zone,
or submerged in seawater.
Marine atmospheres are generally considered to be one of
the more aggressive atmospheric corrosion
environments. Some factors that affect corrosion rates in
marine atmospheres are listed below:
• Humidity
• Wind
• Temperature
• Airborne contaminants
• Location
• Biological organisms
Cathodic protection can be accomplished by either using
an impressed current system or by using sacrificial anode
system. Magnesium, aluminum and zinc alloys are the
most frequently used sacrificial anode systems.
Shaft Failures
Shafts function in wide ranging service conditions,
including corrosive environments, and both very high and
very low temperatures. Shafts may experience a range of
loading conditions. In general, shafts may experience
tension, compression, bending, torsion, or a combination
of these loading conditions. Additionally, shafts may
experience vibratory stresses.
Wear is a common cause of shaft failure. Abrasive wear is one of the forms of
wear failures. Abrasive wear, or abrasion, is caused by the displacement of
material from a solid surface due to hard particles or protuberances sliding along
the surface. Abrasive wear can reduce the size and destroy the shape of a shaft.
Some examples of abrasive wear of shafts are foreign particles such as sand, dirt,
metallic particles, and other debris in the lubricant. This debris can damage a
shaft by wear.
One of the more common causes of shaft failure is due to
fatigue. Fatigue failures commonly start at a stress
raiser. Other forms of fracture also commonly occur at
stress raisers as well. Some typical features in shafts that
act as stress raisers are listed below:
Corners
Keyways
Grooves
Press or shrink fits
Welding defects
Nicks or notches
Splines
Quench cracks
Localized
corrosion
Arc strikes
Failures may occur due to misalignment. One cause of misalignment
is the mismatch of mating parts. Misalignment can be introduced
during original assembly of equipment. Misalignment can be
introduced after an overall or repair of equipment. Deflection or
deformation of supporting components in service may also cause
misalignment. Misalignment can cause vibration resulting in a
fatigue failure of the shaft
Some other causes of shaft failures include the following:
Accidental overload
Corrosion
Creep or stress rupture
Brittle fracture
Stress corrosion cracking
Hydrogen embrittlement
Several factors effect the quality of metal castings. Some of these factors
are listed below:
• Coefficients of thermal conductivity
• Thermal expansion and contraction,
• Chemistry
• Precision of molds and dies
• Shrinkage allowances
• Dryness of molds
• Casting design
• Method of pouring liquid metal
• Design of gates and risers
Casting Discontinuities
Some common casting deficiencies are:
• Inclusions
• Porosity (blow holes, pinholes)
• Cold Cracking
• Hot Cracking
• Cold Shuts
• Surface irregularities
• Distortion
• Improper composition
Casting Failure Analysis
Casting failures can be due to various
causes. Improper loading or environment may
contribute to the cause of failure. Casting
imperfections may or may not contribute to the cause
of failure. Some imperfections may be commonly
occurring discontinuities or anomalies that are
normally expected to be present in castings. Other
imperfections are casting defects that result in failure
of the casting. Failure analysis can determine the
cause of the casting failure and determine if a casting
imperfections was the primary or contributing cause of
failure.
Casting Concerns
Casting issues AMC can evaluate for your organization are listed below:
Mold designs
Mold types
Sand casting
Investment Casting
Ceramic mold casting
Plaster mold casting
Shell mold casting
Permanent Mold Casting
Gating systems
Cores
Solidification of metals
Cast structures
Fluidity of molten metals
Heat transfer
Shrinkage
Casting defects
Die casting types
Centrifugal casting
Turbine blade casting
Heat Treatment
Ferrous alloys
Non-ferrous alloys
Heat treatment processes
Cleaning/finishing methods
Inspection
Testing
Casting quality assurance
Process Capability
Allowance and tolerance
Process capability
Process variation
Cast alloys
Fastener Failure
•The primary function of a fastener is to transfer
load. There are many types of fasteners. Examples
of some requirements for fasteners are listed
below: Higher strength
• Increased high temperature dependability
• Increased low temperature dependability
• Reduced cost
• Easier maintenance
• Improved corrosion resistance
The choice of a fastener is dependent on the design requirements and
environment in which the fastener will be used. Attention to various
aspects of the fastener must be considered. Some of these are listed
below:
• Function of the fastener
• Operating environment of the fastener
• Type of loading on the fastener in service
• Thickness of materials to be joined
• Type of materials to be joined
• Configuration of the joint to be fastened
Mechanical Fasteners
Mechanical fasteners are frequently
grouped as listed below:
• Pin fasteners
• Threaded fasteners
• Rivets
• Blind fasteners
• Special purpose fasteners
• Fasteners for composites
Rivets, pin fasteners, and special-purpose fasteners are usually
designed for permanent or semipermanent installation.
Pin fasteners are fasteners are used in joints in which the load is
primarily shear. Pins can be either solid or tubular. A collar is
sometimes swaged or formed on the pin to secure the joint.
Threaded fasteners are commonly thought of as any threaded part
that, after joint assembly, may be removed without damage to the
fastener or to the members being joined.
Rivets are permanent one piece fasteners one end of the rivet is
mechanically upset during installation.
Blind fasteners are commonly multiple part devices that can be
installed in a joint that is accessible from only one side. Typically a selfcontained mechanism, an explosive, or other device forms an upset on
the inaccessible side when a blind fastener is installed.
Special-purpose fasteners are often proprietary, such as retaining rings,
latches, slotted springs, and studs. These fasteners are frequently
designed to allow easy, quick removal and replacement and commonly
show little or no deterioration with repeated use.
Mechanical fasteners for composites are often used in combination
with adhesive bonding to increase the reliability of highly stressed
joints. The common pins, bolts, rivets, and blind fasteners are used
with composites. However, the numerous problems that have
occurred have motivated the development and testing of many special
purpose fasteners.
Some of the problems with fasteners for
composites are listed below:
•Pullout of the fastener under load
• Drilling damage to the composite
• Installation damage to the composite
• Delamination of the composite material near the hole
• Differences in expansion coefficients between the
composite and the fastener
• Galvanic corrosion between the composite and the
fastener
• Fuel leaks around the fastener
• Fretting
Failure Origins
Frequent locations for fastener failure listed below:
• In the head-to-shank fillet
• Through the first thread inside the nut
• The transition from the thread to the shank
Causes of Fastener Failures
Some causes of fastener failure are listed below:
•
•
•
•
•
•
Shear
Overload
Fatigue
Corrosion
Manufacturing discrepancies
Improper installation
Fastener Failure Analysis
A fastener may experience either static loading or fatigue loading.
Static loading may be tension, shear, bending, or torsion. These
static loading conditions may occur in combination. One example of
fatigue loading is vibration. In addition to overload and fatigue, some
other common reasons for fastener failures include environmental
issues, manufacturing discrepancies, and improper use or incorrect
installation.
Fastener Failure Analysis
A fastener may experience either static loading or fatigue
loading. Static loading may be tension, shear, bending, or
torsion. These static loading conditions may occur in
combination. One example of fatigue loading is vibration. In
addition to overload and fatigue, some other common reasons
for fastener failures include environmental issues,
manufacturing discrepancies, and improper use or incorrect
installation.
Some common questions concerning fasteners are listed below:
• How were the fasteners torqued?
• In what order were fasteners tightened?
• What is the best way to verify the torque on fasteners?
• How does torque value vary over time?
Fatigue is one of the most common failure modes for threaded
fasteners. Fretting failures may result from small movements
between adjacent surfaces. Additionally, atmospheric corrosion,
liquid immersion corrosion, galvanic corrosion, crevice corrosion,
stress corrosion cracking, and hydrogen damage may contribute
to fastener failure
Material selection, heat treatment, cutting or rolling threads,
manufacturing, assembly, and design are some of the factors that
effect fastener failures. Failure analysis can determine the cause
of the fastener failure and determine the primary or contributing
causes of fastener failure.
Roller and Ball Bearings
Roller and ball bearings are commonly used in various
components. The rollers or balls are placed in between two
raceways. This allows relative motion by rotation of these
pieces.
Some common types of bearings used include:
• Radial contact
• Angular contact
• Thrust
• Cylindrical
• Needle
• Tapered
• Spherical
Bearing load ratings are established on the results of laboratory rolling contact
fatigue tests. Real world conditions such as misalignment, vibration, shock
loading, insufficient or inefficient lubrication, extremes of temperature, or
contamination, will decrease the life expectancy of the bearings. If these
conditions are severe, they may lead to premature failure of the bearings.
Some common characteristics of bearing failures are listed
below:
• Wear
• Fretting
• Corrosion
• Indentations
• Electrical pitting
• Smearing
• Cracking
• Flaking
Some of the factors that may lead to bearing failure are
improper lubrication, impact loading, vibration, excess
temperature, contamination, excessive loading, and
misalignment
Gear Failures
Gears can fail in several different ways. Increased vibration and noise
level from the equipment is commonly associated with gear failures.
Cast irons, nonferrous alloys, powdered-metals, and steels are
materials used in gears.
•Worm gears
• Herringbone gears
• Helical gears
• Spur gears
• Bevel gears
• Internal gears
Gear Failure Analysis
Some of the failure modes in gears are listed below:
• Fatigue
• Wear
• Stress Rupture
• Impact
Tooth bending fatigue, contact fatigue, and thermal fatigue are
among some of the types of fatigue failures in gears. Abrasive
wear and adhesive wear are the common modes of wear
failure of gears. Material, manufacturing, engineering, service
environment, and heat treatment are some of the causes of
gear failures.
Wear Failures
Wear may be defined as damage to a solid surface caused by the removal or
displacement of material by the mechanical action of a contacting solid, liquid, or
gas. It may cause significant surface damage and the damage is usually thought
of as gradual deterioration. While the terminology of wear is unresolved, the
following categories are commonly used.
•Adhesive wear
• Abrasive wear
• Erosive wear
Adhesive wear has been commonly identified by the terms galling, or
seizing. Abrasive wear, or abrasion, is caused by the displacement of material
from a solid surface due to hard particles or protuberances sliding along the
surface. Erosion, or erosive wear, is the loss of material from a solid surface due
to relative motion in contact with a fluid that contains solid particles. More than
one mechanism can be responsible for the wear observed on a particular part.