Any change in the size, shape, or material properties of a structure, machine, or machine part that
renders it incapable of performing its intended function must be regarded as a mechanical failure of
the device. It should be carefully noted that the key concept here is that improper functioning of a
machine part constitutes failure. Thus, a shear pin that does not separate into two or more pieces
upon the application of a preselected overload must be regarded as having failed as surely as a drive
shaft has failed if it does separate into two pieces under normal expected operating loads Failure of a device or structure to function properly might be brought about by any one or a
combination of many different responses to loads and environments while in service. For example,
too much or too little elastic deformation might produce failure. A fractured load-carrying structural
member or a shear pin that does not shear under overload conditions each would constitute failure.
Progression of a crack due to fluctuating loads or aggressive environment might lead to failure after
a period of time if resulting excessive deflection or fracture interferes with proper machine function.
A primary responsibility of any mechanical designer is to ensure that his or her design functions
as intended for the prescribed design lifetime and, at the same time, that it be competitive in the
marketplace. Success in designing competitive products while averting premature mechanical failures
can be achieved consistently only by recognizing and evaluating all potential modes of failure that
might govern the design. To recognize potential failure modes a designer must be acquainted with
the array of failure modes observed in practice, and with the conditions leading to these failures. The
following section summarizes the mechanical failure modes most commonly observed in practice,
followed by a brief description of each one.A failure mode may be defined as the physical process or processes that take place or that combine
their effects to produce a failure, as just discussed. In the following list of commonly observed failure
modes it may be noted that some failure modes are unilateral phenomena, whereas others are combined
phenomena. For example, fatigue is listed as a failure mode, corrosion is listed as a failure
mode, and corrosion fatigue is listed as still another failure mode. Such combinations are included
because they are commonly observed, important, and often synergistic. In the case of corrosion
fatigue, for example, the presence of active corrosion aggravates the fatigue process and at the same
time the presence of a fluctuating load accelerates the corrosion process.
The following list is not presented in any special order but it includes all commonly observed
modes of mechanical failure:
1. Force and/or temperature-induced elastic deformation.2. Yielding.3. Brinnelling.4. Ductile rupture.
5. Brittle fracture.6. Fatigue:a. High-cycle fatigueb. Low-cycle fatiguec. Thermal fatigued. Surface fatiguee. Impact fatiguef. Corrosion fatigueg. Fretting fatigue7. Corrosion:a. Direct chemical attackb. Galvanic corrosionc. Crevice corrosiond. Pitting corrosione. Intergranular corrosionf. Selective leachingg. Erosion corrosionh. Cavitation corrosioni. Hydrogen damagej. Biological corrosionk. Stress corrosion
8. Wear:a. Adhesive wearb. Abrasive wearc. Corrosive weard. Surface fatigue weare. Deformation wearf. Impact wearg. Fretting wear9. Impact:a. Impact fractureb. Impact deformationc. Impact wear
d. Impact frettinge. Impact fatigue10. Fretting:a. Fretting fatigueb. Fretting wearc. Fretting corrosion
11. Creep.12. Thermal relaxation.13. Stress rupture.14. Thermal shock.15. Galling and seizure.16. Spalling.
17. Radiation damage.18. Buckling.19. Creep buckling.20. Stress corrosion.21. Corrosion wear.
22. Corrosion fatigue.23. Combined creep and fatigue.
As commonly used in engineering practice, the failure modes just listed may be defined and
described briefly as follows. It should be emphasized that these failure modes only produce failure
when they generate a set of circumstances that interferes with the proper functioning of a machine
or device.
Force and I or temperature-induced elastic deformation failure occurs whenever the elastic (recoverable)
deformation in a machine member, brought about by the imposed operational loads or temperatures,
becomes large enough to interfere with the ability of the machine to perform its intended
function satisfactorily.
Yielding failure occurs when the plastic (unrecoverable) deformation in a ductile machine member,
brought about by the imposed operational loads or motions, becomes large enough to interfere with
the ability of the machine to perform its intended function satisfactorily.
Brinnelling failure occurs when the static forces between two curved surfaces in contact result in
local yielding of one or both mating members to produce a permanent surface discontinuity of
significant size. For example, if a ball bearing is statically loaded so that a ball is forced to indent
permanently the race through local plastic flow, the race is brinnelled. Subsequent operation of the
bearing might result in intolerably increased vibration, noise, and heating; and, therefore, failure
would have occurred.
Ductile rupture failure occurs when the plastic deformation, in a machine part that exhibits ductile
behavior, is carried to the extreme so that the member separates into two pieces. Initiation and
coalescence of internal voids slowly propagate to failure, leaving a dull, fibrous rupture surface.Brittle fracture failure occurs when the elastic deformation, in a machine part that exhibits brittle
behavior, is carried to the extreme so that the primary interatomic bonds are broken and the member
separates into two or more pieces. Preexisting flaws or growing cracks form initiation sites for very
rapid crack propagation to catastrophic failure, leaving a granular, multifaceted fracture surface.
Fatigue failure is a general term given to the sudden and catastrophic separation of a machine
part into two or more pieces as a result of the application of fluctuating loads or deformations over
a period of time. Failure takes place by the initiation and propagation of a crack until it becomes
unstable and propagates suddenly to failure. The loads and deformations that typically cause failure
by fatigue are far below the static failure levels. When loads or deformations are of such magnitude
that more than about 10,000 cycles are required to produce failure, the phenomenon is usually termed
high-cycle fatigue. When loads or deformations are of such magnitude that less than about 10,000
cycles are required to produce failure, the phenomenon is usually termed low-cycle fatigue. When
load or strain cycling is produced by a fluctuating temperature field in the machine part, the process
is usually termed thermal fatigue. Surface fatigue failure, usually associated with rolling surfaces in
contact, manifests itself as pitting, cracking, and spalling of the contacting surfaces as a result of the
cyclic Hertz contact stresses that result in maximum values of cyclic shear stresses slightly below the surface. The cyclic subsurface shear stresses generate cracks that propagate to the contacting
surface, dislodging particles in the process to produce surface pitting. This phenomenon is often
viewed as a type of wear. Impact fatigue, corrosion fatigue, and fretting fatigue are described later. Corrosion failure, a very broad term, implies that a machine part is rendered incapable of performing
its intended function because of the undesired deterioration of the material as a result of
chemical or electrochemical interaction with the environment. Corrosion often interacts with other
failure modes such as wear or fatigue. The many forms of corrosion include the following. Direct
chemical attack, perhaps the most common type of corrosion, involves corrosive attack of the surface
of the machine part exposed to the corrosive media, more or less uniformly over the entire exposed
surface. Galvanic corrosion is an accelerated electrochemical corrosion that occurs when two dissimilar
metals in electrical contact are made part of a circuit completed by a connecting pool or film of
electrolyte or corrosive medium, leading to current flow and ensuing corrosion. Crevice corrosion is
the accelerated corrosion process highly localized within crevices, cracks, or joints where small
volume regions of stagnant solution are trapped in contact with the corroding metal. Pitting corrosion
is a very localized attack that leads to the development of an array of holes or pits that penetrate the
metal. Intergranular corrosion is the localized attack occurring at grain boundaries of certain copper,
chromium, nickel, aluminum, magnesium, and zinc alloys when they are improperly heat treated or
welded. Formation of local galvanic cells that precipitate corrosion products at the grain boundaries
seriously degrades the material strength because of the intergranular corrosive process. Selective
leaching is a corrosion process in which one element of a solid alloy is removed, such as in dezincification
of brass alloys or graphitization of gray cast irons. Erosion corrosion is the accelerated
chemical attack that results when abrasive or viscid material flows past a containing surface, continuously
baring fresh, unprotected material to the corrosive medium. Cavitation corrosion is the accelerated
chemical corrosion that results when, because of differences in vapor pressure, certain
bubbles and cavities within a fluid collapse adjacent to the pressure-vessel walls, causing particles
of the surface to be expelled, baring fresh, unprotected surface to the corrosive medium. Hydrogen
damage, while not considered to be a form of direct corrosion, is induced by corrosion. Hydrogen
damage includes hydrogen blistering, hydrogen embrittlement, hydrogen attack, and decarburization.
Biological corrosion is a corrosion process that results from the activity of living organisms, usually
by virtue of their processes of food ingestion and waste elimination, in which the waste products are
corrosive acids or hydroxides. Stress corrosion, an extremely important type of corrosion, is described
separately later.Wear is the undesired cumulative change in dimensions brought about by the gradual removal of
discrete particles from contacting surfaces in motion, usually sliding, predominantly as a result of
mechanical action. Wear is not a single process, but a number of different processes that can take
place by themselves or in combination, resulting in material removal from contacting surfaces through
a complex combination of local shearing, plowing, gouging, welding, tearing, and others. Adhesive
wear takes place because of high local pressure and welding at asperity contact sites, followed by
motion-induced plastic deformation and rupture of asperity functions, with resulting metal removal
or transfer. Abrasive wear takes place when the wear particles are removed from the surface by the
plowing, gouging, and cutting action of the asperities of a harder mating surface or by hard particles
entrapped between the mating surfaces. When the conditions for either adhesive wear or abrasive
wear coexist with conditions that lead to corrosion, the processes interact synergistically to produce
corrosive wear. As described earlier, surface fatigue wear is a wear phenomenon associated with
curved surfaces in rolling or sliding contact, in which subsurface cyclic shear stresses initiate microcracks
that propagate to the surface to spall out macroscopic particles and form wear pits. Deformation
wear arises as a result of repeated plastic deformation at the wearing surfaces, producing a
matrix of cracks that grow and coalesce to form wear particles. Deformation wear is often caused
by severe impact loading. Impact wear is impact-induced repeated elastic deformation at the wearing
surface that produces a matrix of cracks that grows in accordance with the surface fatigue description
just given. Fretting wear is described later Impact failure results when a machine member is subjected to nonstatic loads that produce in the
part stresses or deformations of such magnitude that the member no longer is capable of performing
its function. The failure is brought about by the interaction of stress or strain waves generated by
dynamic or suddenly applied loads, which may induce local stresses and strains many times greater
than would be induced by the static application of the same loads. If the magnitudes of the stresses
and strains are sufficiently high to cause separation into two or more parts, the failure is called impact
fracture. If the impact produces intolerable elastic or plastic deformation, the resulting failure is
called impact deformation. If repeated impacts induce cyclic elastic strains that lead to initiation of
a matrix of fatigue cracks, which grows to failure by the surface fatigue phenomenon described
earlier, the process is called impact wear. If fretting action, as described in the next paragraph, is
induced by the small lateral relative displacements between two surfaces as they impact together,
where the small displacements are caused by Poisson strains or small tangential "glancing" velocity
components, the phenomenon is called impact fretting. Impact fatigue failure occurs when impact loading is applied repetitively to a machine member until failure occurs by the nucleation and propagation
of a fatigue crack.Fretting action may occur at the interface between any two solid bodies whenever they are pressed
together by a normal force and subjected to small-amplitude cyclic relative motion with respect to
each other. Fretting usually takes place in joints that are not intended to move but, because of
vibrational loads or deformations, experience minute cyclic relative motions. Typically, debris produced
by fretting action is trapped between the surfaces because of the small motions involved.
Fretting fatigue failure is the premature fatigue fracture of a machine member subjected to fluctuating
loads or strains together with conditions that simultaneously produce fretting action. The surface
discontinuities and microcracks generated by the fretting action act as fatigue crack nuclei that propagate
to failure under conditions of fatigue loading that would otherwise be acceptable. Fretting
fatigue failure is an insidious failure mode because the fretting action is usually hidden within a joint
where it cannot be seen and leads to premature, or even unexpected, fatigue failure of a sudden and
catastrophic nature. Fretting wear failure results when the changes in dimensions of the mating parts,
because of the presence of fretting action, become large enough to interfere with proper design
function or large enough to produce geometrical stress concentration of such magnitude that failure
ensues as a result of excessive local stress levels. Fretting corrosion failure occurs when a machine
part is rendered incapable of performing its intended function because of the surface degradation of
the material from which the part is made, as a result of fretting actionCreep failure results whenever the plastic deformation in a machine member accrues over a period
of time under the influence of stress and temperature until the accumulated dimensional changes
interfere with the ability of the machine part to perform satisfactorily its intended function. Three
stages of creep are often observed: (1) transient or primary creep during which time the rate of strain
decreases, (2) steady-state or secondary creep during which time the rate of strain is virtually constant,
and (3) tertiary creep during which time the creep strain rate increases, often rapidly, until rupture
occurs. This terminal rupture is often called creep rupture and may or may not occur, depending on
the stress-time-temperature conditions.
Thermal relaxation failure occurs when the dimensional changes due to the creep process result
in the relaxation of a prestrained or prestressed member until it no longer is able to perform its
intended function. For example, if the prestressed flange bolts of a high-temperature pressure vessel
relax over a period of time because of creep in the bolts, so that, finally, the peak pressure surges
exceed the bolt preload to violate the flange seal, the bolts will have failed because of thermal
relaxation.
Stress rupture failure is intimately related to the creep process except that the combination of
stress, time, and temperature is such that rupture into two parts is ensured. In stress rupture failures
the combination of stress and temperature is often such that the period of steady-state creep is short
or nonexistent.
Thermal shock failure occurs when the thermal gradients generated in a machine part are so
pronounced that differential thermal strains exceed the ability of the material to sustain them without
yielding or fracture.
Galling failure occurs when two sliding surfaces are subjected to such a combination of loads,
sliding velocities, temperatures, environments, and lubricants that massive surface destruction is
caused by welding and tearing, plowing, gouging, significant plastic deformation of surface asperities,
and metal transfer between the two surfaces. Galling may be thought of as a severe extension of the
adhesive wear process. When such action results in significant impairment to intended surface sliding
or in seizure, the joint is said to have failed by galling. Seizure is an extension of the galling process
to such severity that the two parts are virtually welded together so that relative motion is no longer
possible.
Spalling failure occurs whenever a particle is spontaneously dislodged from the surface of a
machine part so as to prevent the proper function of the member. Armor plate fails by spalling, for
example, when a striking missile on the exposed side of an armor shield generates a stress wave that
propagates across the plate in such a way as to dislodge or spall a secondary missile of lethal potential
on the protected side. Another example of spalling failure is manifested in rolling contact bearings
and gear teeth because of the action of surface fatigue as described earlier.
Radiation damage failure occurs when the changes in material properties induced by exposure to
a nuclear radiation field are of such a type and magnitude that the machine part is no longer able to
perform its intended function, usually as a result of the triggering of some other failure mode, and
often related to loss in ductility associated with radiation exposure. Elastomers and polymers are
typically more susceptible to radiation damage than are metals, whose strength properties are sometimes
enhanced rather than damaged by exposure to a radiation field, although ductility is usually
decreased.
Buckling failure occurs when, because of a critical combination of magnitude and/or point of
load application, together with the geometrical configuration of a machine member, the deflection of
the member suddenly increases greatly with only a slight change in load. This nonlinear response results in a buckling failure if the buckled member is no longer capable of performing its design
function.
Creep buckling failure occurs when, after a period of time, the creep process results in an unstable
combination of the loading and geometry of a machine part so that the critical buckling limit is
exceeded and failure ensues.
Stress corrosion failure occurs when the applied stresses on a machine part in a corrosive environment
generate a field of localized surface cracks, usually along grain boundaries, that render the
part incapable of performing its function, often because of triggering some other failure mode. Stress
corrosion is a very important type of corrosion failure mode because so many different metals are
susceptible to it. For example, a variety of iron, steel, stainless-steel, copper, and aluminum alloys
are subject to stress corrosion cracking if placed in certain adverse corrosive media.
Corrosion wear failure is a combination failure mode in which corrosion and wear combine their
deleterious effects to incapacitate a machine part. The corrosion process often produces a hard,
abrasive corrosion product that accelerates the wear, while the wear process constantly removes the
protective corrosion layer from the surface, baring fresh metal to the corrosive medium and thus
accelerating the corrosion. The two modes combine to make the result more serious than either of
the modes would have been otherwise.
Corrosion fatigue is a combination failure mode in which corrosion and fatigue combine their
deleterious effects to cause failure of a machine part. The corrosion process often forms pits and
surface discontinuities that act as stress raisers which in turn accelerate fatigue failure. Furthermore,
cracks in the usually brittle corrosion layer also act as fatigue crack nuclei that propagate into the
base material. On the other hand, the cyclic loads or strains cause cracking and flaking of the corrosion
layer, which bares fresh metal to the corrosive medium. Thus, each process accelerates the other,
often making the result disproportionately serious.
Combined creep and fatigue failure is a combination failure mode in which all of the conditions
for both creep failure and fatigue exist simultaneously, each process influencing the other to produce
failure. The interaction of creep and fatigue is probably synergistic but is not well understood.
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