30079_18a.pdf

CHAPTER 18

FAILURE CONSIDERATIONS

Jack Collins

Department of Mechanical Engineering

Ohio State University

Columbus, Ohio

Steve Daniewicz

Department of Mechanical Engineering

Mississippi State University

Starkville, Mississippi

18.1 CRITERIA OF FAILURE

377

18.5.10 Damage Tolerance and

Fracture Control

436

18.6

CREEP AND STRESS

RUPTURE

18.2 FAILUREMODES

378

437

18.3 ELASTIC DEFORMATION AND

YIELDING

18.6.1 Prediction of Long-Term

Creep Behavior

382

439

18.6.2 Creep under Uniaxial

State of Stress

18.4 FRACTURE MECHANICS AND

UNSTABLE CRACK GROWTH

440

383

18.6.3 Creep under Multiaxial

State of Stress

442

18.5 FATIGUE AND STRESS

CONCENTRATION

18.6.4 Cumulative Creep

442

396

18.5.1

Fatigue Loading and

Laboratory Testing

18.7

COMBINED CREEP AND

FATIGUE

397

443

18.5.2

The S-N-P Curves—

A Basic Design Tool

401

18.8

FRETTINGANDWEAR

449

18.5.3

Factors That Affect

S-N-P Curves

18.8.1 Fretting Phenomena

450

402

18.8.2 Wear Phenomena

456

18.5.4

Nonzero Mean and

Multiaxial Fatigue Stresses 402

18.9

CORROSION AND STRESS

CORROSION 462

18.9.1 Types of Corrosion 463

18.9.2 Stress Corrosion Cracking 467

18.5.5

Spectrum Loading and

Cumulative Damage

410

18.5.6

Stress Concentration

414

18.5.7

Low-Cycle Fatigue

420

18.5.8

Three-Phase Approach for

Fatigue Life Prediction

18.10 FAILUREANALYSISAND

RETROSPECTIVE DESIGN

429

468

18.5.9

Service Spectrum

Simulation and

Full-Scale Testing

435

18.1 CRITERIA OF FAILURE

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.

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

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.

18.2 FAILUREMODES

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 com-

bined 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

1. Force and/or temperature-induced elastic deformation.

2. Yielding.

3. Brinnelling.

4. Ductile rupture.

5. Brittle fracture.

6. Fatigue:

a. High-cycle fatigue

b. Low-cycle fatigue

c. Thermal fatigue

d. Surface fatigue

e. Impact fatigue

f. Corrosion fatigue

g. Fretting fatigue

7. Corrosion:

a. Direct chemical attack

b. Galvanic corrosion

c. Crevice corrosion

d. Pitting corrosion

e. Intergranular corrosion

f. Selective leaching

g. Erosion corrosion

h. Cavitation corrosion

i. Hydrogen damage

j. Biological corrosion

k. Stress corrosion

8. Wear:

a. Adhesive wear

b. Abrasive wear

c. Corrosive wear

d. Surface fatigue wear

e. Deformation wear

f. Impact wear

g. Fretting wear

9. Impact:

a. Impact fracture

b. Impact deformation

c. Impact wear

d. Impact fretting

e. Impact fatigue

10. Fretting:

a. Fretting fatigue

b. Fretting wear

c. 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 (recov-

erable) deformation in a machine member, brought about by the imposed operational loads or tem-

peratures, 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 per-

forming 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 dissim-

ilar 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 dezinc-

ification 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, contin-

uously baring fresh, unprotected material to the corrosive medium. Cavitation corrosion is the ac-

celerated 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 micro-

cracks that propagate to the surface to spall out macroscopic particles and form wear pits. Defor-

mation 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 prop-

agation 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 pro-

duced 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 prop-

agate 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 action.

Creep 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 some-

times 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

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