ductile and brittle metal characteristics

8/2/2019 Ductile and Brittle Metal Characteristics

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Ductile and Brittle Metal CharacteristicsDuctile metals experience observable plastic deformation prior to fracture. Brittle metalsexperience little or no plastic deformation prior to fracture. At times metals behave in atransitional manner - partially ductile/brittle.Ductile fracture has dimpled, cup and cone fracture appearance. The dimples canbecome elongated by a lateral shearing force, or if the crack is in the opening (tearing)mode.Brittle fracture displays either cleavage (transgranular) or intergranular fracture. Thisdepends upon whether the grain boundaries are stronger or weaker than the grains.

The fracture modes (dimples, cleavage, or intergranular fracture) may be seen on thefracture surface and it is possible all three modes will be present of a given fractureface.

Schematics of typical tensile test fractures are displayed above. Brittle FracturesBrittle fracture is characterized by rapid crack propagation with low energy release andwithout significant plastic deformation. The fracture may have a bright granularappearance. The fractures are generally of the flat type and chevron patterns may bepresent.


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Ductile FracturesDuctile fracture is characterized by tearing of metal and significant plastic deformation.The ductile fracture may have a gray, fibrous appearance. Ductile fractures areassociated with overload of the structure or large discontinuities.ASME Pressure Vessel FailuresPressure vessels and pressure piping used in

refineries, chemical processing plants, water

treatment systems of boilers, low pressure

storage tanks commonly used in process, pulp

and paperand electric power plants operate

over a broad range of pressures andtemperatures and experience a variety ofoperating environments. Shell, head,

attachments, and piping are some of thecomponents that commonly fail. Somecommon types of failures are listed below:

Cracking Explosion Rupture Leakage

Hydrogen embrittlrment Creep and stress rupture Fatigue Over pressure
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Faulty design Improper fabrication practice Faulty inspection Damage during shipment and storage Damage during field fabrication and

erection Specifying or using improper materials Corrosion Stress corrosion cracking

Over temperature Weldingproblems Discontinuities Stress raisers Improper heat treatment Caustic embrittlement. Brittle fractures Erosion

Fatigue FailuresMetal fatigue is caused by repeatedcycling of of the load. It is aprogressive localized damage due tofluctuating stresses and strains on thematerial. Metal fatigue cracks initiateand propagate in regions where thestrain is most severe.

The process of fatigue consists ofthree stages:

Initial crack initiation Progressive crack growth

across the part Final sudden fracture of the

remaining cross section Schematic of S-N Curve, showing increase infatigue life with decreasing stresses.

Stress RatioThe most commonly used stress ratio is R, the ratio of the minimum stress to themaximum stress (Smin/Smax).

If the stresses are fully reversed, then R = -1.

If the stresses are partially reversed, R = a negative number less than 1.

If the stress is cycled between a maximum stress and no load, R = zero. If the stress is cycled between two tensile stresses, R = a positive number less

than 1.Variations in the stress ratios can significantly affect fatigue life. The presence of amean stress component has a substantial effect on fatigue failure. When a tensile
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mean stress is added to the alternating stresses, a component will fail at loweralternating stress than it does under a fully reversed stress.

Preventing Fatigue FailureThe most effective method of improving fatigue performance is improvements in

design: Eliminate or reduce stress raisers by streamlining the part Avoid sharp surface tears resulting from punching, stamping, shearing, or other

processes Prevent the development of surface discontinuities during processing.


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Reduce or eliminate tensile residual stresses caused by manufacturing. Improve the details of fabrication and fastening procedures

Fatigue Failure AnalysisMetal fatigue is a significant problem because it can occur due to repeated loads belowthe static yield strength. This can result in an unexpected and catastrophic failure inuse.Because most engineering materials contain discontinuities most metal fatigue cracksinitiate from discontinuities in highly stressed regions of the component. The failuremay be due the discontinuity, design, improper maintenance or other causes. A failureanalysis can determine the cause of the failure.Corrosion Failures

Corrosion is chemically induced damage toa material that results in deterioration of thematerial and its properties. This may resultin failure of the component. Several factorsshould be considered during a failureanalysis to determine the affect corrosionplayed in a failure. Examples are listedbelow:

Type of corrosion Corrosion rate The extent of the corrosion Interaction between corrosion and

other failure mechanismsCorrosion is is a normal, natural process.Corrosion can seldom be totally prevented,but it can be minimized or controlled byproper choice of material, design, coatings,and occasionally by changing theenvironment. Various types of metallic andnonmetallic coatings are regularly used to

protect metal parts from corrosion.Stress corrosion crackingnecessitates a tensile stress, which may be caused byresidual stresses, and a specific environment to cause progressive fracture of a metal.Aluminum and stainless steel are well known for stress corrosion crackingproblems. However, all metals are susceptible to stress corrosion cracking in the rightenvironment.
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Laboratory corrosion testing is frequently used in analysis but is difficult to correlate withactual service conditions. Variations in service conditions are sometimes difficult toduplicate in laboratory testingCorrosion Failures AnalysisIdentification of the metal or metals, environment the metal was subjected to, foreignmatter and/or surface layer of the metal is beneficial in failure determination. Examplesof 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 preliminaryexamination will provide enough information to show a simple analysis is adequate.

Uniform CorrosionUniform or general corrosion is typified by the rusting of steel. Other examples of

uniform corrosion are the tarnishing of silver or the green patina associated with the

corrosion of copper.

General corrosion is rather predictable. The life of components can be estimated based

on relatively simple immersion test results. Allowance for general corrosion is relatively

simple and commonly employed when designing a component for a known

environment.Some common methods used to prevent or reduce general corrosion are listed below:

Coatings Inhibitors

Cathodic protection Proper materials selection

Pitting Corrosion
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Pitting is a localized form of corrosive attack. Pitting corrosion is typified by the

formation of holes or pits on the metal surface. Pitting can cause failure due to

perforation while the total corrosion, as measured by weight loss, might be rather

minimal. The rate of penetration may be 10 to 100 times that by general corrosion.Pits may be rather small and difficult to detect. In some cases pits may be masked due

to general corrosion. Pitting may take some time to initiate and develop to an easily

viewable size.Pitting occurs more readily in a stagnant environment. The aggressiveness of the

corrodent will affect the rate of pitting. Some methods for reducing the effects of pitting

corrosion are listed below:

Reduce the aggressiveness of the environment Use more pitting resistant materials Improve the design of the system

Crevice CorrosionCrevice corrosion is a localized form of corrosive attack. Crevice corrosion occurs at

narrow openings or spaces between two metal surfaces or between metals and

nonmetal surfaces. A concentration cell forms with the crevice being depleted of

oxygen. This differential aeration between the crevice (microenvironment) and the

external surface (bulk environment) gives the the crevice an anodic character. This can

contribute to a highly corrosive condition in the crevice. Some examples of crevices are

listed below: Flanges Deposits Washers Rolled tube ends

Threaded joints


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O-rings Gaskets Lap joints

SedimentSome methods for reducing the effects of crevice corrosion are listed below:

Eliminate the crevice from the design Select materials more resistant to crevice corrosion Reduce the aggressiveness of the environment Stress Corrosion Cracking Stress corrosion cracking is a failure mechanism that is caused by environment,

susceptible material, and tensile stress. Temperature is a significantenvironmental factor affecting cracking.

For stress corrosion cracking to occur all threeconditions must be met simultaneously. The componentneeds to be in a particular crack promoting environment,the component must be made of a susceptible material,and there must be tensile stresses above someminimum threshold value. An externally applied load isnot required as the tensile stresses may be due toresidual stresses in the material. The threshold stresses

are commonly below the yield stress of the material.

Stress Corrosion Cracking FailuresStress corrosion cracking is an insidious type of failureas it can occur without an externally applied load or atloads significantly below yield stress. Thus, catastrophicfailure can occur without significant deformation orobvious deterioration of the component. Pitting iscommonly associated with stress corrosion crackingphenomena.

Aluminum and stainless steel are well known for stress corrosion crackingproblems. However, all metals are susceptible to stress corrosion cracking in theright environment.

Controlling Stress Corrosion Cracking There are several methods to prevent stress corrosion cracking. One common

method is proper selection of the appropriate material. A second method is toremove the chemical species that promotes cracking. Another method is to


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change the manufacturing process or design to reduce the tensile stresses.AMC can provide engineering expertise to prevent or reduce the likelihood ofstress corrosion cracking in your components.

Hydrogen EmbrittlementWhen tensile stresses are applied to a hydrogen embrittled component it may failprematurely. Hydrogen embrittlement failures are frequently unexpected andsometimes catastrophic. An externally applied load is not required as the tensilestresses may be due to residual stresses in the material. The threshold stresses tocause cracking are commonly below the yield stress of the material.High strength steel, such as quenched and tempered steels or precipitation hardenedsteels are particularly susceptible to hydrogen embrittlement. Hydrogen can beintroduced into the material in service or during materials processing.

Hydrogen Embrittlement FailuresTensile stresses, susceptible material, and the presence of hydrogen are necessary tocause hydrogen embrittlement. Residual stresses or externally applied loads resultingin stresses significantly below yield stresses can cause cracking. Thus, catastrophicfailure can occur without significant deformation or obvious deterioration of thecomponent.

Very small amounts of hydrogen can cause hydrogen embrittlement in high strengthsteels. Common causes of hydrogen embrittlement are pickling, electroplating andwelding, however hydrogen embrittlement is not limited to these processes.

Hydrogen embrittlement is an insidious type of failure as it can occur without anexternally applied load or at loads significantly below yield stress. While high strengthsteels are the most common case of hydrogen embrittlement all materials aresusceptible.

Liquid Metal EmbrittlementLiquid metal embrittlement is the decrease in ductility of a metal caused by contact withliquid metal. The decrease in ductility can result in catastrophic brittle failure of anormally ductile material. Very small amounts of liquid metal are sufficient to result in

embrittlement.Some events that may permit liquid metal embrittlement under the appropriatecircumstances are listed below:

Brazing Soldering Welding


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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 thecomponent.

Liquid Metal Embrittlement FailuresThe liquid metal can not only reduce the ductility but significantly reduce tensilestrength. Liquid metal embrittlement is an insidious type of failure as it can occur atloads below yield stress. Thus, catastrophic failure can occur without significantdeformation or obvious deterioration of the component. Intergranular or transgranular cleavage fracture are the common fracture modes

associated with liquid metal embrittlement. However reduction in mechanical propertiesdue to decohesion can occur. This results in a ductile fracture mode occurring atreduced tensile strength. An appropriate analysis can determine the effect of liquidmetal embrittlement on failure.High Temperature Failure AnalysisCreep occurs under load at high temperature. Boilers, gas turbine engines, and ovensare some of the systems that have components that experience creep. Anunderstanding of high temperature materials behavior is beneficial in evaluating failuresin these types of systems.

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 orintergranular. While creep testing is done at constant temperature and constant loadactual components may experience damage at various temperatures and loadingconditions.

Creep of MetalsHigh temperature progressive deformation of a material at constant stress is calledcreep. High temperature is a relative term that is dependent on the materials being

evaluated. A typical creep curve is shown below:


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In a creep test a constant load is applied to a tensile specimen maintained at a constanttemperature. Strain is then measured over a period of time. The slope of the curve,identified in the above figure, is the strain rate of the test during stage II or the creeprate of the material.Primary creep, Stage I, is a period of decreasing creep rate. Primary creep is a periodof primarily transient creep. During this period deformation takes place and theresistance to creep increases until stage II. Secondary creep, Stage II, is a period ofroughly constant creep rate. Stage II is referred to as steady state creep. Tertiarycreep, Stage III, occurs when there is a reduction in cross sectional area due to neckingor effective reduction in area due to internal void formation.Stress RuptureStress rupture testing is similar to creep testing except that the stresses used are higherthan in a creep test. Stress rupture testing is always done until failure of the material.In creep testing the main goal is to determine the minimum creep rate in stage II. Oncea designer knows the materials will creep and has accounted for this deformation aprimary goal is to avoid failure of the component.


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Stress rupture tests are used to determine the time to cause failure. Data is plotted log-log as in the chart above. A straight line is usually obtained at each temperature. Thisinformation can then be used to extrapolate time to failure for longer times. Changes in


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slope of the stress rupture line are due to structural changes in the material. It issignificant to be aware of these changes in material behavior, because they could resultin large errors when extrapolating the data.

Failure AnalysisHigh temperature failures is a significant problem. A failure analysis can identify theroot cause of your failure to prevent reoccurrence. AMC can provide failure analysis ofhigh temperature failures to identify the root cause of your component failure

ductile and brittle metal characteristics

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