different types of load and fatigue failure(2)

7/28/2019 Different Types of Load and Fatigue Failure(2)

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6/6/2013 1Dr .SANJAY CHIKALTHANKAR

Dr. Sanjay ChikalthankarDept. of Mech. Engineering, GECA


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INTRODUCTION :-When a component is

subject to increasing loads iteventually fails. It iscomparatively easy to determinethe point of failure of acomponent subject to a single

tensile force. The strength dataon the material identifies thisstrength. However when thematerial is subject to a number ofloads in different directions some

of which are tensile and some ofwhich are shear, then thedetermination of the point offailure is more complicated.

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TYPES OF LOADS:-STATIC LOAD

1) AXIAL LOAD2) BENDING LOAD3) TORTIONAL LOAD

DYNAMIC LOADFLUCTUATING LOAD

1) TESILE MEAN LOAD2) COMPRESSIVE MEAN

LOAD3) REPEATETED LOAD

SUDDENLY APPLIED LOADIMPACT LOAD

FATIGUE LOAD 6/6/2013 3Dr SANJAY CHIKALTHANKAR


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1)AXIAL LOAD :-

Uniform and Prismatic(straight)bar,rod,tube etc.

Homogenous material.

Load P directed axially along the

centroidal axis of cross section.Elastic loading



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Bending moment :- Uniform prismatic beam (L > 10 b).

Carries load that produce deflectionperpendicular to its longitudal axis.

Bending relative to principal axis only

Linear elastic material.

Applicable to small deflection and as long asdeflection is in circular arc, ie. d2v/dx2 is agood approximation of the beam curvature.



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Torsional load :-Prismatic & circular (solid or hollow but thick)

Torsional member

Homogenous material

Sections at which torques are applied areremote from ends.

Angle of twist is small.



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DYNAMIC LOAD :-

Loads that vary during normal service of the

product produce dynamic stress.Dynamic stress can be cyclic or random.

High cycle fatigue part subject to millions ofstress cycles.

Cyclic loads produce cyclic stress which can lead to mechanicalfatigue failure



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Fluctuating load :-Tensile mean stress (can cycle

between tension and compression orall tension)

Compressive mean stress (can cyclebetween tension and compression orall compression)

Repeated, one-direction stress



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1) TESILE MEAN LOAD :-

smax is tensile and smin is compressive

Advantageous to define briefly the general types offluctuating stresses which can cause fatigue.Maximum and minimum stresses are equalMinimum stress is the lowest algebraic stress in thecycle.Tensile stress is positive , compressive stress isnegative.

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Dr SANJAY CHIKALTHANKAR


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Compressive mean stress :-

smax is tensile and smin is compressive

Maximum and minimum stresses are notequalRepeated stress cycle contains maximumand minimum stresses of opposite sign.



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Repeated, one-direction load :-Airplane structures are subjected repeated loads, called cyclic loads, andthe resulting cyclic stresses can lead to microscopic physical damage to the

materials involved. Even at stresses well below the material's ultimatestrength, this damage can accumulate with continued cycling until itdevelops into a crack or other damage that leads to failure of thecomponent. The process of accumulating damage and finally to failure dueto cyclic loading is called fatigue. An insidious cause of loss of strength.



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IMPACT LOAD :-

A common type of structural analysis results

from an impact load. The impact should becaused by a weight falling on the design objectfalling and striking a hard surface.



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FATIGUE LOAD :-Fatigue loading is primarily the type of loading

which causes cyclic variations in the applied stress orstrain on a component. Thus any variable loading isbasically a fatigue loading .

Fatigue cracking is one of the primary damagemechanisms of structural components. Fatigue crackingresults from cyclic stresses that are below the ultimatetensile stress, or even the yield stress of the material. Thename fatigue is based on the concept that a materialbecomes tired and fails at a stress level below thenominal strength of the material. The facts that the

original bulk design strengths are not exceeded and theonly warning sign of an impending fracture is an oftenhard to see crack, makes fatigue damage especiallydangerous.



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WHAT IS FAILURE ?Ans:- Failure means a part which is

permanently distorted and not functionproperly . And it is separated in two or morepieces, then it is said that part under failure.

There are numbers of machinecomponents, which is subjected to severaltypes of loads simultaneously. For example,a power screw subjected to torsional momentas well as axial force. Crank shaft, propellershaft and connecting rod are examples ofcomponent subjected to complex loads.



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The failures of such component arebroadly classified into two groups- elasticfailure and yielding failure . Elastic failureresults in excessive deformation, whichmakes the component unfit to its functionsatisfactorily yielding result in excessiveplastic deformation after the yield pointstress is reached, while fracture results inbreaking he component into two or more

pieces.



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OBJECTIVES AND SCOPEIn this module we will be discussing on design aspects

related to fatigue failure, an important mode of failure inengineering components. Fatigue failure results mainly due tovariable loading or more precisely due to cyclic variations inthe applied loading or induced stresses So starting from thebasic concepts of variable (non-static) loading, we will bediscussing in detail how it leads to fatigue failure incomponents, what factors influence them, how to accountthem and finally how to design parts or components to resistfailure by fatigue

WHAT IS FATIGUE?Fatigue is a phenomenon associated with variable

loading or more precisely to cyclic stressing or straining of amaterial. Just as we human beings get fatigue when a specifictask is repeatedly performed, in a similar manner metalliccomponents subjected to variable loading get fatigue, whichleads to their premature failure under specific conditions



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Fatigue FailureOften machine members subjected to such repeated or

cyclic stressing are found to have failed even when theactual maximum stresses were below the ultimate strengthof the material, and quite frequently at stress values evenbelow the yield strength. The most distinguishingcharacteristics is that the failure had occurred only afterthe stresses have been repeated a very large number oftimes. Hence the failure is called fatigue failure.

ASTM Definition of fatigue

The process of progressive localized permanentstructural changes occurring in a material subjected toconditions that produce fluctuating stresses at some pointor points and that may culminate in cracks or completefracture after a sufficient number of fluctuations.Let us first make an attempt to understand the basicmechanism of fatigue failure



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Fatigue Failure- Mechanism A fatigue failure begins with a small

crack; the initial crack may be so minute and can

not be detected. The crack usually develops at apoint of localized stress concentration likediscontinuity in the material, such as a change incross section, a keyway or a hole. Once a crack isinitiated, the stress concentration effect becomegreater and the crack propagates. Consequentlythe stressed area decreases in size, the stressincrease in magnitude and the crack propagatesmore rapidly. Until finally, the remaining area is

unable to sustain the load and the component failssuddenly. Thus fatigue loading results in sudden,unwarned failure.



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Fatigue Failure StagesThus three stages are involved in fatigue failure namely-Crack initiation-Crack propagation

-Fracture Crack initiation Areas of localized stress concentrations such as fillets, notches, key ways, bolt holes and even scratches or tool marks are potential zones for crackinitiation. Crack also generally originate from a geometrical discontinuity or

metallurgical stress raiser like sites of inclusions As a result of the local stress concentrations at these locations, the induced stress goes above the yield strength (in normal ductile materials)and cyclic plastic straining results due to cyclic variations in the stresses.On a macro scale the average value of the induced stress might still bebelow the yield strength of the material.

During plastic straining slip occurs and (dislocation movements) results ingliding of planes one over the other. During the cyclic stressing, slipsaturation results which makes further plastic deformation difficult.Machine Design IIIndian Institute of Technology Madras As a consequence, intrusion and extrusion occurs creating a notch like discontinuity in the material.



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Crack propagation

This further increases the stress levels and

the process continues, propagating thecracks across the grains or along the grainboundaries, slowly increasing the crack size.

As the size of the crack increases the cross

sectional area resisting the applied stressdecreases and reaches a thresh hold level atwhich it is insufficient to resist the appliedstress.



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Final fracture

As the area becomes too insufficient to

resist the induced stresses any further asudden fracture results in the component



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The micro mechanism of fatigue fracture :-Cause:-Material body Cyclic stressEffect :-

Atomic Microscopic Microscopic

1.Dislocation movements 1. Slip formation 1.Stable stages

2.Dislocation multiplication 2. Slip saturation 2.Unstable stages

3.Defect interaction 3. Structure deterioration 3.Critical length

4.Cross slip 4. Extrusion intrusion 4.Final fracture5. Engergy changes

6. Crack nucleation and growth

Crystallographically


Fatigue Properties


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Fatigue PropertiesFatigue cracking is one of the primary damage mechanisms of structural components. Fatigue

cracking results from cyclic stresses that are below the ultimate tensile stress, or even the yield stress

of the material. The name fatigue is based on the concept that a material becomes tired and fails

at a stress level below the nominal strength of the material. The facts that the original bulk design

strengths are not exceeded and the only warning sign of an impending fracture is an often hard to seecrack, makes fatigue damage especially dangerous. The fatigue life of a component can be

expressed as the number of loading cycles required to initiate a fatigue crack and topropagate the crack to critical size. Therefore, it can be said that fatigue failure occurs inthree stages crack initiation; slow, stable crack growth; and rapid fracture.

As discussed previously, dislocations play a major role in thefatigue crack initiation phase. In the first stage, dislocationsaccumulate near surface stress concentrations and formstructures called persistent slip bands (PSB) after a large numberof loading cycles. PSBs are areas that rise above (extrusion)or fall below (intrusion) the surface of the component due tomovement of material along slip planes. This leaves tiny steps inthe surface that serve as stress risers where tiny crackscan initiate. These tiny crack (called microcracks nucleate alongplanes of high shear stress which is often 45o to the loading direction.

In the second stage of fatigue, some of the tiny microcracks join together and begin

to propagate through the material in a direction that is perpendicular to the maximumtensile stress. Eventually, the growth of one or a few crack of the larger cracks will dominateover the rest of the cracks. With continued cyclic loading, the growth of the dominate crackor cracks will continue until the remaining uncracked section of the component can nolonger support the load. At this point, the fracture toughness is exceeded and theremaining cross-section of the material experiences rapid fracture. This rapid overloadfracture is the third stage of fatigue failure.



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Fatigue phenomenaFatigue failure occurs when metal is subjected to arepetitive or fluctuating stress and will fail at a stressmuch lower than its tensile strength.Fatigue failures occur without any plastic deformation.Basic factors necessary to cause fatigue failure.Maximum tensile stress of sufficiently high value.A large enough variation or fluctuation in the appliedstress.A sufficiently large Number of cycles of the appliedstress.

Additional factors to cause fatigue failure. Stress concentrationCorrosionTemperatureOverloadResidual stressesCombined stresses 6/6/2013 25Dr SANJAY CHIKALTHANKAR


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Stress cycles :-

Advantageous to define briefly the general types offluctuating stresses which can cause fatigue.Maximum and minimum stresses are equalMinimum stress is the lowest algebraic stress in the cycle.Tensile stress is positive , compressive stress is negative.



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Stress cycles :-

Maximum and minimum stresses are not equalRepeated stress cycle contains maximum and minimumstresses of opposite sign.



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Stress cycles :-

Complicated stress cycleEncounter in a part such as an aircraft wing which is subjected toperiodic unpredictable overloads.



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The S-N curveThe method of presenting engineering fatiguedata is by means of the S-N Curve.Life of specimen is given by N Number ofcycles of failureMaximum applied stress



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The S-N curveProcedure for determining S-N Curve.Testing of 1st Specimen at high stress where failure isexpected to occur in fairly short Number of cycles.Stress is decreased for each succeeding specimenuntil 1 or 2 specimens do not fail in the specifiedNumber of cycles. 10 cycles.The highest stress at which a runout (non-failure) isobtained is taken as the fatigue limit.Materials without the fatigue limit the test is usuallyterminated at a low stress where the life is about 10or 5 x 10 cycles.The S-N Curve is usually determined with about 8 to12 specimens.



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Statistical nature of fatigue


Stress life method plots of alternating stress S vs cycles to


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Stress life method -- plots of alternating stress, S, vs. cycles to

failure, N.

ignores true stress and strain (assumes elastic strains) which

may be significant since initiation of fatigue cracks is plastic

deformation stress life methods should not be used to

estimate lives below 1000 cycles

Endurance Limit: stress for which material has "infinite" life (>1x106 cycles)

existence due to interstitial elements (pin dislocations and

prevent slip) can disappear due to periodic overloads,

corrosive environments or high temperaturesMost nonferrous alloys do not exhibit endurance limit

(some use value at 5x108 cycles or some other number much

higher than the design life)



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Endurance

Endurance strengthis the stress level that a material can

survive for a given number of load cycles.Endurance limit isthe stress level that a material cansurvive for an infinite number of load cycles.

Estimate for Wrought Steel:

Endurance Strength = 0.50(Su)

Most nonferrous metals (aluminum) do not have anendurance limit



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Fatigue Testing

Bending tests

Sontag = Constant stress cantilever beamsGood for flat stock (sheets)

Get shear stress in addition to bending stress.

Specimen

Top View


Fatigue Testing


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Number of Cycles to Failure, N

Stress,

s(ksi)



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Representative Endurance Strengths

Estimated endurance strength of steel is about 0.50 * Su


Factors Effecting for Metal Fatigue


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A number of variables can have a significant impact onfatigue, such as:

Size. Larger components are more likely to have fatiguecracks initiate, due to larger volumes of material subject tohigh stresses, and due to a greater chance of residual stresses(inherent processing difficulty). Effects mainly seen at very

long lives.Type of loading. Endurance limits vary by loading condition(axial, bending, torsion)

Surface finish. Scratches, pits and machining marks add

stress concentrations. Fine grained materials (high strengthsteel) more affected. Large effect, correction factors usuallypresented graphically

Factors Effecting for Metal Fatigue



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Surface treatments. Fatigue cracks initiate at free surface,treatments can be significant

Plating, thermal or mechanical means to induce residual stress

Compressive residual stresses are beneficial, tension is detrimental

Residual stresses not permanent, can be relaxed (temp., overload)

Temperature. Endurance limits increase at low temperature (butfracture toughness decreases significantly)Endurance limits disappear at high temperature Creep is important above

0.5Tm (plastic, stress-life not valid)

Environment. Corrosion has complex interactive effect with fatigue(attacks surface and creates brittle oxide film, which cracks and pits to

cause stress concentrations)



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Surface Finish (Qualitative and quantitative descriptionsof surface roughness)

The condition of the surface is more important for high strength materialsResidual surface stresses can be important (e.g. grinding = residual tension)

Condition of surface at shorter lives dominated by crack propagation

(surface condition less of an effect)Localized surface irregularities (e.g. stamping) can be high stress concentration

J.A. Bannantine, J.J. Comer and J.L. Handrock. Fundamentals of Metal Fatigue Analysis. Prentice-Hall, 1990.



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Surface Treatment -- Review of Residual Stresses




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Surface Treatment -- Plating




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Surface Treatment -- Plating




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General trends for chrome and nickel plating:




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Surface Treatment -- Thermal

Various heat treatment process (e.g. nitriding,carburizing) can produce higher strength materials at thesurface which significantly improves fatigue life




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Surface Treatment -- Thermal

Certain processing operations can have reverse effect. Forexample, hot rolling and forging can cause decarburization(loss of surface carbon atoms), which is very detrimentalto fatigue life.




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Surface Treatment -- Mechanical

Two most important methods: Cold Rolling and Shot Peening

Cold Rolling

Steel rollers pressed to surfaceof component as it is rotated ina lathe

Used on large parts Can produce deep residualstress layer




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Surface Treatment -- Mechanical

Two most important methods: Cold Rolling and Shot Peening

Shot Peening

Surface of component blasted with high velocity steel or glass beads Core of material in residual tension, surface in residual compression Easily used on odd shaped parts, but leaves surface dimpling




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Shot peening can be used to undodeleterious effects of plating,

decarburization, corrosion and grinding




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Environmental Effects




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Fatigue Design Guideline

1. Consider actual stresses, including stress

concentrations, rather than to nominal average stresses.2. Visualize load transfer from one part or section to

another and the distortions that occur during loading to

locate points of high stress3. Avoid adding or attaching secondary brackets, fittings,

handles, steps, bosses, grooves, and openings at locationsof high stress4. Use gradual changes in section and symmetry of design to

reduce secondary flexure

5. Consider location and types of joints (frequent cause of fatigue problem6.

Use double shear joints when possible

7. Do noy use rivets for carrying repeated tensile loads (bolts superior) 8.

Avoid open and loosely filled holes


F ti D i G id li


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9. Consider fabrication methods, specify strict

requirements when needed

10. Choose proper surface finishes, but not overly severe

(rivet holes, welds, openings etc. may be larger drivers)

11. Provide suitable protection against corrosion

12. Avoid metallic plating with widely different properties

than underlying material

13. Consider prestressing when feasible, to include shot

peening and cold working14. Consider maintenance, to include inspections, and

protection against corrosion, wear, abuse, overheating, andrepeated overloading15. Avoid use of structures at critical or fundamental

frequency of individual parts or of the structure as a whole

(induces many cycles of relatively high stress)

16. Consider temperature effects

Fatigue Design Guideline



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A fatigue failure, therefore, is characterized bytwo distinct regions. The first of these is due toprogressive development of the crack, while the

second is due to the sudden fracture. The zone ofsudden fracture is very similar in appearance tothe fracture of a brittle material, such as cast iron,that has failed in tension. The crack propagation

zone could be distinguished from a polishedappearance. A careful examination (by anexperienced person) of the failed cross sectioncould also reveal the site of crack origin

Basic features of failure appearance:-



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Dr. Sanjay ChikalthankarDept. of Mech. Engineering, GECA

different types of load and fatigue failure(2)

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