Mechanical Failures

8/13/2019 Mechanical Failures

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Mechanical Behavior,

Properties and Testing ofMaterials

Umair Bin Asim


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Overview

Mechanical application of material is to keep

the things that are supposed to be stationary,

stand their ground; Static loading, and thethings that are supposed to be moving to

move only in the intended fashion, i.e. not

more not less; Dynamic loading


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Overview

What does standing there ground implies

Bearing the intended (sometime even more)

static loads Keep on bearing loads when its value is

constantly changing; Fatigue loading

Enduring extreme conditions, loads, elevated

temperature and extended time periods;Creep

Bearing the load even when it was suddenly

applied; Impact loading


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Overview

Even when the material is subjected tobattering in the form of key holes, holes forfasteners, sudden changes in cross sections;Stress concentration

And also to face some adverse effects likecorrosion


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Static Loading

Tension Test

Strength

Ductility

Toughness

Elastic Modulus

Test Specimen

Dog-bone shape

ASTM Standard E8

Original Gauge length lo Cross-sectional area Ao


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Tension

Stress-strain curves

Linear elastic: elongation in the specimen that

is proportional to the applied load. Engineering stress: the ratio of the applied

load P, to the original cross-sectional area, Ao,

of the specimen.

Engineering stress equation: = P/Ao

Engineering strain equation: e = (l-lo)/lo


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Tension

Yield Stress: the stress at which

permanent (plastic) deformation occurs.

Permanent (plastic) deformation: stressand strain are no longer proportional.

Ultimate tensile strength (UTS): the

maximum engineering stress.


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Tension

If the specimen is

loaded beyond its

UTS it begins to

neck.

Fracture stress: the

engineering stress at

fracture.


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Tension

Modulus of elasticity: ratio of stress to

strain in the elastic region.

Modulus of elasticity equation: E = /e This linear relationship is known as

Hookes Law.

Poisons Ratio: the ratio of the lateral strainto the longitudinal strain.


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Ductility

Ductility: extent of plastic deformation that

the material undergoes before fracture.

Two measures of ductility: Total elongation: (lf-lo)/lox 100%

Reduction of Area: (Ao-Af)/Aox 100%


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True-Stress and True-

Strain

True-stress: ratio of the load, P, to the

instantaneous cross-sectional area, A, of

the specimen. True-strain: the sum of all the

instantaneous engineering strains.

True-stress equation: = P/A True-strain equation: e = ln(l/lo)


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Construction of Stress-

Strain Curves


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Temperature Effects

As temperatureincreases: Ductility and toughness

increase.

Yield stress and themodulus of elasticitydecrease.


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Other Stress-Strain

Characteristics

Compression, Torsion, and Bending

Hardness, Toughness, and Strength


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Compression Test

Simplistically, compression

testing is the opposite of

tensile testing

Load tends to squeeze orcompact the specimen

Metals and many plastics, are

more efficient at resisting

tensile loads. Therefore, theyare more commonly tested

using tensile loading


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Disk Test

The cylindrical specimenssurface begins to bulge, knownas barreling.

Compression test developed for

brittle materials such asceramics and glass.

Tensile stress from this test canbe calculated with the followingequation: = 2P/dt P is load

at fracture, d is diameter of disk,t is thickness.


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Torsion Test

In addition to tension and compression, a work-piecemay be subjected to shear strains.

Punching holes in sheet metal. Metal cutting.

Torsion test used for determination of properties inshear.Usually performed on a thin tubular specimen.

Shear stress can be calculated with formula: T/2r2t T is torque, r is average radius of tube, t is thickness of tube.

Shear strain is calculated with formula: r/l r is radius of tube, is angle of twist in radians, and l is length of

tube.


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Torsion Test

The ratio of the shear stress to the shear

strain in the elastic range is known as the

shear modulusor modulus of rigidity. The angle of twist, , to fracture in the

torsion of solid round bars and elevated

temp can help estimate forge-ability of

metals.


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Bending

Preparing specimens from brittle materials,

such as ceramics and carbides, is difficult

because of problems in shaping andmachining them to certain dimensions.

The most common test for brittle materials

is the bendor flexure test.


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Bend / Flexure Test

Rectangular specimen

supported at its ends.

Load is applied vertically at 1 or

2 pts. The stress at fracture in

bending is known as the

modulus of rupture, flexural

strength, or transverserupture strength.


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Hardness

Commonly used property which gives indication

of resistance to scratch and wear of a

material/specimen.

Widely used

Relatively inexpensive and simple

Non-destructive

Other mechanical properties can be estimated Hardness is not a fundamental property because

indentation depends on shape of indenter and

load applied.


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Brinell Test

J. A. Brinell 1900

Involves pressing a steelor carbide ball of 10mmagainst a surface withvarious loads. 500, 1500, or 3000 kg

Measures diameter ofindentation.

Harder surfaces havesmall indentation whilesofter surfaces havelarger indentation.


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Rockwell Test

S. P. Rockwell 1922

Test measures depth

rather than diameter of

indentation. Diamond indenter

presses against surface

with minor load and then

major load. The difference in depths ofpenetration is a measure of

the hardness of material.


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Vickers Test

Developed in 1922.

Comparable to Brinell

Test except using a

pyramid shaped

diamond to make

indentation.

Lighter loads thanBrinell Test

From 1 to 120 kg


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Knoop Test

Developed in 1939.

Comparable to Brinell and Vickers test.

Uses an elongated pyramid shaped diamond to make

indentations. Uses very light loads. From 25 g to 5 kg.

Known as a micro-hardnesstest because of the lightsloads.

Suitable for very small or very thin specimens.

Test also used for measuring the hardness of individualgrains and components in a metal alloy.


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Fatigue

FatigueComponents in manufacturing

equipment are subjected to fluctuating

cyclic (periodic) loads and static loads.

Cyclic Stresson wings, landing gears

Thermal Stress -- exhaust nozzle

Both stresses may cause part failure at stresslevels below normal static stress loading


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Fatigue

Fatigue Failure -- Failure

associated with every

stress cycle, propagated

through the material until

critical crack is reached

and material fractures.

Fatigue Testing --

Various stresses, tension

then bending to a

maximum load limit (total

failure.)


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Fatigue

S-N Curves

Stress Amplitude (S) --

Maximum stress

specimen is subjected Number of Cycles (N)

Level of stress a

material tolerates

decreases with an

increase in cycles.


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Fatigue

Endurance (Fatigue Limit) -- Maximumstress material may be subjected withoutfatigue failure.

Aluminum Alloys and similar materials exhibitan indefinite endurance limit.

Fatigue strength is specified at a certainnumber of cycles (10^7.)

Carbon Steels have a proportional endurancelimit and tensile strength, usually 0.4 to 0.5.


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Fatigue

Improving Fatigue Strength -- Fatigue life is

influenced greatly by the method of preparation

to the surfaces of the part or specimen.

Fatigue Strength of manufactured products may beimproved overall by

Inducing compressive residual stresses on

surfaces

Shot Peening or Roller Burnishing

Case hardening (surface hardening) by various

means


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Fatigue

Fatigue Strength of manufactured products

may be improved overall by

Providing a fine surface finish and thereby reducing

the effects of notches and other surfaceimperfections.

Selecting appropriate materials and ensuring that

they are free from significant amounts of inclusions,

voids, and impurities.


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Creep

Permanent elongation of a component under a

static load maintained for a period of time.

Grain-Boundary Sliding -- Mechanism of creep

at an elevated temperature in metals.

In high-temperature applications, gas-turbine blades,

jet engines, and rocket motors.


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Creep

Creep Testing --

Subjecting a specimen to

a constant tensile load

(engineering stress) at a

certain temperature,

measuring the length

changes at various time

increments.

Primary, secondary, andtertiary stages


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Creep

Rupture (Creep Rupture) -- Failure bynecking and fractures

Creep rate increases with specimen

temperature and the applied load. Secondary Linear ranges and slopes aid to

determine reliable design.

A higher melting point generally is related toan increase in creep resistance. Stainless Steels, Super-alloys and Refractory

metals and alloys


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Impact

Testing consists of

placing a notched

specimen in an impact

tester and breaking it with

a swinging pendulum.

Impact or Dynamic

Loading

CharpyTest -- Specimen

supported at both ends.

Izod Test -- Specimen

supported at one end.


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Impact

Impact Toughness -- The energy

dissipated in breaking the specimen may

be obtained from the amount of swing in

the pendulum.

Useful in determining the ductile-brittle

transition temperature of materials.

High Impact ResistanceHigh StrengthHighDuctilityHigh Toughness


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Impact

Notch Sensitivity --

Sensitivities to surface

defects, lowers impact

toughness.


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Mechanical & Aerospace Engr., SJSU

Discontinuity in

Cross Section

Stepped shafts

Discontinuity

Discontinuity


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Bracket with screw holes

High

stress

Discontinuity in

Cross Section


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Infrared analysis reveals the points of

greatest stress on a Renault parts surface

caused by weld points and helps estimate

fatigue limits for those critical areas

(bottom).

Discontinuity in

Cross Section


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Bolt failure at the junction of

the thread and solid section.

Location of higher stresses.

Discontinuity

Discontinuity in

Cross Section


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Maximum stress at the discontinuity

Nominal stress, max stress

with no discontinuity

Ktis used for normal

stresses and Ktsfor

shear stresses.

Discontinuity in

Cross Section


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Stress concentration factor is found using experimental methods.

Photoelasticitya plane polarized light is passed thru aphotelastic material (all transparent plastics) resulting in a

colorful fringe pattern indicating the intensity of the stress.

Discontinuity in

Cross Section


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Brittle Coatinga speciallyprepared lacquers are usually

applied by spraying on the actualpart. After air drying, the part is

subjected to stress. A pattern of

small cracks appear on the surface.

Data could be used to locate strain

gages for precise measurement of

the stress. The method is sensitiveto temperature and humidity.

Discontinuity in

Cross Section


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The CAD model is subdivided into many small pieces of simple

shapes called elements.

FEA program writes the equations governing the behavior of each

element taking into consideration its connectivity to other elements.

These equations relate the unknowns, for example displacements instress analysis, to known material properties, restraints and loads.

The program assembles the

equations into a large set of

simultaneous algebraic

equations (thousands or evenmillions).

These equations are then

solved by the program to

obtain the stress distribution

for the entire model.

Discontinuity in

Cross Section


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Stress Concentration

FactorRound shafts in axial Tension


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Round section

with a fillet

Bending

Torsion

Discontinuity in

Cross Section


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Ken Youssefi Mechanical & Aerospace Engr., SJSU

Flat plate in Tension

with a hole

Flat plate in Bending

with a hole

Discontinuity in

Cross Section


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Failure and Fracture of

Material

Failure -- One of the most important

aspects of material behavior. It directly

influences the selection of a material for a

particular application, the methods of

manufacturing, and the service life of the

component.


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Material

In selecting and processing

materials

Fracture -- Either internal or

external. Sub-classified intoDuctile or Brittle.

Ductile materials - extensive

plastic deformation and energy

absorption (toughness) beforefracture

Brittle materials - little plastic

deformation and low energy

absorption before fracture


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Materials

Ductile Fracture -- Plastic deformation

proceeds failure.

Highly ductile materials neck down to a point

before failing.

Most metals and alloys will neck down to a

finite area and then fail.

Generally ductile fractures take place alongplanes which shear stress is a maximum.


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Failure and Fracture in

Materials

Ductile Fracture -- Plastic deformationproceeds failure.

Close examination of ductile fracture surface

shows a fibrous pattern with dimples. Failure is initiated with formation of tiny voids which

grow and coalesce, developing micro-cracksleading to fracture.

In tension-test, fracture begins at the center ofthe necked region as a result of the growthand coalescences of cavities.


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Material

Cup-and-Cone Fracture -- Due to appearance,the fracture surface of a tension-test specimen.


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Failure and Fracture

in Material

Crack propagation is fast

Propagates nearly

perpendicular to direction ofapplied stress

Often propagates by

cleavage - breaking of

atomic bonds along specificcrystallographic planes

No appreciable plastic

deformation

Brittle fracture in a mild steel


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Material

Defects -- Scratches, flaws, and pre-existing

external or internal cracks.

The high tensile stresses subject the tip of the crack to

propagate the crack rapidly due to the materialsinability to dissipate energy.

Catastrophic failure occurs under tensile stresses

when compared to their strength in compression.


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Material

Fatigue Failure -- Minute external or internal

cracks develop at pre-existing flaws or defects in

the material. The cracks propagate over a

period of time and leads to total and sudden

failure of the part.


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Failure and Fatigue in

Material

Beach Marks -- Term given the fracture surface

in fatigue.

Striations -- On the fracture surface, several

appearing on each beach mark.


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Corrosion Resistance

Corrosion

Typically used to

describe metal or

ceramic deterioration Similar phenomena

occur in plastics

Often referred to as

degradation


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Types of corrosion

Pitting

Intergranular

Crevice

Galvanic cell

Stress-corrosion cracking

Selective Leaching

Oxidation

Passivation


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Pitting

Can occur over the

entire surface or be

localized

Intergranular

Occurs along grain

boundaries


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Crevice Occurs at the interface of

bolted or riveted joints

Galvanic cell Occurs between dissimilar

metals when an electrolyte

is present Not as common in pure

metals or single-phasealloys


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Stress-corrosioncracking Cold worked metals

are most susceptible

Selective leaching Occurs when

metalworking fluidattacks specificelements in tool anddie materials


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Oxidation

A chemical reaction which leaves a small layerof oxidized material on the surface

Resists further corrosion Aluminum & Titanium

Passivation

The development of a protective film bychemical reaction Stainless Steel


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http://www.hardwaresquare.com/category/hardware/nails/

http://www.boeing.com/companyoffices/gallery/images/space/delta_i

v/d4_1st_heavy_24.html

http://en.wikipedia.org/wiki/Density

http://www.nyu.edu/pages/mathmol/textbook/density.html TEXTBOOK

http://www.american-carbide.com/EndMills/DHEM.aspx

http://www.roymech.co.uk/Useful_Tables/Rivets.html

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/conins.html http://en.wikipedia.org/wiki/Oxidation
http://www.hardwaresquare.com/category/hardware/nails/http://www.boeing.com/companyoffices/gallery/images/space/delta_iv/d4_1st_heavy_24.htmlhttp://www.boeing.com/companyoffices/gallery/images/space/delta_iv/d4_1st_heavy_24.htmlhttp://en.wikipedia.org/wiki/Densityhttp://www.nyu.edu/pages/mathmol/textbook/density.htmlhttp://www.american-carbide.com/EndMills/DHEM.aspxhttp://www.roymech.co.uk/Useful_Tables/Rivets.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/electric/conins.htmlhttp://en.wikipedia.org/wiki/Oxidationhttp://en.wikipedia.org/wiki/Oxidationhttp://hyperphysics.phy-astr.gsu.edu/hbase/electric/conins.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/electric/conins.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/electric/conins.htmlhttp://www.roymech.co.uk/Useful_Tables/Rivets.htmlhttp://www.american-carbide.com/EndMills/DHEM.aspxhttp://www.american-carbide.com/EndMills/DHEM.aspxhttp://www.american-carbide.com/EndMills/DHEM.aspxhttp://www.nyu.edu/pages/mathmol/textbook/density.htmlhttp://en.wikipedia.org/wiki/Densityhttp://www.boeing.com/companyoffices/gallery/images/space/delta_iv/d4_1st_heavy_24.htmlhttp://www.boeing.com/companyoffices/gallery/images/space/delta_iv/d4_1st_heavy_24.htmlhttp://www.hardwaresquare.com/category/hardware/nails/


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http://www.materialsengineer.com/G-Pitting-Corrosion.htm

http://www.corrosion-doctors.org/Forms/intergranular.htm

http://www.textronfasteningsystems.com/appsol/galv_corr.html
http://www.materialsengineer.com/G-Pitting-Corrosion.htmhttp://www.corrosion-doctors.org/Forms/intergranular.htmhttp://www.textronfasteningsystems.com/appsol/galv_corr.htmlhttp://www.textronfasteningsystems.com/appsol/galv_corr.htmlhttp://www.corrosion-doctors.org/Forms/intergranular.htmhttp://www.corrosion-doctors.org/Forms/intergranular.htmhttp://www.corrosion-doctors.org/Forms/intergranular.htmhttp://www.materialsengineer.com/G-Pitting-Corrosion.htmhttp://www.materialsengineer.com/G-Pitting-Corrosion.htmhttp://www.materialsengineer.com/G-Pitting-Corrosion.htmhttp://www.materialsengineer.com/G-Pitting-Corrosion.htmhttp://www.materialsengineer.com/G-Pitting-Corrosion.htm

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