Mechanical Design-material Properties

8/3/2019 Mechanical Design-material Properties

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MET 210W

Chapter 2 – Materials in

Mechanical Design



Properties of Materials:

1. Chemical – relate to structure of material,atomic bonds, etc.

2. Physical – response of a material due tointeraction with various forms of energy(i.e. magnetic, thermal, etc).

3. Mechanical – response of a material dueto an applied force. Main focus forMachine Design.



Important Mechanical Properties:



Tension Test

• Most important and common material test for generating mechanical properties.

• Can be load vs displacement or load versus strain. Always convert load to stress.

Example: stress-strain curves:



Stress-Strain Curve for Steel

Sy

Yield Point, Sy

Tensile Strength, SuElastic Limit

Proportional Limit

E

Modulus of Elasticity

Strain,

S t r e s s ,



Stress Strain Curve for Aluminum

Sy

Yield Strength, Sy

Tensile Strength, Su

Elastic Limit Proportional Limit

Parallel Lines

Strain,

S t r e s s ,

Offset strain, usually 0.2%



Ductility

• The degree to which a material will deformbefore ultimate fracture.

– Ductile materials indicate impending failure.

(%E ≥ 5%) – Brittle materials don’t (%E < 5%)

– For machine members subject to repeatedloads or shock or impact, use %E ≥ 12%

%100xL

LLElongation%

o

of



Ductile materials - extensive plastic deformation andenergy absorption (toughness) before fracture

Brittle materials - little plastic deformation and low energy

absorption before failure



Shear Strength Estimates

yus

y

ys

S75.S

2

SS

Yield strength in

shear

Ultimate strengthin shear



Poisson’s Ratio

RANGES

0.25 – 0.27 for Cast Iron

0.27 – 0.30 for Steel

0.30 – 0.33 for Aluminum and Titanium AL LONGITUDIN

TRANSVERSE

o

ofAX

o

ofLAT

LLL

h

hh



Modulus of Rigidity in Shear

• Measure of resistance to sheardeformation.

• Valid within the ELASTIC range of thematerial

)(,

12EGG



Modulus of Elasticity aka Young’s Modulus (psi) – slope of linear region:

12

12 E

σ 2 - σ 1 = difference in tensile stress between points 1 and 2

ε 2 - ε 1 = difference in tensile strain between points 1 and 2

offset useor A

Pystrength yield Sy

y

%2.

Yield Strength (psi) = onset of permanent deformation:

Percent Elongation:

%100

o

o f

L

L L

Lo = original gauge lengthLf = final gauge length

Percent Reduction of Area :

%100

o

f o

A

A A

Ao = original cross-sectional areaAf = final cross-sectional area

A

PuST U Suu ..

Tensile Strength (psi) = max stress or peak stress sustainable:

Poison's Ratio (unit less) = ratio of transverse to longitudinal strain:

allongitudin

transverse

Summary: Key Material Properties:

Modulus of Resilience (psi) = area under stressstrain curve up to elastic limit or yield strength

E U el

elel R22

12

Modulus of Toughness (psi) = total area under stressstrain curve up from 0 to fracture. Related to impactStrength:

curveunder AreaU T Misc: fracture stress, proportional limit,elastic limit, elastic strain, impact

strength, fracture toughness, etc……

•>5% = ductile•<5% = brittle

ModulusShear G

12

12



Ultimate strength in compression:

2

Sy

shear instrength yield Sys y

Yield Strength in shear:

shear instrengthultimateSusu

Ultimate Strength in shear:

Other important material properties specific to Polymers:

ncompressioinstrengthUltimateSucuc

StrengthFlexuralF

ModulusFlexural E F

Also secant strengths, secant modulus,compression set, stress creep, relaxation, etc..

)1(2 G E

Note:

Summary: Key Material Properties:



Example: find yield

strength, ultimate strength

and modulus of elasticity:



Example: find yield strength and ultimate

for material that does not exhibit knee

behavior



Example – DATAgeneratedon MTSmachine:

EX



Stress-Strain Tensile Curve for Specimen 5

0.0

5000.0

10000.0

15000.0

20000.0

25000.0

30000.0

35000.0

40000.0

45000.0

50000.0

0.0000 0.0200 0.0400 0.0600 0.0800 0.1000 0.1200

Strain (in/in)

S t r e s s

( p s i )

Speed of Loading = 0.1 in/min

Temperature = 23 C

RJM 9/5/05

Su = ultimateStrength =47,820 psi

Sy = YieldStrength =44,200 psi

.002 = .2%offset

E = Young’s Modulus = (34,640 – 10,597)/(.0036 - .0011) = 9.6 E6

% Elongation = 11.5%

EX:

EX:




0.0

5000.0

10000.0

15000.0

20000.0

25000.0

30000.0

35000.0

40000.0

45000.0

50000.0

0.0000 0.0200 0.0400 0.0600 0.0800 0.1000 0.1200

Strain (in/in)

S t r e s s

( p s i )


Temperature = 23 C

RJM 9/5/05

Modulus of Resilience =area under stress-straincurve up to elastic limit

psi E

elelel 8.96

)000,000,10(2

)000,44(

22

122

EX:

Elastic strain approx: .005 in/in




0.0

5000.0

10000.0

15000.0

20000.0

25000.0

30000.0

35000.0

40000.0

45000.0

50000.0

0.0000 0.0200 0.0400 0.0600 0.0800 0.1000 0.1200

Strain (in/in)

S t r e s s

( p s i )


Temperature = 23 C

RJM 9/5/05

Approx = 96.8 psi + (46,000)(.115 - .0043) = 5,190 psi

Modulus of Toughness =UT = area under stress-strain curve from 0 tofracture strain.



Hardness

• Resistance of a material to be indented byan indenter.

– BRINELL 3000 kg load

10 mm ball of hole = BHN

– ROCKWELL 100 kg load (B Scale)

1/16” Ball (B Scale)

B-Scale for soft materials

C-Scale for harder metals (Heat treated)(Use 150 kg load with diamond cone indenter)

Hardness calculated directly by machine (depth of indentation)



Hardness Comparison

Hardness

values in theranges HRB

>100 and HRC< 20 are not

recommended



Ultimate Tensile Strength

• Highest level of stress a material candevelop.

• FOR CARBON STEEL ONLY:

Su ≈ 500 * BHN(in PSI, BHN = Brinell Hardness Number)



Toughness

• Toughness is the ability of a material toabsorb energy without failure.

• Parts subjected to impact or shock loadsneed to be tough.

• Testing: Charpy and Izod tests

• Impact energy determined from the testingis used to compare materials



Fatigue

• Failure mode of parts experiencingthousands or millions of repeated loads.

• Endurance Strength - a materialsresistance to fatigue. Determined bytesting.



Creep

• Progressive elongation of a part over time.

• Metals – usually requires a large load

– usually requires high temperature (> .3Tm)

• Plastic – creep occurs at low temperatures

Polymers: Creep vs Stress Relaxation vs. Compression Set – related butmeasured differently!!



Mechanical Property Summary

Property InterpretationCommon or

Related MeasureStrength Ability to resist breaking Yield stress

Stiffness Ability to resist deformation Modulus of elasticity

Ductility Permanent deformationbefore breaking %Elongation

ToughnessAbility to withstand impact orresist breaking

Energy or worknecessary to fracturematerial

Hardness Ability to resistabrasion/scratching

Scores on hardnesstests

CreepGradual, continuingdeformation under anapplied constant stress

Creep strength



Material Selection

• “The materials selected for a design often willdetermine the fabrication processes that can beused to manufacture the product, itsperformance characteristics, and its recyclability

and environmental impact. As a result,engineers should acquire a robustunderstanding of material characteristics and thecriteria that one should use in making material

selections.”

- Voland, Engineering by Design, Addison-Wesley, 1999, pg. 400



Material Categories

• Metals – iron, steel, aluminum, copper, magnesium,

nickel, titanium, zinc

• Polymers – thermoplastics & thermosets

• Ceramics

• Composites – Carbon fiber, Kevlar & fiberglass,

wood and reinforced concrete



Steel

• Widely used for machine elements – High strength

– High stiffness

– Durable – Relative ease of fabrication

• Alloy of Iron, Carbon, Manganese & 1 or

more other significant elements.(Sulfur, Phosphorus, Silicon, Nickel, Chromium,

Molydbenum and Vanadium)



Carbon

• Carbon has huge effect on strength,hardness and ductility of steel.

Carbon Content

Strength & Hardness

Ductility ↓



All these curves are

steels.

What do they have in

common?

What is different?



Steel Designation Systems

• AISI – American Iron & Steel Institute

• SAE – Society of Automobile Engineers

• ASTM – American Society for TestingMaterials



General Designation

• General Form AISI:

AISI XXXX

Carbon Content in

Hundredths of a percent

Specific alloy in thegroup

Alloy group; indicates

major alloying elements

AISI 1020 AISI 4340



Examples:

2350

2550

4140

1060



1. Low Carbon (less than 0.3% carbon)

• Low strength, good formability• If wear is a potential problem, can be carburized

(diffusion hardening)• Most stampings made from these steels• AISI 1008, 1010, 1015, 1018, 1020, 1022, 1025

2. Med Carbon (0.3% to 0.6%)• Have moderate to high strength with fairly good ductility• Can be used in most machine elements• AISI 1030, 1040, 1050, 1060*

3. High Carbon (0.6% to 0.95%)• Have high strength, lower elongation• Can be quench hardened• Used in applications where surface subject to abrasion –

tools, knives, chisels, ag implements.• AISI 1080, 1095

Plain Carbon Steel



Steel Conditions

• Steel properties vary depending on themanufacturing process

• Steel is often rolled or drawn through a die

– Hot-rolled – rolled at elevated temperature

– Cold-rolled – improved strength & surfacefinish

– Cold-drawn – highest strength with goodsurface finish



Heat Treating

• Process for modifying the properties ofsteel by heating

• Processes used most for machine steels:

– Annealing

– Normalizing

– Through-hardening (quench & temper)

– Case hardening



All these curves are

steels.What do they have in

common?

What is different?



RT = Room Temperature

LC = Lower Critical Temperature

UC = Upper Critical Temperature

Annealing

• Full-Annealing: createsuniform composition of the

material. – Soft, low-strength material

– No significant internal stress



Stress Relief Annealing

• Stress Relief Annealing

– Done after welding,machining or cold forming torelieve residual stressesminimizing distortions






Normalizing

• Similar to annealing butat a higher temperature(about 1600°F)

• Higher strength

• Machinability andtoughness are improved

over as-rolled state. RT = Room Temperature


UC = Upper Critical TemperatureAustenite: A nonmagnetic solid solutionof ferric carbide or carbon in iron, used in

making corrosion-resistant steel



Through-hardening• Heated quickly forming

austenite then quicklycooling in a quenchingmedium.

• Martensite – hard form ofsteel is formed

• Quenching mediums:water, brine and special

mineral oils.• Quenched steel that isn’t

tempered is brittle






Tempering

• Reheat steel to 400°F – 1300°Fimmediately after quenching and allowingit to cool slowly.

• As tempering temperature increases,ultimate and yield strengths decrease andductility increases

• Machine parts should be tempered at 700°F minimum after quenching. Quenchingleaves the material brittle.



AISI

1040 WQTHigher Temperingtemps. decreases

strength butincreases ductility

WQT = waterquenched &tempered

Fig. A4-1, Appendix 4, pg. A-8



Case Hardening

• Surface of a part is hardened but coreremains soft & ductile – think m&m’s.

• Usually .010 to .040 thick

• Methods:

– Flame hardening and induction hardening

– Carburizing, nitriding, cyaniding, and carbo-

nitriding

S i l S l



Stainless Steel

• Corrosion resistant steel – 12 to 18%

chromium content• Types

– Austenitic – moderate strength, nonmagnetic,

tempering: 1/4 hard, 1/2 hard, 3/4 hard and fullhard. (200 and 300 series)

– Ferritic – magnetic, good for use at hightemps. Can’t be heat-treated. (400 series)

– Martensitic – magnetic, can be heat-treated.Good toughness and stronger than 200 and300 series. Wide range of uses: scissors, pumparts, airplanes, marine hardware, medical

equipment.

S l S l



Structural Steels

High strength, low carbon alloy steel



Structural Plates and Bars



Gray Iron

• Brittle material, Su from 20 to 60 ksi

• Compressive stress 5X Su

• Excellent wear resistance

• Easy to machine

• Good vibration dampening ability

• Classes: 20, 25, 30, 40, 50, 60Minimum Su



Ductile Iron

• Higher strength than gray iron

• More ductile

• Grade designation:

Tensile

strengthin ksi

Yield strength in ksi

% elongation in

a 2” gage length

GRADE 80-55-06



Malleable Iron

• Heat treatable cast iron

• Moderate to high strength

• High modulus of elasticity

• Good machineability

• Good wear resistance

• Grade designation:GRADE 40010

Yield strength % elongation



Powdered Metals

• Metal powders are placed into a die andcompacted under high pressure.

• Sintering at high temperatures fuses the

powder into a uniform mass.

• Usually brittle – not good for impact

• Sintered bearings – porous and can besaturated with lubricant



• Lightweight material, good corrosionresistance, relative ease of forming &machining.

• Good appearance.• Generally tempered

– O = annealed

– H = strain-hardened – T = heat treated

• 6061-T6

Aluminum

Strain-hardening:controlled cold workingof the alloy – increaseshardness and strength,reduces ductility.



Titanium

• Good corrosion resistance

• High strength to weight ratio

• Modulus of Elasticity 16 x 106 psi

• Specific weight = .160 #/in3

• Strength 25 to 75 ksi

• High cost• Difficult to machine

Designation:

Ti-50A

Yield strength expected in ksi

Pl ti



Plastics

• Thermoplastic – can be repeatedly formed byheating or molding – properties not changed.CAN BE RECYLCED! – Nylon

– ABS

– Polycarbonate – Acrylic

– Commodity plastics: Polypropylene (P), Polyethylene (PE), PolyvinylChloride (PVC), Polystyrene (PS)

• Thermoset – undergoes a chemical changeduring forming. It can’t be reshaped. CAN NOT

BE RECYCLED! – Phenolic

– Polyester

– Epoxy



Ceramics

• Formed by applying high temperatures toinorganic, nonmetallic, and generallyinexpensive material, especially clay.

• Strong, nonconductive and weatherresistant.

• Brittle



Composites

• Two or more materials acting together toprovide material properties that can betailored to specific conditions.

• Often glass or carbon fibers bondedtogether with a matrix material – epoxy,polyester, others.



Material Selection

• A good material is one that works in thegiven application cheaply.

• If wt & size not important use cheap matl

• Size no problem, wt is use hollow matl

• Wt & size important

use $$$ material

Mechanical Design-material Properties

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