Transcript MET 210W - School of Engineering | Penn State Erie, The

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 to
interaction with various forms of energy
(i.e. magnetic, thermal, etc).
3. Mechanical – response of a material due
to an applied force. Main focus for
Machine 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
Yield Point, Sy
Sy
Elastic Limit
Tensile Strength, Su
Stress, 
Proportional Limit
Modulus of Elasticity

E

Strain, 
Stress Strain Curve for Aluminum
Proportional Limit
Elastic Limit
Sy
Tensile Strength, Su
Stress, 
Yield Strength, Sy
Parallel Lines
Offset strain, usually 0.2%
Strain, 
Ductility
• The degree to which a material will deform
before ultimate fracture.
– Ductile materials indicate impending failure.
(%E ≥ 5%)
– Brittle materials don’t (%E < 5%)
– For machine members subject to repeated
loads or shock or impact, use %E ≥ 12%
Lf  Lo
%Elongation 
x 100%
Lo
Ductile materials - extensive plastic deformation and
energy absorption (toughness) before fracture
Brittle materials - little plastic deformation and low energy
absorption before failure
Other properties determined from stress strain curve:
Shear Strength Estimates
S ys 
Sy
2
S us  .75 S y
Yield strength in
shear
Ultimate strength
in shear
Poisson’s Ratio
 TRANSVERSE
 
 LONGITUDIN AL
LAT
h f  ho

ho
 AX
L f  Lo

Lo
RANGES
0.25 – 0.27 for Cast Iron
0.27 – 0.30 for Steel
0.30 – 0.33 for Aluminum and Titanium
Modulus of Rigidity in Shear
• Measure of resistance to shear
deformation.

E
G , G

2(1  )
• Valid within the ELASTIC range of the
material
Summary: Key Material Properties:
Percent Elongation:
Yield Strength (psi) = onset of permanent deformation:

L f  Lo
Lo
Py
 y  Sy  yield strength 
or use .2% offset
A
Lo = original gauge length
Lf = final gauge length
Tensile Strength (psi) = max stress or peak stress sustainable:
•>5% = ductile
•<5% = brittle
Pu
 u  Su  U .T .S 
A
Modulus of Elasticity aka Young’s Modulus (psi) – slope of linear region:
  1 
E 2

 2  1 
σ2-σ1 = difference in tensile stress between points 1 and 2
ε2-ε1 = difference in tensile strain between points 1 and 2
  
G  2 1   Shear Modulus
 2  1 
Poison's Ratio (unit less) = ratio of transverse to longitudinal strain:

 transverse
 longitudinal
Misc: fracture stress, proportional limit,
elastic limit, elastic strain, impact
strength, fracture toughness, etc……
 100 %
Percent Reduction of Area :

Ao  A f
Ao
 100 %
Ao = original cross-sectional area
Af = final cross-sectional area
Modulus of Resilience (psi) = area under stress
strain curve up to elastic limit or yield strength
2
1
U R   el el  el
2
2E
Modulus of Toughness (psi) = total area under stress
strain curve up from 0 to fracture. Related to impact
Strength:
UT  Area under    curve
Summary: Key Material Properties:
Yield Strength in shear:
Sy
 y  Sys  yield strength in shear 
2
Ultimate Strength in shear:
 u  Sus  ultimate strength in shear
Ultimate strength in compression:
 uc  Suc  Ultimate strength in compression
Other important material properties specific to Polymers:
 F  Flexural Strength
EF  Flexural Modulus
Also secant strengths, secant modulus,
compression set, stress creep, relaxation, etc..
Note:
E  2G(1  )
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 –
DATA
generated
on MTS
machine:
EX:
Su = ultimate
Strength =
47,820 psi
Stress-Strain Tensile Curve for Specimen 5
50000.0
Speed of Loading = 0.1 in/min
Temperature = 23 C
RJM 9/5/05
45000.0
40000.0
35000.0
Sy = Yield
Strength =
44,200 psi
Stress (psi)
30000.0
E = Young’s Modulus = (34,640 –
10,597)/(.0036 - .0011) = 9.6 E6
25000.0
20000.0
15000.0
10000.0
5000.0
0.0
0.0000
% Elongation = 11.5%
.002 = .2%
offset
0.0200
0.0400
0.0600
Strain (in/in)
0.0800
0.1000
0.1200
EX:
Stress-Strain Tensile Curve for Specimen 5
50000.0
Speed of Loading = 0.1 in/min
Temperature = 23 C
RJM 9/5/05
45000.0
40000.0
35000.0
Stress (psi)
30000.0
25000.0
20000.0
15000.0
Modulus of Resilience =
area under stress-strain
curve up to elastic limit
 el2
1
(44,000) 2
  el el 

 96.8 psi
2
2 E 2(10,000,000)
10000.0
Elastic strain approx: .005 in/in
5000.0
0.0
0.0000
0.0200
0.0400
0.0600
Strain (in/in)
0.0800
0.1000
0.1200
Stress-Strain Tensile Curve for Specimen 5
50000.0
Speed of Loading = 0.1 in/min
Temperature = 23 C
RJM 9/5/05
45000.0
40000.0
35000.0
Modulus of Toughness =
UT = area under stressstrain curve from 0 to
fracture strain.
Stress (psi)
30000.0
25000.0
20000.0
15000.0
10000.0
Approx = 96.8 psi + (46,000)(.115 - .0043) = 5,190 psi
5000.0
0.0
0.0000
0.0200
0.0400
0.0600
Strain (in/in)
0.0800
0.1000
0.1200
Hardness
• Resistance of a material to be indented by
an 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 the
ranges HRB
>100 and HRC
< 20 are not
recommended
Ultimate Tensile Strength
• Highest level of stress a material can
develop.
• FOR CARBON STEEL ONLY:
Su ≈ 500 * BHN
(in PSI, BHN = Brinell Hardness Number)
Toughness
• Toughness is the ability of a material to
absorb energy without failure.
• Parts subjected to impact or shock loads
need to be tough.
• Testing: Charpy and Izod tests
• Impact energy determined from the testing
is used to compare materials
Fatigue
• Failure mode of parts experiencing
thousands or millions of repeated loads.
• Endurance Strength - a materials
resistance to fatigue. Determined by
testing.
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 but
measured differently!!
Mechanical Property Summary
Common or
Related Measure
Property
Interpretation
Strength
Ability to resist breaking
Yield stress
Stiffness
Ability to resist deformation
Modulus of elasticity
Ductility
Permanent deformation
before breaking
%Elongation
Toughness
Hardness
Creep
Energy or work
Ability to withstand impact or
necessary to fracture
resist breaking
material
Ability to resist
abrasion/scratching
Scores on hardness
tests
Gradual, continuing
deformation under an
applied constant stress
Creep strength
Material Selection
• “The materials selected for a design often will
determine the fabrication processes that can be
used to manufacture the product, its
performance characteristics, and its recyclability
and environmental impact. As a result,
engineers should acquire a robust
understanding of material characteristics and the
criteria 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 Testing
Materials
General Designation
• General Form AISI:
AISI XXXX
Carbon Content in
Hundredths of a percent
Specific alloy in the
group
Alloy group; indicates
major alloying elements
AISI 1020
AISI 4340
Examples:
2350
2550
4140
1060
Plain Carbon Steel
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
Steel Conditions
• Steel properties vary depending on the
manufacturing process
• Steel is often rolled or drawn through a die
– Hot-rolled – rolled at elevated temperature
– Cold-rolled – improved strength & surface
finish
– Cold-drawn – highest strength with good
surface finish
Heat Treating
• Process for modifying the properties of
steel 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?
Annealing
• Full-Annealing: creates
uniform composition of the
material.
– Soft, low-strength material
– No significant internal stress
RT = Room Temperature
LC = Lower Critical Temperature
UC = Upper Critical Temperature
Stress Relief Annealing
• Stress Relief Annealing
– Done after welding,
machining or cold forming to
relieve residual stresses
minimizing distortions
RT = Room Temperature
LC = Lower Critical Temperature
UC = Upper Critical Temperature
Normalizing
• Similar to annealing but
at a higher temperature
(about 1600°F)
• Higher strength
• Machinability and
toughness are improved
over as-rolled state.
RT = Room Temperature
LC = Lower Critical Temperature
Austenite: A nonmagnetic solid solution
of ferric carbide or carbon in iron, used in
making corrosion-resistant steel
UC = Upper Critical Temperature
Through-hardening
• Heated quickly forming
austenite then quickly
cooling in a quenching
medium.
• Martensite – hard form of
steel is formed
• Quenching mediums:
water, brine and special
mineral oils.
• Quenched steel that isn’t
tempered is brittle
RT = Room Temperature
LC = Lower Critical Temperature
UC = Upper Critical Temperature
Tempering
• Reheat steel to 400°F – 1300°F
immediately after quenching and allowing
it to cool slowly.
• As tempering temperature increases,
ultimate and yield strengths decrease and
ductility increases
• Machine parts should be tempered at 700
°F minimum after quenching. Quenching
leaves the material brittle.
AISI
1040 WQT
Higher Tempering
temps. decreases
strength but
increases ductility
WQT = water
quenched &
tempered
Fig. A4-1, Appendix 4, pg. A-8
Case Hardening
• Surface of a part is hardened but core
remains soft & ductile – think m&m’s.
• Usually .010 to .040 thick
• Methods:
– Flame hardening and induction hardening
– Carburizing, nitriding, cyaniding, and carbonitriding
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 full
hard. (200 and 300 series)
– Ferritic – magnetic, good for use at high
temps. Can’t be heat-treated. (400 series)
– Martensitic – magnetic, can be heat-treated.
Good toughness and stronger than 200 and
300 series. Wide range of uses: scissors, pump
arts, airplanes, marine hardware, medical
equipment.
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, 60
Minimum Su
Ductile Iron
• Higher strength than gray iron
• More ductile
• Grade designation:
GRADE 80-55-06
Tensile
strength
in ksi
% elongation in
a 2” gage length
Yield strength in ksi
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 and
compacted 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 be
saturated with lubricant
Aluminum
• Lightweight material, good corrosion
resistance, relative ease of forming &
machining.
• Good appearance.
• Generally tempered
– O = annealed
– H = strain-hardened
– T = heat treated
• 6061-T6
Strain-hardening:
controlled cold working
of the alloy – increases
hardness 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
Designation:
Difficult to machine
Ti-50A
Yield strength expected in ksi
Plastics
• Thermoplastic – can be repeatedly formed by
heating or molding – properties not changed.
CAN BE RECYLCED!
–
–
–
–
–
Nylon
ABS
Polycarbonate
Acrylic
Commodity plastics: Polypropylene (P), Polyethylene (PE), Polyvinyl
Chloride (PVC), Polystyrene (PS)
• Thermoset – undergoes a chemical change
during forming. It can’t be reshaped. CAN NOT
BE RECYCLED!
– Phenolic
– Polyester
– Epoxy
Ceramics
• Formed by applying high temperatures to
inorganic, nonmetallic, and generally
inexpensive material, especially clay.
• Strong, nonconductive and weather
resistant.
• Brittle
Composites
• Two or more materials acting together to
provide material properties that can be
tailored to specific conditions.
• Often glass or carbon fibers bonded
together with a matrix material – epoxy,
polyester, others.
Material Selection
• A good material is one that works in the
given application cheaply.
• If wt & size not important  use cheap matl
• Size no problem, wt is  use hollow matl
• Wt & size important  use $$$ material