Transcript Mechanical Behavior, Testing, and Manufacturing Properties

Mechanical Behavior, Testing,
and Manufacturing Properties
of Materials
Group 4: Brenton Elisberg,
Michael Snider, Michael
Anderson, and Jacob Hunner
Overview
• Metals can be processed into various
shapes by deforming them plastically
under the application of external forces.
The effects of these forces on material
behavior are described in this chapter,
including:
Overview
• Types of tests for determining the mechanical behavior of
materials.
• Elastic and plastic features of stress-strain curves and
their significance.
• Relationships between stress and strain and their
significance, as influenced by temperature and
deformation rate.
• Characteristics of hardness, fatigue, creep, impact, and
residual stresses, and their role in materials processing.
• Effects of inclusions and defects in the brittle and ductile
behavior of metals.
• Why and how materials fail when subjected to external
forces.
Section 2.1 – 2.2.6
• Tension Test
– Strength
– Ductility
– Toughness
– Elastic Modulus
– Strain-hardening
capability
• Test Specimen
– Usually solid and round
– Original Gauge length lo
– Cross-sectional area Ao
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: σ = P/Ao
• Engineering strain equation: e = (l-lo)/lo
Tension
• Yield Stress: the stress at which
permanent (plastic) deformation occurs.
• Permanent (plastic) deformation: stress
and strain are no longer proportional.
• Ultimate tensile strength (UTS): the
maximum engineering stress.
Tension
• If the specimen is
loaded beyond its
UTS it begins to
“neck.”
• Fracture stress: the
engineering stress at
fracture.
Tension
• Modulus of elasticity: ration of stress to
strain in the elastic region.
– Modulus of elasticity equation: E = σ/e
• This linear relationship is known as
Hooke’s Law.
• Poisons Ratio: the ratio of the lateral strain
to the longitudinal strain.
Ductility
• Ductility: extent of plastic deformation that
the material undergoes before fracture.
• Two measures of ductility:
– Total elongation: (lf-lo)/lo x 100%
– Reduction of Area: (Ao-Af)/Ao x 100%
True-Stress and TrueStrain
• 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: σ = P/A
– True-strain equation: e = ln(l/lo)
Construction of StressStrain Curves
• The stress-strain curve can be represented
by the equation: σ = Ken
– K = strength coefficient
– n = strain hardening exponent
• Specific energy: energy-per-unit volume of
the material deformed.
Construction of StressStrain Curves
Strain at Necking in a
Tension Test
• True-strain at necking
is equal numerically to
the strain-hardening
exponent, n, of the
material.
Temperature Effects
• As temperature
increases:
– Ductility and toughness
increase.
– Yield stress and the
modulus of elasticity
decrease.
• Temperature also affects
the strain-hardening
exponent of most metals,
in that n decreases as
temperature increases.
Section 2.2.7 – 2.7
• Rate of Deformation
• Superplasticity
• Effects of Compression, Torsion, and
Bending
• Hardness, Toughness, and Strength
Rate of Deformation
Effects
• Some machines form
materials at low
speeds.
– Hydraulic Presses
• Some Machines form
materials at high
speeds.
– Mechanical Presses
Rate of Deformation
Effects
• Deformation rate: the
speed at which a tension
test is being carried out,
in units of m/s or ft/min.
• Strain rate: a function of
the specimen length.
• Short specimens stretch
more during the same
time period than a long
specimen would.
Effects of Temp and
Strain
 Typical effects that temperature and strain rate
have together on the strength of metals:
– Sensitivity of strength-to-strain rate increases with
temperature.
– Increasing the strain rate increases the strength of
the material (strain-rate hardening).
– The slope of these curves is called the strain-rate
sensitivity exponent.
– The relationship between strength and strain is
represented by:  = Cem
– C is the strength coefficient and e is the true strain
rate. m is the slope of the graph.
Rate of Deformation
Effects
 Ex: You have 2 rubber
bands, one 20 mm and
the other 100mm in
length. You elongate them
both by 10mm in a period
of 1 sec. The engineering
strain in the shorter one is
10/20=.5 while the longer
one is 10/100=.1, thus the
strain rates are .5 s-1 and
.1 s-1
Superplasticity
• Refers to the capability of some materials to
undergo large, uniform elongation prior to
necking and fracture.
• This elongation can be as long as 200% to
2000% of the original length.
• Common items that demonstrate this: bubble
gum, glass (at high temp) and thermo plastics.
– Because of this capability, some materials can be
formed into complex shapes such as beverage bottles
and even neon advertisement signs.
Other Deformation
Effects
• Hydrostatic Pressure: pressure due to weight of a
fluid.
• Exposing some types of metals to high radiation is
known to increase yield stress, tensile strength, and
hardness. However it decreases ductility and
toughness.
• Increasing hydrostatic pressure can increase the
strain at fracture of materials.
– Billet: A semi-finished form of steel that is used for long
products such as bars and channels.
– Creating hydrostatic pressure on a billet can turn 1 m of
billet into 14 km of wire.
hydraulic
press.htm
Compression
• Many operations in manufacturing,
especially with forging, rolling, and
extrusion, are performed with the material
being subjected to compressive forces.
Compression Test
 A specimen is subjected
to a compressive load.
 Carried out by
compressing a solid
cylindrical specimen
between two welllubricated flat dies.
 The cylindrical
specimen’s surface
begins to bulge, known as
barreling.
Disk Test
• Compression test developed
for brittle materials such as
ceramics and glass.
• A disk shaped specimen is
loaded between to solid
platens. Tensile stresses build
up perpendicular to the
centerline along the disk,
fracture begins, and the disk
will split vertically.
• Tensile stress from this test can
be calculated with the following
equation:  = 2P/dt P is load
at fracture, d is diameter of
disk, t is thickness.
Torsion Test
• In addition to tension and compression, a work-piece
may be subjected to shear strains.
– Punching holes in sheet metal.
– Metal cutting.
• Torsion test used for determination of properties in
“shear.” Usually performed on a thin tubular specimen.
• Shear stress can be calculated with formula: T/2r2t
– T is torque, r is average radius of tube, t is thickness of tube.
• Shear strain is calculated with formula: rФ/l
– r is radius of tube, Ф is angle of twist in radians, and l is length of
tube.
Torsion Test
• The ratio of the shear stress to the shear
strain in the elastic range is known as the
shear modulus or modulus of rigidity.
• The angle of twist, Ф, to fracture in the
torsion of solid round bars and elevated
temp can help estimate forge-ability of
metals.
Bending
• Preparing specimens from brittle materials,
such as ceramics and carbides, is difficult
because of problems in shaping and
machining them to certain dimensions.
• The most common test for brittle materials
is the bend or flexure test.
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
transverse rupture
strength.
Hardness
• Commonly used property which gives
indication of the strength and resistance to
scratch and wear of a material/specimen.
– Resistance to permanent indentation.
– Hardness is not a fundamental property
because indentation depends on shape of
indenter and load applied.
Brinell Test
• J. A. Brinell 1900
• Involves pressing a steel
or carbide ball of 10mm
against a surface with
various loads.
– 500, 1500, or 3000 kg
• Measures diameter of
indentation.
• Harder surfaces have
small indentation while
softer surfaces have
larger indentation.
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 of
penetration is a measure of
the hardness of material.
Vickers Test
• Developed in 1922.
• Comparable to Brinell
Test except using a
pyramid shaped
diamond to make
indentation.
• Lighter loads than
Brinell Test
– From 1 to 120 kg
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-hardness test because of the lights
loads.
– Suitable for very small or very thin specimens.
• Test also used for measuring the hardness of individual
grains and components in a metal alloy.
Mohs Hardness Test
• Developed by F. Mohs in
1822.
• Test based on capability
of one material to scratch
another.
• Each material can scratch
all materials below it with
a lesser hardness.
• Based on a scale of 1 to
10.
Scleroscope
• Instrument with
diamond-tipped
hammer.
• Hammer is dropped
from a certain height.
• Hardness is related to
the rebound of the
indenter.
• Small and portable.
Durometer
• Used to test hardness of
plastics, rubbers, and
other soft materials.
• An indenter is pressed
against the surface and
then a constant load is
applied rapidly.
• Hardness is measured
based on depth of indent
after 1 second.
Section 2.7 – 2.12
Fatigue
• Fatigue – Components in manufacturing
equipment are subjected to fluctuating
cyclic (periodic) loads and static loads.
• Cyclic Stress – on gear teeth
• Thermal Stress -- cool die in repeated
contact with hot work pieces
– Both stresses may cause part failure at stress
levels below normal static stress loading
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.)
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.
Fatigue
• Endurance (Fatigue Limit) -- Maximum
stress material may be subjected without
fatigue failure.
– Aluminum Alloys and similar materials exhibit
an indefinite endurance limit.
– Fatigue strength is specified at a certain
number of cycles (10^7.)
– Carbon Steels have a proportional endurance
limit and tensile strength, usually 0.4 to 0.5.
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.
– May occur in tools and dies subjected to constant
elevated temperatures (forging and extrusion.)
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, and
tertiary stages
Creep
• Rupture (Creep Rupture) -- Failure by
necking 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 to
an increase in creep resistance.
• Stainless Steels, Super-alloys and Refractory
metals and alloys
Creep
• Stress Relaxation -- The stresses resulting from
loading of a structural component decrease in
magnitude over a period of time, while the
dimensions of the component remain constant.
– Thermoplastics
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.
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 Resistance – High Strength – High
Ductility – High Toughness
Impact
• Notch Sensitivity -Sensitivities to surface
defects, lowers impact
toughness.
– Heat-treated metals,
Ceramics, and
Glasses
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.
Failure and Fracture of
Material
• In selecting and processing materials
– Fracture -- Either internal or external. Sub-classified
into Ductile or Brittle.
– Buckling -- Longitudinal deformation under
compression, similar to barreling.
• Some products are designed with failure essential for their
function:
– Food and Beverage containers with tear tabs
– Shear pins on shafts to prevent damage if overloaded
– Perforated paper or metal (packaging)
– Metal or plastic screw caps for bottles
Failure and Fracture of
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 along
planes which shear stress is a maximum.
Failure and Fracture in
Materials
• Ductile Fracture -- Plastic deformation
proceeds 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-cracks
leading to fracture.
– In tension-test, fracture begins at the center of
the necked region as a result of the growth
and coalescences of cavities.
Failure and Fracture in
Material
• Cup-and-Cone Fracture -- Due to appearance,
the fracture surface of a tension-test specimen.
Failure and Fracture in
Materials
• Effects of Inclusions -- May consist of
impurities of various kinds and of secondphase particles, such as oxides, carbides,
and sulfides.
– Extent of influence depends on their shape,
hardness, distribution, and fraction of total
volume.
• Higher Volume fraction of inclusions, the lower the
ductility of the material.
Failure and Fracture in
Materials
• Effects of Inclusion cont’d
– Two factors affect void formation
• The strength of the bond at the interface between an inclusion
and the matrix.
• The hardness of the inclusion: a soft inclusion will conform to
the overall shape change of the work-piece.
– Mechanical Fibering from the alignment of inclusions
during plastic deformation.
– Subsequent processing of material must involve
considerations of the proper direction of working for
maximum ductility and strength.
Failure and Fracture in
Material
• Transition Temperature -- Across a narrow
temperature range many metals undergo a sharp
change in ductility and toughness.
– The phenomenon occurs mostly in body-centered
cubic and in some hexagonal close-packed metals,
rarely exhibited by face-centered metals.
– Transition Temperature depends on composition,
microstructure, grain size, surface finish and shape of
the specimen, and deformation rate.
– Transition Temperature raises with high rates of
deformation, abrupt changes in work-piece shape, and
the presence of surface notches.
Failure and Fracture in
Material
• Strain Aging -- Phenomenon in which
carbon atoms in steels segregate to
dislocations thereby pinning them and
increasing the resistance to dislocation
movement. Resulting in increased
strength and reduces ductility.
• Accelerated Strain Aging – Phenomenon
occurs in a few hours at a temperature
higher than room temperature.
Failure and Fracture in
Material
• Blue Brittleness -- Occurs in the blue-heat range
where steel develops a bluish oxide film.
• Blue Fracture -- Occurs with little or no gross
plastic deformation.
• In tension, fracture takes place along the cleavage plane
(crystallographic plane), where the normal tensile stress is a
maximum.
– Low temperature and a high rate of deformation
promote brittle fracture.
– The fracture surface of polycrystalline metal under
tension has a bright granular appearance.
Failure and Fracture in
Material
– Tensile stresses
normal to the
cleavage plane,
initiate and control
the propagation of
fracture.
• Chalk, Gray Cast
Iron, and Concrete
Failure and Fracture in
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 materials
inability to dissipate energy.
– Catastrophic failure occurs under tensile stresses
when compared to their strength in compression.
• Trans-granular -- Crack propagates through the
material.
Failure and Fatigue in
Material
• Inter-granular -- Crack propagates along the
grain boundaries, generally when the grain
boundaries are soft, contain a brittle phase, or
have been weakened by liquid- or solid-metal
embrittlement.
Failure and Fracture in
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.
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.
Failure and Fatigue in
Material
• 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 be
improved overall by
• Inducing compressive residual stresses on
surfaces
– Shot Peening or Roller Burnishing
• Case hardening (surface hardening) by various
means
Failure and Fatigue in
Material
– Fatigue Strength of manufactured products
may be improved overall by
• Providing a fine surface finish and thereby reducing
the effects of notches and other surface
imperfections.
• Selecting appropriate materials and ensuring that
they are free from significant amounts of inclusions,
voids, and impurities.
Failure and Fatigue in
Material
• Factors and processes that may reduce fatigue
strength: decarburization, surface pits
(corrosive) acting as stress raisers, hydrogen
embrittlement, galvanizing, and electroplating.
• Stress-Corrosion Cracking -- Either over a
period of time or soon after being manufactured,
parts free from defects may develop cracks.
Failure and Fracture in
Material
• Stress-Corrosion Cracking -- cont’d
– Crack propagation may be inter- or trans-granular.
– Susceptibility of metals to stress-corrosion cracking
depends mainly on the material, on the presence and
magnitude of tensile residual stresses and on the
environment (Salt water and other chemicals.)
– To avoid Stress-Corrosion cracking stress relieve the
part after it is formed. Full annealing may be done,
but reduces the strength of cold worked parts.
Failure and Fatigue in
Material
• Hydrogen Embrittlement -- The presence of
hydrogen may reduce ductility and may cause
severe embrittlement and premature failure in
many metals, alloys, and nonmetallic materials.
– Especially severe in high-strength steels.
• Melting of Metal, Pickling (removal of surface oxides by
chemical or electrochemical reaction,) Electrolysis in
Electroplating, Water Vapor in the Atmosphere, Moist
Electrodes, and fluxes in Welding
– In copper alloys, Oxygen may also cause
embrittlement.
Residual Stresses
• Work-pieces are subjected to plastic deformation
that is not uniform throughout the part. Stresses
remain within a part after it has been formed and
all the external forces are removed.
– The bending of a metal bar. The elastic and plastic
deformation resulting in a permanent bending.
• The linear load reaches the yield stress, changing nonuniformly. The release of the external force is opposite
the curvilinear load (elastic.) The difference in the two
loads gives the residual stress pattern within the bar.
Compressive residual stresses in ad and oe, tensile
residual stresses in od and ef.
Residual Stresses
• Warping -- Disturbances of residual stresses
acquire a new radius of curvature to balance the
internal forces.
• Temperature Gradients within a body may also
cause residual stresses (cooling or forging.)
– Contractions and Expansions within a material
produce non-uniform deformation (beam or lumber.)
• Tensile residual stresses on a surface are
undesirable due to the reduction in strength
when an external force is applied to the part
(brittle, less ductile.)
Failure and Fatigue in
Material
• Compressive residual stresses on a surface are
desirable.
– Shot Peening and Surface Rolling
• Reduction and Elimination of Residual Stresses - Either by stress relief annealing or by further
deformation of the work-piece.
– Stress Relaxation may occur over time, and may
increase greatly by raising the temperature of the
work-piece.
Work, Heat, and
Temperature
• Almost all of the mechanical work of
deformation in plastic working is converted
into heat.
• Stored Energy -- A portion of work stored
within the deformed material as elastic
energy.
– 5 to 10% of total energy input, in some alloys
may be as high as 30%
Work, Heat, and
Temperature
• Change of Temperature is the ratio of specific
energy to the density and specific heat of
material.
– Higher Temperatures are associated with large areas
under the stress-strain curve and with smaller values
of specific heat.
– Physical properties as specific heat and thermal
conductivity depend on temperature and must be
taken in account during calculations.
– If deformation process is performed rapidly, the heat
losses will be relatively small over that brief period.
– If the process is carried out slowly, the actual
temperature rise will be only a fraction of the
calculated value.
Work, Heat, and
Temperature
– Specific Energy (u)
-- Work of deformation
per unit volume
– Density (d)
– Specific Heat of
Material (c)
Chapter 3
Physical Properties
of Materials
• Engineers must make
conscious function
and performance
decisions based on
the physical
properties of materials
Physical Properties
of Materials – Ch. 3
• Overview
– Density
– Melting point
– Specific heat
– Thermal conductivity
– Thermal expansion
– Electrical, magnetic and optical properties
– Corrosion resistance
Density
• Mass per Unit Volume
– Typical units include
• kg/m^3
• lb/ft^3
• Specific Gravity
– Density with respect to
water
– No units
Density
• Strength-to-Weight
ratio
– Specific Strength
– Tensile strength /
density
• Stiffness-to-Weight
ratio
– Specific Stiffness
– Elastic modulus /
density
• Units of length
Melting Point
• The energy required to separate the atoms of a
material
– Units of temperature
• Important consideration when the material will be
subject to an operating temperature or a thermal
cycle during manufacturing process
– Annealing
– Heat treating
– Hot working
Specific Heat
• The energy required to raise the temperature of
a unit mass by one degree
• Units of J/kg ˚K
• Important consideration in the forming or
machining operations
Thermal
Conductivity
• The rate at which heat
flows within and
through a material
• Units of W/m ˚K
• Very low thermal
conductivity of
Titanium
– Can result in excessive
tool wear during
machine operations
Thermal Expansion
• The expansion or contraction of a material when exposed
to a thermal cycle
• Units of µM/m ˚C
• Hot rivets are installed through holes in steel plate
• When the rivets cool they contract causing an extremely
tight compressive stress on the joint
Electrical, Magnetic
and Optical Properties
• Electrical Properties
– Conductivity
• The ratio of the
current density to
the electric field
strength
– Dielectric Strength
• A materials
resistivity to direct
electrical current
Electrical, Magnetic
and Optical Properties
• Electrical Properties
–
–
–
–
Conductors
Superconductors
Semiconductors
Piezoelectric effect
• A reversible interaction
between an elastic strain
and an electric field
• Typical applications
include pressure
transducers, sensors, and
strain gauges
Electrical, Magnetic
and Optical Properties
• Magnetic Properties
– Ferromagnetism
– Ferrimagnetism
– Magnetostriction
• The expansion or contraction of a material when
subjected to a magnetic field
• The principle behind ultrasonic machining
equipment
Electrical, Magnetic
and Optical Properties
• Optical Properties
– Color
– Opacity
Corrosion Resistance
• Corrosion
– Typically used to
describe metal or
ceramic deterioration
– Similar phenomena
occur in plastics
– Often referred to as
degradation
Corrosion Resistance
• Types of corrosion
–
–
–
–
–
–
–
–
Pitting
Intergranular
Crevice
Galvanic cell
Stress-corrosion cracking
Selective Leaching
Oxidation
Passivation
Corrosion Resistance
• Pitting
– Can occur over the
entire surface or be
localized
• Intergranular
– Occurs along grain
boundaries
Corrosion Resistance
• 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-phase
alloys
Corrosion Resistance
• Stress-corrosion
cracking
– Cold worked metals
are most susceptible
• Selective leaching
– Occurs when
metalworking fluid
attacks specific
elements in tool and
die materials
Corrosion Resistance
• Oxidation
– A chemical reaction which leaves a small layer
of oxidized material on the surface
– Resists further corrosion
• Aluminum & Titanium
• Passivation
– The development of a protective film by
chemical reaction
• Stainless Steel
Physical Properties
of Materials
• Review
– Density
– Melting point
– Specific heat
– Thermal conductivity
– Thermal expansion
– Electrical, magnetic and optical properties
– Corrosion resistance
• 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.materialsengineer.com/G-Pitting-Corrosion.htm
• http://www.corrosion-doctors.org/Forms/intergranular.htm
• http://www.textronfasteningsystems.com/appsol/galv_corr.html