Mechanical Properties of Metals Chapter

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Metallurgy
Fundamentals
Ferrous and Nonferrous

Mechanical Properties of
Metals
Chapter 6

Copyright Goodheart-Willcox Co., Inc. May not be posted to a publicly accessible website.
• Understand that there are multiple ways that a load can be applied
in order to measure the strength of an object.
• Recognize that standard tests of strength and ductility are required
so everyone can communicate and understand the properties of a
metal.
• Describe the difference between elastic and plastic deformation.
• Explain how the modulus of elasticity is an expression of the
stiffness of a material.
Learning Objectives

• Recognize that the yield point is the point on the stress-strain curve
where a metal first experiences permanent deformation.
• Understand the importance of ultimate tensile strength in describing
the maximum amount of stress a metal object can withstand before
breaking.
• Understand how changes in microstructure affect and reflect
changes in shape as a metal undergoes increased strain and
stress.
Learning Objectives

• Describe the characteristics of metals with high ductility.
• Explain how hardness tests can be valuable on a metal production
line.
• Describe the difference between tensile toughness and impact
strength.
• Understand and describe the various ways in which metals can fail.
Learning Objectives

• Mechanical property of a material is seen in response to applied
force.
• Strength and ductility are key mechanical properties.
• Recognized measurements of strength and ductility are needed.
• Important to end users and manufacturers
• Understanding failure modes helps to avoid them.
Introduction to Mechanical Properties

• Strength is ability of an object to
support a load, or force.
• Strength of object depends on its
shape and direction of load it is
supporting.
Strength
Goodheart-Willcox Publisher

• Force can be applied multiple ways to measure strength.
• Always apply force to the part, then measure how much it deforms.
Direction of Load

• ASTM International is an international standards organization.
• Formerly American Society for Testing and Materials
• It publishes testing procedures for many different measurements.
ASTM International Testing Procedures

• Tensile testing is most useful.
• Straightforward to apply
• Can mathematically estimate
strength in other directions
• Standard test specimens are
used.
• Specimen geometry varies with
starting material.
Tensile Test
Goodheart-Willcox Publisher; Ben Church, Advanced Analysis Facility, U-W Milwaukee

• Specimen is loaded.
• Specimen pulled in tension at steady rate to
stretch it
• Extensometer on sample measures amount of
extension.
Tensile Test Machine
ADMET, Inc. Norwood, MA

• Tensile test machine continuously
measures and plots force applied and
extension of sample.
• Known as load-extension plot, or graph
• At small extensions, plot has a linear
relationship.
• This straight-line relationship is called
Hooke’s law.
Load-Extension Plot

• By converting load to stress and extension to strain, you can
report stress and strain that apply to test specimen.
• Stress is force per unit area experienced by material.
• Strain is ratio of change in length of material to original length.
Stress and Strain

• Stress is measured in pounds per square inch (psi) or pascals (Pa).
• Stress = Force/Area
• Find stress for cylindrical sample cut from plate.
• Diameter is 0.3568″ (9.0227 mm).
• Maximum load is 6150 pounds-force (27,357 Newtons).
• Calculate area and divide load by area to get tensile strength.
Calculating Stress 1

• Calculate area
• Area = π × r2 = π × D2/4
• Area = 3.14159 × 0.35682/4
• Area = 0.1000 in2
• Calculate stress to get tensile strength
• Stress = Force/Area
• Stress = 6150 lb/0.1000 in2
• Stress = 61,500 psi (same as 61.5 ksi)
• If reported in pascals, 1 megapascal (MPa) = 1,000,000 pascals (Pa)
Calculating Stress 2

• Extensometer measures distance between two contact points.
• Start 2.000″ apart (original length)
• Test sample fractured at 0.740″ extension.
• Strain = Length of extension/Original length
• Strain = 0.740/2.000 = 0.370
• e = 0.370
• Percent elongation = e × 100 = 0.370 × 100 = 37.0%
Calculating Strain (e)

• Platform truck trailers haul heavy loads, which
stress and strain the trailers.
• Trailers are designed with upward bow, so
loads cause them to flatten out.
• When loads are removed, trailers spring back
to a bowed shape.
Absorbing Stress and Strain: A Platform Trailer
Practical Metallurgy

• Sample loaded in elastic region has
temporary shape change under strain.
• Modulus of elasticity, also called Young’s
modulus, measures a material’s stiffness.
• Property comes from the primary metal in
the alloy.
Modulus of Elasticity

• In tensile test, point where permanent
deformation first occurs is called yield point
(upper yield point).
• Elastic region exceeded, plastic deformation
begins.
• Stress drops to lower yield point.
• Not all materials have well-defined yield point.
• All ductile materials have plastic deformation
at some stress level.
Yield Point

• Stress when material first experiences
plastic (permanent) deformation is
yield strength.
• Value of strain at yield point is called
yield strain.
• Using procedures that result in smaller
grains produces higher yield strength.
Yield Strength

• Important to know maximum load capacity of components
• Overloading a trailer will cause it to permanently deform.
• It has not broken but is bent to wrong shape.
• It is no longer usable.
Plastic Deformation: A Broken Platform Trailer

• Forming of metal parts into usable shapes
should avoid cracks caused by excessive
bending/stretching.
• Applicable properties found in plastic
region of tensile test
• Plastic region occurs after elastic limit, up
to ultimate fracture.
Strength and the Plastic Region

• Key property in plastic region is ultimate tensile strength (UTS).
• Also called tensile strength
• Maximum tensile stress before breaking
Ultimate Tensile Strength (UTS)

• Once sample yields, it uniformly elongates and reduces its cross-
sectional shape.
• Crystals develop dislocation tangles.
• Strength increases.
• Stress-strain curve slopes upward.
• Sample reaches UTS and begins local necking until it fractures.
• Strain at which sample fractures is strain at failure.
Test Sample Shape Change—
Strain in the Plastic Region

• Forged yoke eye bolts are used for heavy lifting.
• Product specifications include “minimum proof load”
and “minimum break load.”
• Minimum proof load is related to load at yield (yield
strength).
• Minimum break load is related to ultimate tensile
strength.
Tensile Strength in Product Specifications
Columbus McKinnon Corporation

• Forming parts requires good ductility.
• Opposite of ductility is brittleness: tendency to stretch or deform little
before fracture.
• Reduction in area at fracture is another measure of ductility.
• Use initial and final diameter of test sample to calculate
• Percent reduction in area = (D2
initial − D2
final)/D2
initial × 100
Strain, Ductility, and Forming Metal
Ben Church, Advanced Analysis Facility, U-W Milwaukee

• Frame rails for platform trailers are made from heavy sheet metal.
• Metal on outside corner is stretched like a tensile sample.
• Cracks can form with metal of insufficient ductility.
Ductility: Frame Rails for a Platform Trailer

• Hardness testing is quick substitute for
tensile test.
• Used if part is too small
• Used if tensile testing is too expensive
• Hardness tests press a small indenter into
the surface of test samples.
• Depth of penetration determines hardness
number.
Hardness: An Indicator of Tensile
Strength

• Several different scales (with different loads and
indenters)
• Most common scales used industrially are B and C
scales.
• Rockwell B scale (units of HRB) for typical steel
• Rockwell C scale (units of HRC) for hardened steels
• Other specialized tests exist, including superficial
hardness.
• Small loads for thin sheet metal
• Leaves small indents
Rockwell Hardness Test
Mitutoyo/MTI Corp

1. Prepare sample.
2. Place on support anvil and raise until it
contacts penetrator.
3. Apply minor load.
4. Apply major load.
5. Remove major load and sample.
6. Penetration depth is reported as hardness
value on machine.
Procedure:
Performing a Rockwell Hardness Test

• Many processes use hardness specifications.
• If hardness falls outside allowable range, process may have drifted
away from correct settings.
• Hardness values provide indication of tensile strength.
• Only for steel
• Inexact
Applications of Hardness Tests

• Brinell hardness test
• Uses large, spherical penetrator
• Averages hardness over many grains in coarse-grained metals
• Vickers test
• Uses pyramid-shaped diamond to make microscopic indents
• File hardness test
• Runs metal file across surface
• Very quick but inaccurate
• Requires experienced technician to perform
Other Hardness Tests

• Toughness is energy required to break sample in slow-moving test.
• Tensile toughness is area under stress-strain curve.
Toughness

• Impact strength of material is energy
required to break a sample.
• Broken in a rapid, sharp blow
• Impact strength test uses swinging arm to
break sample.
• Impact strength is energy absorbed to
break sample.
Impact Strength

• Impact test machines can test specimens in
Charpy and Izod configurations.
• Specimens are held differently.
• Test procedure
• Sample is cut from metal stock.
• A notch is cut into one side.
• Impactor strikes sample to break it through notch.
Charpy and Izod Test Samples

• Ductile fracture
• Characterized by plastic deformation before fracture occurs
• Brittle fracture
• Shows little or no plastic deformation
Ductile and Brittle Fractures
Ben Church, Advanced Analysis Facility, U-W Milwaukee

• Metal can be tested under compressive loads.
• In elastic range, elastic modulus and stress-strain
curve are same in compression and tension.
• Not same after yielding
• Compression samples bulge
• Tensile strength used to estimate compression
strength
• Sample area must be adjusted for bulging
Compressive Strength

• Cutting processes shear metal.
• Several processes shear metal during
production.
• Large cutting shears
• Die and punch
• Press must be large enough to shear metal.
• Varies with shear strength of material
Shear Strength

• Some metal parts are twisted during production.
• Amount part can twist without distorting
permanently is determined by its torsional
strength.
• Bending strength (flexural strength) is stress a
material can take before yielding when bent.
• Equipment must be strong enough to form
metal.
• Must stress parts into plastic region
Torsional and Bending Strength

• Different strengths must be considered during production.
• Shear strength
• Torsional strength
• Bending strength
• During use, these are important.
• Frame members must flex (elastic range).
• Frame members must not permanently distort (plastic range).
Strength-Related Properties of a Platform Trailer

• Critical parts are made to be fail-safe.
• Parts can be weakened during production.
• Understanding common ways metals fail helps
avoid this.
• Ductile failure is preferred to brittle failure.
• Brittle failures often occur along grain boundary
surfaces.
• Intergranular failure is brittle.
• Appears shiny since each grain surface reflects
light
Fracture and Failure Modes in Metals
Jay Warner

• Failure due to repetitive or cyclic loading
• Stress level can be well below yield strength.
• After millions of flexing cycles, small cracks form.
• These can grow and eventually fail.
• This is fatigue failure.
Fatigue Failure
Jay Warner

• Fatigue testing involves cyclic loading of
test samples.
• Constant maximum load applied until
failure
• Below certain level of stress, failure never
occurs.
• Level called fatigue limit
• Exists for steel
Fatigue Failure and Fatigue Strength

• Notch on a part can decrease maximum safe load.
• Machine cuts or accidental gouges
• Poor welds
• Notches should be avoided during production.
• Stress is greater near notch.
• Smaller radius at tip is worse.
• Cracks start there.
• Once cracks start, stress concentration increases.
Stress Concentration at Notches

• Wear resistance is ability of metal to resist metal loss when sliding
against another material.
• Abrasion is wearing away of material by rubbing or scraping.
• During abrasion, minute particles of metal detach.
• Increases abrasion or disrupts surface finish
Abrasive Wear Resistance

• Abrasion reduced several ways
• Reduce force pressing pieces together.
• Use layer of oil to separate materials.
• Use one material whose surface atoms strongly adhere to themselves
(titanium nitride coating is example).
Reducing Abrasion

• Metal-to-metal wear has many names.
• Scratching, scoring, galling, scuffing, seizing, fretting
• Hard materials also cause wear.
• Rocks
• Sand
• Coatings are used to minimize wear.
Metal-to-Metal and Metal-to-Nonmetal
Wear

• Creep is slow plastic deformation of material caused by sustained
load.
• Occurs at high temperatures and stress below yield stress
• Grain boundaries slide past other grains, causing deformation.
• Creep occurs in all metals.
Creep Failure

• At low temperatures, carbon and low-
alloy ferritic steels lose ductility.
• Impact strength and toughness greatly
decrease.
• Ductile-brittle transition temperature
varies with type of steel.
• Notches or weld bead undercuts become
fracture initiation points when too cold.
Ductile–Brittle Transition at Low
Temperatures

• Some things raise ductile-brittle transition
temperature.
• Cold-working processes
• High-strength steels (high-carbon or heat-treated
steels)
• Some things lower ductile-brittle transition
temperature.
• Nickel is common alloy addition for low-
temperature applications.
Key Factors Affecting Ductile–Brittle
Transition
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