1/24/2011
Properties of Metals
Alessandro Anzalone, Ph.D.
Hillsborough Community College
Brandon Campus
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Mechanical Properties
Physical Properties
Metallurgical Microscopy
Nondestructive Testing
References
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http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Graphics/MixedMetals%28mayFranInt.%29.jpe
Tensile Test
The tensile test involves the relationship between tensile stress and
strain. Although these terms are sometimes confused, they have
separate and distinct meanings. Stress is defined as the
resistance of a material to external elements such as force, load,
or weight measured in pounds per square inch (PSI). It is
calculated by dividing the applied load to a specimen by the
cross-sectional area of the specimen. When the stress is applied
it will cause the specimen to deform or stretch. Strain is the
amount of deformation or stretch that occurs over a standard
gage length (usually 2 in.) expressed in inches/inch or as a
percentage. This gage length may also be referred to as “the
original length.”
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The proportional limit is technically the stress value at which the
elastic portion of the curve loses its proportionality between
stress and strain. Since it is so difficult to determine (when is the
line no longer straight’?), it is rarely used in a practical manner.
As the name implies, the elastic limit is the highest stress (at
the limit) at which the specimen will return to its original length.
Because this value can be found only by trial and error it is
valuable as a concept only. Neither of these values is included
among handbook data.
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In Figure 2.6, at the top of the elastic region, the amount of strain
increases with very little increase in stress. That is, the metal
yields, and this yield point is a very obvious end of the elastic
region and is a good value on which to base a design. If the metal
was cold-worked or heat-treated, or was a nonferrous metal, it
would not have a yield point. In Figure 3.24 the curve for the
high-carbon steel does not have a yield point: in that case a line
is constructed parallel to the elastic region at a strain value of
0.002 in. Where that line intersects the stress—strain curve is
known as the yield strength. The yield point or yield strength,
whichever is appropriate,
appropriate is a handbook value for metals and
alloys and is commonly used in design; units are PSI.
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The tensile strength in pounds per square inch is determined by
dividing the maximum load (in pounds) by the original crosssection (in square inches) before testing.
Elongation, or percent elongation, is defined as the value of
strain at failure. It is found by determining the amount the
specimen stretched during the test, dividing by the original
length, and expressing the result as a percentage. The percent
reduction of area is determined in a similar fashion; the
reduction in the cross-sectional area of the specimen (at the
point where it failed)) is divided byy the original
p
g
area and
expressed as a percentage. Both these measures are used as
indicators of ductility (which is the property that allows a metal
to deform permanently when loaded in tension). Any metal that
can be drawn into wire is ductile. Soft steel, aluminum, gold,
silver, and nickel are ductile metals.
Malleability is the ability of metals to be deformed permanently
when loaded in compression. Metals that can be rolled,
hammered, or pressed into flat pieces or sheets are malleable.
D til metals
Ductile
t l are usually
ll also
l malleable,
ll bl b
butt th
there are some
exceptions. Lead is very malleable but not very ductile and
cannot be drawn into wire very easily, but it can be extruded in
wire or bar form. Gold, tin, silver, iron, and copper are malleable
metals. Plasticity, a more general property, is the ability of a
metal to be permanently deformed without failing. On the
stress—strain curve of Figure 2.6 it would mean the capability to
be worked in the region
g
labeled there as “Plastic range.”
g
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Hardness Test
Hardness is generally defined as the resistance to penetration or
indentation. The greater the hardness, the smaller the
indentation with the same penetrator tip and pressure. However,
the property of hardness is related to the elastic and plastic
properties of metals, and certain hardness tests are based on
rebound or elastic hardness. Also, certain scratch tests are used
to determine abrasive hardness or resistance to abrasion.
Hardness is also related to tensile strength, so it can be assumed
that a hard steel is also strong and resistant to wear.
wear
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Impact Test
This test is performed to measure the impact strength of a metal;
since a specimen with a notch (stress raiser) is used, this test is
also said to measure notch toughness. Thus, this test measures
the ability of a metal to resist rupture from impact loading (also
called mechanical shock) when there is a notch or stress raiser
present. The property of toughness is not necessarily related to
ductility, hardness, or tensile strength; however, as a rule, a
brittle metal with low tensile strength such as cast iron will fail
under low shock loads; hardened,
hardened tempered tool steels show a
high impact strength; and coarse-grained metals have lower
shock resistance than fine-grained metals. A notch or groove will
lower the shock resistance of a metal. Test specimens are made
with given dimensions
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Izod-Charpy Testing Machine
Ductile—Brittle Transition Temperature
Another function of the impact test is to determine the effect of
temperature change on various metals. Metals that have the
body-centered cubic atom lattice, for example, iron and steel,
show a temperature range in which ductility and, more
important, toughness drop rapidly. This transition zone can be
seen in Figure 2.22. When the notch-bar specimens show half
brittle and half ductile failures, the transition temperature has
been reached.
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Creep Test
Creep strength is a metal’s ability to resist creep, a continuing
plastic flow at a stress below the yield strength of a metal. For
the most part, creep is a high-temperature phenomenon that
increases as the temperature increases. Table 2.2 gives some
stress loads at various temperatures required to cause elongation
by creep. Some metals have been designed for high-temperature
service such as for aircraft jet turbine engine blades that show no
signs of creep at temperatures that would melt many metals.
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Fatigue Test
Fatigue in metals can occur when metal parts are subjected to
repeated loading and unloading, particularly in cyclic reversals
of stress, such as is seen in a rotating shaft having transverse or
one-side loading. Fatigue failures can occur at stresses far below
the yield strength of a material with no sign of plastic
deformation. When machine parts that are subject to cyclic
loading are designed, their resistance to fatigue may he more
important than yield or ultimate strength. Fatigue is not time
dependent and can he initiated by any number of factors such as
machining tool marks or welding undercuts and localized stress
caused by a welding head. Fatigue can he accelerated by a
corrosive atmosphere and by higher frequency stress reversals.
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Modulus of Elasticity
The modulus of elasticity is actually an expression of the stiffness of a
material. It is also called Young ‘s modulus. If, within the elastic
range, the stress is divided by the corresponding strain at any
given point, the result will he the modulus of elasticity for that
material; thus,
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Conductivity
The thermal conductivity and electrical conductivity of metals
are both related to the fact that metals have free electrons in
their atom lattices. Thus. if a metal such as silver or copper is a
good conductor of electricity, it is also a good conductor of heat.
Pure metals are better conductors than their alloys. For this
reason copper or aluminum used for electrical wiring must be
uncontaminated. Electrical copper grades are 99.97 percent
pure. Resistance to the flow of electricity can be altered by
several factors:
1. Resistance increases as the temperature increases.
2. it increases as a result of cold-working or heat treating the
metal.
3. it increases with the amount of impurities and alloying
elements present.
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Thermal Expansion and Contraction
Most metals expand when heated and contract when cooled. When
heated or cooled each metal expands and contracts at a different
rate that is related to its coefficient of thermal. This physical
property of metals can cause difficulties in construction and
manufacturing. Engineers plan use of expansion joints in paved
highways and bridges to avoid buckling due to heating.
Reflectivity
Some metals have a high degree of reflectivity and are used as light
reflectors and in insulation as heat deflectors. Aluminum, nickel,
chromium, tin, and bronze are among the metals having high
reflectivity, whereas lead has virtually none. In spite of that
exception reflectivity is one of the characteristics that
distinguishes metals from other substances.
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Ferromagnetism
Ferromagnetism is characterized by an attraction between certain
metals that have the ability to retain a residual magnetic force.
The ferromagnetic metals are iron, cobalt, and nickel. Of these,
iron is the most commercially important metal used extensively
in electrical machinery. Soft iron does not retain much residual
magnetism when removed from a magnetic field. This quality
makes it useful for electromagnets and electric motors in which
the field is turned on and off or reversed repeatedly. Powerful
permanent magnets are made by alloying metals such as cobalt,
cobalt
nickel, iron, and aluminum.
Metallurgical Microscopy
Metal surfaces are examined by means of metallurgical microscopes
and associated techniques of surface preparation. Structures in
metals not readily visible to the naked eye may be seen at highpower magnification from 10 times the diameter (lOX) to 3000X
for light microscopes and many more thousands of diameters
with the electron microscope. Techniques of photomicrography
in which a camera is attached to the microscope make possible
photographic illustrations of specimens . Metallurgical
microscopes unlike those used to see through a specimen,
microscopes,
specimen use
reflected light from an internal source. Inverted stage
microscopes are widely used to study metals.
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As the name implies, nondestructive testing in no way impairs the
part for further use. It does not measure mechanical or physical
properties but instead identifies defects such. as voids,
inclusions, or cracks that might later lead to failure of the part.
Also, improper handling of materials that causes changes in the
microstructure can lead to failure. For example, a nick or dent in
a highly stressed part such as a spring can cause early failure. At
present, even these aberrations can be detected on a production
basis. Nondestructive methods can also verify hardness or heat
treatment in many cases. Since testing for defects in the early
stages of the manufacturing process can substantially reduce
rejects, nondestructive testing is closely related to quality
control.
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The following are the most common types of nondestructive testing:
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Magnetic-particle inspection
Fluorescent penetrant inspection
Dye penetrants
Ultrasonic inspection
Radiography (X-ray and gamma ray)
Eddy-current inspection
Magnetic-particle inspection
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Fluorescent penetrant inspection
Dye penetrants
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Ultrasonic inspection
Radiography (X-ray and gamma ray)
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Eddy-current inspection
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R Gregg Bruce, William K. Dalton, John E Neely, and Richard R Kibbe, , Modern Materials and Manufacturing
Processes, Prentice Hall, 3rd edition, 2003, ISBN: 9780130946980
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Properties of Metals
Alessandro Anzalone, Ph.D.
Hillsborough Community College
Brandon Campus
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