Overview of Mechanical Testing
The Role of Testing
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Material test data typically used for:
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Design/construction of new mechanical or structural
elements
Control of established processes
Material development
Scientific knowledge (i.e., understanding how certain
factors affect materials)
Others?????
Engineers must have a general understanding of:
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Common test methods for material properties
What constitutes a valid test
Applicability of data / limitations of tests
“Types” of material testing
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Commercial testing
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Materials research testing
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Concerned mostly with checking acceptability of materials
under purchase specs
Standard procedures are used
Objective to determine if properties of material or part fall
within required limits
To obtain a new understanding of known materials
Characterize properties of new materials
Develop new or refine existing test standards or material
quality standards
Scientific testing

Provide data to support development/verification of models,
analyses, etc.
“Types” of Tests
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Field tests
 Testing done on location (such
as flight line, construction site,
etc)
 Typically have lower precision
than laboratory tests
 May better represent actual use
environment
Laboratory tests
 Tests done in laboratory using
load frames and other
specialized equipment under
controlled conditions
 Typically more expensive and
complicated than field tests
 Usually have greater precision
than field tests.
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Destructive
 Involve breaking or damaging
the sample
 Not applicable for finished
parts
 Examples, tensile testing,
hardness testing, fatigue
testing
Non destructive
 Do not damage the sample
 Often used for quality control
 Examples, proof testing,
radiography, microhardness
testing
Structural – tests done on
components or structures
Coupon – tests done on small
samples of material
Significance of Tests
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Concepts of properties/ testing of materials is oversimplified
(many assumptions)
Properties measured by tests affected by test conditions, method
of testing, quality of test samples, quality of test equipment, etc.
Uncertainties in data
 Those associated with material properties due to manufacturing,
processing
 Those associated with level and type of loading and actual
service/environmental conditions
Significance of test measured by precision
 Reliability - within lab variability
 Reproducibility – between lab variability
Accuracy of test = closeness to true value
** a test can be precise, but not accurate **
Materials Properties
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Mechanical
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Microstructure Sensitive
Describe how a material responds
to an applied force
Physical
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Microstructure Insensitive
Describes a materials response to
an applied field or chemical
Stress conditions
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Fundamental stress conditions describe
mechanical behavior features of components
and assemblies:
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Axial tension or compression
Bending, shear or torsion
Internal/external pressure
Stress concentrations and localized contact loads.
Tensile Loading
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Axial tensile loading
s= F/A
Design such that sapp < s failure
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Where s failurecan be:
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su (UTS) if fracture is criterion for failure
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Ductile material: UTS = stress where necking occurs
Brittle material: UTS = stress where material breaks
so (yield strength) if permanent deformation is
criterion for failure
F
A
Stiffness in Tension
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Elastic deformation governed by stiffness
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DL=eL
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e = strain
L = length of bar
n= et/el = Poisson’s ratio
l
In the elastic range of deformation
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s=Ee
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E = elastic modulus
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Can be considered a physical property because it is fundamentally F
related to bond strength, not affected much by microstructure
Can vary with direction if material has anisotropic structure
Design of stiffness critical applications
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Dl
D L = FL/AE < d
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d = design limit change in length
Load cell
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Strain gages mounted on precision
machined alloy steel elements
Load cell mounted such that specimen
is in direct contact with load cell or
indirectly loaded through the machine
crosshead, table, columns of load frame
Calibrated to provide specific voltage
output signal when a certain force is
detected
Can be used in tension or compression
and available with variety of
temperature compensation capabilities
Source: www.inston.com
Clip on extensometer
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Attached to test sample
Measures elongation or strain as
load is applied
Typically have fixed gage lengths (0.
in – 2 in, etc.)
Used to measure axial strain,
available to measure transverse
strain to determine reduction in width
or diameter
Source: ASM Mechanical Testing and
Eval Handbook
Tensile Testing - Engineering Stress Strain Curve
Tensile Testing - Engineering Stress vs. Strain Curve
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Yield Point
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Ultimate Tensile Strength (UTS): Stress at highest applied force
Breaking Strength: stress at which fracture occurs
Modulus: Slope of elastic portion of stress vs strain curve, E (lb/in2)
Resilience:
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area under stress strain curve in elastic region;
indicates amount of energy a material can absorb in elastic range
Toughness:
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yield strength: stress at which slip becomes noticeable and significant - transition between elastic
and plastic deformation, s y (lb/in2);
yield strain: strain at which slip becomes noticeable and significant, e y (in/in or %)
Offset yield strength (0.2% or 0.002 common): stress at which material changes from elastic to
plastic is not always well defined, therefore define an offset yield strength
area under stress strain curve
usually associated with shock or impact loadings
% Elongation = ((lf-lo)/lo) x 100%, lf = gage length at failure, lo = initial gage length
% Reduction In Area = ((Ao-Af)/Ao) x 100%, Ao= original area; Af=final area
Askeland, Phule The Science and Engineering of Materials
Tensile Testing - Failure Modes
•Ductile Failure
–Cup cone fracture
–Dimpled failure surface
–Significant plastic deformation
–Has “necked” or localized
deformation region
•Brittle Failure
–Flat fracture
–Cleavage(radial lines) failure
surface
–Little to no plastic deformation
–Does not have “necked” region
Compression loading
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Isotropic materials
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Anisotropic materials
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suc equal to suT
suc not equal to suT
Buckling may preceed other forms of failure
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sb = (p 2 E I)/(L2 A)
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I = moment of inertia of cross section of bar
Dynamic Properties
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Impact Loading/Impact loading occurs if time duration is less than the
natural period of vibration of part or structure
Depends on material parameters and geometric factors
Design stress, s = V (Em/Al)o.5
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V = velocity of mass, m
A, l = cross-sectional area and length of bar
E = elastic modulus
Impact tests
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Charpy, Izod, Hopkinson bar, Others
Factors that affect data:
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loading rate:
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specimen size and configuration
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smaller energies might be required to break thicker samples
notch configuration
Impact data should be used comparatively
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faster => less energy, higher transition temperature
slower => more energy, lower transition temperature
materials screening
not appropriate for design data
Temperature
Transition temperature
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Temp at which a
material changes from
ductile to brittle
BCC metals have
distinct transition
temp
FCC metals do not
have distinct transition
temp.
ductile
Absorbed Energy, ft-lb
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brittle
Test Temp, F
Impact Tests
Brittle Material
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mat’ls ability to withstand a sudden intense, blow
Toughness:
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provides a measure of impact resistance;
ability of mat’l to absorb energy prior to failure
area under true stress - true strain curve
low toughness
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True stress, psi
Impact Resistance:
True strain, in/in
clean break
brittle material
little to no plastic deformation
Ductile Material
high toughness
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significant plastic deformation
ductile material
True stress, psi
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True strain, in/in
Impact Tests -Charpy and Izod
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Heavy pendulum of mass, m, is dropped from a height, ho.
Pendulum swings through arc, strikes and breaks specimen, rebounds to
height of hf.
Energy dissipated via elastic, plastic deformation and fracture
Potential Energy difference (ft-lb or Joules) read from impact tester.
Charpy:
Pendulum
Specimen
(10 x 10 x 55 mm)
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Izod
Notch
Pendulum
Specimen
(10 x 10 x 75 mm)
Impact Tests - Izod and Charpy
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Potential Energy Difference
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‘D U = U1 -U2 = mg (ho-hf)
 Where:
 ‘D U = potential energy difference
 m = mass of pendulum
 g = gravity
 h0 = drop height
 hf = rebound height
2
 1 ft -lb = 1.356 joules
hf
1
ho
Hardness Testing
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Not a fundamental property
Provides quick, easy, cheap indication and comparative information
regarding material's strength
Used as quality control technique
Widely used for steels
Many different types of hardness tests
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Macrohardness
 Brinell, Rockwell, etc.
 “Destructive” test
Microhardness
 Vickers, Knoop, etc.
 “Non destructive” test
Macrohardness Testing - Brinell
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Steel or tungsten carbide ball (10 mm dia) pressed against material
P
Load of 500, 1500 or 3000 kg applied for 5 - 10 seconds
Diameter of indention measured using microscope
BHN = P/ ((P D/2) x (D - (D2 - d2)1/2))
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Where:
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P = applied load [kg]
D = diameter of ball [mm]
d = diameter of resultant penetration [mm]
BHN = Brinell Hardness Number (Pa)
d
Advantages:
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D
measure hardness over large area, indifferent to small scale variations in
structure;
simple and easy to conduct
used a lot for steels and irons.
Hardness Strength Relationship (for steels using 3000 kg load):
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UTS (MPa) = 3.5 HB
UTS (psi) = 500 HB
Macrohardness Testing - Rockwell
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Indenter pressed on surface of material with a minor load, then major load
Difference in depth or penetration automatically measured => HR
P=0
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Indenter Types and Scales
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Superficial Hardness
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P = minor load
Rockwell test conducted using light loads
Produces shallow indentions
Useful for evaluating surface treatments and thin materials
P = major load
Rockwell Testing
Disadvantages and Advantages
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Limitations:
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not useful for mat’ls < 1/16 in thick;
not useful for mat’ls with rough surfaces;
not useful for non-homogeneous materials (e.g. gray cast iron)
composition and structure can greatly influence results;
Advantages:
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provides direct hardness reading in a single step;
quick, easy to use;
provides for relatively small indentions that can be easily concealed
or removed via finishing.
Hardness Testing - Miscellaneous Methods
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Vickers
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Knoop
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Sceleroscope:
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Mohs:
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diamond tipped indentor or hammer enclosed in a glass tube
hardness related to rebound of indentor
scratch resistance
Durometer:
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measures hardness of rubbers, plastics and similar soft and elastic
materials.
Hardness Testing Precautions
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Location
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Thickness:
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should be far enough apart to not allow indentions to interact
Resultant Penetration Size:
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should be at least ten times the depth of penetration
Successive indentions:
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should be at least two indenter diameters from specimen edge
should be large enough to give a representative hardness value for the bulk material
Surface Prep
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not critical for Brinell
somewhat important for Rockwell
very important for tests having small indenter size (smaller the indenter size the more
surface prep is required)
polishing surface provides more accurate results.
Bending
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Normal stress
distribution in bending
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s = Mb Z/I
s = stress in bending
Mb = bending moment
Z measured from neutral
axis
I = moment of inertia
Bend Test For Brittle Materials
Shear Loading
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Translational mode of loading
Shear stress acting on shear plane
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t = F / As
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Can extend shear strength of material from tension
test via:
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As = total area of shear planes
F = transmitted load
to = s0 / (3)0.5
Linear shear (translational shear) affected
significantly by microstructural anisotropy and can
require specialized tests
Stress Concentrations
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Irregular geometries => stress concentrations
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Fillet
Radii
Notches
Holes
Simple relation for stress concentrations
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s max = kt s a = (1 + 2a/b) s a
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kt = stress concentration factor
a = dimensions of geometric irregularity perpendicular to hole
b = dimensions of geometric irregularity parallel to hole
Small cracks perpendicular to load, a >> b
Variety of kt developed through extensive
experimentation and analysis
Fracture Toughness
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All materials contain flaws or defects
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material defects (pores, cracks inclusions)
manufacturing defects (machining tool marks, arc strikes, contact damage)
design defects (abrupt section changes, excessively small fillet radii, holes)
Fractures initiate at defects
Defects have sharp geometries (a >> b) => high localized stresses =>
catastrophic failure
Unsteady crack growth occurs when elastic energy released by growth of
defect exceeds energy required to form crack surfaces.
Design equation for stable crack growth:
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K = Y s (p a ) 0.5 < Kc
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K = stress intensity factor
Y = factor depending on geometry of crack relative to geometry of part
s= applied stress
A = crack length (defect size)
Kc = critical value = fracture toughness of material
Fatigue
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Materials under cyclic stress undergo progressive damage which
lowers resistance to fracture
Fatigue failures count for 90% of all mechanical failures
Fatigue caused by simultaneous action of cyclic stress, tensile
stress, plastic strain.
Plastic strain resulting from cyclic stress initiates crack, tensile stress
promotes crack growth
Fatigue cracks typically initiate near or at “defects” that lie on or near
the surface.
Fatigue testing
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Stressed based fatigue testing
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Fatigue endurance limit = se: lower stress limit of S-N curve for which fracture does not occur ~
10 7 cycles
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Does not exist for all materials
Greatly affected by
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Strain based fatigue testing
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Presence of stress risers: (small surface cracks, machining cracks, surface gouges)
Operating temperature (increase in temp => drop in fatigue resistance)
Environment (humidity, atmosphere, interaction with cycle frequency)
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slow frequency - environment has more time to react
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higher frequency - environment has less time to react
Residual Stresses (compressive residual stresses => increase fatigue life)
Cycles to failure measured and plotted versus strain
Very useful in determining conditions for initiation fatigue
Used in designs where a major portion of the total life is exhausted in crack initiation phase of
fatigue.
Fatigue crack growth rates are measured under conditions of cyclic stress intensity
(DK) at subcritical levels (K < Kc)
Fatigue Failure
Beach or clamshell
markings: Formed when
load is changed during
service or when loading is
intermittent
Striations: finer marks
associated with position
of crack tip after each
cycle.
Creep
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Higher temp + s app < s ys => cavitation, creep elongation and
rupture of material
Tensile test subjected to constant load within high temp
environment and measure elongation with time => creep curve
Creep rate: rate of elongation in second stage, de/dt
Time to rupture: total elapsed time
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Strain, in/in
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For design s f is the creep rupture strength
I
II
de/dt
eo
Time, hours
III
Fracture
Creep - Microstructural Mechanism
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Dislocation Climb:
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Movement of dislocation
perpendicular to its slip plane by
diffusion of atoms to or from the
dislocation line
Dislocations escape from lattice
imperfections, continue to slip and
causes additional deformation of
specimen even at low applied stress
Diffusion controlled phenomenon
(therefore occurs more quickly at
higher temperatures)
Arrhenius Relationship - Creep Rate

creep rate = K s n exp (Q c / R T)

Where:

R = gas constant

T = temp, K

c, K, n= material constants

Q = Activation energy related to
self diffusion when dislocation
climb is important
Stress Corrosion Cracking (SCC)
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Combination of applied stress plus
corrosive environment =>
corrosion of part that would
normally be resistant to corrosion
Stress may be result of residual
stresses
Occurs for select metal
environment pairs only such as
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High strength Al alloys in NaCl,
seawater, water vapor
Cu alloys (including brass) in
ammonia, mercury salt solutions,
amines, water
Low carbon steel in NaOH, Nitrate
solutions, acidic Hydrogen sulfide,
seawater
400 and 300 series stainless steels
in various environments
etc,.
Physical Properties
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Thermal Properties
 heat capacity
thermal conductivity
 thermal expansion
Electrical conductivity
Magnetic Response
Weight
Density
Melting/Boiling Point
Optical Properties
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