Overview of Mechanical Testing

1
Overview of Mechanical Testing
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The Role of Testing

• Material test data typically used for
• 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
• Common test methods for material properties
• What constitutes a valid test
• Applicability of data / limitations of tests

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Types of material testing

• Commercial testing
• 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
• Materials research testing
• 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.

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Types of Tests

• 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

• 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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Significance of Tests

• 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

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Materials Properties

• Mechanical
• Microstructure Sensitive
• Describe how a material responds to an applied
  force
• Physical
• Microstructure Insensitive
• Describes a materials response to an applied
  field or chemical

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Stress conditions

• Fundamental stress conditions describe mechanical
  behavior features of components and assemblies
• Axial tension or compression
• Bending, shear or torsion
• Internal/external pressure
• Stress concentrations and localized contact
  loads.

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Tensile Loading

• Axial tensile loading
• s F/A
• Design such that sapp lt s failure
• Where s failurecan be
• su (UTS) if fracture is criterion for failure
• 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
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Stiffness in Tension

• Elastic deformation governed by stiffness
• D L e L
• e strain
• L length of bar
• n et/el Poissons ratio
• In the elastic range of deformation
• s E e
• E elastic modulus
• Can be considered a physical property because it
  is fundamentally related to bond strength, not
  affected much by microstructure
• Can vary with direction if material has
  anisotropic structure
• Design of stiffness critical applications
• D L FL/AE lt d
• d design limit change in length

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Load cell

• 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
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Clip on extensometer

• 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
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Tensile Testing - Engineering Stress Strain Curve
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Tensile Testing - Engineering Stress vs. Strain
Curve

• Yield Point
• 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
• 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
• area under stress strain curve in elastic region
• indicates amount of energy a material can absorb
  in elastic range
• Toughness
• 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 Affinal area

14
Askeland, Phule The Science and Engineering of
Materials
15
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

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Compression loading

• Isotropic materials
• suc equal to suT
• Anisotropic materials
• suc not equal to suT
• Buckling may preceed other forms of failure
• sb (p 2 E I)/(L2 A)
• I moment of inertia of cross section of bar

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Dynamic Properties

• 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
• V velocity of mass, m
• A, l cross-sectional area and length of bar
• E elastic modulus
• Impact tests
• Charpy, Izod, Hopkinson bar, Others
• Factors that affect data
• loading rate
• faster gt less energy, higher transition
  temperature
• slower gt more energy, lower transition
  temperature
• specimen size and configuration
• smaller energies might be required to break
  thicker samples
• notch configuration
• Impact data should be used comparatively
• materials screening
• not appropriate for design data
• Temperature

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Transition temperature

• Temp at which a material changes from ductile to
  brittle
• BCC metals have distinct transition temp
• FCC metals do not have distinct transition temp.

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Impact Tests
Brittle Material

• Impact Resistance
• matls ability to withstand a sudden intense,
  blow
• Toughness
• provides a measure of impact resistance
• ability of matl to absorb energy prior to
  failure
• area under true stress - true strain curve
• low toughness
• clean break
• brittle material
• little to no plastic deformation
• high toughness
• significant plastic deformation
• ductile material

True stress, psi
True strain, in/in
Ductile Material
True stress, psi
True strain, in/in
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Impact Tests -Charpy and Izod

• 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
• Izod

Pendulum
Specimen (10 x 10 x 55 mm)
Notch
Pendulum
Specimen (10 x 10 x 75 mm)
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Impact Tests - Izod and Charpy

• Potential Energy Difference
• 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
• 1 ft -lb 1.356 joules

1
2
ho
hf
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Hardness Testing

• 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
• Macrohardness
• Brinell, Rockwell, etc.
• Destructive test
• Microhardness
• Vickers, Knoop, etc.
• Non destructive test

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Macrohardness Testing - Brinell

• Steel or tungsten carbide ball (10 mm dia)
  pressed against material
• 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))
• Where
• P applied load kg
• D diameter of ball mm
• d diameter of resultant penetration mm
• BHN Brinell Hardness Number (Pa)
• Advantages
• 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)
• UTS (MPa) 3.5 HB
• UTS (psi) 500 HB

P
D
d
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Macrohardness Testing - Rockwell

• Indenter pressed on surface of material with a
  minor load, then major load
• Difference in depth or penetration automatically
  measured gt HR
• Indenter Types and Scales
• Superficial Hardness
• Rockwell test conducted using light loads
• Produces shallow indentions
• Useful for evaluating surface treatments and thin
  materials

P 0
P minor load
P major load
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Rockwell TestingDisadvantages and Advantages

• Limitations
• not useful for matls lt 1/16 in thick
• not useful for matls with rough surfaces
• not useful for non-homogeneous materials (e.g.
  gray cast iron)
• composition and structure can greatly influence
  results
• Advantages
• 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.

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Hardness Testing - Miscellaneous Methods

• Vickers
• Knoop
• Sceleroscope
• diamond tipped indentor or hammer enclosed in a
  glass tube
• hardness related to rebound of indentor
• Mohs
• scratch resistance
• Durometer
• measures hardness of rubbers, plastics and
  similar soft and elastic materials.

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Hardness Testing Precautions

• Location
• should be at least two indenter diameters from
  specimen edge
• Thickness
• should be at least ten times the depth of
  penetration
• Successive indentions
• should be far enough apart to not allow
  indentions to interact
• Resultant Penetration Size
• should be large enough to give a representative
  hardness value for the bulk material
• Surface Prep
• 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.

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Bending

• Normal stress distribution in bending
• s Mb Z/I
• s stress in bending
• Mb bending moment
• Z measured from neutral axis
• I moment of inertia

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Bend Test For Brittle Materials
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Shear Loading

• Translational mode of loading
• Shear stress acting on shear plane
• t F / As
• As total area of shear planes
• F transmitted load
• Can extend shear strength of material from
  tension test via
• to s0 / (3)0.5
• Linear shear (translational shear) affected
  significantly by microstructural anisotropy and
  can require specialized tests

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Stress Concentrations

• Irregular geometries gt stress concentrations
• Fillet
• Radii
• Notches
• Holes
• Simple relation for stress concentrations
• s max kt s a (1 2a/b) s a
• 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 gtgt b
• Variety of kt developed through extensive
  experimentation and analysis

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Fracture Toughness

• All materials contain flaws or defects
• 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 gtgt b) gt high
  localized stresses gt 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
• K Y s (p a ) 0.5 lt Kc
• 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

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Fatigue

• 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.

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Fatigue testing

• Stressed based fatigue testing
• Fatigue endurance limit se lower stress limit
  of S-N curve for which fracture does not occur
  10 7 cycles
• Does not exist for all materials
• Greatly affected by
• Presence of stress risers (small surface cracks,
  machining cracks, surface gouges)
• Operating temperature (increase in temp gt drop
  in fatigue resistance)
• Environment (humidity, atmosphere, interaction
  with cycle frequency)
• slow frequency - environment has more time to
  react
• higher frequency - environment has less time to
  react
• Residual Stresses (compressive residual stresses
  gt increase fatigue life)
• Strain based fatigue testing
• 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 lt Kc)

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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.
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Creep

• Higher temp s app lt s ys gt cavitation, creep
  elongation and rupture of material
• Tensile test subjected to constant load within
  high temp environment and measure elongation with
  time gt creep curve
• Creep rate rate of elongation in second stage,
  de/dt
• Time to rupture total elapsed time
• For design s f is the creep rupture strength

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Creep - Microstructural Mechanism

• Dislocation Climb
• 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

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Stress Corrosion Cracking (SCC)

• Combination of applied stress plus corrosive
  environment gt 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
• 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,.

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Physical Properties

• Thermal Properties
• heat capacity
• thermal conductivity
• thermal expansion
• Electrical conductivity
• Magnetic Response
• Weight
• Density
• Melting/Boiling Point
• Optical Properties