Tension

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Tension Compression Tests

• Tension and compression tests are similar in
  uniaxial loading
• The factors that make the tension or compression
  test the more desirable in a specific case (see
  next slide)

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Compression Test Set up
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Choose between Tension Compression Tests
The factors that make the tension or
compression test the more desirable in a specific
case

• Suitability of the material to perform under a
  given type of loading
• Metals are suitable for resisting tensile loads
  tension tests for metals
• Concrete is suitable for compression
  compression test
• Differences in properties of a material under
  tensile and compressive loading ?Cgtgt?T for
  Concrete.
• Relative difficulties and complications induced
  by the gripping of or end bearing on the test
  pieces in testing.

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Compression Test Set Up

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Special Limitations

• Loading Difficulty to apply a truly axial load
• Specimen Accidental irregularities in alignment
• tendency for bending
• Friction between the heads of the testing
  machine and the end surfaces of the specimen due
  to lateral expansion of the specimen.
• Stability Need large cross-section of specimen
  to obtain a necessary stability
  Large-capacity testing machine.

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Compression Specimens

• Shape a circular section is to be preferred
• Ratio of length to diameter r ?2 (1.5ltrlt10)
• r is high increase tendency of bending
• r is low frictional restraint becomes
  relatively important
• Ends flat and perpendicular to the axis
• Gage length (Length of specimen)-diameter

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Suggested Dimensionsfor metallic materials
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Observations in Compression Tests

• Observations change in dimensions, critical
  loads, type of failure,
• Brittle materials rupture either along a
  diagonal plane, or with a cone (for cylindrical
  specimens) or a pyramidal (for square specimens)
  shaped fracture.
• Ductile materials bulge laterally,
  take a barrel shape

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Failure of Brittle Materials
Types of failure of brittle materials under
compressive loading.
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Failure of Ductile Material?
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Friction forces
bulge
bulge
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Stress Strain Curves
Ductile material
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Failure point
Nominal Stress
True stress
Failure point of brittle material
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Column Tests - Short Columns

• Short Columns
• It may fail in material
  rupture by direct compression.

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Single Column Bent Test
Lateral load through horizontal actuator with
large stroke capabilities Vertical pre-load
through static cylinders and tie rods
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Column Tests - Buckling and Instability
Long, Slender Columns It may deflect
laterally and fail by bending rather than by
direct compression of the material.
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Buckling

• Buckling A slender column under an axial
  compressive load, if lateral bending occurs
  buckling.
• Buckling failure As axial load increasing, the
  lateral deflections will increase, eventually the
  column will collapse.

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More about Buckling

• Buckling is not limited to columns Thin wall
  member, tanks
• Buckling is one of the major causes of failures
  in structures.

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The End
Compression test next week. Have fun!