Material Testing – Engineering Notes

Material Testing – Engineering Notes

Material Properties

Strength: The ability of a material to withstand forces of compression, tension, shear and torsion.

Tensile Strength: The maximum pulling stress a material can withstand without fracture.

Torsional Strength: The ability to resist twisting forces.

Hardness: The ability to resist abrasion, wear, indentation and scratching.

Ductility: Ability of a material to be stretched into a wire (e.g. copper).

Malleability: The ability to be flattened or shaped without rupture.

Toughness: Ability to withstand blows or impact.

Elasticity: Ability to return to original shape when freed from force distorting it.

Conductivity: Ability of a material to allow heat or electricity to flow though it.

General Definitions, Terms & Formulae

Young’s Modulus: A measure of stiffness in a material

stress/strain

Creep: Creep is the slow deformation of a metal over time, resulting from a constant load/force acting on the metal. Factors that contribute to creep include time, temperature and nature of the force. E.g. Lead is prone to creep.

Ultimate Tensile Strength (Max load)

Screen Shot 2013-05-11 at 14.21.10

Proof Stress

Screen Shot 2013-05-11 at 14.21.42

Fatigue: Fatigue is failure due to cyclic stressing (on/off loading). Failure begins as a crack and grows/spreads until failure due to stress. Factors that contribute to fatigue are sharp corners, time of exposure, nature of force and corrosion.

Factor of Safety: Is the degree of structural capacity beyond applied loads.

Non Destructive Testing (NDT)

X-ray Testing

  • Radiation is passed through the material by an X-ray tube.
  • If no faults are present, absorption is uniform on the negative photographic film.
  • If a fault is present in the weld, less radiation is absorbed, thus producing a variation in the intensity of the emergent beam.
  • This is detected on the negative photographic film.
  • Where the defect exists, a dark spot forms.
X-Ray Testing

X-Ray Testing

Eddy Current Testing

  • A coil is charged with high frequency alternating current is placed close to a material.
  • This produces Eddy Currents on the material.
  • A magnetic field is created in the piece by the current.
  • A defect will alter this magnetic field which will then be located by a search coil.
  • This records and displays the faults.

Ultrasound/Ultrasonic testing

  • Based on reflection of signals.
  • High frequency vibrations are sent through the piece.
  • These are reflected back once they reach the opposite surface of piece.
  • If a flaw is present, the vibrations will be reflected back from it(the flaw). The amount of time for which it takes the vibration to return will be shorter than if no flaw were present.

Penetrant Dye test

  • A dye is applied to the piece being tested and allowed to soak in for a sufficient period of time.
  • If surface cracks are present, these will absorb the dye.
  • The piece is then wiped clean.
  • Chalk dust is then placed on the piece. This will absorb the dye in the crack and illustrate where the flaws are present.

Advantages of Non-Destructive Testing

  • More economical as piece can be used if no flaw is present, especially if piece to be tested is expensive to manufacture.
  • Can be applied to every piece.

Destructive Testing

Impact testing

Impact Testing

Impact Testing

Hardness Testing

Brinnel

  • Hardened Steel ball indenter.
  • Not suitable for thin materials.
  • Based on surface area of impression.

Vickers

  • Pyramid indenter.
  • 136° point angle on indenter.
  • Length of Diagonal of impression measured.
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