Impact Testing

Chapter 6 Mechanical Testing and Properties

6.7 The Impact Test impact strength

Yield Strength: A > B Impact Strength: B > A

Need for toughness testing

• The sudden and complete failure of structures at stress well below their yield strength.

• Indicated that other considerations besides the conventional tensile properties must be included during the design of such structures.

• Analysis of these failures and reanalysis of previous failures indicated that the fracture usually initiated at “notches”

• Design failure: Two rigid member attached at a sharp angle(900 or less).

• Fabrication procedure: Weld arc strikes, Tool gouges.

• Flaws in the material: Flakes, porosity.

Brittle cleavage fracture

• The basic factors which contribute to a brittle cleavage type of fracture are:

• The tri-axial state of stress (stress concentration)

• A low temperature (often sub zero)• A high strain rate or rapid rate of loading

(impact testing)• Factors one and two are responsible for

most of the brittle failure

Impact Testing

• Dynamic load is applied suddenly- as from the result of a moving pendulum hammer to determine:

• Notch toughness of the material.

• Ductile to brittle transition temperature.

• Two types of notched bar impact test are:

i) Izod (UK) ii) Charpy (USA)

Charpy test

• 10mm sq steel bar, V notch at 45 is cut across its on edge. 2mm deep with a 0.25 mm root radius.

• The specimen is supported at two ends as a simple beam.

• Struck a blow with a blunt hardened knife edge with a fixed amount of K.E

• On the opposite side and directly behind the notch.

To evaluate the brittleness of a material subjected to a sudden blow.

Izod test

• Either circular or 10 mm sq in x-section and contains a V-notch near one end.

• The specimen is clamped vertically like a cantilever.

• Struck with the pendulum hammer at the opposite end on the side of the notch.

Impact tester

Transition temperature curve

• Notched bar impact test is more meaningful when conducted over a range of temperature.

• The temperature at which ductile to brittle transition takes place can be determined.

Chapter 6 Mechanical Testing and Properties

6.7 The Impact Test impact strength

Impact strength vs. Temperature

Note: BCC metals have transition temperature, but most FCC metals do not.

• A steel ductile and insensitive to a notch at one T may be brittle at a lower temperature.

• A steel that appears ductile and insensitive to notch at slower strain rate may be brittle at high strain rate.

• Small notched specimen may be ductile while geometrically large specimen may be brittle

Metallurgical Factor Affecting Transition Temperature

• Decreasing Carbon lowering Transition Temperature.

• P,N,O,H are undesirable element--- increase the tendency towards brittle fracture.

• Ni improve toughness• A ratio of Mn/C 3:1 is desirable• Coarse grain size higher the transition

temperature.• Inclusions ,oxides, raises the transition

temperature.

Plane stress and Plane strain

• Plane Stress : State of stress in which one or two of the pairs of faces on an elemental cube are free of any stresses.

• e.g Torsion of thin wall tube. Expansion of a thin walled spherical shell under internal pressure.• Plane strain: State of tress where one of the pair of

faces on an elemental cube undergoes zero strain. • e.g Torsion of thin walled tube. Piece of material being

compressed in a die .

Draw ability

• Deep drawing test i.e. Plasticity ability of the material to be formed into desired shape without rupture.

• Ductility : the plasticity exhibited by a material under tension.

• A measure of ductility is elongation

• A measure of ductility is reduction

Cont…

• Both identify ductility.

• However there is no relationship between elongation and reduction in area.

• Elongation: A measure of stretching.

• Reduction: A measure of contraction.

• No ductile material has been reported to have zero values of each.

Test in the workshop

• Closed bend test: the piece is bend on itself and flattened.

• Angle bend test: The metal is bent over a former. The radius of the nose of the former and the angle of bend are fixed .

• Reverse bend test: The metal is bent round a former of specified nose radius either through 900 or 1800 for a specified number of times.

ERICHSEN (ERICKSON) TEST

-For many years there has been need of a rapid test which could be used to differentiate between different sheet metals for use in metal forming operations.

-Cold forging, cold extrusion, cold rolling, tube-sinking or wire-drawing.

- Erichsen test is used to determine the relative capability of deformation of a certain sheet metal.

-Erichsen test is a very popular test which gives an excellent account of current metallurgical practice in producing pressed and deep drawn components.

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Erichsen number

-In the Erichsen test, the punch is pressed into the sheet until fracture occurs.

-This point the test is stopped immediately and the depth of the bulge noted.

-This depth (mm) gives the Erichsen number.

Forming limiting diagram (FLD)

• Sheet blank is marked with a grid pattern of circles or similar pattern (chemical etching or photo printing).

• The blank is stretched over an un lubricated punch and the deformation of the circle is observed and measured.

• The circles are made as small as possible.

• After stretching the original circle has deformed into an ellipse.

• The major axis of the ellipse determine major strain (always positive).

• Minor axis of the ellipse represents the stretching (positive) or shrinking (negative)

• Notch-Toughness• Notch toughness is the ability that a material possesses

to absorb energy in the presence of a flaw. As mentioned previously, in the presence of a flaw, such as a notch or crack, a material will likely exhibit a lower level of toughness. When a flaw is present in a material, loading induces a triaxial tension stress state adjacent to the flaw. The material develops plastic strains as the yield stress is exceeded in the region near the crack tip. However, the amount of plastic deformation is restricted by the surrounding material, which remains elastic. When a material is prevented from deforming plastically, it fails in a brittle manner

To evaluate the brittleness of a material subjected to a sudden blow.