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Impact Testing

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Chapter 6 Mechanical Testing and Properties

6.7 The Impact Test impact strength

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

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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.

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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.

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Brittle cleavage fracture

The basic factors which contribute to abrittle cleavage type of fracture are:

The tri-axial state of stress (stressconcentration)

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

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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)

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Charpy test

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

The specimen is supported at two ends asa 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.

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

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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 acantilever.

Struck with the pendulum hammer at the

opposite end on the side of the notch.

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Impact tester

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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.

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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.

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A steel ductile and insensitive to a notchat one T may be brittle at a lowertemperature.

A steel that appears ductile and insensitiveto notch at slower strain rate may be brittleat high strain rate.

Small notched specimen may be ductilewhile geometrically large specimen maybe brittle

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Metallurgical Factor Affecting

Transition Temperature Decreasing Carbon lowering TransitionTemperature.

P,N,O,H are undesirable element--- increase the

tendency towards brittle fracture. Ni improve toughness

A ratio ofMn/C 3:1 is desirable

Coarse grain size higher the transition

temperature. Inclusions ,oxides, raises the transition

temperature.

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Plane stress and Plane strain

Plane Stress : State of stress in which one or two of thepairs of faces on an elemental cube are free of anystresses.

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 offaces on an elemental cube undergoes zero strain.

e.g Torsion of thin walled tube. Piece of material beingcompressed in a die .

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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

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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 tohave zero values of each.

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Test in the workshop

Closed bend test: the piece is bend onitself and flattened.

Angle bend test: The metal is bent over aformer. The radius of the nose of theformer and the angle of bend are fixed .

Reverse bend test: The metal is bent

round a former of specified nose radiuseither through 900 or 1800 for a specifiednumber of times.

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ERICHSEN (ERICKSON) TEST

-For many years there has been need of a rapid test whichcould be used to differentiate between different sheetmetals 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 capabilityof deformation of a certain sheet metal.

-Erichsen test is a very popular test which gives anexcellent account of current metallurgical practice inproducing pressed and deep drawn components..

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Erichsen number

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

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

-This depth (mm) gives the Erichsen

number.

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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.

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After stretching the original circle has

deformed into an ellipse.

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

Minor axis of the ellipse represents the

stretching (positive) or shrinking (negative)

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Notch-Toughness

Notch toughness is the ability that a material possessesto absorb energy in the presence of a flaw. Asmentioned previously, in the presence of a flaw, such as

a notch or crack, a material will likely exhibit a lower levelof toughness. When a flaw is present in a material,loading induces a triaxial tension stress state adjacent tothe flaw. The material develops plastic strains as theyield stress is exceeded in the region near the crack tip.

However, the amount of plastic deformation is restrictedby the surrounding material, which remains elastic.When a material is prevented from deforming plastically,it fails in a brittle manner

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