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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.