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Tensile Testing
The following MATERIAL
PROPERTIES can be evaluated /
determined by TENSILE TESTING:
•
STRENGTH
•
DUCTILITY
•
ELASTICITY
•
STIFFNESS
Material Properties
STRENGTH - the greatest stress that the material can
withstand prior to failure.
DUCTILITY - a material property that allows it to undergo
considerable plastic deformation under a load before
failure.
ELASTICITY - a material property that allows it to retain its
original dimensions after removal of a deforming load.
STIFFNESS - a material property that allows a material to
withstand high stress without great strain.
The Tensile Tester
A machine which applies a tensile force (a
force applied in opposite directions) to the
specimen, and then measures that force
and also the elongation:
This machine usually uses a hydraulic
cylinder to create the force. The applied
force is determined by system pressure,
which can be accurately measured.
The Tensile Tester
Applied Force is hydraulic pressure times
the area of the cylinder’s piston.
F = PRESSURE x AREA
=
2
0.6N/mm
x
2
1962.25mm
=1177.35 N
Tensile Tester
Data Acquisition Unit
Stress - Strain Curve
THE SAMPLE
Area of measured test
Marking the sample:
A precision punch with
two points exactly
2.000” apart is used to
mark the sample in the
tested region.
This establishes the
original length.
This device, called an
EXTENSOMETER,
measures the distance
between the two prongs
very precisely while the
test is in progress.
Before
After
A Stainless Steel
sample is loaded in
the tester.
Terms
Necking: The localized decrease in diameter in a
specimen near the rupture point.
Elastic Deformation Region: The area of a stress strain curve where the specimen will deform under load, yet
return to its original shape when the load is removed.
Plastic Deformation: Deformation that occurs once
the object has been stressed past its elastic limit. The
deformation is no longer reversible.
Necking
“Necking” occurs as
the sample leaves
the elastic
deformation region
and begins to
deform plastically.
The classic cup &
cone shape of a
fairly ductile tensile
fracture is visible
here.
Upon completion of the test, the sample is
reassembled and final measurements for total
elongation and minimum diameter are made
using a vernier caliper.
For maximum precision, the points of the vernier
caliper must be placed exactly at the center of the
marks made by the punch prior to the test.
Compare the material properties of
these three metal samples.
All three failed under tension
PLASTIC DEFORMATION REGION
ELASTIC DEFORMATION REGION
The point of departure from the elastic
deformation region is known as the “yield
point.”
In the elastic region, the slope of this
line is the material’s Modulus of
Elasticity.
Stress - Strain Plot