Environmental Engineering Reference
In-Depth Information
Figure 3.9
Upper: Standard dog-bone-shaped test piece used in tensile
tests. Lower: Tensile deformation of a rectangular test piece. Notice
shrinking of the width. Direction of strain shown by the
double-headed
arrow at right
.
The behavior of the material in the test is best understood in terms of
the stress-strain curve.
Figure 3.10
shows typical stress-strain curves for
a dog-bone-shaped (a standard ASTM shape) test piece of a series of
glass-bead filled LDPE test pieces and a schematic for a plastic test-piece
undergoing tensile strain illustrating the slow development of a 'neck'
region. The value of
f
at the point of failure is called the tensile strength
(MPa), and the strain
ε
(%) at failure is usually expressed as a percentage
and is called the ultimate extensibility or the elongation at break. As the
strain is increased, the sample extends elastically (or recoverably) initially.
At a higher strain, a maximum force characterized by a maximum in the
stress-strain curve called the “yield point” occurs; the corresponding stress
is called the yield stress. At or about this strain, the sample develops a
“neck” and continues to extend by plastic deformation. The width of the test
piece under strain is drastically reduced, and the stress does not increase
