©
COPYRIGHT 1999 THE ESAB GROUP, INC. LESSON
I, PART A a sharp
abrupt load is applied to it. An impact tester measures the degree of susceptibility
to what is called brittle fracture.
1.6.6.1 The
impact specimen is machined to exact dimensions (Figure 6) and then
notched on one side. Quite often,
the notch is in the form of a "V" and the test in this case
is referred to as a Charpy V-Notch
Impact Test. The specimen is cooled to a predetermined
temperature and then placed in a stationary clamp at the base of the testing
machine. The specimen is in the
direct path of a weighted hammer attached to a pendulum
(Figure 6). 1.6.6.2
The hammer is released from a fixed
height and the energy required to fracture the
specimen is recorded in ft-lbs. A specimen that is cooled to -60°F
and absorbs 40 ft-lbs of
energy is more ductile, and therefore, more suitable for low temperature service
than a specimen
that withstands only 10 ft-lbs at the same temperature. The specimen that
withstood 40 ft-lbs energy is said
to have better toughness or notch toughness. 1.6.7
Fatigue
Strength
- A metal will withstand a load less than its ultimate
tensile strength
but may break if that load is removed and then reapplied several times. For
ex- ample, if
a thin wire is bent once, but if it is bent back and forth repeatedly, it will
eventually fracture
and it is said to have exceeded its fatigue strength. A common test for
this strength
is to place a specimen in a machine that repeatedly applies the same load first
in tension and
then in compression. The fatigue strength is calculated from the number
of cycles the
metal withstands before the point of failure is reached.
FIGURE 6 FRACTURES
CRACKS
DEFORMS CHARPY
V-NOTCH SPECIMEN
ENERGY IN
FT/LBS CHARPY
IMPACT TEST MACHINE CHARPY
V-NOTCH IMPACT TEST
