Introduction to Basic Manufacturing Processes and ... - always yours

Porperties and Testing of Metals 127
V notch in metal is to ensure that the specimen will break as a
result of the impact load to which it is subjected. Without the
notch, many alloys would simply bend without breaking, and it
would therefore be impossible to determine their ability to absorb
energy. It is therefore important to observe that the blow in Charpy
test is delivered at a point directly behind the notch and in the
Izod test the blow is struck on the same side of the notch towards
the end of the cantilever. Fig. 7.10 shows the impact testing set
up arrangement for charpy test. The specimen is held in a rigid
vice or support and is struck a blow by a traveling pendulum that
fractures or severely deforms the notched specimen. The energy
input in this case is a function of the height of fall and the weight
of the pendulum used in the test setup. The energy remaining
after fracture is determined from the height of rise of the pendulum
due to inertia and its weight. The difference between the energy
input and the energy remaining represents the energy absorbed by
the standard metal specimen. Advance testing setups of carrying out
such experiments are generally equipped with scales and pendulumactuated
pointers, which provide direct readings of energy absorption.
Specimen
Fig. 7.10 Schematic impact
testing machine setup
7.3.5 Testing of Fatique
Material subjected to static and cyclic loading, yield strength is the main criterion for product
design. However for dynamic loading conditions, the fatigue strength or endurance limit of
a material is used in main criteria used for designing of parts subjected to repeated alternating
stresses over an extended period of time. Fig 7.11 shows a fatigue test set up determining
the fatigue strength of material. The fatigue test determines the stresses which a sample of
material of standard dimensions can safely endure for a given number of cycles. It is performed
on a test specimen of standard metal having a round cross-section, loaded at two points as
a rotating simple beam, and supported at its ends. The upper surface of such a standard test
specimen is always in compression and the lower surface is always in tension. The maximum
stress in metal always occurs at the surface, halfway along the length of the standard test
specimen, where the cross section is minimum. For every full rotation of the specimen, a
point in the surface originally at the top centre goes alternately from a maximum in compression
to a maximum in tension and then back to the same maximum in compression. Standard test
specimens are tested to failure using different loads, and the number of cycles before failure
is noted for each load. The results of such tests are recorded on graphs of applied stress
against the logarithm of the number of cycles to failure. The curve is known as S-N curve.
Test S pecim en
Ball
Bearing
Shaft
Shut Off Switch
Flexible Coupling
Revolution
Counter
Shaft
Weights
Motor
Fig. 7.11 Schematic fatigue test setup