(4) CHARPY IMPACT
TESTING
Group:
There
is no old definition
NEW DEFINITION
In general, impact tests are
performed to measure the response of a material to dynamic loading. The most
common laboratory test configurations are the pendulum machine and the drop
tower. The results obtained from a standard impact tests are usually a single
value of the impact energy or energy spent on a single specimen. This is of
limited value in describing the dynamic behavior of a particular sample
material. Therefore, instrumenting an impact machine yields information on the
impact forces, impact velocities, displacements, and strain energies of the
striker at any time during the dynamic test. Figure 10.5 shows a conventional
Charpy impact testing machine used to measure fracture toughness of a
three-point bending specimen (3PB) under an impact loading system at low
velocity.
Impact loads
generate high strain rates in solid materials. For instance, con-ventional and
instrumented Charpy impact testing machines impart low strain rates at low
velocity when compared to ballistic impact velocity. The former technique has
been used for characterizing the dynamic behavior of some par-ticular composite
materials [25-32] and the latter technique promotes impact at a high velocity,
which varies according to the type of gun projectile being used. Excellent work
in the ballistic field can be found elsewhere [33-34 The instrumented Charpy
impact machine remains a key means for fracture toughness testing due to its
low cost, convenience, reliability based on certifi-cation standards, and
simple use. A particular instrumented Charpy impact machine is shown in Figure
10.6. Thus, the transient load history during a Charpy test is readily obtained
by placing strain gages on the striker so that it becomes the load cell. Using
software during an impact one can record the displacements by integrating the
acceleration versus time twice with respect to time. The accuracy of these
measurements may be affected by the internal forces in the striker, variations
in the contact force distribution between the striker and the specimen, striker
geometry, and by strain gage location on the striker.
Figure 10.7
illustrates a typical load history for a relevant case.
(Nestor
Perez, Fracture Mechanics, pg.248)
(5) DAMAGE
TOLERANCE (FRACTURE MECHANICS)
Group:
There
is no old definition
NEW DEFINITION
Problems of fracture mechanics
are solved using two different approaches. In the first approach, component
geometry which includes the length, location and orientation of the crack is
given along with boundary conditions. The objective is to find the upper limit
of the applied load that would not cause catastrophic failure of the component.
In the second approach, known
as damage tolerance, the maximum load on a component is known; the objective is
to find the longest length of a crack that remains dormant. Once we know the
length, the structural component can be thoroughly checked with an appropriate
non-destructive test. In the case of fluctuating loads applied on the component,
a fatigue crack may be nucleated even at a surface which was previously crack
free. This crack may grow with fluctuating loads. In such situations, critical
components are checked regularly. If a crack that is likely to grow and become
critical is detected, then the component is repaired or replaced. On the other
hand, detection of a small crack should not cause panic because its length may
be much smaller than the maximum length of crack allowed in damage tolerance
analysis.
These days many companies have started
believing in avoiding a likely catastrophic failure by regular non-destructive
tests of critical components. For example, a chemical company making urea in
Kanpur city maintains an excellent nondestructive test department. Its
engineers mostly face problems at the pipe-joints and thus they regularly'
check the joints, identify cracks and take necessary actions. In the long run,
it saves considerable expenses because a catastrophic failure through the
growth of a crack may cause extensive damage to other parts, besides causing
shut-down of the plant and loss of human lives in some cases.
Damage tolerance of metals
Typically, damage tolerance
design/analysis involves assessing the longevity of a structure that contains a
preexisting flaw (or crack) in a critical location. Assessing the damage
tolerance of a given design against its anticipated use has become an integral
step during design/development and structural sizing. For parts already in
service, periodic reassessment during the service life is essential. In the
case of an aircraft, for example, fatigue loads spectra are updated periodically
using loads data collected from the in-flight load recorder, If no crack is
found during a scheduled inspection of a given part, the structural life of
that part will be reassessed using the updated fatigue loads spectrum (for that
part) and the original assigned initial crack size. If a crack of any size is
found, the newly detected crack size will be used as the analytical initial
crack size. The useful life of that part will be the recalculated life plus the
life already in-service. This revised life is used to check against the
in-tended service life. Corrective actions should be taken as required. A
fracture-mechanics-oriented damage tolerance design/analysis procedure consists
of the following steps:
·
Establish
design criteria suitable to the expected use of the vehicle, machinery etc.
·
Identify
structural elements and the expected loading conditions.
·
Develop
a systematic means of identifying the criticality of these structural elements.
·
Check
the actual criticality of the questionable structural elements by using the
best available fatigue and fracture mechanics methodology, and perform trade
studies regarding safety, weight, and cost. The fracture mechanics analysis
also helps to establish reliable in-service inspection intervals and to
periodically update the fracture critical parts list.
(Kumar,Kumar
Prashant, Elements Of Fracture Mechanics, pg.7)
(A.
F. Liu, Mechanics and Mechanisms of Fracture: An Introduction, pg.215)
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