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MPM Pendulum 9000 Test
Machines
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| MPM Pendulum Impact Test Machine |
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| Close-up Photograph Showing Test
Specimen Support |
The MPM Charpy/Izod 9000 Series impact test machines can be
used with pendulums in the 100 ft-lb to 700 ft-lb energy capacity range.
Since the machine frame has been designed for up to 700 ft-lbs, any lower
capacity can be accommodated. Therefore, you can upgrade your machine capacity
later by purchasing a higher capacity pendulum without the need to buy an entire
test machine. MPM will provide the test machine with the
customer-specified energy capacity at the time of purchase. As discussed
later, the accuracy of the energy measurement for higher capacity machines is
equivalent to that for low capacity machines through the use of advanced encoder
technology.
Research at MPM
over the past decade has shown that there are differences between the optical
encoder energy and the instrumented striker determined energy. This web site
contains journal articles (see
Charpy Testing Articles)
on this subject and the research findings will not be reviewed here other than
to note two key elements in test machine design that must be addressed to ensure
accurate energy determination. The first is reduction of pendulum vibration,
and the second is elimination of post-fracture test specimen interaction
with the striker. Vibration in the arm is undesirable because it is incorrectly
recorded by the dial/encoder as energy absorbed by the test piece. MPM has
performed extensive finite element design calculations to develop a design that
has natural frequencies which are substantially different from those generated
during the impact event. This design ensures that modes are not excited in the
pendulum, which would affect the energy measurement. The second important
design issue, interaction of the test specimen with the hammer, is discussed in
the (next web page) section below.
MPM's Z-Hammer™ Design
Specimen contact with the striker or hammer after
fracture of the test piece is highly undesirable because it removes energy
from the pendulum, which is then incorrectly recorded by the
dial/encoder as absorbed energy. In U-hammer test machines, this presents a
problem for tests conducted in the transition region. High speed photography
has confirmed that the specimen halves that exit the front of the test machine
rebound off the U-hammer exit channel and can interact with the striker
several times after the fracture event has been completed. The C-hammer
design has a related problem, experienced when ductile test specimens
exit the front of the test machine. In some designs, the specimen rides on
the lower portion of the hammer and adds energy to the dial.
MPM has developed the Z-hammer™ design to overcome
problems associated with existing C- and U-hammers. The Z-hammer™ and C-hammer have
the advantage that there is no material on either side of the swing plane, and
therefore the broken specimen halves move away from the striker after the
fracture event. In addition, the lower Z-hammer™ portion has been designed so
that it is impossible for a test specimen to drop down and ride on the hammer
as occurs with the C-hammer design.
Energy and Impact Velocity
Energy and
impact velocity are two key measurements made during an impact test. MPM
pendulum impact test machines measure the energy absorbed in fracturing a test
specimen using an optical encoder with 36,000 divisions per revolution. This
is the finest division encoder ever used in Charpy testing history and
is capable of resolving energy to a much finer resolution than can be achieved
with any dial. For example, the MPM encoder resolves energy to within 0.03
ft-lbs on 400 ft-lb pendulum machines. Experience
at MPM has shown that it is not possible to resolve a dial indicator to within
better than about 0.25 ft-lbs. Therefore, the 100 year-old dial has been
eliminated from the MPM test machines. The encoder data is acquired by the ImpactTM v 4.3 software and used to determine the absorbed energy
by calculating the height of the striker contact point before release and the
maximum height attained after impact. In addition, the velocity of the
striker is recorded from release up to termination of the impact event.
Therefore, the exact velocity at impact is recorded in the test record for
each test. The software provides a printed test record which includes key
information such as the test specimen ID, the date of the test, name of the
operator, test temperature, impact velocity, measured energy, and much
more.
Sample Charpy Impact Data
Sample Charpy
impact data for a Charpy pendulum impact test conducted on a steel specimen is
given below. The encoder is used to measure the velocity from release to
impact. The report summarizes the key test parameters and energy
measurement. Additional data can be obtained by instrumenting the striker.
Please refer to the
Instrumented Impact Testing web page for an example of instrumented
test results.
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Click below to enlarge and enhance photo |
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| Sample Charpy Impact Data |
Windage And Friction Correction
Another important benefit
associated with the encoder technology is that the windage and friction
correction is much more accurate than can be achieved using a dial. The MPM
system automatically corrects for windage and friction on every test. A free
swing is performed to determine the windage and friction correction without a
test specimen. During an actual test, the software applies the correction
only to the height of the swing after specimen impact. The program contains a
geometric function which properly scales the correction.
Continuous Velocity Adjustment
MPM pendulum
and drop weight test machines are also equipped with a continuous hammer
release height adjustment. Some pendulum manufacturers refer to this as a
“low-blow” fixture. Adjusting the hammer release height enables testing at
velocities from 0 to v = (2gh)0.5, where v is the impact velocity,
g is the gravitational constant, and h is the release height. The software
has an equation to determine the local gravitational constant based on
elevation above sea level and latitude.
Determination Of Radius To Cop
An important
element in pendulum machine design is to ensure that the test specimen is
impacted with the center-of-strike (COS) coincident with the
center-of-percussion (COP). This results in very little (theoretically zero)
shock and absorbed energy being transmitted into the test machine frame. The
MPM ImpactTM v 4.3 software has an automatic algorithm for
measuring the radius to COP. The radius to COP measurement is extremely
accurate, with a standard deviation on the order of 0.001 inches. MPM
calibrates the test machine to be certain that the radius to COP is within
0.01 inches of the radius to COS. The program uses this exact radius in the
energy measurements to determine the peak height of the strike point before
and after impact. This algorithm has been shown to significantly improve the
accuracy of the encoder energy measurement. MPM also offers a service for
older pendulum machines to measure and correct the COP.
Automatic Specimen Identification and
Grouping
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The scanner and push button
consolette provide a
streamline high specimen volume throughput. |
MPM offers a bar code scanner which can automatically
read the specimen ID for a single specimen and for a group of specimens (see
figure below). In addition, MPM offers a camera system for both bar code
reading as well as for alpha numeric character identification. These scanners
can be used with the specimen grouping option to streamline testing and to
produce automatically calculated means and standard deviations for a
predefined number of test specimens.
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| Label ID Dialog Options Box |
The Impact™ software basic group testing is performed by
scanning or manually entering the base sample ID in the text box located on
the “Run Test” screen. Then, when the first test for the group of specimens
is to be performed, the “Drop Impactor” button is depressed. The Sample ID
then has a numeric designator appended to it. The designator is a dash
followed by a sequential numbering (e.g. “SampleID-1”). This designator is then
incremented as each test in the group is performed until the total number of
samples in the group have been tested. After the entire group has been
processed, a summary report is generated that includes key values for the
individual tests (absorbed energy, and/or lateral expansion, and/or percent
shear) and the mean and standard deviations for the group of tests. During
the group test sequence, the mouse or the push button on the consolette can be
used to release the pendulum.
Options
The pendulum impact test machine can be provided with several options:
System Upgrades
Existing test machines can be upgraded by addition of the
MPM instrumented striker system, encoder energy/velocity system, in-situ
heating and cooling system, and automatic hammer return system. MPM will
assemble the system and provide a field installation and calibration if
desired. Shown below is an example of an upgraded test machine (Tinius Olsen
Model 84) that includes the MPM encoder system, instrumented striker system,
and in-situ heating and cooling system.
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Upgraded Test Machine Showing
MPM Enhancements |
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