Charpy Machines

MPM Pendulum 9000 Test Machines

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.

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

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.

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:

• Instrumented striker system (Instrumented Impact Testing)

• Automatic pendulum return system (Automatic Pendulum Return)

• In-situ heating & cooling system (In-situ Heating & Cooling)

• Automatic specimen transfer system (Automatic Specimen Transfer)

• Precision tongs (Tongs)

• Image analysis system for percent shear determination

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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