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How do I Verify Specimen Alignment on my VFD Automatic Machine?

ASTM C39 requires "Verification of Alignment When Using Unbonded Caps." This article explains how to perform this check on a VFD automatic machine.

According to ASTM C39, when using neoprene unbonded caps (i.e., pad caps and retaining rings), you need to verify proper alignment of the specimen within the testing machine (sometimes called a "perpendicularity check"). To do this, you apply some load to the specimen, but less than 10% of the expected strength of the specimen.  After applying this load, you check alignment and centering of the specimen.  If the specimen does not meet the requirements of the specification, you need to release the load, correct the issue, and check again. If the specimen meets the requirements, you can run the test.  Here are the steps involved in doing this on a Forney VFD automatic machine:

  1. Properly insert the specimen and unbonded caps into the testing machine.
  2. Press and hold the Jog Advance button to close the gap between the specimen and the upper breaking head.
  3. Release the Jog Advance button when the specimen is close to contacting the upper breaking head, but there is no load on the specimen.
  4. Tare, or zero, the load at this point.
  5. Press and hold the Jog Advance again to bring the specimen in contact with the upper breaking head. Release the button when there is some load on the specimen, but prior to achieving 10% of expected strength of the specimen.
  6. Verify that the specimen is centered in the retaining rings.
  7. Verify that the axis of the specimen does not depart from vertical by more than 0.5°.
  8. If the specimen is not centered in the rings, or does meet the verticality requirement, press the "Retract" button to release the load and adjust. After adjusting, repeat the taring, loading, and checking process.
  9. If the specimen meets the requirements, press Start Test.