MTS DCPD Measurement System - Manual (Page 33)

Foil Specimen
After the DCPD Measurement System is installed and the Controller is setup, the foil specimens is used
to verify the DCPD unit is functioning properly. The following figure shows the cabling connections for the
foil specimen. This section assumes the DCPD unit has been connected to the controller and that the
controller has been set up for DCPD operation; refer to the Installation section for information.
The cables have clips on the ends that are attached to the foil specimen. The clips on the cables allows
you to easily disconnect the cable ends when cutting the foil specimen.
Parts
• EMT shears
• Cables
• Clips
Theory of Operation
The theory of DCPD is that as the crack grows in the specimen, the resistance increases. Given that the
current through the specimen is constant, as the resistance increases, the voltage drop measured across
the specimen also increases. By measuring the voltage drop, specimen crack length can be calculated
and used in data acquisition. Cutting the foil to increasing increments as printed on the foil specimen will
result in an incremental increase in specimen voltage.
The DCPD unit is extremely accurate, therefore the voltage values shown on the foil specimen for each
cut length are approximate. This is because the cut length can not be exactly to the line and the clips will
not be in the exact same position following each cut. Depending on the accuracy of the cut and the
placement of the clips on the foil specimen, the +/- 10% voltage reading tolerance printed on the foil
specimen might be exceeded. But if care is taken in placing the clips in the same position and cutting the
foil specimen as close as possible to the lines indicated, voltage readings should be close to those printed
on the foil specimen.
DCPD Measurement System | 33
Installation