ASTM D876
Polymer Tube Tensile Test
Standard Test Methods for Nonrigid Vinyl Chloride Polymer Tubing Used for Electrical Insulation
Theory:
ASTM D876 is extremely similar to ASTM D2671 for Heat-Shrinkable Tubing. Both tests specify several types of tests for nonrigid PVC tube and pipes. The tensile test references other standards such as D412 and requires the use of specific sample specimen dies. Tubes can also be tested as long as their psi is below a certain threshold. Another test under D876, the heat-shrink test, uses a special type of mandrel which is inserted into the end of the pipe to maintain its inner diameter (ID). The third test of note is a peel test, wherein an adhesive tape or adhesive layer of tubing is peeled away from the core tube utlizing a special fixture.
Geometry:
The ID and OD are the two most critical factors. By subtracting OD minus ID, a cross sectional area can be calculated. The ID value is important in order to specify the size of the mandrel. In the video for ASTM D2671, you will see the mandrels which are slightly lighter and whiter in color. A wire clamp collar is wrapped around the mandrel and tube to secure the specimen.
The peel test fixture is a different apparatus. The tube is held perpendicular in the machine by a specialized fixture.
Solution:
The mandrels used for tensile testing the nonrigid tube are a custom item and each are machined individually. The mandrels must be just thinner than the ID of the specimen when it is at its standard conditioning temperature and ready for testing. The technician must be careful not to jam the mandrel into the sample because it can cause stretching and corrupt the results. Once the mandrel is situated a wrap collar is tightened around the sample to secure it.
The D876 peel fixture holds the sample orthogonally to the two machine posts. The sample is able to rotate freely when it is secured in the yoke. A piece of tape or adhesive is wrapped around the sample and pulled. As the tape is pulled the sample will rotate as the tape is peeled away and a reading of peel strength can be taken.
Analysis:
There are several calculations that can be listed as ASTM D876. It is important that the customer as well as the specifying engineers are clear on the type of test, test procedure, and specific calculation.
For the tensile tests, the standard tabulations of tensile strength and elongation can be taken. Advanced analyses such as offset yield strength can also be performed.