What is a Bell Telephone Test?
There are several ways of measuring the stress crack resistance of a polyethylene, but the most commonly used method is that set out in ASTM D 1693, the Bell Telephone Test. There are several names for this test including:
- Bell Telephone Test (BTT) because of its original use in the Bell Laboratories
- Bent strip (BS) ESCR test
The diagrams above show how the test involves a short notched plate of material, which is bent and held in a holder and immersed in the test fluid. Typically, the test fluid is a detergent solution (very specifically Igepal CO630) to accelerate cracking and the test takes place at elevated temperature, e.g. 50°C, by immersing the test assembly in a heated bath. However depending on the application the plastic will be used for, the temperature can be varied, and the solution used varied to simulate the expected conditions that the plastic will have to endure.
The photograph above shows the typical mode of failure in the Bell Telephone Test, which must be observed and recorded as a function of time. The material is regarded as having failed when 50% of the test specimens show some incipient failure. There are different specifications of test geometry according to the classification of the material, i.e. LDPE or MDPE/HDPE. The method is simple and gives good indications of performance, but it is notoriously unreliable due to being highly operator and equipment sensitive.
Other stress crack testing other than the Bell Telephone Test?
For this reason, Impact Solutions prefers another form of stress crack resistance testing, ISO16770, or the Full Notch Creep Test. The full notch creep test according to ISO 16770 is rapidly becoming the preferred way of measuring the stress crack resistance of a polyethylene. Follow this link to learn more about this form of stress crack resistance testing.