Environmental stress cracking

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If a plastic is mechanically stressed below its yield point in air , stress cracks can occur after a certain period of time, which can be very long . The cause can be internal or external or a combination of both types of tension . The simultaneous exposure to a chemical medium can lead to a drastic reduction in the time to breakage. This phenomenon is known as environmental stress cracking ( environmental stress cracking referred = ESC).

mechanism

ESC is usually attributed to the following processes:

  1. Formation of micro-cavities in test specimens due to microscopic stress concentration after application of mechanical stress.
  2. Formation and subsequent growth of macro-voids caused by the breaking of intermolecular bonds in neighboring voids caused by the chemical environment and the formation of crazes .
  3. Hairline crack growth caused by the rupture of the fibrils as a result of the applied tension and contact with the chemical environment.
  4. Finally, a crack spreads at the tip of the hairline crack, which leads to the brittle fracture. The cracks can traverse the thickness of the material completely and separate it into two or more pieces, or they can stop as soon as they enter areas with less tension or different material morphology.

Measurement methodology

Determining the ESC is complicated because it is influenced by many parameters. This can u. a. Specimen condition (orientation, structure, residual stresses ) and production, thermal history of the specimen, chemical environment.

Five different methods for ESC determination are standardized in DIN EN ISO 22088:

  1. Method in which a test specimen is subjected to a constant tensile load while it is immersed in a stress-cracking medium at a specified temperature.
  2. Process in which plastic strips are arranged under constant flexural strain and exposed to a medium causing stress cracks for a predetermined period of time.
  3. Method in which a hole with a specified diameter is drilled in the test specimen and a ball or an oversized round pin is pressed into the bore while the test specimen is brought into contact with a medium that causes stress cracks.
  4. Method in which a constant tensile deformation force is applied to a specimen while it is immersed in a stress-cracking medium at a temperature selected for the test.
  5. Process in which a slow rate of strain is applied to a specimen while it is immersed in a stress-cracking medium.

The subject matter is the determination of the relative effects of exposure to chemical media (environment) on plastics (test specimens and finished parts). However, it is often not possible to establish a connection between the results of short-term ESC measurements on test specimens and the actual usage behavior of finished parts.

Full notch creep test

The Full Notch Creep Test (FNCT) is one of the test methods for determining the slow crack growth behavior of polyethylene under the action of a wetting agent that is used to accelerate the test. The FNCT is regulated internationally in the ISO 16770 regulation "Plastics - Determination of environmental stress cracking (ESC) of polyethylene - Full-notch creep test (FNCT)" and also in German as plastics - Determination of the stress cracking resistance of polyethylene under the influence of media (ESC) - Creep test on specimens with circumferential notch (FNCT) available. The current status is as of February 1, 2004.

Experimental principle

Rectangular manufactured samples, preferably in the size 10 × 10 × 100 mm³, made from pressed plates or finished products (e.g. from a pipe wall) are used as samples. These are provided with a circumferential notch (full notch), which is used to initiate cracks. To accelerate the test, tests are carried out at elevated temperatures (50–95 ° C) and under the action of a wetting agent (ARKOPAL N100 or ARKOPAL N110). The test specimens are exposed to a constant load under these environmental conditions and the time to break is detected. These loads are chosen in such a way that tensions of 4 to 5  MPa usually act in the notched residual cross-section of the specimen and slow crack growth (SCG) takes place. Since the test takes place in a wetting agent, it is also included in the environmental stress cracking.

See also

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