Test method stress cracking

The established standard test method for CF crack growth rate is ASTM E647 (30). In this constant load test method, the crack length is measured visually as a function of elapsed cycles followed by numerical analysis of the data to obtain the crack growth rate. Crack growth rates are then expressed as a function of crack tip intensity range Ak-, which is based on linear-elastic stress analysis. 

In the usual tests on stress crack resistance (see Sect. 5.3.2), e.g. the pin impression test EN ISO 4600 1998 Plastics - Determination of Environmental Stress Cracking (ESC) - Ball or Pin Impression Method, or in the so-called bend strip test or Bell-test as per ASTM D1693-97a Standard Test Method for Environmental Stress-Cracking of Ethylene Plastics, no failure of HDPE geomembranes is observed under the test conditions required by the standards. These index tests therefore cannot be used for the assessment of stress crack resistance of plastic geomembranes. The requirements are simply too low. 

It is, however, desirable to be able to test the stress crack resistance on the geomembranes themselves using a laboratory method. M. FleiBner suggested an index test method in 1987 for thermoplastic pipes which could also be transferred to geomembranes (FleiBner 1987). In FleiBner s method a perimeter notch was cut with a razor-blade on a quadratic paral-... 

Standard Test Methods for Use ofMattsson s Solution of pH 7.2 to Evaluate the Stress Corrosion Cracking Susceptibility of Copper—Zinc Alloys, ASTM G 37-85, American Society for Testing and Materials, Philadelphia, Pa., 1992. 

Recommended practice for examination and evaluation of pitting corrosion Test method for determining susceptibility to stress corrosion cracking of high-strength aluminium alloy products Test method for pitting and crevice corrosion resistance of stainless steels and related alloys by the use of ferric chloride solution Recommended practice for preparation and use of direct tension stress corrosion test specimens... 

In TPE, the hard domains can act both as filler and intermolecular tie points thus, the toughness results from the inhibition of catastrophic failure from slow crack growth. Hard domains are effective fillers above a volume fraction of 0.2 and a size <100 nm [200]. The fracture energy of TPE is characteristic of the materials and independent of the test methods as observed for rubbers. It is, however, not a single-valued property and depends on the rate of tearing and test temperature [201]. The stress-strain properties of most TPEs have been described by the empirical Mooney-Rivlin equation... 

ISO 8796 2004 Polyethylene PE 32 and PE 40 pipes for irrigation laterals - Susceptibility to environmental stress cracking induced by insert-type fittings - Test method and requirements ISO 9625 1993 Mechanical joint fittings for use with polyethylene pressure pipes for irrigation purposes... 

TMO 198-98, Slow Strain Rate Test Method for Screening Corrosion Resistant Alloys for Stress-Corrosion Cracking in Sour Oil Field Service, NACE International, Houston, TX, 1998. 

Chemical resistance test for rubber linings by Atlas blind flange test Determination of effects of liquid chemicals on plastics Determination of environmental stress cracking (ESC) by the bent strip method... 

Determination of environmental stress cracking (ESC) by the constant tensile test method... 

ISO 4600 details a ball or pin impression method for determining the ESCR. In this procedure, a hole of specified diameter is drilled in the plastic. An oversized ball or pin is inserted into the hole, and the polymer is exposed to a stress cracking agent. The applied deformation, given by the diameter of the ball or pin, is constant. The test is multiaxial, relatively easy to perform, and with not very well-defined specimens, and the influence of the surface is limited. Drawbacks are the small testing surface and the undefined stress state. After exposure, tensile or flexural tests may be performed on the specimens. This leads to the determination of either the residual tensile strength or the residual deformation at break. 

A constant tensile-stress method is outlined in ISO 6252, in which a test specimen is exposed to a constant tensile force while immersed in a stress cracking agent so as to determine the time to rupture under a specified stress. This uniaxial test leads to the determination of the lifetime of the specimen with accuracy, but it is time consuming and requires complex equipment. Variations of this test include a tensile creep test that monitors the strain and a monotonic creep test that uses a constant stress rate instead of a fixed stress [1]. 

Another bent-strip method for evaluating the ESCR is presented in ISO 4599. In this test, strips of a plastic are positioned in a fixed flexural strain state and exposed to a stress cracking agent for a predetermined period. The test is uniaxial and simple to perform, and the deformation is constant. Because of the molecular chain relaxations, the stress state is well defined only at the beginning of the test. After exposure to the medium, the strips are removed from the straining rig, examined visually for changes in appearance, and then tested for some indicative property such as tensile strength. 

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