Long-term stress crack resistance

New materials placed In new service conditions always raise the question of lifetime performance. In this case, permeation performance, stress-crack resistance, and long-term mechanical Integrity are particularly relevant in situations where the plastic containers are expected to be reused. In the case where hazardous materials are transported In plastic containers, regulatory decisions (Including the ruling on requests for exemptions) on these containers may be facilitated by using data on the material properties of the polymer. 

Several accelerated long-term tests for assessing the stress-cracking resistance (ESC) to be less time consuming than conventional creep tests were developed such as the full notch creep test (FNCT) as one of the most important methods using notched specimens (an overview about accelerated methods is given in Chapter 4.7, Stress Cracking Resistance )- Moreover a couple of methods for components or semi-finished products (such as plastic pipes) are available where the full-scale pipe test and small-scale steady state test (S4 Test) are the most used ones. 

The short term, chemical resistance of most plastics is well known. What is not so well known is the long term chemical resistance or the environmental stress cracking resistance (ESC). 

Stress crack resistance of textured geomembranes (long-term tensile test) DVS 2203-4 Section 3.2.16... 

The requirement for stress crack resistance of the HDPE material used for geomembranes formulated in this way has been included in the BAM certification guideline for geomembranes in landfill liners (Muller 1992, 1999b) since it is based on genuine long-term tests. This evaluation procedure is very conservative since it assumes that the long-term available residual stress at an imposed deformation initiates a stress crack, in the same way as an imposed durable external stress does (see Sect. 5.4.1). 

Thus the long-term tensile test on plain specimens cannot be used to characterise stress crack resistance of smooth geomembranes. In particular,... 

In the pipe pressure test, NCTL test and long-term tensile test, a well-defined load condition (internal pressure and tensile force) is specified under which the stress crack develops. In reality, however, geomembranes are predominantly stressed by imposed deformations. Therefore, methods which characterise stress crack resistances under constant strain have also been used for geomembrane tests. 

Stress crack resistance of PE materials is eharacterised by times-to-failure in long-term tensile tests on notched specimens not only in the above mentioned but also in many other technical papers. The phenome-... 

Fig. 5.18. Time-to-failure t in the long-term tensile test (NCTL test) on cold drawn test specimen as a function of the degree of orientation X. Circles tensile stress parallel to the direction of stretching and the notch across it, rectangle applied tensile stress across the direction of stretching and the notch parallel to it. In the first case the time-to-failure increases exponentially with the degree of orientation, in the second case it decreases exponentially. Orientation has a drastic effect on stress crack resistance. Source (Zhou and Brown 1994)...

With the textures applied in a second process step, the base geomembrane of the textured product has identical properties to a smooth geomembrane. However, the above mentioned questions arise of how effective the adhesion of the texture particles is and of their speeific long-term behaviour, i.e. the question of the long-term shear strength of the textured surfaces (Sect. 3.2.18). The problem of stress crack resistance must also be... 

Stress-crack resistance (longterm tensile test) Time to failure Geometric mean Long-term tensile test of times to failurebased on DVS R2203-700 h 4 on at least 5 test specimens at 80 °C und 4 N/mm tensile stress in a 2 % surfactant solution ... 

Environmental stress crack resistance (ESCR) is the ability of the material to resist long-term exposure to stress-cracking agents. Materials with ESCR values in excess of 1000 h are typically considered acceptable based on one of the common test procedures. 

The environmental stress cracking is the most common failure reason of polymer parts during their use. There are already many tests to verify the stress crack resistance. Most methods use combinations of an external strain and aggressive liquids to achieve a quick test result. The extrapolation to longer time periods is only successfiil with the help of the expert knowledge of the raw material producers. The development of a new testing method enables a simulation of the material long-term behavior on the basis of a short time test of plastics under the influence of a medium. 

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