Stress crack formation

This pol5mieric material exhibits many advantageous features similar to those of HDPE (for example resistance to chemicals), it does not, however, show the disadvantages of HDPE, which is initially a tendency towards stress crack formation. As the density of this material is relatively low, it is difficult to put it into any of the above categories and it has been therefore termed Linear Low Density Polyethylene (LLDPE) (Bork 1984). It should also be noted that the terms MDPE and LMDPE are hardly used any... 

Fig. 2.4. Sketch of the basic stractural components in HDPE geomembrane morphology - based on our understanding as of today. Folded polymer chains form extended lamellae (top left). Twisted stacks of lamellae create longitudinal fibrils (bottom). The sphere-shaped spheralite (top right) is composed of radial fibrils pointing outwards with amorphous areas in between. This stmctural model enables the interpretation of basic processes, such as stress crack formation, see Sect. 5.3.4...

The carbon black particles are dispersed within the material composed of spherulites of various sizes, joined and eoimected by the cement of the amorphous areas. Low-molecular polymer fractions, antioxidants and other low-molecule additives are dissolved in the amorphous areas. This is the place where contaminants or oxygen may dissolved and difiuse and oxidative degradation takes place. Microcracks within the amorphous areas, eaused by disentanglement of the tie-molecules between the crystalline areas while the material is exposed to stress, are the initiators of stress crack formation, see Sect. 5.3.4. 

Fig. 3.14a. Stress-rapture characteristic of HDPE pipes made of Hostalen GM 5010 T2. Compared to modem materials this outdated resin had very low resistance to stress crack and oxidative degradation. Therefore, the various failure modes could be observed within manageable testing times even at 60 °C. Zl, Z2, Z3 designate the sections of the hoop stress versus time-to-failure curve, which result from different failure modes Ductile failure with excessive deformation (Zl), stress crack formation and brittle failure (Z2) and oxidative degradation (Z3). The figure has been taken from reference (Koch et al. 1988)...

The effect of liquid surfactants can powerfully accelerate stress crack formation. Nevertheless, stress crack formation in plastics must be distinguished from stress crack corrosion as known in particular in metallic materials. Corrosion is understood as the erosion of atoms from the material by chemical processes and in metals particularly by electro-chemical reactions. Additional influence by stresses leads to crack formation and brittle fracture which often resembles of the failure of stress cracks in plastics. Stress crack formation in thermoplastics is, however, a purely physical process. No chemical changes take place in the material even under the influence of surfactants. The terminology is nevertheless not completely uniform. The accelerating effect of liquids on stress crack formation in plastics is occasionally described as stress crack corrosion although no real corrosion process is connected with it. 

Damage by stress crack formation arose occasionally in HOPE geomembranes in the past, in particular in weld seam areas (EPA 1992 Hsuan et al. 1993). Stress erack resistance is therefore a substantial criterion in the selection of polyethylene resins for geomembranes. For this reason, in the following, test methods used will be dealt with in detail. At the same time the phenomenon should be specified more precisely. Table 5.2 gives an overview of the test methods. 

The same critical remarks must be made in connection with this test method with un-notched specimens as concerning long-term tensile tests on smooth geomembrane specimens stress crack formation proceeds from random weak points in the boundary region of the specimen. The type of speeimen preparation will have a significant influence on the test results. 

Before the theoretical concepts for the description and understanding of stress crack formation will be discussed in detail, some fracture-mechanics terms and relationships should be reviewed and explained to help understand these theories (Knott 1973). 

The so-called structure particle model explains craze formation and crack formation by microcracks along the interface of structural units ( particles ) of the morphology. The formation of microcracks is determined by the interface energy between these particles. Microcracks develop if a critical deformation limit, which depends on the interface energy, is exceeded. This model primarily provides a quantitative description of the effect of liquid or gaseous media on stress crack formation. 

In the NCTL test (see Sect. 3.2.13) ductile failure occurs at tensile stresses above 50 % of the stress at yield. Under about 30 % of the stress at yield the specimen fails through a brittle fracture. There is a range between 30 % and 50 %, where both mechanisms - creep and plastic deformation of the entire material cross-section as well as stress cracking - determine the fracture behaviour. This intermediate range and the range of ductile failure will be briefly dealt with below. The attention is first directed to stress crack formation that results in an unambiguous brittle failure. 

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