Crazing under internal stress

ESC is mostly a surface-initiated failure of multiaxially stressed polymers in contact with surface-active substances. These surface-active substances do not cause chemical degradation of the polymer, but rather accelerate the process of macroscopic brittle-crack failure. Crazing and cracking may occur when a polymer under multiaxial stresses is in contact with a medium. A combination of external and/or internal stresses in a component may be involved. 

Solvent cements should be chosen with approximately the same solubUity parameter as the plastic to be bonded. Table S.l (under solvent cementing) Usts typical solvents used to bond major plastics. It is common to use a mixture of fast-drying solvent with a less volatile solvent to prevent crazing. The solvent cement can be bodied to 25 percent by weight with the parent plastic to fill gaps and reduce shrinkage and internal stress during cure. 

Fig. 29. Times to failure of HD PE water pipes under internal pressure p at different stresses and temperatures A, 20°C B, 40°C C, 60°C D, 80°C. 1 = ductile failure (see Fig. 2) 2 = creep crazing (see Fig. 28). Circumferential stress a = d 2s, where = average diameter and s = wall thickness. From Ref 19.

Fig. 1.5. Times-to-failure of high-density polyethylene (HDPE) water pipes under internal pressure p at different stresses and temperatures, d average diameter, s wall thickness A ductile failure, B creep crazing (specimens are shown in Figs. 1.6—1.8 after 1141).

Volume initiation limited crazing. Consider a large volume V of a polymer with internal heterogeneities of approximately spherical shape with a radius R and an initial density of Hq/V (m ). The rate of increase of craze front length under a distant stress of in this case is... 

When the critical limiting strain for microcrack formation is exceeded under such dangerous loads , the brittle failure of the plastic material will occur. Only where the loads are temporary and sufficiently small and slow, e.g. in common machining processes or application conditions with limited strains, can microcracks be stopped by structural particles lying crosswise which take over the stress and ensure a sufficient internal cohesion of the material. When only a limited number of microcracks and associated crazes are formed under such harmless loads , the serviceability of the plastic material is not impaired. Therefore, depending on the type of load-... 

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