Semicrystalline thermoplastics, crazing

Crazing inhibition by crosslinking explains the better fatigue resistance of thermosets compared with thermoplastics (this is not valid for semicrystalline thermoplastics such as polypropylene or poly (ether ether ketone). 

Figure 12 represents all steps of craze formation in crystalline polymers in a single model. It is based on Hornbogen s model for a crack tip in a polymer crystal, under the utilization of individual block drawings by Schultz for the fine scale nature of plastic deformation in semicrystalline thermoplastics. The classification into four regions A to D (after ) helps to describe and imderstand the influence of molecular parameters on craze strength and craze breakdown. 

Factoid Affecting Crazing in Semicrystalline Thermoplastics 6.1 Effect of Ambient Temperature... 

It is well known that the mechanical behavior of glassy amorphous polymers is strongly influenced by hydrostatic pressure. A pronounced change is that polymers, which fracture in a brittle manner, can be made to yield by the application of hydrostatic pressure Additional experimental evidence for the role of a dilatational stress component in crazing in semicrystalline thermoplastics is obtainai by the tests in which hydrostatic pressure suppresses craze nucleation as a result, above a certain critical hydrostatic pressure the material can be plastically deformed. 

It has been demonstrated that under appropriate conditions crazing is a comparable mode of plastic deformation in semicrystalline thermoplastics. Typically such a condition is plane strain, which is favored by thick sections or by notches. In the interior of the sample near the notch, local plastic deformation under plane strain grows into... 

Shear yielding in a form of homogeneous bulk process can contribute substantially to the crack resistance of a polymeric solid. On the other hand, however, localized shear yielding in the form of shear microbands is believed to be a precursor of brittle fracture in many semicrystalline thermoplastics. Localized shear yielding is thus a major contributor to the initiation of cracks along with crazing, which, however, is believed not to operate in semicrystalline thermoplastics. 

---