Attempt to Explain Crazing in Amorphous Thermoplastics and Adhesion Fractures in Semicrystalline Thermoplastics and Filled Polymers

Abstract

In stressed amorphous thermoplastics, cracklike phenomena are observed to develop perpendicularly to the highest normal strain. In semicrystalline thermoplastics, adhesive cracks occur mainly at spherulite boundaries which are positioned perpendicularly to the highest normal strain. Corresponding results were found with multiphase materials, e.g., filled elastomers, thermosets, and thermoplastics.

From the phenomenological similarity of all these observations, the conclusion is drawn that crazes in amorphous thermoplastics are adhesion fractures at particle boundaries. Such particle boundaries are, for instance, the boundaries of raw-material grains which are not destroyed during the plasticizing and processing of a material. Consequently, similar conditions exist as with multiphase materials; the properties of the particles, however, differ less. As “craze material”, “soft” particles must be regarded as those which are stretched during the propagation of the craze.

The surprising existence of constant threshold values of normal strain for the formation of adhesion cracks in all the plastic materials mentioned is explained by the power-law dependence of secondary valence forces (adhesion forces) on distance. After a certain deformation, adhesion cracks develop at numerous places on account of the statistical distribution of critical phase boundaries. This leads to the formation of microcracks. These cracks grow until they reach particles which stop them. These considerations suggest that the strain limit for debonding at critical phase boundaries be estimated by means of an equation which is based on Griffith’s theory and solved with respect to strain. Also, an attempt is made to explain the dependence of strain at fracture on strain rate.