Fracture in crazes

Kramer, E. J. and Berger, L. L. (1990) Fundamental processes of craze growth and fracture, in Crazing in Polymers II, edited by Kausch, H. H., Advances in Polymer Science, vols. 91/92, Berlin Springer-Verlag, pp. 1-68. 

E. J. Kramer, L.L. Berger, Fundamental processes of craze growth and fracture, in Crazing in... 

Before discussing specific aspects of micro deformation and fracture in bulk polyolefins, some basic notions of microdeformation and the micromechanics of fracture mediated by generation and breakdown of cavitated or fibrillar deformation zones or crazes are introduced. SCG in PE and rate-depen-dent fracture in iPP are then considered in more detail. 

At the fundamental level, current understanding of crazing and fracture in semicrystalline polymers remains less advanced than in glassy polymers. Even in these latter, phenomena such as disentanglement are generally subject to unverified assumptions concerning their kinetics, or even their exis-... 

A shortening in relaxation time in the critically strained region makes some materials tough. The shift of relaxation time is attributed to strain-induced dilatation and can reach as much as five decades. Thermal history, on the other hand, dictates the initial state from which this dilatation starts and may be expressed in terms of excess entropy and enthalpy. The excess enthalpy at Tg is measurable by differential scanning calorimetry. Brittle to ductile transition behavior is determined by the strain-induced reduction in relaxation time, the initial amount of excess entropy, and the maximum elastic strain that the material can undergo without fracturing or crazing. 

Failure Mechanisms. BPF polycarbonate develops crazes at ascending stresses and fractures in a pseudo-brittle manner similar to polystyrene or PMMA. At room temperature the block polymers develop few separate crazes. As the yield is approached, shear bands grow from the edges. Fracture initiates at an edge from a point where the two shear bands initiated. When a neck forms, the plastic strain in the neck is ca. 80% however fracture occurs shortly after the neck is formed so that the ultimate elongation of the specimen is only 10 or 12%. The shear bands and necks show some stress whitening (Figure 9). 

Fig. 39 a. Schematic comparison of the deformation processes in crazes and shear bands. Corresponding fracture surfaces are shown in b for amorphous PS and in c for semicrystalline PP (craze) and PB-1 (shear band)... 

Friedrich, K., Karsch, U. A. The Influence of Molecular Weight on Crazing and Fracture in Polypropylene, in Proc. 27th Int. Symp. on Macromolecules, Vol. 2, p. 1035, Strasbourg, France, July 1981... 

In the sections immediately following we will review briefly the state of understanding of processes governing the initiation and inactivation of craze fronts, the velocity of craze fronts, and the time dependent fracture of craze matter under stress. 

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