Yield and fracture

However, these simple empirical expressions are far from universal, and fail to account for effects specific to nonlinear behavior, such as the appearance of finite first and second normal stress differences (Tyy = Ni(y) and 7yy — Tzz = 2(7) in steady shear flow. (For a linear viscoelastic material in shear, ctxx, Cyy and a-zz are equal to the applied pressure, usually atmospheric pressure.) TTiese may be linked to the development of molecular anisotropy in polymer melts subject to flow, and are responsible for the Weissenberg effect, which refers to the tendency for a nonlinear viscoelastic fluid to climb a rotating rod inserted into it, as well as practically important phenomena such as die swell [20]. 

The driving force for recovery is entropic, as in a cross-linked rubber. There are no chemical cross-links in a thermoplastic, but chain entanglements ( knots ) and other physical interactions between molecules have a similar effect in causing the material to behave like a network. 

A few plastics appear to fracture in a brittle manner, with no sign of ductility (polystyrene is a familiar example). Close examination, however, reveals that brittle crack propagation in plastics is invariably accompanied by a certain amount of localized yielding over a restricted region near the crack tip. In polystyrene and in other glassy thermoplastics, this takes the form of craze formation, which is illustrated in Fig. 5.1, and is discussed more fully later in this chapter. 

To the naked eye, a craze looks like an extension of the crack, but electron microscopy reveals that load-bearing fibrils about 20 nm in diameter span the gap between the surfaces of the polymer. A network of open holes of similar diameter runs through the craze. Molecular entanglements are essential, since without them there would be little to stabilize the loaded fibrils. If the polymer chains are too short to form effective entanglements, the material is extremely fragile. 

Although it is brittle in tension, polystyrene is ductile in compression, and the same is true of other apparently brittle thermoplastics, and also of lightly cross-linked thermosetting resins. Very tightly cross-linked resins show little 

The balance between these competing mechanisms is afTected by temperature, strain rate, type of loading, component geometry, and the presence of aggressive liquids. 

1 A craze in polystyrene the arrow indicates direction of tensile stress (after R. P. Kamlsour). 

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Monnerie, L., Halary,. L. and Kausch, H.-H. Deformation, Yield and Fracture of Amorphous Polymers Relation to the Secondary Transitions. Vol. 187, pp. 215-364. 

Breach, C.D. Donald, A.M. Jones, R.A.L. In 9th International Conference on Deformation, Yield, and Fracture of Polymers (Conference Papers). The Institute of Materials London, 1994. 

Cook N, Dawson D, Thomas K. Deformation Yield and Fracture of Polymers, Cambridge, Proceedings, 1991, pi 7/1... 

The consequences of secondary transition motions on the mechanical properties (deformation, yield and fracture) of amorphous polymers are addressed in a second paper in this volume [1]. 

Deformation, Yield and Fracture of Amorphous Polymers Relation to the Secondary Transitions... 

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