M. Bevis

The morphology of crazes in polystyrene

Abstract Thin sections of pre-crazed bulk specimens of polystyrene and thin sections containing crazes which were obtained by straining fresh films of polystyrene have been examined in the electron microscope. The use of reinforcing impregnants, a cold stage or a diamond knife was not required for ultramicrotomy. The microstructure of crazes in polystyrene has been shown in the later stages of craze deformation to consist of a well-defined fibrillar structure. The fibrils have a characteristic diameter of 200 to 400 A and in the electron microscope the fibrils exhibit a beaded contrast. The beaded contrast can be considered as evidence for the existence of a domain structure in amorphous polymers. Some preliminary observations of the structure of crazes formed by straining in the electron microscope and some observations on the fracture surfaces of crazes are also reported.

Stress-induced twinning and phase transformations in polyethylene single crystals

Abstract A detailed study has been made of the deformation modes which operate in polyethylene single crystals when drawn on a Mylar substrate. The orientation relationships associated with the stress-induced twinning and monoclinic phase transformations have been determined by electron diffraction and compared with predictions based on recently developed theories of the crystallography of martensitic transformations. The agreement between theory and experiment is good and the identification of the deformation modes has enabled the criteria which govern the operation of these modes to be established. These criteria and the crystallography of the transformation processes are discussed in detail.

A comparison of the morphology of crazes formed in thin films and in bulk specimens of polystyrene

The microstructure of crazes formed in solvent-cast thin films have been studied by transmission electron microscopy. The results have been compared with the microstructure of crazes determined from the examination of replicas obtained from the fracture surfaces of bulk specimens. The structure of crazes formed in thin films and in the bulk have been shown to be similar with the exception that much finer fibril structures are observed in thin films at large deformations. A model of the microstructure of a craze is presented.

An investigation of the relationships between processing conditions, microstructure and mechanical properties of an injection moulded semi-crystalline thermoplastic

The semi-crystalline polymer TPX has been injection moulded into standard tensile bars using a range of barrel temperatures, mould temperatures and injection pressures. Acid etching, microhardness testing and selected volume X-ray pole figures were used to characterize the changes in microstructure and crystalline texture which occurred throughout the range of mouldings. A strong correlation was shown to exist between microstructure and crystalline texture and processing conditions. Three-point bend tests on the injection moulded bars indicated that a ductile-to-brittle transition in failure mode could be induced by changing the moulding conditions, and that this change could be correlated with microstructural and dimensional changes.

Fracture processes in polystyrene

The model for the craze controlled fracture process in polystyrene has been developed further by taking into consideration the micromorphology of the crazes in which the nucleation and propagation of cracks occurs. The micromorphology of crazes formed in thin films of polystyrene, some of which had fractured, has been characterized by means of transmission electron microscopy. The observed micromorphological detail has been shown to be consistent with the micromorphology of the fracture surfaces of bulk specimens. In particular, the slow and fast regions of crack propagation which result in distinctly different fracture surface morphologies have been shown to be associated with differences in micromorphology which occur along the length of a craze.

Deformation processes in thin melt-cast films of high-density polyethylene: I. Experimental methods and deformation processes in the equatorial regions of spherulites

Abstract Selected area electron diffraction is described as a method for making quantitative studies of intralamellar deformation processes in micropherulitic structures. The technique is applied to characterize stress-induced twinning and phase transformations in the equatorial regions of spherulites in melt-cast spherulitie films after drawing while in contact with Mylar substrates. The sequence of deformation processes which occurs with increasing applied strain and with position in a spherulite has been identified, and is described in detail and compared with previously reported results obtained from experiments with single crystal and bulk forms of high-density polyethylene.

Crazing and fracture of polystyrene

An attempt has been made to gain a more detailed understanding of crazing and craze controlled fracture processes in polystyrene, by studying the deformation behaviour of a range of commercially available poly- and monodisperse polystyrenes. The basis of the investigation was the characterization of the microstructures of crazes formed in thin film specimens, and studies of the remnants of crazes retained on the fracture surfaces of bulk specimens. Marked differences in the morphology of crazes and craze-controlled fracture surfaces were observed. Additives and impurities appeared to have more effect than a wide range of molecular weight distributions on the crazing and fracture process.

Electron microscopy studies of fracture processes in amorphous thermoplastics

Abstract The electron microscopy techniques which can be used to provide definite information about fracture processes in amorphous thermoplastics are summarized. A comparison is made of the micromorphology of crazes formed in thin films of polystyrene and the micromorphology of the fracture surfaces of bulk specimens. This shows that the fracture behaviour of thin films simulates closely the fracture behaviour of the bulk in craze controlled fracture and that the former procedure may therefore be used for high resolution studies of fracture processes. A brief description is also given of the precautions that have to be taken in the study of electron beam sensitive materials and of the developments in electron optical instruments which should result in an improvement in the facilities available for electron microscopy studies of polymers.

An investigation of the microstructure and mechanical properties of high density polyethylene spherulites

Scanning transmission electron microscopy and related techniques have been used to characterize the micromorphologies of high-density polyethylene spherulites produced under different crystallization conditions. The experimental techniques used in the investigation are summarized, as are some relationhips between spherulite morphology, crystallization conditions and mechanical properties. A preliminary model for the micromorphology of banded polyethylene spherulites is proposed.

Transformation Strains in Lattices

A generalized theory of transformation strains in lattices is developed which incorporates earlier analyses of both deformation twinning and martensite crystallography in addition to other new transformation mechanisms. In the formal analysis two different lattices are related by an invariant plane strain, a rotation and a further strain which characterizes the particular transformation being considered. A general solution is then obtained for the invariant plane strain in terms of this characteristic strain. The special cases of twinning shears, invariant plane transformation strains and martensite crystallography theories are examined in detail for mechanisms involving either single or double strains. In these cases the characteristic strain consists of appropriate combinations of shears, pure strains, invariant plane strains and lattice invariant deformations. Applications of the particular mechanisms examined to transformations of technological interest in crystalline materials are discussed and further special cases and extensions of the analysis considered.