D. Hull

Crack propagation in single crystals of tungsten

Abstract The effect of pre-existing micro-cracks on the fracture of [010] tungsten single crystas has been investigated. Cleavage cracks on (010) planes were introduced into the edge of the specimens by a spark-machining technique. Tensile tests at 77°K showed that the fracture stress was dependent on the crack length according to the Griffith criterion. At this temperature the specimens failed in a completely brittle manner. A surface energy of 6300 ergs cm−2 was obtained in close agreement with the theoretical value for the true surface energy. Metallographic examination of the fracture surfaces showed that the markings on the surface depend on the stress level at fracture. One test was carried out to determine the effect of prestraining at 295°K on the subsequent fracture behaviour at 77°K. The effect of temperature on the fracture of specimens containing a constant crack length was determined between 77 and 478°K. As the temperature increased, the crack propagation mechanism changed from fast cleavag...

Vacancy trapping in quenched aluminium alloys

Abstract Foils of some aluminium-based alloys have been given various quenching treatments and the size and distribution of the vacancy clusters observed in an electron microscope. It is concluded that the solute atom trap the vacancies and that their binding energies determine both the male of the clusters and the number of vacancies they contain.

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.

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.

Microscopy of failure mechanisms in filament-wound pipe

Filament-wound pipes pressurized in one of two modes, to give biaxial or uniaxial stress conditions, have been examined using standard microscopic techniques. Whitening of the pipes which develops during testing has been correlated with different types of cracking. A mechanism for weepage in filament wound pipes by the crossing of transverse cracks in adjacent laminae is described. The types of crack present in the pipe at loads up to final failure were found to be consistent with predictions of the stresses acting in the pipe walls.

The effect of polymerization conditions and crystallinity on the mechanical properties and fracture of spherulitic nylon 6

The molecular and structural parameters controlling the mechanical properties, deformation and fracture of spherulitic nylon 6 have been investigated. The nylon was prepared by the anionic polymerization of ε-caprolactam and the polymerization conditions were varied to give samples having a range of spherulite diameter, molecular weight and degree of crystallinity. The tensile properties and fracture mode of the nylon varied considerably with degree of crystallinity and polymerization temperature. High crystallinity and low polymerization temperatures below 423 K gave a brittle material. Polymerization above 423 K resulted in a ductile material which showed a yield drop. In this material final fracture was preceded by the formation of inter and trans spherulitic cracks which coalesced to form a large cavity that led to final failure. In nylon having a low degree of crystallinity, fracture was fibrillar in nature and occurred by the ductile drawing of the material to strains greater than 250%.

Propagation of stress corrosion cracks in aligned glass fibre composite materials

A modified fracture toughness test has been used to measure the growth of cracks in unidirectional glass fibre/polyester resin composite materials in the presence of a dilute hydrochloric acid. Crack growth rates perpendicular to the fibre axis have been measured over a range of stress intensities. Scanning electron microscope studies of the fracture surfaces have shown that the micromechanisms of nucleation and propagation are dependent on stress intensity. The use of crack growth data to predict component lifetimes and the existence of inherent flaws in the material are discussed.

The effect of mixed mode I/III on crack evolution in brittle solids

A modification of Sommers classical experiment [4] has been used to fracture rods of a brittle epoxy resin in mixed mode I/III stress conditions. The nucleation and growth of cracks in an increasing KIII/KI stress field have been investigated, particularly in relation to the formation of river lines and the evolution of multiple cracks between river lines to form smooth helicoid surfaces. The fractographic features associated with the progressive development of river line patterns involving crack bowing at river line steps, interaction between arrays of cracks and the coalescence of river lines are described. As KIII/KI increases the scale of the river line patterns increases but the patterns remain self similar. Using principles from differential geometry it is shown that helicoid surfaces can be generated entirely by crack evolution involving ‘tilting’ without ‘twiting’.

Craze yielding and stress-strain characteristics of crazes in polystyrene

Abstract The crazing behaviour of a grade of polystyrene which crazes profusely when tested in tension has been studied at 293°K using quantitative optical, electron and electron scanning microscopy. Tensile tests on this material showed that a high density of crazes was produced at room temperature. The crazes formed in a zone which spread progressively down the specimen. The plastic strain associated with crazing was sufficient to cause craze yielding and the phenomenon is compared with discontinuous yield in mild steel. Craze-yielded specimens were used to determine the stress—strain curve of craze material. Microstrain techniques were developed for this work and the total volume of craze was determined from the density, distribution and thickness of the crazes. It was found that the stress—strain characteristics of polystyrene crazes are similar to those observed by Kambour and Kopp (1969) for solvent-induced crazes in polycarbonate.

Tilting cracks: the evolution of fracture surface topology in brittle solids

For a growing crack the conventional definitions of tilting and twisting are inadequate. New definitions are proposed, based on differential geometry, and it is shown that, in homogeneous, isotropic, brittle solids, non-planar crack growth must occur entirely by tilting movements. Examples are given of the growth of cracks on curved surfaces which illustrate that the no-twist condition produces significant constraints on the path of fracture. The development of fracture surfaces when cracks are subject to mixed-mode loading conditions are described with particular reference to the influence of mode III, twisting conditions. It is shown that the requirement that only tilting can occur leads to many characteristic fractographic features.