B. D. Caddock

Microporous materials with negative Poisson's ratios. I. Microstructure and mechanical properties

A microporous, anisotropic form of expanded polytetrafluoroethylene has been found to have a large negative major Poissons ratio. The value of Poissons ratio varies with tensile strain and can attain values as large as -12. The microporous structure of the material is described and the mechanisms that lead to this large negative Poissons ratio are identified. Micro-rotational degrees of freedom are observed, suggesting that a micropolar elasticity theory may be required to describe the mechanical properties.

Microporous materials with negative Poisson's ratios. II. Mechanisms and interpretation

For pt.I see ibid., vol.22, p.1877-82 (1989). In a previous paper the morphology of a microporous material made from expanded poly(tetrafluoroethylene) was described and results presented for its mechanical behaviour. The material was shown to be highly anisotropic and exhibited a large negative Poissons ratio. In this paper a simple model for the microstructure is described to account for this effect. The model is based on an interconnected array of anisotropic particles that deforms so as to produce a large transverse displacement under longitudinal tensile loading. Very good agreement is found between the model and experimental results, providing an explanation for the variation of Poissons ratio with tensile strain, in terms of changes in material morphology.

Negative Poisson ratios and strain-dependent mechanical properties in arterial prostheses

The elastic properties of two artificial arterial prostheses have been measured. One is a fibrillar polyurethane (PU) material, the other is an expanded polytetrafluoroethylene (ex-PTFE). It is shown that the conventional assumption of incompressibility is invalid for both materials and that changes in the thickness of the prosthetic artery wall must be measured directly. The ex-PTFE shows highly strain-dependent properties, the most significant being that of large negative through-thickness Poisson ratios, up to -11. These results have important consequences for matching the properties of prostheses to natural materials.

Simulation of prismatic cracking of cooling basalt lava flows by the drying of sol-gels

The progressive development of primary arrays of straight and curved cracks during the drying of thin films of aqueous silica sol-gel is investigated experimentally by direct observations using light microscopy. Three main test configurations are used to generate a range of drying profiles in the sol-gel: (i) films between a glass plate and a flexible glass cover slip; (ii) films between two glass plates held apart by a rigid spacer, and; (iii) uncovered films on a glass plate. Cracks in the primary arrays grow normal to the iso-concentration water profiles. The arrays of primary cracks observed in basalt lava flows and described by Spry (J. Geol. Soc. Aust. 8 (1962) 191) i.e. parallel straight columns, chevrons, rosettes, fans, inverted fans and basins, are reproduced in the pseudo two-dimensional films of sol-gel. The development of both primary and secondary cracking patterns is strongly dependent on the constraints associated with the bonding of the sol-gel to the glass and the subsequent splitting of the gel-glass interfaces.

Statistical changes during the corrosion of glass fibre bundles

The mechanical properties of E-glass fibre bundles have been measured after corrosive attack by hydrochloric acid of various concentrations for various times. The effective stiffness of the fibre bundles is seen to be proportional to the effective cross-sectional area of the fibres as identified with the characteristic core-sheath geometry found in fibres exposed to long-term acid attack. However, the strength of the fibre bundles is not simply related to the effective area of the fibres and the statistics of fibre strength vary considerably with time. In particular the Weibull shape parameter is seen to increase rapidly at short times, before core-sheath formation is observed, and then fall slowly with core-sheath formation. Hence we have a shortterm narrowing of the strength distribution followed by a long-term broadening.

The diffusion of hydrochloric acid in polyester thermosetting resins

The possibility of diffusion of corrosive media in polyester resins is considered as a contributor to the process of stress corrosion cracking in glass reinforced polyester composites. It is shown that neither the saturation levels reached nor the rate of diffusion achieved for hydrochloric acid is sufficient for this mechanism to be considered a significant contributor to the process of low stress induced corrosion. It is believed that the corrosive medium will attack the fibres by a process of percolation through stress induced microcracks.

Influence of humidity on the cracking patterns formed during the drying of sol-gel drops

Optical microscopy has been used to study the sequence of events associated with drying of a silica-based sol-gel. Drops of sol containing 50 wt% silica were deposited on glass slides and dried in cells containing sulphuric acid as a drying agent. The drops, on the slide, were about 3.5 mm in diameter and before the onset of drying had a spherical cap shape. At the start of drying the rim of the drops became pinned to the glass surface. The humidity of the drying environment had a strong effect on shrinkage within the drop and the processes that occurred during drying. In very dry environments evaporation occurred primarily from the edges of the drop and resulted in a radial concentration gradient in the gel. Shrinkage resulted in a pattern of radial cracks that spread from the outside in a direction orthogonal to the iso-concentration profiles. Cracking was preceded by shear deformation. Drying was more uniform in humid environments and occurred from both the rim and the top surface of the drop. Wrinkling of the skin of gel that formed on the surface of the drop was followed by radial and circumferential cracking, except in very humid environments. The results are attributed to the influence of the radius of curvature of the drop surface on the rate of evaporation and the relative rates of flow and diffusion in the drying sol-gel.

Stress-corrosion failure envelopes for E-glass fibre bundles

Tests have been conducted on stressed and unstressed fibre bundles under attack by aqueous hydrochloric acid of various concentrations. Without an applied stress, fibre weakening occurs at long times by core-sheath development. With an applied stress, failure occurs more rapidly, following a fatigue law with an exponent that is a function of acid concentration. The value of this exponent and the time required for bundle failure are in agreement with data obtained from experiments on the stress corrosion of composites.

Diffusion behaviour of the core-sheath structure in E-glass fibres exposed to aqueous HCl

The existence of a characteristic core-sheath morphology in E-glass fibres undergoing acid corrosion is well known. This effect is attributed to the removal of calcium and aluminium ions from the glass structure. Sheath growth measurements indicate that this abstraction involves a two-stage diffusion process, with a room-temperature diffusion coefficient of 1.4×10−16m2 sec−1 initially, slowing to 1.5×10−17 m2 sec−1 at later times. This two-stage process is related to fibre structure. It is also shown that there is an initial delay, during which the core-sheath structure does not develop.

Diffusion of aqueous hydrochloric acid into polyester resins

The influence of an applied stress on the solubility and diffusivity of aqueous hydrochloric acid in polyester resins has been examined. A radioactive tracer method, involving the use of3H and36Cl, enabled the mobility of both the water and the acid in the solution to be followed simultaneously. The results obtained were in accordance with those obtained previously with unstressed resins and showed that whereas water would diffuse freely in stressed polymers, hydrochloric acid would not. The results have important implications in relation to the mechanism of crack propagation in E-glass/polyester composites.