A. N. Gent

A new constitutive relation for rubber

Abstract A simple, two-constant, constitutive relation, applicable over the entire range of strains, is proposed for rubber networks. Behavior in simple extension is derived as an example.

Internal rupture of bonded rubber cylinders in tension

An unusual rupture process is described. It consists of the sudden appearance of internal cracks in bonded rubber cylinders at a well-defined and comparatively small tensile load. The cracks occur in all the vulcanizates examined, and in particularly weak rubbers are found to cause failure. Their appearance may also lead to marked changes in the load-deflexion relationship. A theoretical treatment is presented on the basis of a proposed criterion for crack formation. It is assumed that they form when the negative hydrostatic pressure component of the applied stress reaches a critical value. It is shown by means of the theory of large elastic deformations that a critical value of the negative pressure exists at which any cavity in the rubber will burst, and the calculated value is shown to be in satisfactory agreement with experiment. Moreover, the theory successfully predicts the observed dependence of the cracking stress on the Young’s modulus of the rubber, its virtual independence of the rubber strength and extensibility, and the general form of the observed variation of cracking stress and crack disposition with the thickness of the test-piece.

Effect of Wetting Liquids on the Strength of Adhesion of Viscoelastic Material

Abstract The effect of a variety of wetting liquids on the resistance to peeling separation for a lightly crosslinked rubbery adhesive in contact with a Mylar substrate has been studied over a wide range of peeling rates and at two temperatures. Although the magnitude of the peel strength is much greater than the thermodynamic work of detachment, it is reduced by alcohols and alcohol/water mixtures in good agreement with calculated reduction factors. It is concluded that the measured strength is a product of two terms: the thermodynamic work, and a numerical factor, generally large, denoting inefficiency. The latter term is strongly dependent on peel rate and temperature for viscoelastic adhesives. Two anomalies are pointed out: particularly low adhesion is observed at low rates of peel for certain liquids, attributed to swelling of the adhesive, and smaller effects are found for some other liquids than predicted.

Engineering with rubber : how to design rubber components

This book describes the principles of rubber science and technology. It describes what rubber consists of, how it behaves, and how to design engineering components with rubber. The sections include: an introduction (D L Hertz); materials and compounds (G R Hamed); elasticity (A N Gent); dynamic mechanical properties (A N Gent and K W Scott); strength (G J Lake and A G Thomas); mechanical fatigue (M D Ellul); durability (A Stevenson and R Campion); design of components, including shear and compression bearings, and vibration and noise control (P M Sheridan, F O James and T S Miller); finite element analysis (R H Finney); and tests and specifications (J G Sommer and Oon Hock Yeoh). An appendix provides tables of physical constants.

The Compression of Bonded Rubber Blocks

Experimental measurements are described of the load-deflection relations for rubber blocks having a wide range of thicknesses and varied shapes of cross-section. An approximate theoretical treatment is presented for classically small compressions of circular discs and infinitely long rectangular bars. The measured stiffnesses for small compressions are shown to be in good agreement with the theoretical predictions for a vulcanizate containing no carbon black, and, when an empirically obtained modification is made for thixotropic effects, for vulcanizates containing various quantities of carbon black. The behaviour under moderately large compressions is also examined.

Nucleation and Growth of Gas Bubbles in Elastomers

An experimental study is described of the formation and growth of gas bubbles in crosslinked elastomers. A critical condition for bubble formation is found to hold in most cases: The gas supersaturation pressure must exceed 5G/2, where G is the shear modulus of the elastomer. The kinetics of bubble growth are shown to be in good accord with a simple diffusion‐controlled growth relation for a variety of gases, elastomers, temperatures, and pressures. The number of bubbles depends strongly upon the degree of supersaturation above the critical level. This effect is attributed partly to kinetic factors and partly to the presence initially of internal holes of a range of sizes.

Failure Processes in Elastomers at or Near a Rigid Spherical Inclusion

A systematic experimental study has been carried out of two distinct failure phenomena, cavitation and debonding, in an elastomer containing a rigid spherical inclusion. Several elastomers were employed containing glass beads of various diameters, ranging from 60 to 5000 μm, and with chemically different surfaces. The critical stress for cavitation was found to depend upon both Youngs modulus, E, of the elastomer and the diameter of the bead. By extrapolation, it was found that the stress for cavitation near an infinitelylarge bead is given by 5E/12, as predicted by theory. In contrast, the critical stress for debonding decreased somewhat with increasing Youngs modulus of the elastomer. This is attributed to a concomitant decrease in the strength of adhesion between the elastomer and the bead surface, due to rheological effects. The stresses for both cavitation and for debonding were found to vary approximately with the negative half-power of the bead diameter. This suggests that a similar Griffith mechanism governs both failure processes when the bead size is small. A study of cavitation and debonding in the presence of two glass beads was also carried out. As predicted from theoretical considerations, both stresses were found to decrease as the distance between the two beads was decreased, irrespective of the diameter of the bead and Youngs modulus of the elastomer. At higher strains, however, a second cavitation process was found to take place at a point midway between the beads. Tensile fracture of the specimen resulted from the unrestrained lateral growth of the second cavity.

Mechanics of Foamed Elastic Materials

Abstract The deformation of a foamed elastic material, both in tension and compression, and its resistance to tearing and to tensile rupture, have recently been derived on the basis of a model consisting of a large number of thin threads joined at their ends to form a three-dimensional network. A general account of this theoretical treatment, and the evidence for it, is presented. The theory is extended to deal with small deformations of closed-cell foams; relations for Youngs modulus and Poissons ratio are derived. The viscous damping of open-cell foams due to air flow through the network of threads is also discussed.

Fracture mechanics and cavitation in rubber-like solids

Conditions for propagation of a pressurized crack within a rubber-like solid are derived in terms of the elastic properties of rubber, the fracture energyGc and the initial radiusro of the crack. A previously proposed criterion, that the critical internal pressurePc for crack growth is given by 5E/6, whereE is the tensile (Young) modulus of elasticity, is shown to be inadequate both for small cracks, when the stiffening of rubber at high strains must be taken into account, and for large cracks, when the critical degree of inflation is so small that the assumptions leading toPc=5E/6 do not apply. However, this simple criterion is found to remain a useful guide for cracks having initial radii lying in an intermediate range, such thatroE/Gc lies between about 0.0005 and 1. For representative rubber-like solids, this corresponds to the rangero=0.5 μm to 1 mm.

Cavitation in Rubber: A Cautionary Tale

Abstract What can we conclude from this brief history of cavitation in rubber-like solids? At first, the condition for internal cracking appeared to be particularly simple. Whenever a stress was se...