Measurements are described of the dynamic properties of rubber, loaded with various amounts and types of filler, when subjected to mechanical vibration in simple shear at amplitudes from 0 to 3 per cent shear in the frequency range 20 to 120 c.p.s. The decrease of dynamic modulus with increasing amplitude is shown, for a wide range of filler types and concentrations, to be determined by the amount of stiffening produced by the filler. This relationship is not influenced by variations in the vulcanizing ingredients, reasonable variations in state of vulcanization, addition of softener, or imposition of static shear strain. Rubber compounds stiffened by mixture with, or chemical combination of, other polymers exhibit a smaller order of nonlinearity than that described above and also exhibit much lower hysteresis values within the amplitude range 0 to 3 per cent shear.
Comparatively few organic peroxides meet the rather exacting requirements of the ideal peroxidic curing agent for rubber. Only di-tertiary butyl peroxide has yet been found to give satisfactory vulcanization in black stocks. Pure gum vulcanizates of a high degree of transparency and good color have been obtained with two commercially-available non-hazardous peroxides which have also been used successfully with some non-black fillers. The incorporation of resorcinol overcomes a tendency to surface tackiness in peroxide cures and has a beneficial influence on tensile strength.
An apparatus is described which subjects a rubber test-piece to a force in simple shear, varying sinusoidally with time in the frequency range of 0.0017– 17 c/s, the instantaneous values of force and displacement being measured by photoelectric pickups. From the display on the screen of a cathode-ray tube of the mechanical hysteresis loop described by the vibrating rubber, measurements are made which make possible calculation of the dynamic shear modulus and hysteresis. Typical results are given.
Richard Rose, Senior Lecturer at University College, Northampton, and Will Fletcher and Gaynor Goodwin, Watling View School, St Albans, argue that although many schools have attempted to enable pupils with special educational needs to play an active part in their own planning procedures and assessment, few have identified and analysed the skills required by both pupils and teachers. Theydescribe a one‐year small‐scale action‐research project, in a school for children with severe learning difficulties, which resulted in the development of procedures for the assessment of ‘pupil readiness’ for full involvement in the target‐setting process.
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