Tensile Stiffness of a Thermoplastic Reinforced with Glass Fibres or Spheres

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Abstract
Stiffness under uniaxial tension of injection-moulded polypropylene was found to increase much more rapidly with the addition of glass fibres than with the same amounts, by weight, of glass spheres and to decrease in both cases with strain. The observed increases in stiffness due to the fibres correlated closely with the results obtained from a simple model of a two-phase material, when allowances were made for the apparently low degree of alignment of the fibres which decreased with increasing fibre content. The stiffening effect of the fibres was virtually constant over the strain range considered. The lower increases in stiffness due to the glass spheres also correlated with the results obtained from the simple model but only for strains of less than 0.5 per cent: at higher strains the inclusion of glass spheres reduced the stiffness of the polypropylene.
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