EFFECTS OF INTERFACE, VOLUME FRACTION AND GEOMETRY ON STRESS REDISTRIBUTION IN POLYMER COMPOSITES UNDER TENSION

The interfacial and fracture characteristics of high-modulus carbon-fi bre/epoxy-vesin composites have been investigated Three different coup on geometries were employed, namely 20 microcomposite tapes, fibre tow s and fill composite tensile coupons. In all cases, the point-by-point stress in the fibre was measured by the technique of remote laser Ram an microscopy (ReRaM). The composite specimens were loaded incremental ly in tension and the stress transfer profiles emanating from the fibr e, were closely monitored. At each applied stress level, the interfaci al shear stress (ISS) distribution was derived by means of a balance o f shear-to-axial forces argument. The redistribution of stress in fibr es adjacent to a filament break in all geometries was determined as a function of distance from the fibre fracture. The values of stress con centration were found to depend upon the number of nearest neighbours and the radial distance from the fibre fracture. Thus, the apparent di screpancy between measurements obtained from 20 tapes and those front full unidirectional composites was resolved. A phenomenological equati on was derived to relate the fibre stress concentration to interfibre distance for both geometries. This equation has also been employed to estimate the stress concentration in the bulk of a composite for a hex agonal array of fibres. Finally, the interfacial shear stress in the n eighbouring fibres as a result of the shear perturbation induced by an adjacent fibre fracture has been quantified for the first time. (C) 1 997 Elsevier Science Limited.