"THE EFFECT OF INTERNAL STRESSES ON THE FIBRE-MATRIX INTERFACE IN A POLYETHYLENE FIBRE-EPOXY RESIN COMPOSITE"
Ricardo H. Cruz-Estrada
JUNE, 1997-SEPTEMBER, 1997
The work in this project was focused in the area of internal stresses in fibre einforced composites, especially reinforced laminates manufactured with thermosetting resins.
Internal microstresses in fibre reinforced composites (FRC) produced due to shrinkage stresses during cure and thermal stresses due to differences between the thermal expansion coefficients of the matrix and fibre are often sufficient to produce microcracking even in the absence of external loads. External factors from the surrounding environment, susch as exposure to elevated temperatures may also contribute to create adverse internal stresses within the material. The microstresses generated may cause loss of adhesion or debonding at the fibre-matrix interface, which can be followed by diffusion of water or other fluids into this area. Therefore, the appreciation of the structure and properties of the interface is essential for the understanding of the mechanical and physical properties of an FRC since that plays a major role in controlling these properties.
The main object of the present work was to analyse the influence of thermally induced stresses on the microstructure and mechanical properties of the fibre-matrix interface in model composites based on single polyethylene (PE) fibres embedded in a low temperature curing epoxy resin. The specimens were subjected to a programme of heat treatment such that controlled levels of internal stresses were produced. The microstructure of the interface was analysed by using polarized optical microscopy. Pull-out tests were carried out to estimate the bond strength between the fibre and the matrix. Experimental results show that when the cured specimens are subjected to heat treatment, the area around the fibre-matrix interface shows an increase in stress concentrations. Consequently, the interfacial shear strength diminishes providing evidence that the difference between the thermal expansion coefficients weakens the bond.