FAILURE PROCESSES IN THE FIBER-POLYMER INTERPHASE

The role of friction in tests used to measure the strength of the fibre-polymer interface is critically examined. It is shown that it is very difficult to obtain results that are untainted by friction, and the different tests probably involve different ratios of frictional shears to pure debonding shears. This could account for the large discrepancies observed in tests with these methods. Another difficulty with reported values for debonding shear stresses is that they are occasionally much higher than the probable shear strength of the polymer. This is unlikely to be due to constraints arising in the test. Instead, the energy of debonding may be the cause; quite low debonding energies require large shear stresses to overcome them.