Stress-relaxation behaviour in composites based on short oil-palm fibres and phenol formaldehyde resin

The stress relaxation in phenol-formaldehyde composites reinforced with short oil-palm empty fruit bunch fibres has been studied as a function of fibre loading, fibre treatment, physical ageing and strain level. Maximum stress relaxation was observed for 30 wt.% fibre loading. Treated composites showed higher relaxation except in composites treated with alkali and toluene 2, 4 diisocyanate (TDI). Water ageing increased the rate of relaxation. The effect of hybridisation of the oil-palm fibres with glass fibres on the relaxation behaviour was examined. The stress-relaxation rate of the composite was lowered upon hybridisation. The rate of relaxation for different time intervals and crossover time was calculated in order to explain the relaxation mode of the composites. Master stress-relaxation curves were drawn to explain the long-term behaviour of the composites by superimposing the stress values at different strains by a horizontal shift along the logarithmic time axis. The relaxation modulus values for the composite show a trend similar to that of relaxation of stress in the composites.