Recently, the demand for reinforced plastics from natural, sustainable, biodegradable, and environmentally friendly fibers has been rising worldwide. However, the main shortcoming of natural fibers reinforced plastics is the poor compatibility between reinforcing fibers and the matrix. Hence, it is necessary to form a strong attachment of the fibers to the matrix to obtain the optimum performance. In this work, chemical treatments (acid pretreatment, alkali pretreatment, and scouring) were employed on jute fibers to modify them. The mechanical properties, surface morphology, and Fourier transform infrared spectra of treated and untreated jute fibers were analyzed to understand the influence of chemical modifications on the fiber. Then, jute fiber/epoxy composites with a unidirectional jute fiber organization were prepared. Basic properties of the composites such as the void fraction, tensile strength, initial modulus, and elongation at break were studied. The better interfacial adhesion of treated fibers was shown by scanning electron microscope (SEM) images of fractured coupons. Hence, the chemical treatment of jute fiber has a significant impact on the formation of voids in the composites as well as the mechanical properties of jute fiber composites.

The durability of the notch and un-notch composites is mainly affected by environmental ageing, the condition of mechanical loading and mechanical ageing. Due to the dissimilarity of two components of fibre mat-reinforced thermosetting composites (e.g. fibre and resin), thermal stress and strain can be generated and raised by moisture, temperature variation, thermal ageing and mechanical ageing in any working environment. This study aims at investigating the effect of thermal ageing fatigue and mechanical ageing fatigue on the mechanical durability and notch sensitivity properties of glass fibre mat unsaturated polyester resin composites and kenaf fibre mat/unsaturated polyester composites. Tensile tests of smooth and notched specimens, low cycle tensile fatigue tests, dynamic mechanical analysis, stress distribution by point-stress failure criteria and notch sensitivity analysis were performed to both types of the specimens. Both composites were previously taken under up to 100 number of freeze-thaw ageing cycles (from −25 to + 58℃). It is found that the tensile modulus and un-notched strength of kenaf fibre mat/unsaturated polyester were decreased by 30 and 27%, respectively, which was more severe than that of glass fibre mat/unsaturated polyester composites. After the thermal ageing, notch (open hole) did not diminish the strength property of kenaf fibre mat/unsaturated polyester, and there was an absolute increase of characteristic distance (d0) which indicates the lower notch sensitivity of kenaf fibre mat/unsaturated polyester. Although glass fibre mat/unsaturated polyester had superior mechanical properties, after thermal ageing, while kenaf fibre mat/unsaturated polyester presented stable low cycle fatigue property, better energy absorption and notch sensitivity properties.

The mechanical property and long-term durable life of short fiber composites are inevitably subjected to internal and external influences during molding and afterwards, such as weld, temperature, stress type, and so on. This study was concerned with the influence of weld on mechanical properties of short glass fiber reinforced polyethylene terephthalate injection molding composites and the investigation of their tensile properties at different temperatures and tensile speeds with varying fiber contents. The weld strength was about 50–60% less than the tensile strength at different temperatures and tensile speeds. As the fiber content increased from 15 wt% to 30 wt%, the weld strength was reduced by 10%. The tensile and weld strength were in general inversely proportional to the temperature and linearly proportional to the tensile speed. Tensile modulus showed an inverse association with temperature and a mostly non-linear relation with the tensile speed. The weld line integrity in tensile strength was independent of tensile speed and temperature below the glass transition temperature level. Morphology evaluation testified that the higher test speed emanated better fracture surface fiber–resin adhesion properties with comparatively brittle fracture tensile behavior. The time-temperature superposition principle was applied to find the scope of long-term durability, lifetime prediction and to describe the viscoelastic properties of the welded glass fiber reinforced polyethylene terephthalate composites. An Arrhenius type of shift factor was obtained to fit the tensile strength data which was independent of the fiber contents of the welds. The experimental results can provide a valuable reference for the design and manufacture of short fiber-reinforced composites in long-term durability applications.