The effects of fibre prestressing have been investigated on the flexural properties of glass-epoxy composites. Fibre prestressed composites were made by applying and maintaining a known amount of tension on the fibres during the curing process of the epoxy resin. In the next step, bending tests were conducted on a tensile machine using four point bending. In this study the modulus and strength of the fibre prestressed composite increased up to 33%. The experimental data also indicated that there existed a fibre prestressing level at which the flexural properties reached their maximum values. This prestressing level was found to be a function of the curing temperature. A mechanism has been proposed to explain the improvement of the flexural properties by the use of fibre prestressing during the cure process.

The glass fiber (GF) reinforced epoxy composite samples used in this study are prepared by four different standardized methods, designated as Methods A, B, C, and D. The GF that is modified with silica particles and prestressed during curing in the composite system (Method D) is proved to be significantly superior as a potential candidate material for use in high impact applications. The impact property of the so-finished composite samples is compared, in turn, with other composite samples prepared by the other methods. There are clean fiber reinforced epoxy composite (Method A), modified GF reinforced epoxy composite (Method B), and prestressed clean GF reinforced epoxy composite (Method C). The results indicate that the impact property of the composites prepared by this novel Method D is greater by up to 100% in comparison with Method A and by lesser but, significant, amounts by the other two methods.

By applying the appropriate tension to the fibers during the curing process, the impact strength of glass-epoxy composites is improved up to 33%. Charpy tests show that the increase of the fiber prestressing increases the impact strength up to a particular level. Beyond this level, however, the increase of prestressing reduces the impact strength. The fiber prestressed samples indicate splitting breakage within the polymer that creates more new surfaces as compared with unprestressed composites. This results in the sample absorbing more energy during the impact. The splitting breakage is explained by the formation of the residual stresses in the polymeric matrix. Those stresses are controlled and promoted by fiber prestressing during the curing process.

Thermoplastic matrix composites have recently emerged as promising engineering materials because of their desirable properties such as high service temperatures, high impact resistance, and processing advantages. However, residual stresses in composites introduced during fabrication are cited as one of the most significant problems in the processing of composites. In some instances these stresses have been shown to significantly degrade the strength of the material, resulting in matrix cracking, debonding, reduced fracture toughness, and delamination. In this work, studies have been carried out on glass fiber reinforced polypropylene composites formed by compression molding process from co‐mingled fabrics. The fibers were pre‐stressed during the process to produce high performance composite products with low residual microstresses, which are harmful to the properties of the composite. Mechanical tests showed that pre‐stress can increase the tensile, flexural and interlaminar shear properties of the composites, and there exists an optimum pre‐stress level to gain best properties for each external loading condition.

The effects of curing conditions on the mechanical properties of glass fiber reinforced epoxy composites under different fiber prestressing levels were quantitatively studied. The composite samples were prepared with silica particle modified continuous E-glass fibers, epoxy resin matrix, fiber prestressing and step curing processing. Room temperature cured and isothermal high temperature cured composite samples, with the same content and structure, were made to study the difference of three curing conditions on a quantitative level. Based on the test results of flexural, shear, and impact properties of the composite samples, it was found that the step curing procedure generates composite samples with up to 47% and 14% increased properties than room temperature cured and isothermal high temperature cured samples, respectively, when manufactured under the same fiber prestressing level. The possibility of generating an optimum residual stress within the samples during processing is proposed and discussed to explain the contribution of the step curing process to the composite properties.