The mechanical properties of fiber-reinforced polymer composites depend on several aspects such as the characteristics of constituents, fiber volume fraction, and manufacturing techniques. Fiber prestressing is considered a very attractive manufacturing technique that can be used to produce fiber-reinforced polymer composites with high mechanical properties. This technique has the potential to eliminate or reduce some manufacturing problems like fiber waviness. In the present study, a new approach was used to prepare prestressed fiber-reinforced polymer composites. Unidirectional E-glass fiber-stitched mats were impregnated with epoxy matrix through vacuum-assisted resin transfer molding process. Once the infusion was done, a pre-calculated tensile force was applied to the fiber mats through a hydraulic tensile machine. The impregnated fiber mats were left under tension and vacuum during curing of the epoxy matrix (24 h). Five prestressed samples were prepared by using five different prestressing levels 20, 40, 60, 80, and 100 MPa. In addition, non-prestressed (control) sample was prepared for the purpose of comparison. The influence of fiber prestressing on fiber waviness, fiber volume fraction, and void content was investigated. Flexural, tensile, and compression tests were performed to observe the effect of fiber prestressing on the mechanical properties. The results have shown the success of this new approach in producing prestressed fiber-reinforced polymer composites with high mechanical properties comparing to non-prestressed composites. The microstructure analysis has shown dramatical reduction in fiber waviness for the prestressed samples over control sample. All prestressed samples have shown higher fiber volume fraction and lower void content comparing to the control sample. Also the results have shown as the prestressing level increases, fiber volume fraction increase and void content decreases. Prestressing levels of 40 and 60 MPa were found to be the best candidates, they have led to an increase in tensile strength, compressive strength, and flexural strength by 24.2%, 72.5%, 28% and 28.6%, 100.4%, 26.1%, respectively, comparing to the non-prestressed sample. Ease of implementation and promising results of this new approach would attract the attention toward it. Automotive industry is one potential nominee to apply this approach during manufacturing of fiber-reinforced polymer leaf spring.

The utilization of composite materials has nowadays increased in aerospace applications due to their less weight and superior mechanical properties. Nevertheless, machining of composite materials without damage is quite challenging through conventional system due to their inherent heterogeneity, anisotropy, and thermal sensitivity. To overcome this problem, abrasive water jet machining process can be employed. It is a non-conventional machining processes with high accuracy, high flexibility and with no heat generation. However, there are more challenges in cutting fiber reinforced plastics with this technique. Hence, this work deals with the assessment of the optimum process parameters in abrasive water jet cutting of carbon fiber reinforced plastic composite. Cutting experiments were conducted by varying input parameters such as the traverse rate, standoff distance on three laminates of different thickness. Analysis of variance through response surface methodology technique was used to study the effect of each input parameters on the output responses such as kerf taper and surface roughness. Optimum parameters that provide the best machining quality were found using numerical and graphical optimization techniques. The results showed that increasing the traverse rate results in increased surface roughness and taper angle of the cut kerf. Hence lower traverse rate is preferable when machining quality is of high importance.

The undesired residual stresses in fiber reinforced polymer composites are developed during their manufacturing processes due to the thermal and chemical shrinkage of the polymer matrix, which negatively affect the performance of the composites. Applying tensile stress to the reinforcement fibers during the curing of the matrix can reduce or eliminate the undesired residual stresses. Furthermore, the undesired tensile residual stress within the matrix can be replaced with beneficial compressive residual stress which can improve the mechanical properties of the composites. In this study, theoretical stress analysis was performed in order to determine the value of the optimum compressive residual stress that is associated with the highest tensile strength. This value was determined based on the tensile testing result of prestressed composites with a 40% fiber volume fraction. For the composites with different fiber volume fractions, a new approach was introduced to estimate the optimum fiber prestressing level which generates the optimum compressive residual stress within the matrix and consequently, the highest tensile strength can be achieved. In order to validate this approach experimentally, prestressed composites with 25 and 30% fiber volume fraction were prepared by applying the estimated optimum fiber prestressing levels during curing of matrix and tensile testing was performed. The result of the stress analysis showed that the value of the optimum compressive residual stress corresponding to the highest tensile strength is 2.5 MPa approximately. The tensile testing results confirmed the validity of the new approach in estimating the optimum fiber prestressing level for the composites with different fiber volume fractions. For both prestressed composites with 25 and 30% fiber volume fraction, the highest tensile strength was achieved when the theoretically estimated optimum fiber prestressing level was applied to the fibers during the curing of the matrix.