Effect of biaxial fabric prestressing on the mechanical properties of plain–weave E–glass/polyester composites

It is of interest whether induced residual stresses would affect the mechanical properties of fibre–reinforced composites. One of the methods that can be used for altering the induced residual stresses within the matrix is the method of fibre prestressing. Although this method was previously used for developing the mechanical properties of unidirectional composites, its application to the woven composites was very rare. There are many applications of composite materials where woven fabric has been used instead of unidirectional fibre such as for helmets, armours, boats, and the automotive components. The mechanical properties of woven composite may be improved without increasing its volume and/or weight. Therefore, this study emphasizes on improving the mechanical properties and fatigue behaviour of the plain–weave composite by applying biaxial fabric prestressing. Firstly, the induced residual stresses within the composite’s constituents due to fibre prestress was calculated theoretically by developing the macro-mechanics theory. Secondly, numerical modelling of the prestressed composites was implemented using ANSYS® software for validating the theoretical results and estimating the full distribution of the residual stresses within the composite’s constituents. The biaxial prestressing frame was used for providing biaxial fabric pretension load. Prestressed composites were manufactured with different levels of prestressing ranging from 25 to 100 MPa and prepared at different fibre orientation angles such as 0, 15, 30 and 45°. Lastly, experimental tests such as tensile, flexural and fatigue were conducted on the E–glass plain–weave/polyester resin composite in order to assess the advantages that might result from applying biaxial fabric prestressing. Theoretical results showed that the level of the induced residual stresses within the composite’s constituents depends on fibre prestress level, fibre volume fraction, and the elastic properties of the composite’s constituents. Residual stresses calculated by the developed macro–mechanics theory were in agreement with those obtained by the numerical modelling and previous studies of no less than 1.53%. Numerical simulation of the prestressed composite showed that the maximum induced residual stresses due to fibre prestressing were located at the fibre–matrix interface. Increasing the fibre prestress level increases both the induced compressive residual stresses within the matrix and the fibre–matrix interfacial shearing stress. Experimental results showed that prestressing level of 50 MPa offered the highest improvement in the quasi–static properties and fatigue life behaviour. Enhancements in the tensile and flexural properties were about 20% (from 3.74 to 4.4 GPa of tensile modulus and from 35 to 42 MPa of critical stress) and 15% (from 2.54 to 2.96 GPa of flexural modulus and from 87.11 to 99.88 MPa of flexural strength), respectively. Fatigue cycles to failure were prolonged up to 43% (from 19949 to 28594 cycles) at 0.4 normalised peak stress in comparison with non–prestressed counterparts. The levels of improvement were reduced with increasing the fibre orientation to 45°. Empirical functions were estimated to include the prestress effect in the tensile, flexural and fatigue behaviours. Prestressed composite specimens with 50 MPa showed a decline in the improved tensile strength, flexural strength and fatigue cycles to failure which were about 3.56 % (from 42.07 to 40.56 MPa), 1.96% (from 99.88 to 97.92 MPa) and 14.55% (from 28594 to 24432 cycles) after six months since they were manufactured, respectively. These declines resulted from the stress relaxation effect within the matrix. Considering all findings, it was concluded that the proposed prestressing method enhanced the mechanical properties of the plain–weave composite. This improvement resulted from increasing the composite resistance against quasi–static and fatigue loadings by reducing both fibre waviness and the tensile residual stresses induced within the matrix. The fibre prestress method enhanced the mechanical properties of the plain–weave composite in both on–axis and off–axis directions. These improvements still existed after complete redistribution of the induced residual stresses within the matrix.

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