METHODS FOR REDUCING NOTCH SENSITIVITY OF HYBRID PSEUDO-DUCTILE POLYMER COMPOSITES WITH FABRIC REINFORCEMENT: EXPERIMENTAL STUDY

Abstract


Composite materials reinforced with synthetic fibres have been used in aviation and space technology for more than half a century. Fibre-reinforced composites with high specific strength and corrosion resistance are an attractive alternative to traditional structural materials, including steels, aluminium and titanium alloys. At the same time, composites based on carbon and glass fibres are inherently brittle structural materials with high strength sensitivity to stress concentrations due to the design features of the structures or defects that occur in operation. One way to solve this problem is hybridisation which makes it possible to increase the nonlinearity of the composite stress-strain diagram and reduce sensitivity to notches. Hybrid composites combine several types of reinforcing filler with different fracture strains and exhibit a pronounced pseudo-ductile plateau in tension. Such material behaviour ensures the redistribution of stresses near the concentrator and potentially reduce notch sensitivity. When designing hybrids, it is necessary to take into account the influence of different factors including the ratio between the components and their lay-up, using various technological methods, and the specific strength of the finished material. This paper presents the results of an experimental study on the strength of hybrid composites based on glass and carbon fabrics in the open hole tests. It was found that hybrids with an extended hardening area after the pseudo-yield plateau are were more notch sensitive. A low elongation component layers rotation on angles up to 10°, as well as the use of thin polymer veils, also reduce the sensitivity of the composite strength to the presence of the defects.

Full Text

The stress concentration has a significant effect on the performance of composite structures due to the brittle mechanical behaviour of traditional layered polymer composite materials (PCM). The increased sensitivity to local stresses leads to the necessity of large normative safety factors and reduces the overall weight efficiency. One of the possible solutions to this problem is to expand the area of nonlinear deformation of the material, which ensures the redistribution of stresses in the defect zone and increases the allowed loads. Traditional composite materials often do not have a pronounced nonlinearity of deformation and, in most cases, are considered linearly elastic up to failure. Researchers and engineers proposed several ways to obtain a pronounced non-linear behaviour on traditional composites, for example, using reinforcement with metal [1–4] or high-strength polymer fibres [5–7]. Despite significant inelastic deformations for composite materials (up to 15 %), these methods are not applied in modern critical highly loaded structures, since they do not allow to meet high requirements for mass efficiency, specific strength and/or stiffness. Hybridisation is one of the promising methods for expanding the region of nonlinear deformation (up to pseudo-ductility) by combining at least two types of reinforcing fillers with low and high elongation in failure (LE and HE) in one composite material. There are interlayer, intralayer and intrayarn hybridisation [8–13] (Fig. 1, a). Interlayer hybridisation seems to be the most technologically advanced, as it focuses on the use of known materials and technologies like contact or autoclave moulding of packages from LE and HE prepregs. The LE component fails earlier, while HE component continues to bear the applied load. In [14–16], emphasis is placed on ensuring a stable accumulation of damage in LE component which avoids a sharp drop in the stresses level on the stressstrain diagram and reduces the risk of sudden failure (Fig. 1, b). The sharp stresses drop is associated with a single fragmentation of the LE component and its subsequent delamination (Fig. 1, c). In this case, LE layers are unloaded which can lead to an overload of HE component layers and a general decrease in the strength. It should be noted that in most studies the authors considered hybridisation of unidirectional composites, whereas only a few works are devoted to hybridisation using fabrics with plain, twill, or satin weave structures [17]. The topic of pseudo-ductility as a way to reduce the sensitivity of composite materials to the presence of holes and notches is poorly covered in literature [18–21]. The published studies are different in methods of assessing the influence of stress concentrators on the material strength, and there is no comparison of the hybrids strength with the strength of the hybrid components. This allows to speak only about the effectiveness of a definite hybrid composite within a series of comparative tests of a number of hybrid PCMs. The comparison is strongly necessary if the possibility of replacing a non-hybrid composite material with a hybrid one is assessed. In most cases, the strength of the hybrid under uniform stress (rectangular specimens) is lower than that of the composite based on LE component only. For this reason, the resulting benefit of reducing the concentrator effect on the strength of the overall structure by hybridisation might be negligible. In [22] to assess the strength of the specimens with stress concentrators, it was proposed to use notch sensitivity factor kF = Fn/F0, where Fn is nominal failure stress in a weakened section of a specimen with a concentrator, а F0 – ultimate tensile strength of the rectangular specimen. The value of the notch sensitivity factor typically is within the range of, whereas in critical aerospace structures it is important to create conditions for obtaining insensitivity to holes and notches (kF → 1). The article considers the effect of lay-up engineering of a layered hybrid composite on the sensitivity to stress concentrators. Hybrid composites were composed of LE and HE components based on carbon and glassfabrics, respectively. The authors studied such technological methods as influence of LE and HE layers ratio, the change in the stacking sequence and the angle of LE layers in the package, and the modification of the interface between LE and HE layers with thin polymer veils as well. To assess the effectiveness of the above mentioned methods, notch sensitivity factor and the final weight efficiency of hybrid composites were used.

About the authors

E. V. Leshkov

South Ural State University

N. A. Olivenko

South Ural State University

O. A. Kudryavtsev

South Ural State University

S. B. Sapozhnikov

South Ural State University

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Copyright (c) 2023 Leshkov E.V., Olivenko N.A., Kudryavtsev O.A., Sapozhnikov S.B.

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