Torsional and compressive behaviours of a hybrid material: Spiral fibre reinforced metal matrix composite
Graphical abstract
Introduction
Fibre-reinforced metals are an important group of metal–metal composites [1] and used widely in a range of applications [2], [3], [4]. Armouring a metal with stronger fibres or wires is a traditional way of enhancing its strength [5]. A novel approach to producing metal–metal composites with enhanced ductility was recently suggested by Bouaziz [6]. The concept is based on embedding architectured reinforcements in a metal matrix, which impose a geometrically-induced strain hardening. This postpones the onset of necking, thereby increasing the tensile ductility of the material. A particular realisation of this concept is through embedding helical reinforcement wires that acts as springs and provides the composite with additional strain hardening capability [6]. Fabricating ‘architectured’ composite metals with nanostructured components is particularly attractive, and traditional methods of severe plastic deformation (SPD) [7] and their recent developments (e.g. [8], [9], [10]) appear particularly suitable for that. Indeed, as suggested in [6], SPD techniques make it possible to produce a desired inner architecture of the composite, while at the same time imparting an ultrafine (and sometimes nano scale) grain structure to the constituents of the composite. Not only does SPD processing change the mechanical characteristics of the material, such as tensile strength and fatigue limit, but it can also modify a range of physical properties [7]. However, there are limitations on the industrial applications of most SPD techniques due to the batch character of the processes involved [11]. By contrast, the proposed process is continuous in nature and does not cause such scale-up issues in fabrication of the hybrid materials.
Recently, we proposed to manufacture composite, or hybrid, materials with helical reinforcement structures by employing well established techniques of severe plastic deformation (SPD) [12]. With these techniques, physical bonding of the metallic matrix and the reinforcing fibres is achieved via twisting of a pre-assembled metal composite. In a recent article, the suitability of one of the SPD techniques, known as twist extrusion [13], for producing metal–metal composites with a spiral structure was demonstrated [14]. What is especially attractive in the adaptation of SPD techniques to the fabrication of fibre-reinforced metal–metal composites is the possibility to architecture them in a desired way, while simultaneously imparting to them very substantial grain refinement. It is therefore not surprising that further processing techniques, such as axisymmetric forward spiral composite extrusion (AFSCE) [15], [16], which achieve this dual goal, are emerging. The AFSCE process bears certain similarities with TE, but has the advantage that the formation of dead zones in the corners of the die is avoided owing to its axisymmetric nature. Besides, AFSCE is simpler than TE in terms of tooling and processing. An interesting characteristic of TE and AFSCE is that shear deformation occurs only in the transient regions, including entry-twist zones and twist-exit zones. The deformation in the twist zone itself is almost zero and only rigid body motions occur therein. This is conceptually very similar to the archetypal SPD technique of equal-channel angular pressing, in which the shear strain is localised in a very narrow region where the entry and the exit channels meet. A variable pitch version of AFSCE was proposed later [17] to extend the transient zone and increase grain refinement.
In some structures, such as energy absorption systems and crash cushions, a combination of a soft matrix and reinforcing steel wires is required. To design such structures, it is critical to know the combined behaviour of the structure under different loading conditions.
A simple example of the concept presented in [6], viz. that of a metal–metal composite with a spiral structure of embedded armour wires, is proposed here. We consider a metal–metal hybrid which is composed of a cylindrical metallic matrix with a number of reinforcing wires distributed regularly along a pitch circle concentric with the metallic matrix cylinder. We present an analytical model enabling us to study the impact of the design parameters on the plastic torsional and compressive response of the composite.
A metallic sample with embedded armouring wires aligned with its axis is depicted in Fig. 1. This assembly was used in numerical simulations and the actual experiments. In Section 2 we present a model that makes it possible to describe the torsion process transforming the straight wires to spiral ones and then move on to predicting the plastic mechanical response of the hybrid material thus produced to compression loading. Subsequently, in Section 3, the results of experiments on copper armoured with steel wires are presented, which validate the model predictions.
Section snippets
Concept of the composite
The composite metal sample proposed here is fabricated in two steps. The first step is to produce an assembly composed of a metallic matrix and reinforcing wires, all parallel to the cylinder's centreline. The second step consists in twisting the assembly plastically to produce a physical bonding between the metallic matrix and the wires. Next, the process parameters are defined, followed by an analytical solution describing the torsional behaviour of the composite metal sample.
Fig. 1(a) shows
Mathematical description
Once the hybrid material with spiral architecture of the armour wires is fabricated as described in the previous section, one would be interested to know its plastic behaviour under axial loading. We consider its response to compressive loading and use the compression test as a way to verify the model predictions. To develop and solve a semi-analytical model that describes this behaviour, one should consider the presence of spirally shaped reinforcing fibres in the sample and also account for
Discussion and conclusions
The outcomes of this work are two-fold. First, the viability of producing metal–metal hybrids with spiral spring architecture of the reinforcement by torsional deformation was demonstrated. Second, a semi-analytical tool for calculating the mechanical response of such hybrid materials was proposed and successfully tested. In the computational exercise for the case of a pair “copper matrix/steel wire reinforcement”, subdivision of the specimen in ten concentric shells was used. Obviously, a
Acknowledgements
This work was supported by the Ministry of Education and Science of the Russian Federation under grant #14.A12.31.0001 and the National Research Foundation of Korea (NRF) through grant #2014R1A2A1A10051322 funded by the Korean government (MSIP).
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