Three-dimensional Wire Structures

"Super-lightweight materials" by 3d-Kagome-wire structures

In nature many structures are known where material is only used at points where it is absolutely necessary. One example is bone, where a customized strength and low weight is realised by a well directed use of pores. The transfer of such optional characteristics to metallic construction materials has been a long time the focus of numerous research activities. The main objective of the developments is the reduction of material use in vehicles, machinery, plant and equipment. A reduction especially of moving masses means reduction of resources and with it energy and cost reduction. This is also an important environmental contribution.

Because of that, in the recent years there had been strong efforts, both nationally and internationally, to construct new lightweight materials. A promising class are cellular metallic materials (ZMW), where the weight reduction is happening through the installation of pores. In addition to a drastic weight and hence material reduction there are more application specific characteristics that can be realised by ZMW, in particular through the cell structure, such as sound absorption, thermal insulation, energy absorption, mechanical cushioning, chemical and energy transport or catalytic effects. For these characteristics there are more fields of application, in addition to the ones mentioned above, such as the thermal, mechanical and chemical engineering, biotechnology and environmental technology.

Typical representatives of cellular metallic materials are metal foams, hollow sphere structures, open-metallic materials, fibre structures or materials manufactured by screen printing. A completely new variant are 3-dimensional wire structures. These are periodic spatial bar structures.

A special form are so-called "Kagome" structures consisting of wires, which intersect at an angle of 60° or 120°, with each intersection consisting of three wires, see Figure 1.

Examples of completely built Kagome-wire structures (WBK - Wire-woven bulk Kagome) are shown in Figure 2. Previously, such WBK structures were manufactured on the laboratory scale from different materials, such as low alloyed steel, stainless steel, high / ultrahigh hardened steel, copper and aluminium-based materials. Wire strengths of about 0.2 - 3.0 mm could be successfully manufactured to WBK structures. The cell size is currently adjustable in the range of about 5 to 25 mm. After the production of the "green structure" soldering is necessary, which combines the triplepoints of the wires physically. The so-produced WBK structures as ultra light structures can be used in sandwich structures (Figure 2 below).

Depending on the used material, the wire strength and the cell size, the density as a significant material indicator can be adjusted in the range between 0.03 - 0.5 g / cc, but the goal is to realize density values well below 0.15 g / cc.

Another important characteristic of WBK structures is the very high specific strength and stiffness. Figure 3 shows an example of the compressive strength of WBK in comparison to other lightweight metallic materials, depending on the density. Compared with the well-known lightweight aluminium foam structures WBK have a by a factor of 7 to 10 higher specific compressive strength! The pressure tests (Figure 4) showed that the strength of the Kagome-wire structure is about 85% of the ideal Kagome structure (just wires)!

Studies on the strength of Kagome-wire structures in Figure 5 show characteristics that nearly approximate to the bending strength that is theoretically ideal for a Kagome structure.

Unlike for example honeycomb structures WBK continue to have an almost isotropic behaviour, which can be used well targeted in specific applications.

Due to the high material flexibility, the wide ranges in the wire diameter and the cell size WBK structures have a big application potential. Especially for the structural lightweight construction fields of application are in the automotive construction and aerospace. Another interesting field of application could be biomaterials, where an adaptation of the mechanical characteristics of natural bone, for example by a spongy bone, could be well realised by a titanium based WBK structure.

In the course of a project a basis technology for the production of WBK structures has to be developed (until now there is only a manual production). It is therefore necessary to work out a machine technology for 3-dimensional weaving of wire structures and an associated soldering technology including evaluating the image of characteristics of WBK structures. Furthermore, WBK prototypes and demonstrators have to be built and application relevant testing has to be done.

Literature:

Wadley, H.N.G. u.a. „Cellular Truss Core Sandwich Structure“, Adv. Eng. Mat. 2002,4, No. 10, 759-764

Wadley, H.N.G. „Multifunctional Periodic Cellular Metals“, Phil. Trans. R. Soc., A2006, 364, 31-68

Lim, J.-H., Kang, K.-J. „ Mechan. behaviour of sandwich panels with tetrahedral and Kagome truss cores fabricated from wires“, Int. Journ. of Solids and Structures 42 (2006), 5228-5246