A lightweight aggregate concrete-filled steel tube (LACFST) spatial truss beam was tested under bending load. The performance was studied by the analysis of the beam deflection and strains in its chords and webs. According to the test results, several assumptions were made to deduce the bearing capacity calculation method based on the force balance of the whole section. An optimal dimension relationship for the truss beam chords was proposed and verified by finite element analysis. Results show that the LACFST spatial truss beam failed after excessive deflection. The strain distribution agreed with Bernoulli-Euler theoretical prediction. The truss beam flexural bearing capacity calculation results matched test evidence with only a 3% difference between the two. Finite element analyses with different chord dimensions show that the ultimate bearing capacity increases as the chord dimensions increase when the chords have a diameter smaller than optimal one; otherwise, it remains almost unchanged as the chord dimensions increase.
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In case of forming tailored blanks consisting of sheets with different thickness, the forming tools have to be designed with such a shift and the weld-line movement has to be taken into account to avoid failures such as wrinkles or cracks [5]. Weld-line movement occurs especially when forming tailored blanks with high differences in the stress strain behavior and with a high thickness ratio [6] that facilitate the movement of the weld line. The investigation of the stretch-forming behavior of tailored blanks made out of dissimilar material combinations using dual-phase (DP) steels are carried out by Panda et al. [7]. It was concluded that, due to the non-uniform strain distribution, the weaker high-strength low-alloy (HSLA) sheet metal failed close to the weld, resulting in a decrease in the limiting dome height of tailored blanks. Weld-line movement in deep drawing of cylindrical cups has been investigated, and the typical failures like cracks and wrinkles are pointed out when using materials with different thickness [8,9]. The weld-line movement and the formability in general are described analytically by Kinsey and Cao [9]. Several investigations exist on the improvement of the formability of tailor-welded blanks and the reduction of weld-line movement. An application of draw bead in the die is proposed by Heo et al. [10], who added restraining force to control the flow of thinner material during deformation, resulting in a significant reduction in weld-line movement. However, the higher restraining force results in early thinning on the thinner side and subsequently leads to the failure of the tailored blanks during flange drawing. The control and adjustment of the blank holder force can lead to a minimization of the weld-line movement [11]. The die cushion of the press is replaced by a nitrogen cylinder system consisting of six nitrogen cylinders. The system thus behaves like a multipoint pressure control system capable of adjusting the blank holder force around the periphery of the sheet. Kinsey and Cao showed the possibility of a reduction of weld-line movement by clamping the blank locally with the use of hydraulic pressure in a segmented tool [12,13]. Variable blank holder pressure is also suggested by Kinsey and Wu [6] to control the movement of the weld line.
The use of segmented tools for forming tailored blanks enables the application of different blank holder forces on the sheets. Although usually segmented, the tool materials do not differ and allow withstanding high loads. It means that, in one region, the lifetime of the tool is overestimated. Three benefits mentioned for the use of light tools are the reduction of cycle time, lower energy consumption, and handling during the installation of the tool in the press [14]. A major benefit of the servo press technology is that the ram can be stopped and accelerated without reaching the dead center, and the cycle time can therefore be reduced [15]. By reducing the weight of the tools, the effectiveness of the servo presses can be increased. To improve the forming behavior of tailored blanks, to decrease the weld-line movement, and to make the process more energy efficient, the aim of this work is to replace the tool material of one segment by using hybrid tools as described in the studies of Witulski et al. [16]. With the use of such tools, the forming of parts with DP600 material is possible, and the lifetime of such tools can reach a number of 1000 parts without the appearance of failures on the tool or on the formed sheet [17]. Another known quantity-optimized tooling technology is the use of nickel-shell-based tools [18]. This is a hybrid tool system that consists of a wear-resistant active tool surface (nickel) and a polymeric base. This strategy has a potential of cost savings of up to 40% over conventional tool technologies. Mennecart et al. [19] pointed out that the use of polymer (polyurethane) and hybrid tools (polyurethane strengthened by fiber reinforcement) has some other advantages too, such as the homogenization of pressure distribution or the capability to include elements to control the local blank holder pressure. As described in the studies of Endelt, Tommerup and Danckert [20,21], the use of media to apply a load in different regions can improve the forming results of high-strength steels.
(a) Segmented die setup with steel and reinforced polymer type K2; (b) spring-reinforced die setups and axial symmetric cutting plan; (c) screen-printed grid on initial tailored blank with sections in two orientations related to the weld line.
Figure 5 shows the reason of the minimization of weld-line movement due to elastic deformation of the die and especially the die entrance in comparison with Setup 1 (steel/steel) (Figure 5a). Figure 5b,c show the pressure distribution in the middle of the punch travel for the tool for Setups 3 and 4. Here, Setup 4 (steel/polymer K3) (Figure 4c) shows higher pressure and a more homogeneous pressure distribution in the die entrance than Setups 1 and 3 (Figure 5b). This can be explained by the different contact angles in this area due to the deformation. When the zone of contact was larger, the restraining force was caused by friction increases.
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