*Article* **On the Performance of Thin-Walled Crash Boxes Joined by Forming**

**Diogo F. M. Silva 1, Carlos M. A. Silva 1, Ivo M. F. Bragança 2, Chris V. Nielsen 3, Luis M. Alves <sup>1</sup> and Paulo A. F. Martins 1,\***


Received: 7 June 2018; Accepted: 28 June 2018; Published: 29 June 2018

**Abstract:** A new joining by forming process that combines lancing and shearing with sheet-bulk compression is utilized to assemble thin-walled crash boxes utilized as energy absorbers. Process design and fabrication of the new crash boxes are analyzed by finite elements and experimentation. Axial crush tests were performed to compare the overall crashworthiness performance of the new crash boxes against that of conventional crash boxes assembled by resistance spot-welding. Results show that the joining process is a good alternative to resistance spot-welding because the new crash boxes can absorb the same crushing energy, and because the new process helps to overcome typical manufacturing problems of welding.

**Keywords:** crash boxes; joining by forming; resistance spot-welding; crashworthiness

#### **1. Introduction**

An energy absorber is an important element of a vehicle because it protects the lives of passengers by managing the absorption of energy and collapse of its structure during an accident. One of the strategies currently employed by vehicle manufacturers to meet the increasing requirements on safety and the diminishing weight targets is the utilization of energy absorbers made of high-strength materials in lightweight body structures. However, the search for more effective energy absorbers is wider and includes new geometries and materials with the potential to enhance the crashworthiness performance of the structures under different types of loading. These trends are comprehensively discussed in two recently published state-of-the-art reviews on energy absorbers [1,2].

The search for new processes to manufacture energy absorbers has so far received little attention. Most of the publications in the field make use of energy absorbers produced by conventional extrusion [3,4] or by combination of forming and welding [5,6] (or, adhesive bonding) with or without hydroforming [7]. However, the design of energy absorbers with higher performance and wider applicability than the existing ones requires the development of new fabrication processes that allows for a combination of dissimilar materials with different thicknesses and types of surface coatings, as well as a combination of adhesives.

Joining by forming [8] can also be successfully utilized to fabricate efficient, low-cost, energy absorbers. Lee et al. [9], for example, made use of self-piercing riveting to assemble thin-walled crash boxes with double hat-shaped sections made from steel and aluminum formed panels. They analyzed the overall crashworthiness performance and concluded that the energy absorbed by the self-piercing

riveted crash boxes is higher than that of the adhesive-bonded. Gronostajski and Polak [10] utilized two different clinching processes to assemble thin-walled crash boxes with double hat-shaped sections made of steel formed panels and concluded that clinching can be successfully applied to fabricate energy absorbers.

Table 1 summarizes the main advantages and disadvantages of the fabrication processes that are commonly utilized to assemble crash boxes with top-hat and double-hat shaped sections made from individual formed panels.

**Table 1.** Main characteristics of the fabrication processes that are commonly utilized to assemble thin-walled crash boxes with top-hat or double-hat shaped sections.


This paper is focused on the assembly of thin-walled crash boxes with double-hat shaped sections made from individual formed panels. The aim and objective is to present a new fabrication process that combines lancing of the tenons, in which specific areas of the panels are sheared and bent in a single press operation (Figure 1a), shearing of the mortises, in which holes are cut out of the opposite panels (Figure 1b), and sheet-bulk compression of the tenons during which the two panels are assembled by means of "mortise-and-tenon" joints placed along the flanges (Figure 1c,d).

The new fabrication process draws from two previous investigations on joining by forming in which "mortise-and-tenon" joints were successfully utilized to fix two sheets longitudinally in position by sheet-bulk compression. The two sheets were joined either perpendicular [11] or parallel [12] to each other. The reason behind the utilization of "mortise-and-tenon" joints as an alternative to resistance spot-welded or adhesive bonded joints is because they offer the same advantages of self-piercing riveted and clinched joints (refer to Table 1) without having the constraints related to organic coatings and lubricants. Moreover, the "mortise-and-tenon" joints have the ability to be easily combined with adhesives in order to increase stiffness and are also capable of ensuring higher-peel strength than self-piercing riveted and clinched joints due to the protrusion of the flat-shaped surface heads of the tenons above the adjacent sheet panels, after compression.

**Figure 1.** New fabrication process to assemble the individual formed panels of thin-walled crash boxes with double-hat shaped sections: (**a**) lancing (shearing and bending) of the tenon; (**b**) shearing of the mortise; (**c**) sheet-bulk compression of the tenon with a tapered punch; (**d**) mechanical locking by sheet-bulk compression of the tenon with a flat punch; (**e**) cross-section of a "mortise-and-tenon" joint. Note: the lancing in (**a**) is upside-down in order to replicate the press movement.

The main challenge of applying the "mortise-and-tenon" joint concept in the assembly of crash boxes (Figure 1e) derives from the need to compress thin-walled formed panels in the direction perpendicular to thickness. However, the authors solved the problem by introducing a novel two-stage variant of the sheet-bulk compression process that makes uses of a tapered heading punch in the first stage (Figure 1c), and a flat heading punch in the second stage (Figure 1d). The tapered heading punch ensures a better balance of material displacement and diminishes the risk of buckling during the initial compression of the panels. The flat heading punch ensures the mechanical locking of the two individual panels.

The crash boxes assembled with the new proposed "mortise-and-tenon" joint concept are subjected to static and dynamic axial crushing and its overall crashworthiness performance is compared against that of resistance spot-welded. Results show that the new crash boxes are a good alternative to those assembled by resistance spot-welding.
