Design and Analysis of an Automotive Crash Box Using Strut-Based Lattice Structures

Road accidents represent a significant global crisis with the potential for catastrophic outcomes. The automotive industry has placed the utmost emphasis on safety in the present day for paramount reasons. Car accidents result in a considerable toll of injuries and, tragically, fatalities. Statistics reveal an alarming figure of approximately 1.3 × 10⁶ lives lost and 39 × 10⁶ individuals harmed on a global scale each year [1]. Consequently, numerous safety enhancements have been incorporated into vehicles, such as airbags, parking sensors, stability control, cruise control, traction control, and more. Among these safety measures, crash boxes are specifically designed components positioned at the front end of cars, trucks, and various vehicles, serving as a protective barrier between the bumper and the chassis [2,3]. Crashes are contingent on the presence of kinetic energy. As a vehicle moves, it possesses a certain quantity of kinetic energy. When a collision occurs, this kinetic energy initially undergoes a conversion into the energy for elastic deformation and further dissipated through plastification process and reaches a state of equilibrium [4]. The crash box is designed to autonomously deform and absorb the impact's energy, mitigating harm to both the vehicle and its occupants. In 2005, Braymand pioneered the concept of the crash box and was granted a U.S. patent (no. US20050016807). During that era, the crash box featured multiple concave formations, referred to as crash beads, which played a pivotal role in initiating the controlled deformation of the crash box during a collision event [5]. Traditional crash boxes are limited by their substantial weight. There is a pressing need to innovate crash boxes that can achieve two essential goals: reducing weight and enhancing crash safety. These innovations should focus on efficiently absorbing the maximum amount of energy using thin materials. In nature, examples like bones and bamboo, which have evolved over millions of years, offer valuable insights into the creation of lightweight yet strong structures [6]. These flimsy constructions are seen in natural cellular structures combined with hierarchical structures. The high stiffness-to-weight ratio of the cellular formations is the key factor that contributes to the structure's lightweight design. A lattice structure is formed when a unit cell is repeated and packed together tightly, with no gaps between the edges or faces. Lattice structures have a much lower density and high specific strength than solid materials, which makes them lightweight and easy to transport. They have high specific stiffness, which means they are stiff for their weight. Lattice structures can absorb a lot of energy before they fail [7]. The capacity of lattice structures to absorb impact energy renders them valuable for scenarios where impact resistance holds significance, such as in the automotive and aerospace industries. Additionally, lattice structures offer insulation properties for heat and sound, making them pertinent for situations where temperature regulation and noise reduction are critical. However, a review of existing literature reveals that the available lattice configurations are relatively limited, with most being only minor adaptations of other cell designs [8]. Lattice structures have found application in the automotive industry, and the aeronautics and aerospace sectors have also harvested their benefits. These structures contribute to improving the performance-to-weight ratio of components and enhancing the efficiency of vehicles in the aeronautical and aerospace domains [9]. A high-energy absorption crash box designed for automobiles has been created. Through the application of finite element analysis (FEA), Nakazawa et al. have identified the body component responsible for absorbing collision energy. In addition, they have introduced a novel design strategy for the crash box's cross-sectional shape, as presented in their work [10]. Kumar et al. conducted a crush box analysis using the Finite Element Method, following the fundamental principles outlined in federal motor vehicle safety standard. This analysis resulted in the generation of stress and energy graphs. Their research findings indicate that corrugated shapes with a thickness of 1.8 mm are the most effective in absorbing energy for automotive crush box, as demonstrated in their study [11]. The thin-walled tube's impact shape has been optimized using finite element analysis and design optimization. Li et al. have introduced the most effective configuration, a thin-walled square cross section tube with double concave and bulgy grooves, for superior energy absorption and reduced peak loads in their research [12]. The computer aided engineering of thin-walled structures with various cross sections exposed to axial and oblique loads was conducted. This study by Tarlochan et al. introduced a triggering mechanism and explored foam filling. The hexagonal tube proved to be the most effective in energy absorption. However, the addition of foam filling did not significantly improve crash performance. They also introduced a circular notch, increasing crash force efficiency by 7% [13]. Through material optimization, the enhancement of passenger safety in vehicle crashes is achievable. Devendra Kantilal et al. performed analysis and designed a vehicle crash box with a circular shape, exploring a range of materials. Their findings indicated that 1018 mild steel exhibited less deformation compared to the 6061 Aluminum Composite [14]. Ciampaglia et al. performed experimental investigations and numerical optimization to assess the impact response of an origami crash box when subjected to axial impact. They validated their findings by utilizing a combination of finite element analysis and experimental testing. Their study revealed that the upper module exhibited a triggering effect and gradual failure, while the lower module contributed added stiffness to prevent early failure. This research resulted in an enhanced crush efficiency compared to standard crash boxes [15]. Baroutaji et al. [16] explore improving impact resistance in structures, emphasizing the use of thin-walled tubular components for energy absorption during collisions. It provides an up-to-date overview of developments in the last decade, including crashworthiness optimization and the performance of unconventional components (multicell, foam-filled tubes) under various loads. Krishna et al. [17] aimed to devise an optimal cross-sectional configuration for an automobile's crash box, with a strong focus on enhancing energy absorption capabilities while eliminating the requirement for a crash bead. Tao et al. [18] conducted a comprehensive exploration of lattice structures in additive manufacturing. Their research encompassed various facets, including the development of design methodologies, the mechanical behavior of lattice structures, and the critical factors influencing their performance, such as topology optimization, porosity, architectural design, and material selection for the components. Saleh et al. [19] offer a comprehensive review of manufacturing techniques for functionally graded materials. This critical analysis outlines the challenges and benefits gleaned from over 30 years of research and provides insights into various applications and upcoming research trends essential for the accurate design and production of functionally graded materials with smoothly graded surfaces. Du Plessis et al. [20] performed a morphological analysis and image-based simulations using design files for comparative purposes. They specifically focused on standard strut-based lattices with minimal surface patterns. While previous studies had individually investigated these two lattice types, this work marked the first direct comparison. The results revealed a marginal performance advantage for minimal surface patterns in angular load simulations. Peng [21] developed a numerical model capable of forecasting the mechanical and fatigue properties across various relative densities and topologies of lattice systems. They explored the relationship between relative densities and geometric features in four distinct lattice structures. To assess the Young's modulus and yield strength of these lattices, the study employed FEA in simulating uniaxial compression tests. The study conducted by Simpson et al. [22] investigated the response of square tubes filled with lattice structures when subjected to quasi-static compression. The research tested three tube variations: auxetic lattice-filled, honeycomb-filled, and empty tubes. Results showed that honeycomb-filled tubes had superior energy absorption. The study also pointed out the underexplored potential of auxetic foam and lattice types, emphasizing the need for further research in this area. Hou et al. [23,24] additively manufactured four distinct lattice structures, including hexagonal, Kagome, reentrant, and triangular honeycombs. Experimental compression test data aligned well with simulation results. Notably, the Kagome lattice structure demonstrated the highest specific energy absorption (SEA) values in the numerical analysis. It's important to note that this study solely focused on assessing the crashworthiness of these four lattice structures, indicating the necessity for future investigations into the crashworthiness performance of additional lattice configurations.

Lattice structures offer benefits like lightweight design, energy absorption, and improved heat dissipation, but their use in industrial applications is still limited [25]. While there has been extensive research focused on enhancing the crashworthiness of crash boxes, it often comes at the cost of increased weight [26]. Notably, there is a scarcity of literature addressing the incorporation of lattice structures into crash boxes, and no previous studies have explored the application of strut-based lattice cells in this context. The primary aim of this research is to assess the effectiveness of integrating lattice structures into crash box designs to enhance their energy absorption capacity. The crash tests are conducted on crash boxes with and without lattice structures and results are compared. Additionally, parametric analysis has been done for a lattice-filled crash box to optimize its design for maximum energy absorption. The findings from this study offer valuable insights into the potential benefits of lattice structures to enhance vehicle safety during collision.

During the initial phase, various geometric shapes for crash boxes were considered, including circular, square, hexagonal, octagonal, decagonal, and dodecagonal forms. The choice was made to focus on rectangular and square shapes, as they are conducive to accommodating lattice structures [1]. Both crash boxes depicted in Fig. 1 share identical dimensions, featuring a length of 350 mm, a thickness of 2 mm. The perimeter of the crash box, roughly 300 mm, was designed based on the typical dimensions of locally sold sedans and small automobiles [13]. The square-shaped crash box has sides of equal length, measuring 75 mm in height and width. In contrast, the rectangular crash box had a height of 60 mm and a width of 90 mm.

Simulations were conducted for both crash boxes, and the findings were analyzed. Significantly, the square crash box demonstrated a higher energy absorption rate of 23.42 KJ in comparison to its rectangular counterpart, which registered 19.02 KJ. These findings are consistent with the results reported in a prior research paper [13]. Consequently, the present study advanced to its subsequent phase with a specific focus on the Square Crash box.

Multiple homogeneous lattice patterns are integrated into the selected conventional square crash box, leading to the development of a diverse range of crash box configurations. These configurations encompassed a variety of patterns as shown in Table 1, including simple cubic (SC), body centered cubic (BCC), simple cubic body centered cubic (SCBCC), reinforced body centered cubic (RBCC), simple cubic reinforced body centered cubic (SCRBCC), face centered cubic (FCC), face centered body centered cubic (FCBCC), modified face centered cubic (MFCC), Octet-Truss (OT), and simple cubic face centered cubic (SCFCC) patterns, as outlined in Ref. [10]. A single unit cell has been modeled and linearly patterned to achieve the shape of a crash box.

All the crash boxes share identical dimensions, with unit cell measurements set at 25 mm for length, height, and width. The crash boxes themselves have a consistent width of 350 mm and measure 75 mm in both length and height. Subsequently, the research advanced to its third phase, which concentrated on optimization techniques and aimed at improving SEA through modifying the strut diameter and unit cell size.

To design the unit cell in Abaqus software, it is necessary to assign three dimensional (3D)-deformable point coordinates, referred to as datum coordinates. These coordinates serve as reference points within the unit cell and are vital in determining its shape and characteristics. For a unit cell with dimensions of 25 mm in length, height, and width, the following datum coordinates can be assigned: (0,25,0), (25,0,−25), (0,0,0), (25,25,−25), (0,25,−25), (25,0,0), (0,0,−25), and (25,25,0). Once the datum coordinates have been established, wire elements are systematically linked between these reference points, forming the structural framework of the unit cell, as visually represented in Fig. 2. These wire elements define the lattice pattern by linking the datum points. During the assembly phase, the unit cell is linearly patterned in Abaqus software, replicating it in a specified direction to achieve the desired shape, such as a square crash box. By arranging and assembling multiple unit cells, a repetitive lattice pattern can be formed. After the assembly of unit cells, they are merged to create a single part. In the property module of Abaqus, beam elements are assigned to the merged lattice structure. This lattice pattern can be designed to fit within a square crash box, contributing to the development of new crash box designs.

The assembly begins with the modeling of individual components. In this process, a total of four parts are modeled, as depicted in Fig. 3. The first component is the square crash box, which is a 3D deformable shell measuring 350 mm in length, 75 mm in both height and width, and a thickness of 2 mm. Another component follows a BCC lattice pattern, while the other two are rigid plates, 3D-Discrete Rigid and planar, with dimensions of 100 mm length and breadth.

The assembly module integrates components created within the part modules. It also serves as a platform for executing Boolean operations like addition, subtraction, and union to facilitate the seamless design of intricate parts. As depicted in Fig. 4, the individual components previously designed are combined in this process

A prominent Multiphysics simulation program within the SIMULIA suite is Abaqus. Abaqus Standard is utilized for problems resolved through implicit techniques, while Abaqus Explicit is employed for addressing high-speed dynamic challenges [27]. The primary material used for its excellent energy-absorbing properties is steel. Specifically, A36 steel, also known as Mild steel, has been chosen for this purpose, including its Johnson Cook parameters detailed in Table 2. In the United States, A36 steel is a widely accepted structural alloy, governed by ASTM International standards since 1960. Pre-1960, A7 (used until 1967) and A9 (utilized until 1940) were the primary structural steel standards. Notably, SAE/AISI A7 and A9 tool steels and the outdated ASTM A7 and A9 structural steels are not interchangeable. A36 steel is characterized by a Young's Modulus of 200 GPa and a Poisson's ratio of 0.26. Additionally, the moving rigid plate used in simulations possesses a mass of 275 kg and has an impact velocity of 56 kmph.

Abaqus-Explicit module is employed for accurately replicating the crash analysis. A 0.05-s time period is set for this analysis. Key constraints included are the creation of tie contacts between the fixed plate and the crash box, treating plates as rigid bodies, and defining boundary conditions within the load module. In this scenario, one end of the crash box has been fixed, while the other end has been impacted by a load of 275 kg traveling at 56 kmph. Additionally, a friction coefficient of 0.2 has been introduced between the contact bodies. Mesh convergence analysis has been performed to ensure the reliability of the simulation. It was observed that there were no discernible changes in results with a 5 mm mesh size. Consequently, the “explicit-linear” element type (Quad) and a “Quad-Structured” mesh configuration were selected as shown in Fig. 5, as this configuration provided optimal simulation efficiency while ensuring result stability and accuracy, as confirmed through rigorous mesh convergence assessment. The shell and the other units are connected by tie and rigid body constraints. In the Abaqus job module, the analysis task is initiated and submitted for processing. After the simulation has been completed, users can access and view the results using the visualization module.

A set of simulations were performed on a range of crash box designs, including the SC, BCC, SCBCC, RBCC, SCRBCC, FCC, FCBCC, MFCC, OT, and SCFCC. The visuals of crash box after impact analysis are shown in Table 3.

Initially, the kinetic energy is converted into the energy for elastic deformation and further dissipated through plastification process and reaches a state of equilibrium [28]. The energy absorption characteristics of lattice crash boxes were determined using the trapezoidal method and the results were graphically represented in Fig. 6. The energy absorption of the crash box is calculated up to half of its length, following the standard given in the Ref. [13]. SEA, a measure of the energy absorption relative to the crash box's mass, was calculated through dividing the energy absorbed in crash box by its mass. Figure 7 displays a bar graph that provides a comparative analysis of SEA values across multiple crash box designs. This graph effectively visualizes the relative energy absorption capacities of these crash boxes, allowing for a meaningful comparison of their performance in this regard.

After conducting a comprehensive analysis of the specific energy absorption capabilities across a range of lattice crash boxes, it was unequivocally determined that the BCC (Body-Centered Cubic) crash box excelled in performance, as succinctly summarized in Table 4. Marginal performance difference between BCC and MFCC necessitated a closer examination of both their mechanical properties and production technologies. BCC structures often make more efficient use of material, resulting in a higher strength-to-weight ratio, which is crucial in applications where weight is a critical factor [29]. Additionally, BCC structures are generally easier and more cost-effective to produce using simple manufacturing processes due to their simpler geometry [30]. In contrast, MFCC structures require advanced manufacturing techniques, leading to higher production costs and longer fabrication times. Considering these factors, BCC structures offer significant advantages in terms of material efficiency, production cost, and mechanical performance. This pivotal decision marks the advancement of the research project into its subsequent phase, driven by the confidence in the selected design's robust performance in enhancing safety during high-impact scenarios.

In the third phase of the project, square crash box of dimension 75 mm and 80 mm is selected and filled with BCC unit cells, each characterized by different dimensions, specifically 5 mm, 10 mm, 15 mm, 20 mm, 25 mm, and 40 mm. These unit cells were meticulously modeled and systematically arranged in a linear fashion to collectively form a crash box with prescribed dimensions of 350 mm in length. Additionally, the strut diameters within these configurations are varied from 0.5 mm to 3 mm (0.5, 1, 1.5, 2, 2.5, and 3) to explore the impact of these alterations on crash box performance. Following the simulations, it was discovered that the 15 mm unit cell with a 1.5 mm strut diameter as shown in Fig. 8, stood out as the most promising configuration. This combination displayed exceptional performance in terms of Specific energy absorption, with an impressive value of 24.30 kJ/kg and has an energy absorption of 30.484 kJ.

A set of simulations were performed on a range of crash box designs, and the visuals after impact simulations are shown in Fig. 9.

Through multiple iterations and adjustments to the unit cell's dimensions and the diameter of the struts, it was determined that specific energy absorption could be enhanced in certain configurations. Notably, the following combinations yielded superior results in terms of energy absorption: a unit cell size of 10 mm with a 1 mm strut diameter, a unit cell size of 15 mm with a 1.5 mm strut diameter, a unit cell size of 20 mm with a 1.75 mm strut diameter, a unit cell size of 25 mm with a 2 mm strut diameter, and a unit cell size of 40 mm with a 3 mm strut diameter. These designs outperformed others in specific energy absorption.

Figure 10 displays a bar graph that provides a comparative analysis of SEA values across multiple crash box designs. This graph effectively visualizes the relative energy absorption capacities of these crash boxes, allowing for a meaningful comparison of their performance in this regard.

Table 5 presents the computed SEA for the most efficient lattice crash boxes. Among all the crash box setups, the linear patterned crash box, utilizing a 15 mm BCC unit cell and a 1.5 mm strut diameter, exhibits the highest SEA value in comparison to the rest of the crash box and have an energy absorption of 30.48 kJ.

The honeycomb lattice structure has gained renown for its exceptional energy absorption capabilities. Extensive compression tests have indicated their suitability for energy absorption applications. However, real-world crash scenarios have necessitated impact tests. Honeycomb structure has been modeled [2] as shown in the Fig. 11 and has been inserted into the square crash box and simulations having been carried out using different thickness values.

These impact tests have revealed that crash boxes integrated with honeycomb structure generate peak loads five times higher than crash boxes integrated with BCC lattice structures shown in Fig. 12(a). These elevated peak loads have raised concerns about potential neck injuries to passengers. Figure 12(b) also demonstrates that the crash box with BCC lattice yields better specific energy absorption compared the other two honeycomb based crash box. Thus, the simulation results, in fact, have cast doubt on the suitability of honeycomb crash boxes for crashworthiness applications.

This research encompassed the evaluation of various crash box designs to assess their energy absorption capabilities. Initially, a square crash box outperformed other options, recording the highest energy absorption at 23.42 kJ. Consequently, it was chosen as the preferred outer shell for further investigation. In the subsequent phase, different lattice structures were incorporated into the crash box design based on recommendations from existing literature. Among the options tested, the BCC crash box emerged as the leading performer in terms of specific energy absorption, achieving an impressive 23.975 kJ/kg. This outcome led to the selection of the BCC pattern as the inner structure for the intended application, advancing the research project to the next stage. In the third phase, optimization efforts were carried out on the crash box to maximize its specific energy absorption. This process involved fine-tuning the dimensions of the unit cells and adjusting the strut thickness within the lattice structure. It was revealed that specific energy absorption could be significantly improved through specific configurations. Notably, the most favorable combinations were identified as a 15-mm unit cell size with a 1.5 mm strut diameter, resulting in the highest specific energy absorption at 24.30 kJ/kg and a total energy absorption of 30.484 kJ.

As honeycomb structure is known for its better load bearing applications, an attempt has been made to compare its performance for crash box applications. Simulations has been carried out on crash box with honeycomb structure with different thickness values. Even though the peak load taken by the honeycomb based crash box is very high, the energy absorption behavior is not attractive for automotive applications in the perspective of customer safety.

When evaluating the performance of the standard square crash box against the optimized version, a notable enhancement of 30.16% in energy absorption capacity was evident. This significant improvement underscores the effectiveness of the optimization procedure and underscores the potential for increased crash safety in the designated application.

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.