As the global automotive industry shifts toward electrification, the safety of electric vehicle (EV) battery packs has become a paramount concern. The placement of the EV battery pack beneath the vehicle floor, while optimizing center of gravity and passenger space, exposes it to hazards such as road debris, protruding obstacles, and impact during high-speed travel. Such events can induce mechanical damage, leading to thermal runaway, fires, or explosions. Therefore, developing robust bottom protective structures for the EV battery pack is critical to ensuring vehicle safety and reliability. In this study, I focus on the design and experimental evaluation of a lightweight protective structure aimed at enhancing the crashworthiness of the EV battery pack under static indentation loading.
Traditional protective solutions for EV battery packs often employ solid steel or aluminum plates. However, these materials suffer from high mass, low specific stiffness, and limited energy absorption efficiency. Sandwich structures, characterized by two thin face sheets separated by a lightweight core, offer superior specific strength and energy absorption capabilities, making them ideal for impact protection applications. Among various core geometries, corrugated cores provide excellent compressive strength and energy dissipation through plastic deformation and folding mechanisms. This research investigates a trapezoidal corrugated sandwich structure specifically designed for the bottom protection of an EV battery pack. The core and face sheets are connected using two distinct methods—adhesive bonding and riveting—to analyze their influence on the structural performance under quasi-static indentation.
The protective structure for the EV battery pack comprises three main components: an upper face sheet, a lower face sheet, and a trapezoidal corrugated core. To achieve an optimal balance between weight and performance, the face sheets are fabricated from 1.5 mm thick 6061 aluminum alloy, known for its good strength-to-weight ratio and corrosion resistance. The core is made from 1.5 mm thick Q235 carbon steel, selected for its high yield strength and ductility, which facilitate energy absorption through plastic deformation. The overall dimensions of the protective structure for the EV battery pack are 400 mm in width, 800 mm in length, and 14.3 mm in total thickness, tailored to fit standard EV battery pack geometries.
The trapezoidal corrugated core is manufactured via precision bending to form a periodic waveform profile. The geometric parameters of the corrugation, such as wavelength and amplitude, are designed to maximize compressive strength and energy absorption. The face sheets are laser-cut to ensure dimensional accuracy. Two connection methods are employed to assemble the sandwich structure for the EV battery pack: (1) adhesive bonding using a high-strength epoxy resin, which ensures full-area contact between the core and face sheets, and (2) mechanical riveting using steel rivets spaced uniformly across the interface, which introduces localized connections with inherent gaps. These methods are chosen to evaluate the effect of interface continuity on the protective performance of the EV battery pack structure.
The mechanical behavior of the trapezoidal corrugated sandwich structure for EV battery pack protection can be modeled using sandwich theory. The overall bending stiffness \( D \) of the sandwich structure is given by:
$$ D = \frac{E_f t_f h^2}{2(1-\nu_f^2)} + \frac{E_c t_c^3}{12(1-\nu_c^2)} $$
where \( E_f \) and \( E_c \) are the Young’s moduli of the face sheet and core materials, respectively; \( t_f \) and \( t_c \) are the thicknesses of the face sheet and core; \( h \) is the distance between the midplanes of the face sheets; and \( \nu_f \) and \( \nu_c \) are the Poisson’s ratios. For the indentation response, the local crushing strength of the core plays a dominant role. The mean crushing stress \( \sigma_m \) for a corrugated core under compression can be estimated as:
$$ \sigma_m = C \sigma_y \left( \frac{t_c}{l} \right)^{2/3} $$
where \( C \) is a constant dependent on core geometry, \( \sigma_y \) is the yield strength of the core material, and \( l \) is the characteristic length of the corrugation unit cell. These formulas highlight the importance of geometric optimization for enhancing the protective capacity of the EV battery pack structure.
To quantify the material properties and geometric parameters, Table 1 summarizes the key data used in this study for the EV battery pack protective structure.
| Component | Material | Thickness (mm) | Young’s Modulus (GPa) | Yield Strength (MPa) | Poisson’s Ratio |
|---|---|---|---|---|---|
| Upper Face Sheet | 6061 Aluminum Alloy | 1.5 | 68.9 | 276 | 0.33 |
| Lower Face Sheet | 6061 Aluminum Alloy | 1.5 | 68.9 | 276 | 0.33 |
| Core | Q235 Carbon Steel | 1.5 | 210 | 235 | 0.30 |
| Geometric Parameters | |||||
| Overall Dimensions | 400 mm × 800 mm × 14.3 mm | ||||
| Corrugation Wavelength | 50 mm | ||||
| Corrugation Amplitude | 10 mm | ||||
| Core Relative Density | Approx. 0.15 | ||||
The static indentation test is conducted to simulate a grounding event where the EV battery pack impacts a protruding object. The test protocol follows standardized procedures for EV battery pack safety assessment. A spherical indenter with a diameter of 150 mm is used to represent a typical road obstacle. The indentation speed is set to 0.25 mm/s to ensure quasi-static conditions, minimizing dynamic effects. The test is performed on an electronic universal testing machine, with the specimen securely fixed in a box-shaped fixture to simulate constrained boundary conditions akin to those in an actual EV battery pack installation. The fixture incorporates linear displacement sensors to measure the deflection of the lower face sheet, which directly indicates the intrusion threat to the EV battery pack.
The loading criterion for the EV battery pack protective structure is based on industry standards: the indentation process continues until the applied force reaches 25 kN, which corresponds to 110% of the full vehicle load, or until structural failure occurs. This force threshold ensures that the EV battery pack can withstand severe grounding scenarios without compromising integrity. During the test, force-displacement data are recorded at a high sampling rate to capture the detailed mechanical response of the protective structure for the EV battery pack.
The experimental results for both adhesive-bonded and riveted trapezoidal corrugated sandwich structures are analyzed. The force-displacement curves, shown in Figure 1 (represented conceptually), reveal distinct behaviors. For the adhesive-bonded EV battery pack structure, the curve exhibits an initial linear elastic region, followed by a plateau as the core undergoes plastic crushing, and finally a sharp drop indicating adhesive failure at the interface. In contrast, the riveted EV battery pack structure shows a more gradual load increase with multiple small fluctuations due to localized buckling and rivet engagement. The key performance metrics are extracted and summarized in Table 2.
| Connection Method | Peak Force at 25 kN (kN) | Displacement at 25 kN (mm) | Energy Absorbed to 25 kN (J) | Lower Face Sheet Max Deformation (mm) | Specific Energy Absorption (J/kg) |
|---|---|---|---|---|---|
| Adhesive Bonding | 25.0 | 10.97 | 274.3 | 10.97 | 12.5 |
| Riveting | 25.0 | 19.40 | 485.0 | 19.40 | 10.8 |
The data clearly demonstrate that the adhesive-bonded structure for the EV battery pack outperforms the riveted one in terms of protective efficiency. At the required force of 25 kN, the maximum deformation of the lower face sheet is 10.97 mm for adhesive bonding, compared to 19.40 mm for riveting. This represents a 43.45% improvement in resistance to intrusion for the EV battery pack when using adhesive bonding. The energy absorbed up to 25 kN is higher for the riveted structure (485.0 J versus 274.3 J), but this is primarily due to the larger displacement; the specific energy absorption (energy per unit mass) is actually lower for riveting (10.8 J/kg) than for adhesive bonding (12.5 J/kg), indicating that the bonded structure is more efficient in utilizing material for protecting the EV battery pack.
The deformation mechanisms observed post-test provide further insight. For the adhesive-bonded EV battery pack structure, the upper face sheet exhibits a large plastic dent at the indentation site, while the lower face sheet shows a smooth bulge without rupture. The core experiences progressive crushing, and adhesive debonding occurs at the interface between the core and face sheets, leading to the sudden load drop in the force-displacement curve. This debonding is governed by the shear strength of the adhesive, which can be expressed as:
$$ \tau_b = \frac{F_b}{A_b} $$
where \( \tau_b \) is the adhesive shear strength, \( F_b \) is the debonding force, and \( A_b \) is the bonded area. For the riveted EV battery pack structure, the upper face sheet also dents plastically, but the lower face sheet deformation is more diffuse with multiple small wrinkles. The rivets remain intact without fracture, but the gaps between the core and face sheets allow for local buckling, resulting in a less stiff response. The load transfer in riveted joints is concentrated at the rivet points, leading to stress concentrations that reduce overall efficiency for the EV battery pack protection.
To quantify the structural performance, the effective stiffness \( K \) of the EV battery pack protective structure during indentation can be calculated from the initial linear portion of the force-displacement curve:
$$ K = \frac{\Delta F}{\Delta \delta} $$
where \( \Delta F \) is the force increment and \( \Delta \delta \) is the corresponding displacement increment. For the adhesive-bonded case, \( K \) is approximately 3.45 kN/mm, while for the riveted case, it is about 1.85 kN/mm. This significant difference underscores the superiority of adhesive bonding in maintaining structural integrity for the EV battery pack under impact.
The protective performance of the EV battery pack structure can also be evaluated using the intrusion index \( I \), defined as the ratio of lower face sheet deformation to the initial core height:
$$ I = \frac{\delta_{lower}}{h_c} $$
where \( \delta_{lower} \) is the maximum deformation of the lower face sheet and \( h_c \) is the core height. For the adhesive-bonded structure, \( I = 0.77 \), whereas for the riveted structure, \( I = 1.36 \). A lower intrusion index indicates better protection for the EV battery pack, as it implies less deformation into the battery compartment.
The advantages of adhesive bonding for EV battery pack protection stem from several factors. First, the continuous interface provided by the adhesive ensures uniform stress distribution across the entire contact area, minimizing stress concentrations and delaying failure. Second, the adhesive layer adds damping, which can dissipate energy through viscoelastic effects. Third, the bonded interface prevents relative sliding between the core and face sheets, enhancing composite action. In contrast, riveted connections introduce discontinuities that act as initiation sites for local buckling, reducing the effective bending stiffness and compromising the EV battery pack safety.
However, adhesive bonding also has limitations, such as susceptibility to environmental degradation (e.g., moisture, temperature cycles) and potential for brittle failure. To address this, future designs for EV battery pack protective structures could explore hybrid joining methods, such as adhesive bonding combined with spot welding or optimized rivet patterns. Additionally, the core geometry can be further optimized using computational tools like finite element analysis (FEA) to maximize energy absorption for the EV battery pack. For instance, the crushing response of the trapezoidal corrugated core can be modeled using the following empirical relation for mean crushing force \( P_m \):
$$ P_m = A \sigma_y t_c^{5/3} b^{1/3} $$
where \( A \) is a constant dependent on corrugation angle, \( \sigma_y \) is the core material yield strength, \( t_c \) is core thickness, and \( b \) is the width of the crushing zone. Such models aid in tailoring the design for specific EV battery pack requirements.
In terms of manufacturing considerations for EV battery pack protection, adhesive bonding requires precise surface preparation and curing control, which may increase production complexity compared to riveting. Nevertheless, the performance benefits justify the additional effort, especially for high-safety applications like EV battery packs. Table 3 compares the two connection methods across multiple criteria relevant to EV battery pack protective structures.
| Criterion | Adhesive Bonding | Riveting |
|---|---|---|
| Protective Performance (Lower Deformation) | High (10.97 mm at 25 kN) | Moderate (19.40 mm at 25 kN) |
| Specific Energy Absorption | 12.5 J/kg | 10.8 J/kg |
| Structural Stiffness | 3.45 kN/mm | 1.85 kN/mm |
| Interface Continuity | Full-area contact | Discrete points with gaps |
| Stress Distribution | Uniform | Concentrated at rivets |
| Manufacturing Complexity | Moderate (surface prep, curing) | Low (mechanical fastening) |
| Environmental Durability | Potential degradation | Generally robust |
| Weight Impact | Negligible (adhesive mass low) | Low (rivet mass small) |
| Suitability for EV Battery Pack | Highly suitable | Less suitable |
The experimental findings have direct implications for the design of EV battery pack systems. By implementing adhesive-bonded trapezoidal corrugated sandwich structures, vehicle manufacturers can achieve significant improvements in bottom protection without substantial weight penalties. This is crucial for EVs, where battery pack mass directly affects driving range and performance. Moreover, the enhanced crashworthiness reduces the risk of thermal incidents, addressing a key safety concern for EV battery packs.
To generalize the results, the performance metrics can be normalized for different EV battery pack sizes. The normalized intrusion resistance \( R_n \) is defined as:
$$ R_n = \frac{F_{req}}{\delta_{lower} \cdot A_{pack}} $$
where \( F_{req} \) is the required force (e.g., 25 kN), \( \delta_{lower} \) is the lower face sheet deformation, and \( A_{pack} \) is the planform area of the EV battery pack. For the adhesive-bonded structure, \( R_n \) is approximately 7.12 kPa/mm, while for the riveted structure, it is 4.03 kPa/mm. This normalized measure facilitates comparison across different EV battery pack designs.
In conclusion, this study demonstrates that trapezoidal corrugated sandwich structures offer excellent protective capabilities for EV battery pack bottom protection. Adhesive bonding significantly outperforms riveting in terms of reducing intrusion deformation, with a 43.45% improvement at the standard load of 25 kN. The continuous interface provided by adhesive bonding ensures better stress distribution and higher structural stiffness, which are critical for safeguarding the EV battery pack against grounding impacts. Future work should focus on optimizing core geometries, exploring advanced adhesives with improved durability, and validating performance under dynamic impact conditions representative of real-world scenarios for EV battery packs. The insights gained contribute to the development of safer, lighter, and more efficient protective solutions for the next generation of electric vehicles.