In the modern automotive industry, lightweight design is a critical technological trend, particularly for electric vehicles (EVs) where reducing mass directly enhances energy efficiency, range, and performance. As an integral component, the EV battery pack often constitutes a significant portion of the vehicle’s weight, and optimizing its structure through advanced materials is paramount. We explore the use of pultrusion composite materials—specifically, polyurethane resin reinforced with 60% glass fiber—as a superior alternative to traditional metals like aluminum alloys in applications such as the expansion beam within an EV battery pack. This article delves into the material properties, manufacturing processes, performance testing, and simulation-based analysis, emphasizing the advantages for EV battery pack designs. Through detailed comparisons and technical evaluations, we aim to provide a comprehensive reference for adopting pultruded composites in automotive lightweighting initiatives.
The push for lightweighting in EVs stems from the direct correlation between vehicle mass and energy consumption. Studies indicate that a 10% reduction in mass can lead to a 5–7% decrease in energy usage, thereby extending the driving range of an EV battery pack. For instance, aluminum alloys have been widely adopted, offering weight savings of 20–30% compared to steel, but composites like glass fiber-reinforced polymers (GFRP) can achieve over 50% weight reduction while maintaining high strength and stiffness. In the context of an EV battery pack, which requires robust structural integrity to protect battery cells from mechanical stresses and thermal expansion, materials must balance lightweight characteristics with durability. Pultrusion composites, produced via a continuous manufacturing process, present an ideal solution due to their high specific strength and design flexibility. We focus on their application in the expansion beam, a component that mitigates internal forces within the EV battery pack, ensuring safety and longevity.
Pultrusion composite materials are typically composed of a thermosetting polymer matrix, such as polyurethane resin, and reinforcing fibers, like glass fibers, which constitute 60% by weight in our case. The manufacturing process involves pulling continuous fiber strands through a resin bath for impregnation, followed by a heated die for curing and shaping, resulting in profiles with constant cross-sections. This method yields high fiber alignment and volume fractions, enhancing mechanical properties. The key advantages include: high production efficiency (0.5–2.0 m/min), excellent dimensional stability, and the ability to tailor properties by adjusting fiber orientation and content. For an EV battery pack, these attributes translate to components that are not only lighter but also resistant to corrosion and fatigue, crucial for the harsh operating environments of electric vehicles.
To quantify the material performance, we conducted extensive testing on pultrusion composites with polyurethane resin and 60% glass fiber. The mechanical properties are summarized in Table 1, based on standard test methods. These values are critical for designing reliable components for an EV battery pack, as they dictate how the material will respond to static and dynamic loads.
| Test Item | Value | Standard |
|---|---|---|
| Tensile Strength | 1150.94 MPa | GB/T 3354-2014 |
| Tensile Modulus | 45.11 GPa | GB/T 3354-2014 |
| Elongation at Break | 2.49% | GB/T 3354-2014 |
| Flexural Strength | 1211.68 MPa | GB/T 3356-2014 |
| Flexural Modulus | 45.21 GPa | GB/T 3356-2014 |
| Shear Strength | 68.02 MPa | JC/T 773-2010 |
| Fiber Volume Content | 67.09% | GB/T 2577-2005 |
| Resin Content | 18.92% | GB/T 2577-2005 |
| Density | 2.14 g/cm³ | GB/T 1463-2005 |
The high tensile and flexural strengths, coupled with a low density, underscore the suitability for lightweight structures in an EV battery pack. For instance, the specific strength (strength-to-density ratio) can be calculated as: $$\text{Specific Strength} = \frac{\sigma_t}{\rho}$$ where $\sigma_t$ is the tensile strength and $\rho$ is the density. For our composite: $$\text{Specific Strength} = \frac{1150.94 \, \text{MPa}}{2.14 \, \text{g/cm}^3} \approx 537.82 \, \text{MPa·cm}^3/\text{g}$$ This value significantly exceeds that of typical aluminum alloys (e.g., around 100 MPa·cm³/g for cast aluminum), highlighting the weight-saving potential. Additionally, the fiber volume fraction, $V_f$, is crucial for composite performance, given by: $$V_f = \frac{\text{Volume of fibers}}{\text{Total volume}} \times 100\%$$ With $V_f = 67.09\%$, the composite achieves optimal load-bearing capacity, essential for an EV battery pack subjected to vibrations and impacts.
Beyond mechanical properties, electrical insulation is vital for components in an EV battery pack to prevent short circuits and ensure safety. We performed insulation resistance and withstanding voltage tests, as detailed in Tables 2 and 3. The results confirm that pultrusion composites exhibit excellent dielectric properties, making them safe for use near high-voltage battery cells.
| Test Item | Result | Criteria |
|---|---|---|
| Withstanding Voltage (2800 VAC, 60 s) | No breakdown, leakage current ≤ 0.00 mA | Pass (≤ 1 mA) |
| Insulation Resistance (1000 VDC, 60 s) | 6.43 × 10¹³ Ω | Pass (≥ 500 MΩ) |
Furthermore, fire resistance was evaluated using a flame test at 1200°C for 30 minutes, simulating thermal runaway scenarios in an EV battery pack. Post-test, the material maintained insulation integrity with leakage current below 2 mA under 1000 VDC, demonstrating robustness under extreme conditions. This aligns with the safety requirements for an EV battery pack, where materials must withstand thermal events without compromising electrical isolation.
For application in an EV battery pack expansion beam, we conducted finite element analysis (FEA) simulations to compare pultruded composite joints with traditional aluminum joints. The expansion beam is designed to absorb internal pressures from battery cell swelling, a common issue in lithium-ion batteries. Under a static uniformly distributed load of 30,000 N, simulating operational forces, the composite beam showed no failure, whereas the aluminum beam, while also intact, exhibited higher stress concentrations. The stress in a material under load is given by: $$\sigma = \frac{F}{A}$$ where $F$ is the force and $A$ is the cross-sectional area. For the aluminum joint, the maximum stress was computed as 130 MPa, while for the composite joint, the foam core stress was only 0.1 MPa, and the laminate skin had a maximum failure factor of 0.011 based on Tsai-Hill criterion: $$f = \left( \frac{\sigma_1}{S_1} \right)^2 + \left( \frac{\sigma_2}{S_2} \right)^2 – \frac{\sigma_1 \sigma_2}{S_1^2} + \left( \frac{\tau_{12}}{S_{12}} \right)^2$$ where $\sigma_1, \sigma_2$ are principal stresses, $\tau_{12}$ is shear stress, and $S_1, S_2, S_{12}$ are strengths. A value below 1 indicates no failure, so 0.011 signifies high safety margins.
The safety factor (SF) for each joint was calculated as: $$SF = \frac{\sigma_{\text{ultimate}}}{\sigma_{\text{working}}}$$ For aluminum with ultimate tensile strength of 330 MPa: $$SF_{\text{aluminum}} = \frac{330 \, \text{MPa}}{130 \, \text{MPa}} \approx 2.54$$ This corresponds to a maximum load-bearing capacity of approximately 9,144 N, derived from rearranging the stress formula. For the composite joint, using the foam tensile strength of 0.5 MPa: $$SF_{\text{composite}} = \frac{0.5 \, \text{MPa}}{0.1 \, \text{MPa}} = 5.0$$ yielding a capacity around 18,000 N. Thus, the composite joint offers nearly double the strength under tensile and compressive loads, enhancing the reliability of the EV battery pack. These simulations underscore the superiority of pultruded composites in critical applications within an EV battery pack, where weight reduction and high strength are paramount.
To further illustrate the benefits, consider the weight savings. The density of aluminum alloys is about 2.7 g/cm³, compared to 2.14 g/cm³ for our composite. For a typical expansion beam in an EV battery pack with volume $V$, the mass reduction $\Delta m$ can be estimated: $$\Delta m = (\rho_{\text{Al}} – \rho_{\text{composite}}) \times V$$ Assuming $V = 1000 \, \text{cm}^3$: $$\Delta m = (2.7 – 2.14) \, \text{g/cm}^3 \times 1000 \, \text{cm}^3 = 560 \, \text{g}$$ This translates to a weight reduction of approximately 20.7%, aligning with the 25–30% range mentioned in literature. For an EV battery pack, such savings accumulate, improving overall vehicle efficiency.
The design of the expansion beam joint also plays a crucial role. The composite joint features a sandwich structure with 4 mm thick glass fiber laminate skins and a foam core, bonded to end faces, whereas the aluminum joint uses eight bolts for fixation. The stress distribution is more uniform in the composite due to its anisotropic nature, which can be tailored via fiber orientation. We modeled this using classical lamination theory, where the stress-strain relationship for a lamina is: $$\begin{bmatrix} \sigma_1 \\ \sigma_2 \\ \tau_{12} \end{bmatrix} = \begin{bmatrix} Q_{11} & Q_{12} & 0 \\ Q_{12} & Q_{22} & 0 \\ 0 & 0 & Q_{66} \end{bmatrix} \begin{bmatrix} \epsilon_1 \\ \epsilon_2 \\ \gamma_{12} \end{bmatrix}$$ Here, $Q_{ij}$ are stiffness coefficients dependent on material properties and fiber angle. By optimizing these angles, we can minimize stress concentrations in the EV battery pack expansion beam, enhancing durability.
In addition to static analysis, dynamic performance is vital for an EV battery pack subjected to road vibrations. The natural frequency $f_n$ of a beam can be approximated as: $$f_n = \frac{1}{2\pi} \sqrt{\frac{k}{m}}$$ where $k$ is stiffness and $m$ is mass. For the composite beam, higher specific stiffness (modulus-to-density ratio) leads to increased $f_n$, reducing resonance risks. The modulus $E$ for our composite is 45.11 GPa, so specific stiffness is: $$\frac{E}{\rho} = \frac{45.11 \, \text{GPa}}{2.14 \, \text{g/cm}^3} \approx 21.08 \, \text{GPa·cm}^3/\text{g}$$ compared to about 26 GPa·cm³/g for aluminum (e.g., $E = 70 \, \text{GPa}, \rho = 2.7 \, \text{g/cm}^3$). While aluminum has a higher absolute value, the composite’s lower mass compensates, resulting in comparable or better dynamic response for the EV battery pack.
We also evaluated long-term performance through fatigue testing. For composites, the S-N curve describes fatigue life, often modeled as: $$\sigma_a = \sigma_f’ (2N_f)^b$$ where $\sigma_a$ is stress amplitude, $N_f$ is cycles to failure, $\sigma_f’$ is fatigue strength coefficient, and $b$ is exponent. Preliminary data suggest pultruded composites exhibit excellent fatigue resistance, crucial for an EV battery pack enduring cyclic loads from charging-discharging cycles and vehicle motion. This extends the service life of the expansion beam, reducing maintenance needs.
From a manufacturing perspective, the pultrusion process offers scalability for mass-producing components for EV battery packs. The line speed $v$ can be optimized based on cure kinetics, described by: $$\frac{d\alpha}{dt} = k(1-\alpha)^n$$ where $\alpha$ is degree of cure, $k$ is rate constant, and $n$ is order. By controlling temperature and pull speed, we achieve consistent quality, essential for standardizing parts across multiple EV battery pack models. Moreover, the process minimizes waste, as fibers are used continuously, aligning with sustainability goals in electric vehicle production.
In conclusion, pultrusion composite materials, with their high strength-to-weight ratio, excellent electrical insulation, and design flexibility, present a compelling alternative to metals for components like the expansion beam in an EV battery pack. Our analysis demonstrates that composite joints outperform aluminum joints in terms of strength and safety factors, while offering significant weight reductions. This contributes to enhanced energy efficiency and performance of electric vehicles. Future work should focus on optimizing fiber architectures for multi-axial loading in an EV battery pack and integrating smart materials for real-time health monitoring. As the EV industry evolves, pultruded composites are poised to play a pivotal role in advancing lightweight, durable, and safe battery pack designs, ultimately supporting the global transition to sustainable transportation.
To summarize key findings, we present a comparative table of aluminum and composite properties relevant to an EV battery pack expansion beam:
| Property | Aluminum Alloy (Cast) | Pultrusion Composite (PU + 60% GF) |
|---|---|---|
| Density (g/cm³) | 2.7 | 2.14 |
| Tensile Strength (MPa) | 330 | 1150.94 |
| Specific Strength (MPa·cm³/g) | ≈122 | ≈537.82 |
| Safety Factor in Joint | 2.54 | 5.0 |
| Weight Reduction Potential | Baseline | 20–30% |
| Insulation Resistance (Ω) | Conductive | 6.43 × 10¹³ |
| Fire Resistance | Melts at high temperature | Maintains integrity after flame test |
These attributes underscore the transformative potential of pultruded composites in revolutionizing EV battery pack architectures. By embracing such materials, manufacturers can achieve lighter, stronger, and safer vehicles, paving the way for next-generation electric mobility.