## 1 Introduction  
The automotive industry is currently experiencing a pivotal shift to tackle essential issues in vehicle safety, energy conservation, and ecological sustainability affecting consumers and the global environment [1; 2; 3]. An effective approach to addressing the aforementioned issues involves reducing the weight of vehicles to decrease energy consumption. Research indicates that a 10% reduction in the overall vehicle equipment leads to a 6-8% decrease in energy consumption, resulting in a reduction of 1 g/km in carbon emission [1]. Therefore, substantial efforts are being made to develop vehicles that are both lightweight and crashworthy. To address the seemingly contradictory goals of lightweight design and enhanced crashworthiness, composite materials, especially carbon-fiber-reinforced plastic (CFRP), have garnered considerable attention in the scientific community [4; 5; 6; 7; 8]. [PERSON] et al. [4] designed a CFRP-based electric vehicle structure, achieving a 28% reduction in weight compared to glass-fiber-reinforced plastic (GFRP) while also significantly improving the crashworthiness of the vehicle body. Under the constraints of bending stiffness, [PERSON] et al. [9] designed, manufactured, and validated the feasibility of a CFRP roof panel. By adhering carbon fiber to the roof structure components of passenger vehicles, [PERSON] [10] has managed to increase the strength of the roof, effectively doubling its strength-toweight ratio. [PERSON] and [PERSON] [11; 12] have also conducted extensive research in a similar vein. However, the aforementioned studies primarily focus on the redesign of vehicle parts; research on complex vehicle subsystems, such as the door system, is less common. [PERSON] et al. [13] designed a CFRP side-door impact beam that, compared to high-strength steel, achieves a 50% weight reduction while ensuring collision safety. [PERSON] et al. [14] conducted a discrete optimization of the CFRP layup for the back door, achieving a more rational optimal solution compared to commercial software. [PERSON] and [PERSON] [15] employed fiber-reinforced thermoplastic in the lightweight conceptual design of a multi-material car door, successfully achieving a 20% reduction in weight. An important feature of CFRP is the ability to shape its microstructure during the manufacturing process by selecting the type of components (i.e., reinforcement and matrix), the number and orientation of layers [16; 17]. However, while the aforementioned studies have focused on the number and orientation of layers, they have not sufficiently addressed the potential for designing the microstructure. [PERSON] et al. [18] implemented a microscale homogenization and macroscale optimization method, achieving a 60% weight reduction in the lightweight design of the body-in-white floor. Through a synergistic approach combining multiscale numerical simulations with experimental testing, [PERSON] et al. [19] conducted detailed studies on the mechanical characteristics of single-ply woven composite materials. [PERSON] et al. [20] implemented a three-scale multiscale modeling approach, which was applied to a woven composite laminate under three-point bending and to cured woven bias extension tests. Although multiscale methods have been proven effective, current studies mainly focus on simple samples, with limited multiscale design applied to door systems. Additionally, most of these designs address only car body structure replacements and seldom consider the process constraints related to CFRP layup angles. This often results in optimization outcomes that fail to meet these constraints, necessitating further refinement for practical engineering applications. This paper proposes a material-structure integrated design and optimization approach for a CFRP car door design applied in real-world door development. The outline of the paper is as follows: Section 2 presents the analysis methods for CFRP car doors at the microscale and mesoscale and provides a detailed parameter analysis.